https://www.dieselnet.com/tech/sensors_air-mass-flow.php. ... [45] F. Laurantzon, âFlow measurements related to gas exchange applications,â KTH Mechanics, ..... The calibration of the sensors is offered as a service in addition to the product or ...
Contents Engine Flow and Efficiencies ............................................................................................................................................. 3 Introduction .................................................................................................................................................................. 3 Engine Flow Measurement Problems ............................................................................................................................... 3 Engine Flow Measurement Specifications ........................................................................................................................ 4 Computational Modelling of Engine Flow......................................................................................................................... 4 Temperatures.................................................................................................................................................................... 4 Humidity............................................................................................................................................................................ 5 Air Flow ............................................................................................................................................................................. 5 Factors in Engine Air Flow Measurement ..................................................................................................................... 5 Useful Oxygen Concentration ................................................................................................................................... 5 Air Mass..................................................................................................................................................................... 5 Types of Air Flow Measurement Sensors and Instrumentation ................................................................................... 5 Proposed Design ........................................................................................................................................................... 6 Fuel Flow ........................................................................................................................................................................... 6 Factors in Engine Fuel Flow Measurement ................................................................................................................... 6 Fuel Flow Measurement Techniques ............................................................................................................................ 6 Proposed Design ........................................................................................................................................................... 6 Pressure ............................................................................................................................................................................ 6 Factors in Pressure Measurement ................................................................................................................................ 7 Location and Angle of Sensors .................................................................................................................................. 7 Resonant Frequencies ............................................................................................................................................... 7 Pressure Measurement Techniques ............................................................................................................................. 7 Bibliography ...................................................................................................................................................................... 8 Appendix *** - Applications of Engine Flow Measurements ......................................................................................... 12 Air-Fuel Ratio (AFR) ..................................................................................................................................................... 12 Air–Fuel Equivalence Ratio ( λ ) .................................................................................................................................. 12 Fuel–Air Equivalence Ratio ( ϕ ) .................................................................................................................................. 12 Specific Fuel Consumption (SFC) ................................................................................................................................. 12 Volumetric Efficiency (VE) ........................................................................................................................................... 12 Volumetric Efficiency tables.................................................................................................................................... 13 Pressure-Volume Diagrams......................................................................................................................................... 13 Mean Effective Pressures............................................................................................................................................ 14 Introduction ............................................................................................................................................................ 14 Brake Mean Effective Pressure (BMEP) .................................................................................................................. 14
Indicated Mean Effective Pressure (IMEP) ............................................................................................................. 15 Frictional Mean Effective Pressure (FMEP) ............................................................................................................. 15 Ignition Timing Diagrams ............................................................................................................................................ 15 Appendix *** - Classification of Methods of Investigating Flow in Engines ................................................................... 16 Appendix*** - Yanmar 1GM10 Specifications for Engine Flow Measurements ............................................................ 17 Fuel Flow Measurement Specifications ...................................................................................................................... 18 Air Flow and Pressure Measurement Specifications .................................................................................................. 19 Appendix *** - Proposed Full Engine Flow Measurement Specifications ...................................................................... 20 Appendix ** - Properties of Engine Flow Instrumentation Proposals ............................................................................ 21 Airflow Measurement - Properties of Bosch HFM-8 Hot Film Air Flow Meters ......................................................... 21 Fuel Flow Measurement - UK Flowtechnik Oval Gear Positive Displacement Flowmeters........................................ 21 Pressure Measurement - Properties of Kistler Pressure Sensors ............................................................................... 22 Appendix *** - Problems in Engine Flow Modelling ...................................................................................................... 22 Cyclical Variations ....................................................................................................................................................... 22 Behaviour and Causes of Cyclical Variations........................................................................................................... 