energies Article
Vibration Characteristics of Compression Ignition Engines Fueled with Blended Petro-Diesel and Fischer-Tropsch Diesel Fuel from Coal Fuels Tiantian Yang
ID
, Tie Wang *, Guoxing Li, Jinhong Shi and Xiuquan Sun
Department of Vehicle Engineering, Taiyuan University of Technology, Taiyuan 030024, China;
[email protected] (T.Y.);
[email protected] (G.L.);
[email protected] (J.S.);
[email protected] (X.S.) * Correspondence:
[email protected]; Tel.: +86-0351-6010580 Received: 4 July 2018; Accepted: 30 July 2018; Published: 7 August 2018
Abstract: Fischer-Tropsch diesel fuel synthesized from coal (CFT) is an alternative fuel that gives excellent emission performance in compression ignition (CI) engines. In order to study the vibration characteristics, which are important for determining the applicability of the fuel, CFT-diesel blends were tested on a CI engine to acquire vibration signals from the engine head and block. Based on the FFT and continuous wavelet transformation (CWT) analysis, the influence of CFT on the vibration was studied. The results showed that the root mean square (RMS) values of the vibration signal decrease as the proportion of CFT in the blends increases. The CWT results indicated that the vibration energy areas motivated by the pressure shock of transient combustion were weak with increasing CFT proportion for the different frequency bands. The blend of 90% pure petro-diesel and 10% CFT registered the largest RMS value for piston side thrust response, and the RMS of high-frequency pressure oscillation response is greater than that of the main response of combustion, for FT30. Therefore, CFT has the potential to reduce the combustion vibration of the engine at all frequency bands, and there is a possibility that the proportion of blended fuel can be modified to satisfy the vibration characteristics requirements in different frequency bands. Keywords: Fischer-Tropsch diesel fuel synthesized from coal (CFT); vibration characteristic; compression ignition (CI) engine; T-F analysis
1. Introduction Diesel engines are widely used in transportation, industry, agriculture, and construction owing to their high efficiency, reliability and durability. However, compared with gasoline engines, a compression ignition (CI) engine emits more pollutants, especially nitrogen oxides (NOx ) and particulate matter (PM) [1]. The trade-off relationship that exists between the two main pollutions makes traditional emission-reduction techniques inefficient. The application of alternative fuels is one of the effective ways to address this requirement, while helping to alleviate the shortage of fossil fuels. Among the alternative fuels, Fischer-Tropsch (F-T) fuel is a promising option for CI engines, and is produced to liquid by F-T synthesis from a wide range of natural materials such as coal, natural gas, and biomass. Some researches indicate that F-T fuel has a higher mass heating value and cetane number (CN) than commercial diesel fuel [2,3], which are important properties for fuel used in CI engine. The F-T fuel synthesis from coal is called CFT, and the F-T fuel synthesis from natural gas is known as GTL (Gas to Liquid), which both have been studied extensively with regard to combustion [4,5], emissions, and power output. Lapuerta et al. [6] investigated the emissions of GTL fuel derived from a low temperature F-T process, and found that GTL has the potential to reduce the PM emissions. The research of Torregrosa et al. [7] reached a similar conclusion. Li et al. [8] found
Energies 2018, 11, 2043; doi:10.3390/en11082043
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that GTL significantly reduced regulated emissions, with average reductions of 21.2% in CO, 15.7% in HC, 15.6% in NOx , and 22.1% in smoke compared to diesel, as well as average reductions of 85.3% and 43.9% in the unregulated emission of total ultrafine particles number and mass, respectively. Hao et al. [9] investigated the unregulated emissions from CFT, for instance, carbonyl emissions, and concluded that CFT could reduce the level of the total and individual carbonyl compounds. The marked reduction in pollution suggested that F-T fuel is more environmentally friendly than diesel fuel. In addition, GTL exhibited almost the same power output and torque output as diesel, while improving fuel economy [10]. Based on the above research results, it can be revealed that there are a similar combustion and emission characteristics between CFT and GTL, but from the perspectives of resource abundance, environmental and economic constraints, there are some differences between CFT and GTL [11]. With the continued development of the F-T process, CFT can be designed to meet or even exceed the required specifications of fossil diesel [12,13]. At present, CFT is commercially produced in countries where coal reserves are abundant, such as South Africa and China [3,14,15]. CFT has been selected as the alternative fuel used in CI engine for this study, considering China’s energy reserves are coal rich, with less gas. So far, power performance, economy, and emission characteristics are the main optimization targets of alternative fuel design for CI engines. The combustion-induced vibration response can significantly affect the fatigue life and maintenance cost of engine structures. If fuel design of alternative blended fuels could improve the combustion characteristic and its responding vibration response of CI engines, it may also be of benefit in improvement of service life and operational reliability of CI engines. However, the study on the difference of vibration and noise caused by the combustion behavior of different alternative fuels has not been sufficiently deep and extensive concerned. The mechanical vibration signal of a CI engine is the result of multiple excitations arising from factors such as gas combustion pressure shock, piston side thrust force excitation, valve seating shock, and the inertia force in the crank-connecting rod mechanism. These excitations have cyclical and non-stationary time-varying characteristics. The study and analysis of the vibration signal are helpful for the fault diagnosis [16–18], knock detection [19,20], identification of combustion characteristics [21–25], and improving the noise vibration and harshness (NVH) performance of the whole engine. The two main excitation source of engine vibration are the pressure shock caused by rapid combustion in the chamber and impact of piston assembly driven by the fuel combustion. Therefore, the change in the combustion state of the air-fuel mixture has great influence on the engine vibration signal. The vibration states of engine fueled with diesel fuel and biodiesel are markedly different due to the differences in the rise rate and oscillation of in-cylinder pressure [26–28]. Li et al. [29] found that the advanced ignition of biodiesel is the main cause of the nonlinear increase in root mean square (RMS) of vibration response near the combustion top dead center (TDC).The study by Taghizadeh-Alisaraei et al. concluded that the RMS and kurtosis values of engine body vibration increase when ethanol is added to diesel [30]. Meanwhile, the RMS value of dual-fuel engine vibration is lower than that of diesel engine, and the frequency component distribution of dual-fuel engine vibration is narrow [31]. Different alternative fuels have different combustion behaviors due to their diverse physical and chemical properties, which gives the engine different vibration behaviors. Therefore, for the evaluation of alternative fuels, in addition to the combustion and emission characteristics, a certain understanding of the corresponding vibration characteristics, including typical amplitudes and frequency distributions, must be developed. The diffusive combustion and multi-point ignition mode of CI engine necessitates high compression ratios, resulting in vibration and noise that is greater than that of spark ignition engines. Hence, the vibration performance of CI engine fueled with alternative fuels should be given special attention. Especially for light-duty high-speed diesel engines used in passenger vehicles, vibration and noise reduction are very important for the comfort experience of passengers. At present, there are a few studies about CFT as the fuel of CI engine, and most of those few studies focus on the performance, combustion, and emission performance of CFT,
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while the vibration characteristics of CFT have been largely unstudied. In this study, based on the combustion characteristics of a CI engine fuelled by CFT, the influence of CFT and CFT-diesel blends on the vibration performance is investigated, using acceleration signal analysis of the engine head and block. 2. Materials and Methods 2.1. Experimental Setup and Data Acquisition All tests were performed on a direct-injection light-duty diesel engine whose specifications are given in Table 1. The engine was coupled with an electric eddy current dynamometer (DW160, Sichuan, Chengdu, China), and engine operation was controlled using a measurement and control system made by Sichuan Chengbang (Chengdu, China). An in-cylinder gas pressure sensor is mounted on the fourth cylinder to detect the combustion pressure. In this study, the research object is the engine vibration, which is the main source of vibrations for entire vehicle. The main moving parts of the engine are the piston assembly, connecting rods, and crankshaft. Vibration in a reciprocating piston engine is mainly caused by the changes in gas pressure in the cylinder and the alternating inertia force on the different parts. Two accelerometers are used to obtain the vibration signal of the engine. One accelerometer is installed on the engine head to measure the vibration along the direction of piston motion, and another one is installed on the engine block between the second and the third cylinders in the vertical piston movement direction, as shown in Figure 1. The TDC pulse signal is measured by the Hall sensor and the crankshaft rotation angle was recorded accordingly. The Hall sensor is connected to the front of the crankshaft. Vibration signals and TDC pulse signal are recorded by signal acquisition instrument, and transmitted to the computer through the universal serial bus (USB) port. The schematic diagram of the test engine is shown in Figure 1. Table 1. Main technical specifications of diesel engine. Item/Parameter
Details
Engine type Bore × stroke Displacement Compression ratio Intake valve opening Intake valve closing Exhaust valve opening Exhaust valve closing Rated power Rated torque Firing order
Horizontal, four-stroke, water-cooled 100 mm × 105 mm 3.3 L 17.5 24 ◦ CA 1 before TDC 48 ◦ CA after BDC 2 65.3 ◦ CA before BDC 29.3 ◦ CA after TDC 70 kW @3200 rpm 240 Nm @2000~2200 rpm 1-3-4-2-1
1
CA indicates crank angle; 2 BDC indicates bottom dead center.
Data sampling begins after a steady operation condition has been maintained for more than one minute. The data acquisition time was set as 30 s in order to obtain enough vibration data for multiple engine operation cycles and the sampling frequency was 96 kHz. The data acquisition started from the fourth cylinder TDC during the intake stroke. During tests, all the operating parameters of the diesel engine should be consistent, including the fuel supply advance angle, supply pressure of the injector and phase of intake and outlet. CFT fuel can be blended with commercial petrodiesel in any ratio and fueled in engine without modifying the engine because its properties are similar to those of petrodiesel. In this study, CFT fuel and petro-diesel fuel were blended by volume ratio. Four fuel samples with the following nomenclature and compositions were tested: D100 (100% petro-diesel), FT10 (90% pure petrodiesel and 10% CFT), FT30 (70% pure petrodiesel and 30% CFT), and FT100 (100% CFT). The fuels were
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blended and homogenized using ultrasonic stirring. The experimental study was carried out for engine operation conditions with different loads (10, 50, 100, 150 and 200 Nm) and different speeds (1200, 1600, Energies 2000, and 2400 2018, 11, x rpm). 4 of 15 Accelerometer
Cylinder Pressure Sensor
Dynamometer
Signal acquisition system
Engine
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Measurement and control system
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Figure 1. Schematic usedininthe the experiments. Figure 1. Schematicdiagram diagramof of the the setup setup used experiments. 2.2.Fuels Test Fuels 2.2. Test Fuel quality directly affects the ignition, combustion and emission performance of the engine. Fuel quality directly affects the ignition, combustion and emission performance of the engine. The The fuel quality parameters that affect the combustion and emission characteristics of diesel engine fuel quality parameters that affect the combustion and emission characteristics of diesel engine include include sulfur content, CN, density, and distillation temperature. Table 2 shows the comparison of a sulfurfew content, CN, density, and distillation temperature. Table 2 shows the comparison of a few of the of the relevant physical and chemical properties of CFT and petro-diesel based on the ASTM relevant physical and chemical properties of CFT and petro-diesel based on the ASTM standard. standard.
Table 2. 2. Comparison specifications. Table Comparison of of fuels fuels specifications. Properties Properties
CFT CFTFuel Fuel
Density3 (kg/m ) Density (kg/m ) CN CN Low heating value (MJ/kg) Low heating value (MJ/kg) Kinematic viscosity at 30 °C (mm2/s) Kinematic viscosity at 30 ◦ C (mm2 /s) Flashpoint (°C) Flashpoint (◦ C) Initial boiling◦point (°C) Initial boiling point ( C) Final boiling point (°C) Final boiling point (◦ C) 3
759.9 759.9 78.4 78.4 44.2 44.2 2.029 2.029 52 52 157.6 157.6 306.1 306.1
Petrodiesel Petrodiesel 831.2 831.2 55.8 55.8 42.6 42.6 2.3–6 2.3–6 152 152 180 180 370 370
Fuel physical and chemical properties are closely related to its density, which can influence the
Fuel and pressure chemical to its density, can influence time physical at which high is properties produced inare theclosely fuel. Asrelated can be seen from Table which 2, the density of CFT the 3. Under the same plunger stroke used in this test pressure is lower than that of petrodiesel, which 759.9 time at which high is produced in the fuel. Asiscan bekg/m seen from Table 2, the density of CFT 3 . Under and fueltest supply advance thepetrodiesel, higher the density, earlierkg/m the actual fuel injection (except used in this is lower thanangle, that of which the is 759.9 the sametime plunger stroke for high pressure common rail system). The CN value is related to the ignition performance. For a and fuel supply advance angle, the higher the density, the earlier the actual fuel injection time (except fuel with a higher CN, it is easier to be ignited. The more stable the working of engine, and the less for high pressure common rail system). The CN value is related to the ignition performance. For a prone it is to knocking. In comparison to petro-diesel, the CN of CFT is higher (78), indicating that fuel with a higher CN, it is easier to be ignited. The more stable the working of engine, and the less CFT has excellent compression combustion capability. The heating value is usually expressed in proneterms it is to In comparison petro-diesel, of CFT is indicating of knocking. energy per unit mass of fuel.to According to thethe dataCN calculation in higher Table 2,(78), the low heating that CFT has excellent compression combustion capability. The heating value is usually expressed inisterms value (LHV) of CFT is 4% higher than that of petrodiesel of the same quality. The LHV by volume of energy unit mass of ofLHV fuel.by According to the data calculation in Table 2, the equalper to the product mass and density, and hence, the energy released bylow CFT heating is 5% lessvalue (LHV)than of CFT is 4%volume higherofthan that of petrodiesel of the same quality. The LHV analysis by volume is equal to the same petrodiesel, which is a small difference. For performance of engine, the LHV by volume is more significant, and hence, all the LHV mentioned below refers to the the product of LHV by mass and density, and hence, the energy released by CFT is 5% lessLHV than the volume. same by volume of petrodiesel, which is a small difference. For performance analysis of engine, the LHV by volume is more significant, and hence, all the LHV mentioned below refers to the LHV by volume. 3. Results and Discussion
3. Results and Discussion
3.1. Analysis of Combustion Pressure
3.1. Analysis of Combustion Figure 2a shows thePressure in-cylinder combustion pressure curve of the engine at a speed of 2000 rpm and torque of 100 Nm. The peak value of pressure decreases gradually with increasing CFT
Figure 2a shows the in-cylinder combustion pressure curve of the engine at a speed of 2000 rpm proportion in the blended fuels. The peak pressure value increases with the growth of speed and load, and torque 100 Nm. Themore peakobvious value of pressure decreases gradually withinincreasing CFT proportion and theoftrend becomes with the increase in load, as illustrated Figure 2b. When using in theblended blended fuels. The peak pressure value increases with the growth of speed and and the fuels, there is an observable decrease in the peak value of combustion pressureload, with the trend becomes more obvious with the increase in load, as illustrated in Figure 2b. When using blended
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x 5 of 15in the fuels,Energies there 2018, is an11,observable decrease in the peak value of combustion pressure with the increase proportion of CFT. This is mainly because the LHV of CFT is smaller than that of petro-diesel, and so, increase in the proportion of CFT. This is mainly because the LHV of CFT is smaller than that of petroCFT used dieselx engine provides less heat under the same conditions. Energiesin 5 of 15 diesel, 2018, and 11, so, CFT used in diesel engine provides less heat under the same conditions. Figure 2a also indicates that the amplitude of combustion pressure oscillation is relatively high Figure 2a also indicates that the amplitude of combustion pressure oscillation is relatively high the proportion of CFT. This is the mainly because of CFT is smaller than that of petrowhenincrease D100 isinused in the engine, while for other fuels,the theLHV oscillation amplitude decreases gradually when D100 is used in the engine, while for the other fuels, the oscillation amplitude decreases diesel, and so, CFT used in diesel engine provides less heat under the same conditions. as thegradually proportion of CFT increases. Theincreases. oscillation amplitude the smallest when FT100 is used, which as the proportion of CFT The oscillationisamplitude is the smallest when FT100 Figure 2a also indicates combustion that the amplitude of combustion pressure oscillation is relatively high means that CFT has a smooth feature due to the satisfactory evaporation characteristic is used, which means that CFT has a smooth combustion feature due to the satisfactory evaporation when D100 is used in the engine, while for the other fuels, the oscillation amplitude decreases allowcharacteristic the formation of an mixture. allow theeven formation of an even mixture.