22 Methods of Addressing Cyclical Variations ............................................................................................................. 23 Pulsation ..................................................................................................................................................................... 23 Residual Gases ............................................................................................................................................................ 24 Appendix *** - Engine Flow Modelling........................................................................................................................... 24 Thermodynamics Modelling ....................................................................................................................................... 24 Fluid Dynamics Modelling ........................................................................................................................................... 25 Appendix*** - Engine Flow Computational Methods .................................................................................................... 26 Machine Learning Techniques .................................................................................................................................... 26 Appendix*** - Detailed Methods of Air Flow Measurement ......................................................................................... 28 Methods Using Exhaust Gases Analysis ...................................................................................................................... 28 Methods Measuring Speed ......................................................................................................................................... 29 Hot Wire and Hot Film Anemometry and Air Mass Flow Sensors .......................................................................... 29 Laser Doppler Anemometry and Air Mass Flow ..................................................................................................... 30 Methods Measuring Air Volume ................................................................................................................................. 30 Vane (Flap) Air Mass Flow Meters .......................................................................................................................... 30 Methods Measuring Pressure ..................................................................................................................................... 31 ∆p method .............................................................................................................................................................. 31 Laminar Flow Meters .............................................................................................................................................. 32 Appendix*** - Detailed Methods of Measuring Fuel Flow ............................................................................................ 32 Positive Displacement Meters .................................................................................................................................... 32 Appendix *** - Detailed Methods for Measuring Pressure............................................................................................ 33 Piezoelectric Transducers ........................................................................................................................................... 33 Shock Tubes ................................................................................................................................................................ 34
Appendix*** - Standards and Regulations in Engine Flow Measurement ..................................................................... 34 Engine Testing ............................................................................................................................................................. 34 Flow Measurement ..................................................................................................................................................... 35
Engine Flow and Efficiencies Introduction The parameters involved in engine efficiency cannot be measured accurately directly in most cases and therefore decisions have to be made as to what parameters to measure directly, under what conditions, and what models to use to derive the relevant and accurate parameters to provide useful feedback. Therefore to design the engine test bed’s study of engine flow, decisions have to be made regarding:
Sensor measurement(s) Mathematical model to derive accurate measurement of relevant parameter(s) from sensor measurement(s) Computational method of implementing mathematical model (Computational method of optimising mathematical model)
Although there are a lot of different methods of modelling engine flow behaviour, the most common mathematical model approaches can generally be classed into 2 groupings:
Fluid Dynamics Modelling (multidimensional) Thermodynamic Modelling (one dimensional)
Both approaches used for various engine process features (pressure, air flow, fuel flow etc). For the purpose of this study, we have chosen to try and use the engine test bed to analyse the engine flow through the behaviour of the following parameters:
air flow; fuel flow; pressure.
The possible sensors and mathematical models are discussed in each sub section for the parameters. There are certain phenomena that cause difficulties or additional complexities to measuring an modelling flow in engines and these are discussed in sections ****. As the computational methods of both implementing and, if necessary, optimising are very similar for the different engine flow parameters studied, they are looked at more generally in section ****. Other parameters involved in engine flow are briefly discussed in sections ****.
Engine Flow Measurement Problems There are phenomena involved with the measurement of engine flow that introduce complexities such as non-linear, stochastic, or chaotic behaviours to the measurement and modelling of parameters. The main problems involved with small marine, single cylinder diesel engines are:
Pressure drop - Pressure losses occur during the cylinder filling process, normally increasing with engine speed and even generating compressibility effects in the intake valve when high enough [1]. Cyclical variations - The complexity of the parameters involved ensure variation between cycles that are difficult to predict, and must be accounted for during measurement through filtering or smoothing. See Appendix *** for more detail Pressure pulsation - The pressure is not **** See Appendix *** for more detail. Residual gas fractions – The amount of gases present during intake that are left over from the previous cycle [1] Heat transfer - Heat is transferred across the cylinder surfaces (intake manifold, engine runners, cylinder wall, cylinder head, piston and intake and exhaust valves) creating difficulties in thermodynamic measurements. Diesel knock
o
When some of the injected fuel fails to burn, it is left behind and accumulated until eventually the mass burns in one sudden burst after a long delay creating huge uncontrolled peak pressures and shock waves. Temperature drift in sensors o The temperature variations involved in the cycles create stresses on the sensor components which create inaccuracies in measurements [2]. There are two types: o Thermal Shock /Short Term Drift Temperature drift that occurs due to the heat changes in each cycle Generally mild drifts cause higher pressure readings during combustion and lower pressure during the rest of the expansion, and the effect is difficult to identify. It strongly depends on the thermal load at the transducer location, influenced by the intense flows during the gas exchange process and so by the proximity to the fuel jet. [2] o Load Change Drift /Long Term Drift Drift resulting from temperature changes across operating conditions. It creates “a slow instability in the baseline for transducer signal, whose consequences and control depend on the circuitry chosen for its polarization“. [2].