gradually as the proportion of CFT increases. The oscillation amplitude is the smallest when FT100 is used, which means that CFT has a smooth combustion feature due to the satisfactory evaporation 1200 rpm 1600 rpm 2000 rpm 2400 rpm characteristic allow the formation of an even mixture. 1200 rpm
1600 rpm
(a)
2000 rpm
2400 rpm
(b)
Figure 2. (a)2.the pressure fuelunder underthe thecondition condition of 2000 100 Nm. Figure (a) combustion the combustion pressurecurves curvesof of all all fuel of 2000 rpmrpm and and 100 Nm. (b)pressure the pressure value of all fuels underall alltest test conditions. conditions. (b) (b) the peakpeak value of all fuels under (a) Figure 2. (a) the combustion pressure of all fuel under condition 2000 rpm andof100 Nm. Combustion pressure shock is the curves main excitation sourcethe that causesof the vibration mechanical
Combustion pressure main excitation source that causes the vibration of mechanical (b) the pressure peak shock value ofisallthe under all test conditions. structures in an engine. The higher fuels the change rate of pressure, the greater the influence on the engine structures in anInengine. higherdistinguish the changethe rate of pressure, the greaterpressure the influence on the engine structure. order toThe effectively differences in combustion when these four Combustion pressure shock is the mainthe excitation sourcein that causes the vibration ofwhen mechanical structure. distinguish combustion these fuels In areorder used, to theeffectively pressure rise rate (PRR) is differences calculated for extracting thepressure phase information of four structures in an engine. The higher the change rate of pressure, the greater the influence on the engine fuels ignition are used, thethe pressure risePRR rateaccurately. (PRR) is calculated for extracting the phase ignition and maximum Figure 3a details the variation of PRRinformation at a speed ofof2000 structure. In order to effectively distinguish the differences in combustion pressure when these four rpmmaximum and load of 100accurately. Nm. With the increase in CFTthe proportion, peak at value of PRR gradually and the PRR Figure 3a details variationthe of PRR a speed of 2000 rpm and fuels are used, the pressure rise rate (PRR) is calculated for extracting the phase information of decreases, andWith the phase of the peak PRR isproportion, gradually advanced because of an increase in the CN of load ignition of 100 Nm. the increase in CFT the peak value of PRR gradually decreases, and the maximum PRR accurately. Figure 3a details the variation of PRR at a speed of 2000 blended fuels. The circle drawn with dotted lines in Figure 3a illustrates that the start timing of and the of the peak PRR is gradually because of increase of blended rpmphase and load of 100 Nm. With the increaseadvanced in CFT proportion, theanpeak value in of the PRRCN gradually combustion is advanced with the increase in the proportion of CFT. Figure 3b indicates that the fuels.decreases, The circle drawn withofdotted lines 3a advanced illustratesbecause that the timinginof and the phase the peak PRRinisFigure gradually of start an increase thecombustion CN of maximum PRR changes dramatically with the load, that is, the maximum PRR first increases blended with fuels.the Theincrease circle drawn dotted lines in Figure 3a 3b illustrates thatthat thethe start timing of PRR is advanced in thewith proportion of CFT. Figure indicates maximum gradually and then reduces with load. combustion is advanced the withload, the increase themaximum proportionPRR of CFT. Figure 3b indicates that the then changes dramatically that is, in the first increases gradually The CFT fuel iswith easier to be compression ignited compared with petro-diesel due to its highand CN maximum PRR changes dramatically with the load, that is, the maximum PRR first increases reduces with load.shorten the ignition delay period, and result in a less combustible gas mixture. In which could gradually and then reduces with load. The CFT due fuel is CFT easier to be compression ignited compared petro-diesel due its high addition, to would release fewer heat energy due to its with low LHV. In the end, thetophase of CN The CFT fuel is easier to be compression ignited compared with petro-diesel due to its high CN whichmaximum could shorten thebe ignition delay period, and result inbe a less combustible gas mixture. In addition, PRR will advanced, and the peak value will reduced. which could shorten the ignition delay period, and result in a less combustible gas mixture. In due to CFT would release fewer heat energy due to its low LHV. In the end, the phase of maximum addition, due to CFT would release fewer heat energy due to its low LHV. In the end, the phase of PRR will be advanced, the peakand value reduced. maximum PRR will and be advanced, the will peakbe value will be reduced.
(a )
1200 rpm
1600 rpm
1200 rpm
1600 rpm
( b)
2000 rpm
2400 rpm
2000 rpm
2400 rpm
Figure 3. (a) PRR curves of all fuels under the condition of 2000 rpm and 100 Nm. (b) PRR peak value of all fuels under all test (conditions. a) ( b) Figure (a) PRR curves underthe the condition of rpm and 100100 Nm. (b) PRR peakpeak value Figure (a)3. PRR curves of of allall fuels condition of2000 2000 rpm and Nm. (b) PRR value In3.the vibration analysis offuels CIunder engine, the excitation produced by combustion pressure of the of all fuels under all test conditions. of all fuels under all test conditions. fuel-air mixture in the cylinder is the main source of engine vibration. The gas pressure excitation is
mainly composed of the compressing force, pressure increment generated by combustion and high In the vibration analysis of CI engine, the excitation produced by combustion pressure of the
frequency oscillation of the of gas pressure. The two components subject the engine chamber andof the In the vibration engine, thefirst excitation by combustion pressure fuel-air mixture inanalysis the cylinder CI is the main source of engineproduced vibration. The gas pressure excitation is all its connected components to the strong impact of dynamic loads, which produces complex fuel-air mixture in theofcylinder is the main source of engine vibration. The pressureand excitation is mainly composed the compressing force, pressure increment generated bygas combustion high mainly composed of theof compressing force, pressure generated combustion frequency oscillation the gas pressure. The first twoincrement components subject theby engine chamberand and high all its connected components to the strong impact of dynamic loads, which produces complex
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frequency oscillation of the gas pressure. The first two components subject the engine chamber and all its connected components to the strong impact of dynamic loads, which produces complex vibration response of engine body. The third component is generated mainly by the shock of gas pressure oscillation due to the uneven distribution of temperature and pressure of the gas in the chamber during Energies 2018, 11, x 6 of 15 the combustion process. These pressure waves undergo multiple reflections within the combustion chamber to form a high-frequency gasThe pressure oscillation.is generated mainly by the shock of gas vibration response of engine body. third component Combustion pressure in cylinder has typical acoustic characteristics whichofinvolves abundant pressure oscillation due to the uneven distribution of temperature and pressure the gas in the time-domain and frequency-domain information. For awaves betterundergo understanding of vibration feature chamber during the combustion process. These pressure multiple reflections within thewith combustion chamber form a high-frequency gas pressure oscillation. related combustion, thetospectrum of the in-cylinder pressure would be analyzed. Subsequently, pressure cylinder of hasthe typical acoustic characteristics involves the soundCombustion pressure level (SPL)inspectrum cylinder pressure signal is which calculated, andabundant the reference time-domain and frequency-domain information. For a better understanding of vibration feature sound pressure is selected as 20 µPa. Figure 4 illustrates the comparison of the combustion pressure related with combustion, the spectrum of the in-cylinder pressure would be analyzed. Subsequently, spectrum when using different fuels at 2000 rpm and 100 Nm. In the low frequency band of less than the sound pressure level (SPL) spectrum of the cylinder pressure signal is calculated, and the 500 Hz, the amplitude decreases rapidly, which reflects the frequency characteristics of the smooth reference sound pressure is selected as 20 μPa. Figure 4 illustrates the comparison of the combustion curvepressure of gas combustion pressure in the cylinder. The higher the peak pressure values in the cylinder, spectrum when using different fuels at 2000 rpm and 100 Nm. In the low frequency band of the greater the amplitude of the low frequency component becomes. In the middle frequency less than 500 Hz, the amplitude decreases rapidly, which reflects the frequency characteristics of theband of 500–4000 Hz, the decreasing trend becomes slow, theThe amplitude gradually smooth curve of gas combustion pressure in the cylinder. higher the decreases peak pressure values inwith the the increase in thethe proportion of CFT, and the difference betweencomponent each fuel is more obvious than in the cylinder, greater the amplitude of the low frequency becomes. In the middle frequency band. band of 500–4000 Hz, band the decreasing trend slow, decreases The low frequency This frequency reflects the peakbecomes PRR value of the the amplitude gas in the chamber. gradually with the increase in the proportion of CFT, and the difference between each fuel higher the peak PRR value, the greater the energy in the middle frequency band becomes, is asmore shown in obvious than in the low frequency band. This frequency band reflects the peak PRR value of the gas Figure 3a. In the high-frequency band above 4 kHz, the amplitude oscillation becomes strong, and the in the chamber. The higher the peak PRR value, the greater the energy in the middle frequency band energy concentration phenomenon occurs approximately at the frequencies of 4–7, 13 and 16 kHz. The becomes, as shown in Figure 3a. In the high-frequency band above 4 kHz, the amplitude oscillation high-frequency components result from the cylinder pressure oscillation in the cylinder, which also becomes strong, and the energy concentration phenomenon occurs approximately at the frequencies reflects the maximum acceleration of combustion pressure change. of 4–7, 13 and 16 kHz. The high-frequency components result from the cylinder pressure oscillation In pressure is the main of source of the high-frequency in conclusion, the cylinder, combustion which also reflects the oscillation maximum acceleration combustion pressure change.component of CI engine vibration. combustion The strength of the oscillation pressure oscillation relatedoftothe thehigh-frequency combustion PRR, In conclusion, pressure is the mainissource pressure rise acceleration, peak value of combustion pressure, and is crank angle of these component of CI engine vibration. The strength of the pressure oscillation related to thephases combustion pressureThe rise vibration acceleration, peak value of combustion pressure, crank angle of theseto the threePRR, quantities. caused by pressure oscillation willand eventually be phases transmitted three quantities. The vibration caused by pressure oscillation will eventually be transmitted to the engine surface. engine surface. vibration of the engine is closely associated with the pressure change resulting The mechanical The mechanical vibration of the engine is closely associated with the pressure change resulting from the mixture combustion in chamber. At the same time, because of the damping effect of the from the mixture combustion in chamber. At the same time, because of the damping effect of the mechanical components on the gas pressure spectrum in the CI engine, the vibration spectrum caused mechanical components on the gas pressure spectrum in the CI engine, the vibration spectrum caused by combustion is quite different fromfrom the SPL in theincylinder, but the of theofSPL by combustion is quite different the spectrum SPL spectrum the cylinder, butlevel the level the spectrum SPL has an important influence on the vibration signal of combustion. The in-cylinder combustion pressure spectrum has an important influence on the vibration signal of combustion. The in-cylinder of a CI engine is relatively high, and the vibration amplitude is also obvious. combustion pressure of a CI engine is relatively high, and the vibration amplitude is also obvious.
Figure for different differentfuels. fuels. Figure4.4.SPL SPLspectra spectra for
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3.2. Time Domain Analysis of Vibration Signal Energies 2018, 11, x 7 of 15 The excitations acting on the engine mechanical components will vary with the movement. For the engine head, the main excitation sources include the gas pressure shock, transient impact due 3.2. Time Domain Analysis of Vibration Signal to seating of the intake and exhaust valves, and gas throttling shock during exhaust valve opening. The excitations on the engine components will vary with thethe movement. For These excitations act onacting the engine head inmechanical accordance with crank angle. Under steady operation the engine the main excitation sources gas pressure transient dueshock to condition, the head, excitation sources acting on theinclude enginethe block mainly shock, include the gasimpact pressure in seating of the intake and exhaust valves, and gas throttling shock during exhaust valve opening. the chamber, inertia force of the piston-connecting rod mechanism, and piston side thrust force. Under These excitations act on the engine head in accordance with crank angle. Under the steady operation each test condition, the influence of other engine operating parameters on engine vibration is the same, condition, the excitation sources acting on the engine block mainly include the gas pressure shock in except for the difference of fuel type. the chamber, inertia force of the piston-connecting rod mechanism, and piston side thrust force. Figureeach 5a,btest show the time-domain signals collected from the head andis block, Under condition, the influence vibration of other engine operating parameters onengine engine vibration respectively, during one complete working cycle at the speed of 2000 rpm and load of 10 Nm. In the the same, except for the difference of fuel type. two figures, the5a,b vibration of each cylindervibration has beensignals showncollected according to the the engine firing order. Theblock, amplitude Figure show the time-domain from head and respectively, duringin one working is cycle the speed compared of 2000 rpmwith and load of 10 Nm. the of the vibration signal thecomplete fourth cylinder theatmaximum the other threeIncylinders0 two the figures, the vibration cylinder been shown according the order firing of order. Theengine because acceleration sensorofiseach located close has to the fourth cylinder. The to firing the test amplitude of combined the vibration signal in the timing fourth cylinder is the maximum compared with the other left to is 1-3-4-2-1, and with the valve angle, the sequence of events in Figure 5a from three cylinders0 because the acceleration sensor is located close to the fourth cylinder. The firing right is that TDC of the first cylinder, exhaust valve closing (EVC) of the fourth cylinder, TDC of the order of the test engine is 1-3-4-2-1, and combined with the valve timing angle, the sequence of events third cylinder, inlet valve closing (IVC) of the fourth cylinder, TDC of the fourth cylinder, EVC of the in Figure 5a from left to right is that TDC of the first cylinder, exhaust valve closing (EVC) of the fourth cylinder, and TDC of the first cylinder. In practice, the vibration characteristic of these events fourth cylinder, TDC of the third cylinder, inlet valve closing (IVC) of the fourth cylinder, TDC of the can be altered whenEVC a failure a certain andfirst thiscylinder. failure can be identified by detecting fourth cylinder, of theoccurs fourth at cylinder, andposition, TDC of the In practice, the vibration the change in theofphase intensity arisingwhen of abnormal vibration signalposition, from theand failure. Compared characteristic these and events can be altered a failure occurs at a certain this failure withcan petro-diesel, theby influence thechange different combustion ofof alternative used in CI be identified detectingofthe in the phase andcharacteristics intensity arising abnormal fuel vibration signal fromvibration the failure. Compared with petro-diesel, the influence of the different combustion engine on the signal has a similar effect. characteristics alternative fuel usedother in CI cylinders engine on the vibration signal a similar effect. The vibrationofamplitudes of the decrease with the has increase of distance from the The vibration amplitudes of the other cylinders decrease with the increase of distance from thewhich fourth cylinder in Figure 5a. From the vibration signal near the TDC of the fourth cylinder, fourth cylinder in Figure From the vibration signal near the TDC of the fourth cylinder, which is is approximately 350 ◦ CA, 5a. it can be seen that with increasing CFT proportion, the amplitude of the approximately 350 °CA, it can be seen that with increasing CFT proportion, the amplitude of the vibration signal decreases. In summary, the application of CFT has the potential to reduce the vibration vibration signal decreases. In summary, the application of CFT has the potential to reduce the energy of a CI engine. The same conclusion can be obtained from the engine block vibration signal in vibration energy of a CI engine. The same conclusion can be obtained from the engine block vibration the time shown in Figure which5b, is which approximately 540 ◦ CA. signaldomain in the time domain shown 5b, in Figure is approximately 540 °CA.