For further information see Appendix ***.
Engine Flow Measurement Specifications The parameters defining the specifications of designing engine flow measurements in this project are those for a small marine engine, taking a Yanmar 1GM10 as a typical use case. For more information about the 1GM10 engine flow specifications see Appendix ***. A quantitative specification, assessing priority and cost factors were developed and can be seen in Appendix ***.
Computational Modelling of Engine Flow The importance of incorporating computer modelling in the testing of engines has long been important because the majority of the processes involved in engine flow are complex. Computer modelling not only increases efficiency in producing consistent results for simple mathematical calculations but they also enable the use of more detailed algorithms that are important for engines where many of the parameters are too difficult to model simply from fundamental mathematical models alone as the processes are not yet understood sufficiently to produce accurate corresponding mathematical descriptions [3] and it is “becoming more and more critical as the number of control variables is increasing with engine complexity” [4]. The most significant computational methods of algorithm for solving and optimising engine flow measurements are:
Direct Approximations Machine Learning Methods Monte Carlo Methods
A more detailed discussion of the application of the different methodologies for use in engine flow analysis can be found in Appendix ***.
Temperatures Lower temperatures allow leaner AFRs for combustion as well as being important to understand air and fuel flows. Affected by:
External temperatures to cylinder
o o
Temperature on air drawn into cylinder Temperature outside the cylinder during processes This is mainly determined by the coolant temperature which for most analysis is assumed to “not show important variations with the engine load” [1], however in our specification direct seawater cooling is used which can induce variations across both cycles and operating conditions that should be accounted for when modelling. Materials and shapes of cylinder – Affect heat distribution within the cylinder, and insulation. o “In the combustion chamber of an engine, one can discern three very different surfaces: the cylinder liner, the fire deck and the piston.” [5] Combustion reactions occurring AFR –lack of homogeneity creates non-linear relations but often higher AFR means lower temperature [6] Cycle Timings – These influence the time available to draw and dissipate heat in each stage.
Humidity More humidity generally means less oxygen in a useful form for combustion, less brake torque produced, and less nitrogen oxides emitted, but more brake specific fuel consumption, carbon monoxide and sulphur dioxide emissions [7]. The effects of humidity become more pronounced with higher temperatures. Affected by:
External humidity – More humidity drawn into the cylinder, means more inside. Fuel composition – The chemical reactions occurring in the cylinder are influenced by the chemicals present in the fuel and the outputs of these can change humidity. Exhaust – Affects how effectively humidity from previous cycles are left behind in cylinder.
Air Flow It is useful to know the specific features involved in air flow through the engine, so as to build more accurate models of the actual AFRs involved in the combustion process.
Factors in Engine Air Flow Measurement Useful Oxygen Concentration The lower the concentration, the less available to combust and so less power generated. Even if available in the cylinder, it is important to assess and understand factors affecting how much of this is “useful” i.e. actually able to be used in the combustion reaction, in order to distinguish the performance of the engine in the test bed from the influence of the environment and inputs. Typically around 21% oxygen in the air, but this can vary significantly. Less oxygen available to enter, means less for the engine to input to the cylinders. Air Mass In this project we will be looking to estimate the levels of air mass involved in the combustion by estimating the air mass present in the cylinder during the combustion processes, which means generally assessing the flows in and out.
Types of Air Flow Measurement Sensors and Instrumentation The types of air flow measurement:
Methods using exhaust gas analysis Methods measuring air volume Methods measuring air speed Methods measuring pressure
See Appendix *** to *** for more detailed discussion of different types.
Proposed Design Hot film air mass flow sensor, Bosch HFM-8 model – see Appendix ** for more details.
Fuel Flow It is useful to know the specific features involved in fuel flow through the engine, so as to build more accurate models of the actual AFRs involved in the combustion process.
Factors in Engine Fuel Flow Measurement The fuel system can broadly be divided into the low-pressure and the high-pressure sub-systems:
Low-pressure system components are: the fuel storage tank, fuel filters, the fuel flow detector, low pressure pump, pressure regulator and the low-pressure inlet/outlet of the Fuel Injector Pump (FIP). High-pressure system components are the FIP output, common rail, connecting fuel lines and the injector.
In our design, it should be simple enough to measure the fuel mass flow rate on the input line alone.