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③
④
(a )
②
①
( b)
Figure 5. Engine vibration acceleration signal in time domain: (a) Head; (b) Block.
Figure 5. Engine vibration acceleration signal in time domain: (a) Head; (b) Block. In order to intuitively compare the effects of blended fuel used in the engine on vibration characteristic, RMS values compare of engine the headeffects and block vibrationfuel signals areinshown in Figure In order to intuitively of blended used the engine on 6a,b, vibration respectively,RMS which summarize the average RMSblock values of vibration acceleration signals for all 6a,b, characteristic, values of engine head and vibration signals are shown in Figure blended fuels in the mentioned RMS is theacceleration root mean square of the respectively, which summarize theoperation averagecondition. RMS values ofvalue vibration signals for signal all blended and is used to represent the energy in the signal. It can be clearly seen from the two figures that the fuels in the mentioned operation condition. RMS value is the root mean square of the signal and is change of the RMS value of the four fuels is almost the same under all operation conditions. used to represent the energy in the signal. It can be clearly seen from the two figures that the change of The RMS values show a rising trend as the load and speed increase. For higher loads and speeds, the RMS value of the four fuels is almost the same under all operation conditions. the engine combustion becomes more powerful, and the vibration is influenced by the changes in the
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The RMS values show a rising trend as the load and speed increase. For higher loads and speeds, the engine combustion becomes more powerful, and the vibration is influenced by the changes in the 2018, 11,pressure x 8 of 15 high Energies combustion in the chamber. The RMS value rises with the increase in CFT concentration in diesel fuel, and the engine structure vibration is weakened. Therefore, although using CFT will high combustion pressure in the chamber. The RMS value rises with the increase in CFT reduce the engine power due to its lower LHV, the vibration energy is smaller. CFT helps to reduce concentration in diesel fuel, and the engine structure vibration is weakened. Therefore, although the vibration level of the vehicle, which could also be seen from the PRR curve in Figure 3. It also can using CFT will reduce the engine power due to its lower LHV, the vibration energy is smaller. CFT be seen from Figurethe 6 that the difference invehicle, RMS values fouralso fuels insignificant under low-load helps to reduce vibration level of the which of could be is seen from the PRR curve in operating Figureconditions. 3. It also can be seen from Figure 6 that the difference in RMS values of four fuels is CFT has a higher CN and a shorter ignition delay period compared with petro-diesel. As insignificant under low-load operating conditions. proportion of has the apre-mixed decreases, the PRR andcompared combustion peakAs values CFT higher CNcombustion and a shorter ignition delay period withpressure petro-diesel. proportion of the pre-mixed combustion decreases, the PRR and combustion pressure peak values drop, resulting in smoother combustion and less combustion-related excitation. Therefore, the fuel resulting in smoother in combustion and less combustion-related excitation. Therefore, thecondition, fuel with drop, high CN is advantageous reducing vibration. In short, under the low speed and load with highofCN advantageous in reducing vibration. short, under the the lowhigh speed and load the influence fuelis quality on engine vibration is small.InHowever, under speed and load condition, the influence of fuel quality on engine vibration is small. However, under the high speed condition, CFT fuel for diesel engine will significantly reduce the vibration intensity of diesel engine. and load condition, CFT fuel for diesel engine will significantly reduce the vibration intensity of Therefore, according to the characteristics of CFT, the vibration of diesel engine can be reduced by diesel engine. Therefore, according to the characteristics of CFT, the vibration of diesel engine can be improving fuel quality. fuel quality. reduced by improving 2400 rpm 2000 rpm
2000 rpm 2400 rpm 1200 rpm
1200 rpm
1600 rpm
1600 rpm
(a )
( b)
Figure 6. (a) RMS value of engine head vibration signal under different working conditions; (b) RMS
Figure 6. (a) RMS value of engine head vibration signal under different working conditions; (b) RMS value of engine block vibration signal under different working conditions. value of engine block vibration signal under different working conditions. 3.3. Frequency Domain Analysis of Vibration Signal
3.3. Frequency Domain Analysis of Vibration Signal
The frequency characteristic of the engine vibration signal is important for the NVH
performance of characteristic the vehicle, passengers, and drivers. CI signal engine is is important rotary power and the The frequency of the engine vibration formachinery, the NVH performance rotation of the parts supplies a certain excitation which generates an amplitude response in the engine of the vehicle, passengers, and drivers. CI engine is rotary power machinery, and the rotation structure during normal operation. During a complete working cycle (720°), the crankshaft rotates of the parts supplies a certain excitation which generates an amplitude response in the engine two revolutions for the test engine. In each revolution (360°), combustion events happen twice, and structure during normal operation. During a complete working cycle (720◦ ), the crankshaft rotates intake and exhaust valve seating events happen once in each cylinder for the engine. For example, ◦ two revolutions for the test engine. In each revolution (360 ), combustion events happen twice, when the engine speed is 2000 rpm, the rotational frequency is 33.3 Hz, the frequency of combustion and intake and exhaust valve seating events happen once in each cylinder for the engine. For events is 66.6 Hz (2000/60/2 × 4 = 66.6 Hz), the frequency of valve seating events is 133.3 Hz (2000/60/2 example, when theasengine speed 2000 rpm, the rotational is frequencies 33.3 Hz, the frequency × 8 = 133.3 Hz), shown in Figureis7a,b. The amplitude at the twofrequency characteristic (66.6 and of combustion is 66.6 Hz (2000/60/2 × 4 = 66.6 frequency of valve seating events is 133.3 Hz) isevents the largest, indicating that combustion shockHz), and the valve seating impact are the two main 133.3excitation Hz (2000/60/2 = 133.3 Hz), as shown in Figure 7a,b. The amplitude at the two characteristic sources× for8 the engine head. In Figure 7b, a large amplitude at 333.3 Hz that (10 times of rotational frequency) can be frequencies (66.6 and 133.3 Hz) is thefrequency largest, indicating combustion shock and valve seating observed addition the two characteristic Analysis of the forces on the engine body impact are theintwo mainto excitation sources for frequencies. the engine head. suggests accelerometer thatfrequency measures engine vibration sensitivefrequency) to the piston In Figurethat 7b,the a large amplitude at 333.3block Hz (10 timesisofalso rotational can be side thrust impact because the reciprocating motion of the piston assembly and the inertial forces observed in addition to the two characteristic frequencies. Analysis of the forces on the engine body resulting from combustion act in the horizontal direction. From the principles of CI engine dynamics suggests that the accelerometer that measures engine block vibration is also sensitive to the piston side [17], the frequency of the piston side thrust impact event during the up and down motion of the thrustpiston impact because the reciprocating ofTherefore, the pistonthis assembly the