Fuel Flow Measurement Techniques Common methods of fuel flow measurement are:
Positive Displacement Meters o Reciprocating or oscillating piston o Gear Oval gear Helical gear o Nutating disk o Rotary Vane o Diaphragm Ultrasonic Flow Meters Pressure Differential Meters
See Appendix *** for more detailed discussion of the different types.
Proposed Design Positive displacement sensor, Oval Gear Type, UK Flowtechnik OEM004 model – see Appendix *** for more details.
Pressure Measurement of pressure within the cylinder is useful for a number of reasons:
Mean Effective Pressures – See section *** for more detail. Ignition Timings – See section *** for more detail. Air and Fuel Flow Measurements – They allow more accurate modelling of the conditions of the air and fuel flow in the cylinder, see sections *** and *** for more detail. Misfire Detection - “misfire detection is currently performed by monitoring the flywheel speed signal. At high engine speeds or low engine loads, this approach suffers from a poor signal-to-noise ratio. The online analysis of the cylinder pressure signal is one of the most promising approaches to substitute for the flywheel speed analysis.” [8] Feedforward Control of Exhaust – Indicative of the fuel mass burning rate.
o
“Therefore, using the cylinder pressure for feedforward control of the individual exhaust production is very likely to improve overall exhaust emissions, in particular, in transient operation.” [8] o For example using a diesel pressure departure ratio (PDR) algorithm such as used by **** Mass Fraction Burned (MFB) Estimations - “The energy conversion during a combustion cycle can be described by the mass fraction burned (MFB), which can be approximated by [8]:”
𝑀𝐹𝐵 ≈
𝑝𝑐𝑦𝑙,𝑓𝑖𝑟𝑒𝑑 𝑝𝑐𝑦𝑙,𝑚𝑜𝑡𝑜𝑟𝑒𝑑
−1
Factors in Pressure Measurement Location and Angle of Sensors The pressure sensors ideally need to be flush with the combustion chamber surface or otherwise can be “severely affected by the pressure wave action within the narrow access passages, resulting in high frequency pressure pulsations that may obscure the actual combustion characteristics” [9]. The angle influences the pressures measured. Resonant Frequencies The resonant frequency of the cylinder component can be given approximately by Equation *** below where fn is the resonant frequency. L the length of the component, and c is the speed of sound. This needs to be filtered out of pressure analysis results in modelling choices. Equation***
𝑐
𝑓𝑛 = 4𝐿
Pressure Measurement Techniques Methods for Cylinder Pressure Sensor Modelling:
Approximated Motor Pressure Method o Assumes a polytropic compression to calculate the motored pressure at a specific crank angle – often requires the calculation of several different sets of points during the compression stroke and averaging of the results to reduce noise and provide higher precision data, or use of a numerical function approximation such as used by [8] Difference Pressure Method o It can sometimes be easier to measure difference in pressure with cheaper sensors than directly measuring pressure itself.
The most common instrumentation used to measure in-cylinder pressure:
Piezoelectric transducers Shock tubes Laminar Flow Meters
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Appendix *** - Applications of Engine Flow Measurements Air-Fuel Ratio (AFR) Air–fuel ratio is the ratio between the mass of air and the mass of fuel in the fuel–air mix at any given moment. The mass is the mass of all constituents that compose the fuel and air, whether combustible or not. They can be classified into:
Rich - AFR numbers lower than stoichiometric. These are less efficient, but may produce more power and burn at lower temperatures. Lean - AFR numbers higher than stoichiometric. These are more efficient but may cause engine damage or premature wear and produce higher levels of nitrogen oxides.
Even if most of the injected fuel is a vapour at the start of combustion, in a typical diesel only 10 to 35% of it is mixed within the combustion limits, and so the engine effectiveness is more influenced by the effectiveness of this mixing than by the simple presence of the two components [3].
Air–Fuel Equivalence Ratio ( λ ) Air-fuel equivalence ratio is the ratio of actual AFR to stoichiometry for a given mixture. λ = 1.0 is at stoichiometry, rich mixtures λ < 1.0, and lean mixtures λ > 1.0. Equation ***.1 is often used as the definition of λ: 𝐴𝐹𝑅
Equation ***.1
𝜆 = 𝐴𝐹𝑅
𝑠𝑡𝑜𝑖𝑐ℎ
Fuel–Air Equivalence Ratio ( ϕ ) The Fuel-Air equivalence the ratio of the fuel-to-oxidizer ratio to the stoichiometric fuel-to-oxidizer ratio, as shown in Equation ***.2 below. It is useful because it uses the mass and molar values for the mixtures. Equation ***.2
𝜙 = (𝑚
𝑚𝑓𝑢𝑒𝑙 ⁄𝑚𝑜𝑥
𝑓𝑢𝑒𝑙 ⁄𝑚𝑜𝑥 )𝑠𝑡𝑜𝑖𝑐ℎ
Ratios greater than one indicate more fuel in the mixture than required for stoichiometric combustion, irrespective of the fuel and oxidizer being used, while ratios less than one define a deficiency of fuel or equivalently excess of oxygen components [10] [11]. The system can be characterized by temperature, pressure, and fuel/air equivalence ratio alone as these are sufficient to define the internal energy, density and enthalpy for use in one dimensional modelling.
Specific Fuel Consumption (SFC) It is defined as the fuel flow rate per unit power output, and can be expressed as in Equation ***.3 Equation ***.3
𝑆𝐹𝐶 =
𝑚̇𝑓 𝑃
SFC is a measure of how efficiently the fuel supplied to the engine is used to produce power, the Brake SFC (BSFC) defined as the SFC when the power is that produced in terms of the braking torque. BSFC generally decreases with engine size because of reduced cylinder surface to volume ratio meaning less proportional heat losses. At lower engine peeds the BSFC increases due to friction but at higher speeds, the increases are more dependent on the increased time for heat losses from the gas to the cylinder and piston wall.
Volumetric Efficiency (VE) The volumetric efficiency, ηv, can be define as the “ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure (during the intake stroke) to the mass density of the same volume of air in the
intake manifold” [12] or by using Equation ***.4 below where the Vd is the engine displacement, N is the engine speed and the 2 is a result of being a 4 stroke engine. 2 𝑚̇𝑎𝑖𝑟,𝑖𝑛𝑙𝑒𝑡
Equation ***.4
𝜂𝑣 = 𝜌
𝑎𝑖𝑟,𝑖𝑛𝑙𝑒𝑡 𝑉𝑑 𝑁
It can be used for indirectly determining the air mass flow charge when used with pressure and temperature measurements in the intake manifold as shown in Equation ***.5 below where R is the gas constant and pint and Tint are the intake manifold pressure and temperature respectively, and the 2 is used for four stroke engines. [1] 𝑃
Equation ***.5
𝑚̇𝑡𝑜𝑡𝑎𝑙 = 𝜂𝑣 2𝑅𝑇𝑖𝑛𝑡 𝑁𝑉𝑑 𝑖𝑛𝑡
The percentage increase in fuel required, ∆𝑚𝑓𝑢𝑒𝑙 [%], can be determined by the percentage difference in the product of the engine speed, vengine, the cylinder pressure, P, and the volumetric efficiency, ηv, for that engine under those conditions. This leads to Equation ***.6 shown below: Equation ***.6
𝑣
×𝑃 ×𝜂
(𝑣𝑒𝑛𝑔𝑖𝑛𝑒𝑎 ×𝑃𝑎 ×𝜂𝑣,𝑎 ) 𝑚𝑓𝑢𝑒𝑙 = 𝑚𝑓𝑢𝑒𝑙 + ∆𝑚𝑓𝑢𝑒𝑙 𝑒𝑛𝑔𝑖𝑛𝑒𝑏
𝑏
𝑣,𝑏
Volumetric Efficiency tables There are two types of VE table:
Alpha-N tables - Maps how throttle opening percentage (%) and engine speed relate to VE in spark-ignition engines. MAP (Manifold Absolute Pressure) sensor tables – Maps absolute pressure (Pa) inside the manifold sensor and engine speed relate to VE.
Both can additionally have other factors mapped onto them or included such as the effects of temperature and humidity on VE for a given combination of engine speed and throttle or pressure. MaP pressure sensors can be placed anywhere after compressor, before or after throttle for wide open throttle, or after the throttle for partial opening. Then using the pressure measured, the engine speed, Volumetric Efficiency, and optionally temperature and external pressures, are used with maps similar to those of Alpha-N to monitor air flow behaviour.