inertial forces resulting is 333.3 Hz (2000/60/2 × 5 × 4 = motion 333.3 Hz). value isand the characteristic frequency from of combustion act in the horizontal direction. From the principles of CI engine dynamics [17], the the piston side thrust impact events during the up and down motion of the piston assembly. Figure frequency of the piston side thrust impact event during the up and down motion of the piston is 7b also indicates that the engine block mainly bears the action of three excitations: the combustion 333.3pressure Hz (2000/60/2 × 5seating × 4 = 333.3 this value is the characteristic frequency of the shock, valve impact,Hz). and Therefore, piston side thrust. Besides, amplitude the frequency at 333.3 Hz motion decreases with the increase piston side thrust the impact eventsofduring the up and down ofsignificantly the piston assembly. Figure in 7b also CFT proportion as observed in Figurebears 7b. This because the piston side thrust the result ofpressure the indicates that the engine block mainly the is action of three excitations: theiscombustion combination of the combustion force and reciprocating inertia force of the piston assembly. The shock, valve seating impact, and piston sidethe thrust.
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Besides, the amplitude of the frequency at 333.3 Hz decreases significantly with the increase in CFT proportion as observed in Figure 7b. This is because the piston side thrust is the result of the combination of the combustion force and the reciprocating inertia force of the piston assembly. The Energies 2018, 11, x 9 of 15 inertia force is equivalent under the same speed, so the smaller the peak in-cylinder pressure (Figure 2), inertia force is equivalent under the the excitation smaller theresponse peak in-cylinder pressure (Figure the smaller the piston side thrust, andsame the speed, smallersothe amplitude. 2), the smaller the piston side thrust, and the smaller the excitation response amplitude. It can be seen from the above analysis that there are differences among the main excitation forces can be seen from the above that thereanalysis, are differences among the excitationparts forces acting onItdifferent engine parts. In analysis the subsequent the vibration of main the different should acting on different engine parts. In the subsequent analysis, the vibration of the different parts should be analyzed with due emphasis. The characteristic frequencies of the routine events in the engine be analyzed with due emphasis. The characteristic frequencies of the routine events in the engine are are not affected by the fuel combustion behavior in the cylinder. However, the combustion of fuel not affected by the fuel combustion behavior in the cylinder. However, the combustion of fuel in the in the chamber will directly affect the combustion excitation force and the piston side thrust force, chamber will directly affect the combustion excitation force and the piston side thrust force, which which affect dynamic response and high-frequency noise of the engine. affect the the dynamic response and high-frequency radiationradiation noise of the engine. Varying the Varying engine the engine condition will change the frequency and amplitude of the corresponding frequency condition will change the frequency and amplitude of the corresponding frequency in the spectrumin the spectrum diagram. diagram.
①
②
66.6 Hz
133.3 Hz
①
(a )
66.6 Hz
②
③
133.3 Hz
333.3 Hz
( b)
Figure 7. Low frequency band of engine vibration: (a) Head; (b) Block.
Figure 7. Low frequency band of engine vibration: (a) Head; (b) Block. 3.3.1. Engine Head Vibration Signal Analysis
3.3.1. Engine Head Vibration Signal Analysis
In order to study the influence of CFT and CFT blends on the vibration characteristics of CI In orderthe toamplitude study the spectrum influence curve of CFT blends the vibration of CIofengine, engine, ofand the CFT engine head on vibration signal atcharacteristics a rotational speed the amplitude spectrum curve engine head 5a vibration signal at afast rotational speed of 2000 rpm 2000 rpm and load of 100 Nmof asthe shown in Figure is obtained by the Fourier transformation The transformation results are in Figure 8. The frequency bandFourier of vibration acceleration of the The and (FFT). load of 100 Nm as shown in Figure 5a is obtained by the fast transformation (FFT). engine head is relatively wide. There are high amplitudes in the frequency range of 1~9 kHz and transformation results are in Figure 8. The frequency band of vibration acceleration of the engine head approximately and 16 withinthe spectrumrange of the of combustion pressure curve is relatively wide. 13 There arekHz. highCombined amplitudes theSPL frequency 1~9 kHz and approximately in Figure 4, it is found from Figure 8 that there are four frequency components in the highlighted 13 and 16 kHz. Combined with the SPL spectrum of the combustion pressure curve in Figure 4, it is response area: 5, 6.48, 13.2 and 16 kHz, which are the dynamic response frequencies of engine head found from Figure 8 that there are four frequency components in the highlighted response area: 5, 6.48, induced by combustion. 13.2 and In 16 order kHz, to which are the dynamic response frequencies of engine head induced by combustion. facilitate the identification and analysis of the time at which a particular frequency In order to facilitate the identification and analysis of the time at which a particular frequency event occurs, a continuous wavelet transformation (CWT) is applied to the raw acceleration signal of event occurs, a continuous wavelet transformation (CWT) is applied to the raw acceleration the engine head (as shown in Figure 5a) is shown in 2D in Figure 9. The mother wavelet used by thesignal of the engine headis(as Figure 5a) is contents shown in in Figure 9. Theby mother wavelet used by transformation theshown Morlet,inand the critical are2D clearly highlighted the CWT analysis. For the subsequent analysis, valve-close is also indicated because,by compared the transformation is the Morlet, and the timing criticalinformation contents are clearly highlighted the CWTwith analysis. exhaust valve opening (EVO)valve-close and inlet valve opening (IVO), the dominant response isbecause, still caused by For the subsequent analysis, timing information is also indicated compared the combined IVC and EVC [32]. At 360 °CA, the prominent response energy areas are in frequencies with exhaust valve opening (EVO) and inlet valve opening (IVO), the dominant response is still of 5, 6.48, 13.2, and 16 kHz. These four frequencies are the excitation responses of the combustion and caused by the combined IVC and EVC [32]. At 360 ◦ CA, the prominent response energy areas are pressure oscillation from the time-frequency distribution. The response energy area at the frequency in frequencies of 5, 6.48, 13.2, and 16 kHz. These four frequencies are the excitation responses of the content of 5 kHz remains basically unchanged for all fuels, but the response energy areas of the other combustion and pressure oscillation from thethe time-frequency Theprincipal responsefrequency energy area at three frequency contents are weaken with increase in CFTdistribution. proportion. The the frequency content of 5 kHz remains basically unchanged for all fuels, but the response energy content induced by the combustion pressure impact is approximately 5 kHz; meanwhile, the high- areas of the other three frequency are weaken with the increase CFT proportion. Thefuels principal frequency response energycontents is significantly reduced, especially aroundin 6.48 kHz, when blended are burned. frequency content induced by the combustion pressure impact is approximately 5 kHz; meanwhile, the
high-frequency response energy is significantly reduced, especially around 6.48 kHz, when blended fuels are burned.