Pressure-Volume Diagrams Mapping the changes in pressure against volume in the cylinder allows the estimation of the net work done by a thermodynamic cycle by studying the areas between the processes, an example can be seen in pink in Figure ***.1. Figure ***.1 - A typical theoretical P-V diagram of a Diesel engine [13]
Figure ***.2 - A theoretical P-V diagram of an engine showing the possible identification of process errors in the shapes of the diagram [14]
The precise shapes on the diagrams can show more details about the processes involved, for example the pressure drop in the intake of the naturally aspirated engine shown in the negative work done across the bottom, and the influence of the pre-chambers as well as aiding identification of errors, such as shown in Figure ***.2 above.
Mean Effective Pressures Introduction Mean effective pressure (MEP) is “a fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle” [15]. This allows comparison between engines of the same size as the larger the MEP, the more work per cycle and so the more effective the engine. It can be defined using Equation ***.7 below where ng is the number of crank revolutions per power stroke per cylinder, N is the engine crankshaft speed, Vd is the displacement volume, and P is power: Equation ***.7
𝑀𝐸𝑃 =
𝑃 𝑛𝑔 𝑉𝑑 𝑁
Brake Mean Effective Pressure (BMEP) The BMEP is “the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output” [16] i.e. the MEP when the value of power P is set to that of the brake power output. This is therefore not an actual measure of pressures in the cylinder at any point but simply an indicator of efficiency. BMEP values can therefore be estimated for the 1GM10 example use case engine for a few key engine speeds, with results based on the data provided in Appendix *** shown in Figure ***.3 below, showing a typical profile for a diesel engine. Figure ***.3 - Table of Properties for BMEP Specifications Engine Speed [RPM] 3600 2600 1800
Approximate BMEP [kPa] 702 726 628
Indicated Mean Effective Pressure (IMEP) This is the MEP as defined for particular components of the process. For 4 stroke engines, there are two common forms of IMEP: -
Gross IMEP (IMEPG) – IMEP only for the work done during the compression and expansion processes. Net IMEP (IMEPN) – IMEP for the cumulative work output over all the four strokes, including intake and exhaust processes (pumping loop).
Generally the net IMEP is more useful as it is considered good practice to include the pumping loop in the analysis, especially during design and testing [9]. Frictional Mean Effective Pressure (FMEP) This is the MEP whereby the power is that lost through the frictional work done in the processes. It can be defined as shown in Equation ***.8 as the losses between the total IMEPG and the useful output BMEP. Equation ***.8
𝐵𝑀𝐸𝑃 = 𝐼𝑀𝐸𝑃𝐺 − 𝐹𝑀𝐸𝑃
Ignition Timing Diagrams The mapping of pressure allows estimations of heat release per crank angle that allow studies of the timings involved in the combustion process as shown for example in Figure ***.4 below. Figure ***.4 - Theoretical combustion timing of typical Direct-Injection Diesel engine [3]
The phases of the process can generally be classified into the following phases:
Ignition delay – period between start of start of fuel injection into combustion chamber and start of combustion as determined by change in slope of p-angle diagram. fuel mixes with air to within flammability limits Premixed or rapid combustion phase – combustion occurs within a few crank angle degrees in the fuel and air mixture premixed in the previous phase, generating high heat release, and the burning mixture is added to the other fuel Mixing-controlled combustion phase – Once the premixed fuel is used up, the rate is determined alone by the remaining fuel. Heat release rate may or may not reach a second peak. Rate is determined mainly by rate of fuel-air mixing.
Late combustion phase – When some heat release rate continues into the expansion phase.
In engines with swirl chambers, there is no initial high peak as shown in direct injection engines, generating graphs such as shown in Figure ***.5 where C is the process of fuel distributed near the wall: mixing proceeds during the delay but at a rate smaller than direct fuel injection and after ignition, mixing is accelerated by evaporation becoming rapid and radial mixing introduced by differential centrifugal forces. Figure ***.5 - Theoretical combustion timing of typical swirl chamber Diesel engine [3]
Appendix *** - Classification of Methods of Investigating Flow in Engines From researching literature and techniques for this project it was found that the approaches used to study air flow in engines could be generally classified into groups: Purpose The approaches to air flow measurement could be useful for two main different uses:
Methods for Optimal Use – Those methods which can be used to study the flow in an engine test bed to adjust or monitor the input or environmental parameters for control of an engine in use presently, or to be used in the future. This could be done in situ or in a dedicated test bed space. Methods for Optimal Design – Methods which are used to study the flow in engines in order to influence the design parameters of developing an engine. These can often, but not necessarily, be time consuming or destructive as there is no need to rely upon the quality of the engine after testing.