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② ②
③ ③
④ ④
⑤ ⑤
Figure 8. Frequency domain curve of cylinder head vibration acceleration.
Figure 8. Frequency cylinderhead headvibration vibration acceleration. Figure 8. Frequencydomain domaincurve curve of of cylinder acceleration.
16 kHz 16 kHz Ⅱ Ⅱ
① ①
13.2 kHz 13.2 kHz 6.48 kHz 6.48 kHz EVC-4 EVC-4 IVC-3 IVC-3
③ ③
Ⅰ Ⅰ
④ ④
D100 D100
② ②
① ① FT10 FT10
EVC-2 EVC-2 IVC-4 IVC-4
EVC-1 EVC-1 IVC-2 IVC-2
EVC-3 EVC-3
FT30 FT30
IVC-1 IVC-1
5.0 kHz 5.0 kHz
FT100 FT100
Figure 9. CWT transformation of engine head vibration signal. Figure 9. CWT transformation of engine head vibration signal.
Figure 9. CWT transformation of engine head vibration signal. 3.3.2. Engine Block Vibration Signal Analysis 3.3.2. Engine Block Vibration Signal Analysis 3.3.2. Engine Vibration Signal Analysis The Block amplitude spectrum curve of the engine block vibration signal (shown in Figure 5b) is The amplitude spectrum curve of the engine block vibration signal (shown in Figure 5b) is obtained by FFT transformation, and the transformation result is shown in Figure 10. It is evident The amplitude spectrum curve thetransformation engine blockresult vibration signal (shown inisFigure obtained by FFT transformation, andofthe is shown in Figure 10. It evident5b) is that the engine block vibration is mainly distributed in the frequency band below 8 kHz. The that by the FFT engine block vibration mainly distributed result in the is frequency band below 8 is kHz. The obtained transformation, andis the transformation showninin2.5–3.5, Figure 10.and It evident frequency bands of engine block vibration acceleration is concentrated 4–5 6–7 kHz. that frequency bands of engine block vibration acceleration is concentrated in below 2.5–3.5,84–5 andThe 6–7frequency kHz. the engine block vibration is mainly distributed in the frequency band kHz. Due to its complexity, different parts of the CI engine have different response frequencies and modal Due its complexity, different parts of the CIisengine have different response and modal bands of to engine block vibration concentrated 2.5–3.5, 4–5frequencies andis6–7 Due frequencies. The first frequencyacceleration band in the signal spectrum ofin the block vibration notkHz. obvious in to its frequencies. The first frequency band in the signal spectrum of the block vibration is not obvious in complexity, different parts of head the CI engine The haveamplitude differentofresponse frequencies modal frequencies. the signal spectrum of the vibration. three frequency bandsand tends to a decrease the signal spectrum of the head vibration. The amplitude of three frequency bands tends to a decrease withfrequency the increase in CFT. Meanwhile, the amplitude corresponding to isFT10 a frequency The first band in the signal spectrum of the block vibration not at obvious in theofsignal with the increase in CFT. Meanwhile, the amplitude corresponding to FT10 at a frequency of
spectrum of the head vibration. The amplitude of three frequency bands tends to a decrease with the increase in CFT. Meanwhile, the amplitude corresponding to FT10 at a frequency of approximately 6.5 kHz is higher than that of petro-diesel. Combined with the conclusions from the analysis of engine
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approximately 6.5 kHz is higher than that of petro-diesel. Combined with headapproximately vibration acceleration, it can be speculated that the frequency bandthe at conclusions 5 and 7 kHzfrom maythe be the 6.5 kHz is higher than that of petro-diesel. Combined with the conclusions from the analysis of engine head vibration acceleration, it can be speculated that the frequency band at 5 and response to the combustion event excitation. analysis of engine head vibration acceleration, it can be speculated that the frequency band at 5 and 7 kHz may be the response to the combustion event excitation. To verify above speculation, the time-frequency transformation of the vibration signal as 7 kHz maythe be the response to the combustion event excitation. To verify the above speculation, the time-frequency transformation of the vibration signal as shown inTo Figure is above carriedspeculation, out, and the is shown transformation in 2D in Figure vibration response verify5bthe theresult time-frequency of11. theThe vibration signal as shown in Figure 5b is carried out, and the result is shown in 2D in Figure 11. The vibration response ◦ CA in Figure 5b isnear carried out, and theposition result is shown in 2Danalyzed in Figure because, 11. The vibration response of theshown second cylinder the 540 is mainly as the engine of the second cylinder near the 540 °CA position is mainly analyzed because, as the engine blockblock of theaccelerometer second cylinderhas near the 540 °CAdistance position from is mainly analyzed because, as the engine block vibration a minimum thesecond secondcylinder, cylinder, vibration response vibration accelerometer has a minimum distance from the thethe vibration response ◦ vibration accelerometer has a minimum distance from the second cylinder, the vibration response energy at 540 CA is the largest. energy at 540 °CA is the largest. energy at 540 °CA is the largest. ① ①
② ②
③ ③
Figure 10. Frequency domain curve of engine block vibration acceleration.
Figure 10.10. Frequency of engine engineblock blockvibration vibration acceleration. Figure Frequencydomain domaincurve curve of acceleration.
6.65 kHz 6.65 kHz
Ⅳ Ⅳ
D100 D100
Ⅴ Ⅴ
Ⅲ Ⅲ
FT10 FT10
4.6 kHz 4.6 kHz
① ①
③ ③ 3.0 kHz 3.0 kHz
④ ④
② ②
① ① FT30 FT30
FT100 FT100
Figure 11. CWT transformation of engine block vibration signal. Figure 11. CWT transformation of engine block vibration signal.
Figure 11. CWT transformation of engine block vibration signal. At the frequency of 6.7 kHz, there are two highlighted response energy areas at 180 °CA and At the frequency of 6.7 kHz, there are two highlighted response energy areas at 180 °CA and 540 °CA for petrodiesel fuel, and the one that is closer to the TDC is caused by the combustion At of 6.7fuel, kHz, there are two highlighted response energyby areas at 180 ◦ CA and 540the °CAfrequency for petrodiesel and the one that is closer to the TDC is caused the combustion excitation. It is found from the longitudinal comparison of response energy of the two positions for ◦ 540 excitation. CA for petrodiesel fuel,the and the one that is closer the TDC is caused by positions the combustion It is found from longitudinal comparison of to response energy of the two for different fuels, that the response areas are weakened rapidly with the increase in CFT proportion. different fuels, thatfrom the response areas are weakened rapidly with the energy increaseof inthe CFTtwo proportion. excitation. It is found the longitudinal comparison of response positions The right-side highlighted response energy area at 6.7 kHz should be the piston side thrust impact for The highlighted response energy area at 6.7 kHz should be the the piston sideinthrust different right-side fuels, that the response areas are weakened rapidly with increase CFT impact proportion.