Although the primary focus of this report will be on methods for optimal use, it is worth noting that others are still investigated as the development of technology and sensors allows the potential to incorporate the results and data obtained during the research done for optimal design in order to optimise results of use measurement. For example, the results of Particle Image Velocimetry studies could be used to incorporate the behaviour of individual engine, environment, and fuel types in instrumentation using machine learning techniques to provide more reliable or readings of Volumetric Efficiency. In order to do this effectively, an understanding of the other methods and therefore the appropriateness of their data, is needed – for example to avoid or accommodate for the PIV results that provide “reliable values” but were generated using inappropriate assumptions of steady state behaviour [17]. Direct and Indirect Measurement The methods could be classified by which factors they are directly measuring and which factors they were deriving:
Conditions in the Cylinder(s): o For example:
Volume of air in cylinder(s) Oxygen content of air in cylinder(s) Oxygen used in combustion reaction Humidity in cylinder(s) Motion of air in cylinder(s) Volumetric Efficiency (VE) Brake Mean Effective Pressure (BMEP) Environmental/External Factors to Cylinder(s): o For example: Coolant temperature Manifold design
Often methods for optimal use are a combination of measuring several different factors directly, and either deriving the remaining ones through algorithms or calculations using historical data for that specific engine and fuel measured in test bed, or using generic data for that type of engine and conditions. Methods for optimal design will focus on direct measurement of fewer factors.
Appendix*** - Yanmar 1GM10 Specifications for Engine Flow Measurements As we have a Yanmar 1GM10 engine available in the Mechanical Engineering Teaching Laboratory for practical experimentation purposes, we can use this model to represent and focus our parameters around. The important features of the engine model as relevant to engine flow analysis are outlined below. Key features described in the operation manual are [18]:
“It is recommended that new vessels be propped so the engines can operate at 100 to 200 min-1 above the Maximum Rated Power Output engine speed (3700 to 3800) to allow for some added weight and hull resistance. The engine must be able to reach the Maximum Rated Power Output (3600 min-1) under full load at all times” “…coolant temperature reaches the maximum allowable temperature (65°C “ “The exhaust gas is mixed with seawater in the mixing elbow.”
Fuel specifications listed include [18]:
Diesel fuel specifications: o ASTM D975 No. 2-D S15, No. 1-D S15 (USA) o EN590-2009 (EU) The fuel cetane number should be 45 or higher. Water and sediment in the fuel should not exceed 0.05% by volume. Total aromatics content should not exceed 35% by volume. Less than 30% is preferred. PAH (poly cyclic aromatic hydrocarbons) content should be below 10% by volume.
Figure ****.1 - Properties of the Yanmar 1GM10 Engine Relevant to Air Flow Measurement Feature Continuous Rated Output Maximum Rated Output (Fuel 25° at Inlet) Maximum Rated Output (Fuel 40° at Inlet) Continuous Rated Engine Speed Maximum Rated Engine Speed Displacement
Values [19], [20], [18] 5.9 kW (8 hp metric) 6.7 kW (9.1 hp metric) 6.7 kW (9.1 hp metric) 3400 rpm 3600 rpm 0.318 L [19.4 cu in]
Cylinders Bore x Stroke Type Combustion System Aspiration Starting System Cooling System Emission compliance Fuel density at 15° Fuel Injection Timing (Before Top Dead Centre) Fuel Injection Pressure
1 in-line 75mm x 72mm 4-stroke, vertical, water cooled diesel engine Indirect injection [special swirl type pre-combustion chamber] Natural Aspiration Electric starting 12V - 1.0 kW with manual combination Direct seawater cooling by rubber impeller seawater pump EU: RCD 2, BSO II, EMC 0.84 g/cm3 15°±1° 16.7±0.5 MPa
Figure ***.2 - Rate of Fuel Consumption of Yanmar 1GM10
Using the properties described above we can calculate parameters needed for developing engine flow measurement specifications.