The right-side highlighted response energy area at 6.7 kHz should be the piston side thrust impact response, which weakens with the increase in CFT proportion. However, its attenuation is not as fast as the left-side one caused by the combustion excitation. Therefore, the response area is driven
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response, which weakens with the CFTthrust proportion. However, its attenuation is not as fast by the piston side thrust impact. Theincrease piston in side is affected by combustion excitation to some as the left-side one caused by the combustion excitation. Therefore, the response area is driven by the degree, and the block vibration accelerometer is more sensitive to the piston slap events, so that a clear piston side thrust impact. The piston side thrust is affected by combustion excitation to some degree, piston side thrust response area could still be seen, while under the same conditions, the response area and the block vibration accelerometer is more sensitive to the piston slap events, so that a clear piston driven by combustion was relatively weak. In addition, the frequency bands around 3 and 4.6 kHz are side thrust response area could still be seen, while under the same conditions, the response area combustion excitation responses as the highlighted response area is closer to TDC. driven by combustion was relatively weak. In addition, the frequency bands around 3 and 4.6 kHz are combustion excitation responses as the highlighted response area is closer to TDC.
3.3.3. Evaluation of Vibration Signals for Diesel-CFT Fuel Blends
3.3.3.numbers Evaluation Vibration Diesel-CFT Blends The I, IIofmarked in Signals Figure for 9 and III, IV, VFuel marked in Figure 10 represent the main vibration events related to combustion. I isincaused theIII,vibration energy of combustion pressure-driven The numbers I, II marked Figure 9by and IV, V marked in Figure 10 represent the main vibration events related for to combustion. I is caused by the vibration of combustion pressurehigh-frequency oscillation the engine head. II denotes the mainenergy response induced by combustion driven oscillation for IV the represent engine head. denotes the mainresponses response induced pressure forhigh-frequency the engine head. III and twoII main vibration excited by by the combustion pressure for the engine head. III and IV represent two main vibration responses excited combustion pressure on the engine block. V signifies the response inspired by the piston side thrust combustion on values the engine block. V marked signifies the response inspired by were the piston side forceby onthe engine block.pressure The RMS of the five time-frequency areas calculated in thrust force on engine block. The RMS values of the five marked time-frequency areas were calculated order to ascertain the effects of the blended fuel on the areas, and the results are shown in Figure 12. in order to ascertain the effects of the blended fuel on the areas, and the results are shown in Figure It can be perceived that the response energy of each area attends to decrease with the increase 12. of CFT proportion, overall.thatBut RMS-Ienergy of FT30 is significantly than the of II. This It can be perceived thethe response of each area attends toless decrease withvalue the increase means that proportion, the vibration response FT30ofisFT30 greater around 5.5 kHz, theofvibration response of CFT overall. But theofRMS-I is significantly less thanwhile the value II. This means around is smaller. Compared ofvibration FT30 is response more powerful that14 thekHz vibration response of FT30 is with greaterFT100, aroundthe 5.5 combustion kHz, while the around due kHz is smaller. with FT100,the thecombustion combustion ofofFT30 is more powerful to the higher to the14higher heatingCompared value. Moreover, FT30 presents onlydue moderate concerns heating value. Moreover, the combustion of FT30 presents only moderate concerns regarding thefewer regarding the atomization [30] because CFT is more volatile relative to D100. So, FT30 generates atomization [30] because CFT is more volatile relative to D100. So, FT30 generates fewer highhigh-frequency contents. In addition, the RMS of FT10 is maximum for V. It may be that the addition frequency contents. In addition, the RMS of FT10 is maximum for V. It may be that the addition of a of a small proportion of CFT has less impact on the cylinder pressure. However, it may weaken the small proportion of CFT has less impact on the cylinder pressure. However, it may weaken the attenuation effect on the side thrust of the piston by diluting the oil film due to its low viscosity [33]. attenuation effect on the side thrust of the piston by diluting the oil film due to its low viscosity [33]. Thus,Thus, the accelerometer could receive more excitation energy induced by the piston side thrust force. the accelerometer could receive more excitation energy induced by the piston side thrust force. For other blended fuels, RMS-V withthe theincrease increase CFT proportion that the For other blended fuels, RMS-Vvalues valuesdecrease decrease with inin CFT proportion suchsuch that the effecteffect of cylinder pressure reduction ononthe is more moresignificant. significant. of cylinder pressure reduction theside sidethrust thrust is
4.6 kHz 6.65 kHz
14 kHz 5.5 kHz
Figure 12. RMS of the five marked areas in CWT diagrams.
Figure 12. RMS of the five marked areas in CWT diagrams. 4. Conclusions
4. Conclusions
In this study, the combustion and vibration characteristics of a CI engine using CFT blended
In thiswere study, the combustion and vibration of aand CI 70% engine CFT blended fuels investigated. The four fuels used arecharacteristics diesel, 10%, 30%, CFTusing in petro-diesel, and fuels CFT, respectively. four-cylinder operated at loads 10,in 50,petro-diesel, 100, 150, andand 200 pure were pure investigated. The fourAfuels used areengine diesel,was 10%, 30%, and 70% of CFT (except 1200 rpm) Nm loads and speeds of 1200, 1600, 2000, and 2400 rpm. CFT, respectively. A four-cylinder engine was operated at loads of 10, 50, 100, 150, and 200 (except 1200 rpm) Nm loads and speeds of 1200, 1600, 2000, and 2400 rpm. The peak values of the combustion pressure and PRR decrease with the increase in CFT proportion. The phases of peak value and ignition timing also advance gradually. There are three frequency
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contents of energy concentration caused by cylinder pressure oscillation at approximately 4–7, 13, and 16 kHz from the SPL pressure spectrum. Maximum amplitudes of vibration signals of the engine head and block in the time domain reduce with the increase of CFT proportion. RMSs of the signals enlarge with the increase of rotational speed, increasing at first and then decreasing as the load is augmented. RMSs reduce with the increase of CFT proportion under most operation conditions. The frequency components of head vibration signals are abundant and the responses induced by combustion are quite notable. There are four frequency contents at 5, 6.48, 13.2 and 16 kHz from the spectrogram and CWT diagram. The highlighted areas in the CWT diagram of the larger three frequency contents are weaken with the increase in CFT proportion, and the highlighted area at 5 kHz is basically unchanged. The engine head vibration signal is more sensitive to high-frequency combustion excitation. The frequency component of block vibration signal is mainly distributed below 8 kHz. There are three frequency contents around 3, 4.6 and 6.65 kHz in spectrogram and CWT diagrams. The left-side highlighted areas of 6.65 kHz and the responses of frequency contents at 3 and 4.6 kHz are related to combustion excitation, and these responses are weakened rapidly with the increase in CFT proportion. The right-side highlighted area of 6.65 kHz is impacted by the piston side thrust. These responses are also weakened with increasing in the CFT proportion, except for FT10. In brief, CFT has the ability to reduce CI engine vibration. The results and conclusion of this study can pave the way for the optimal design and development of fuel blends based on vibration analysis. Author Contributions: The contributions of the individual authors to this research work are as follows: T.W. designed the bench test for measuring the vibration signals; T.Y., J.S. and X.S. performed the experiments; T.Y. and G.L. analyzed the data; T.Y. reviewed previous research work and wrote the paper. Acknowledgments: Thanks are due to Ruiliang Zhang, Zhifei Wu, Jing Peng, and Zhen Zhao of the Taiyuan University of Technology for the valuable discussions, and to Fengshou Gu of the University of Huddersfield for his help with the data processing method. Conflicts of Interest: The authors declare no conflict of interest.
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