Fuel Flow Measurement Specifications Firstly, if we consider measuring fuel flow analysis, the key parameters can be selected as shown in Figure ***.3. Figure ***.3 - Table of Properties for Fuel Flow Specifications Feature Values Fuel density at 15° [g/cm3 ] 0.84 Fuel Injection Timing (Before Top Dead Centre) 15°±1° Minimum Fuel Cetane Number 45 Diesel typically has a thermal expansion coefficient of around 0.00083 /°C which means that from 15°C to 40°C the diesel density goes from 0.84 g per cm3 to 0.84g per 1.02075cm3 which is 0.823 g per cm3 . From the graph shown in Figure ***.2 it can be found that the maximum fuel volume flowrate is approximately 2.5 litres per hour or 0.694 mL/s and the so the maximum fuel mass flow rate could be estimated at 0.58 g/s using the
larger density at 15°C and this volumetric flowrate. This is a value that whilst rarely practically used by the engine, can be considered a maximum necessary for the fuel flow measurement system to accommodate for, ignoring additional safety values. The fuel flow rate can be calculated using the same procedure for a few key engine speeds to generate the results shown in Figure ***.4 below. Figure ***.4 - Table of Properties for Fuel Flow Specifications Engine Speed [RPM]
Fuel Volumetric Flowrate [cm3/s]
3800
0.69
3600
0.64
2600
0.26
Temperature at Inlet [°C]
Fuel Density [g/ cm3]
Fuel Mass Flowrate [g/s]
15 40 15 40 15 40
0.84 0.82 0.84 0.82 0.84 0.82
0.58 0.57 0.54 0.53 0.22 0.21
Air Flow and Pressure Measurement Specifications And so similar analysis can be done for developing air flow rates and pressure measurements, with the relevant specifications of the engine displayed in Figure ***.5 below. Figure ***.5 - Table of Properties for Pressure and Air Flow Specifications Feature Combustion System Aspiration Fuel Injection Timing (Before Top Dead Centre) Maximum Rated Engine Speed [RPM] Maximum Rated Engine Speed [Hz] Displacement Cylinders Fuel Injection Pressure
Values Indirect injection [special swirl type pre-combustion chamber] Natural Aspiration 15°±1° 3600 60 0.318 L 1 in-line 16.7±0.5 MPa
From this a maximum total volume displacement rate can be seen to be 19.08 L/s. Naturally aspirated means the air flow in is taken to be at atmospheric pressure which is approximately 1.01325 bar and therefore at 15°C it is a density of 0.001225 g/cm3 , and at 40°C it is a density of 0.001127 g/cm3 . If the total displacement volume was occupied with air with a volumetric efficiency of 100% then the mass air flow rate at the inlet could be found to be 0.01075 kg/s or 10.75 g/s. This is could be considered the maximum that measurement equipment should have the capacity to accommodate. In practice because of the nature of the naturally aspirated engine, the actual volume of air during intake is always lower than the displacement volume. Additionally the air flow rates measured entering the cylinder are not necessarily representative of the behaviour in cylinder as it is a small engine with pre swirl chamber injection and “in medium and smaller size DI engines, the air flow is usually swirling about the cylinder axis at up to 10 times the crankshaft rotational speed; this air-flow pattern increases the rate of entrainment of air into the fuel jet to increase the fuel-air mixing rate” [3]. Also there are gas flows between the chambers that result in non-uniform composition and temperature distributions, and heat flows between the chambers as well [3]. Generally though only during combustion is the pressure difference across orifice or nozzle large enough to be sufficient to model.
However, taking a very simplistic model for diesel engines, the stoichiometric air fuel ratio (AFR) is around 14.5:1 which means that if the fuel mass flow rate for 40°C at 60Hz is 0.53 g/s then the air mass flowrate required for stoichiometric combustion is 7.69 g/s. However, in diesel engines the air fuel equivalence ratio is limited (as discussed in Appendix ***) to lean ratios, meaning that the air mass flowrate should be in practice larger than that generated from the stoichiometric AFR, so 7.69 g/s could be considered a minimum for that engine speed. However, the swirl type precombustion chamber means that the peak pressure will be lower than typical and that fuel is injected at low pressures into swirl chamber during air compression, approximately 167 bar in this case, and once there it is mixed with a rich AFR, and only then injected into the main chamber for the lean mixing. In terms of pressure specifications, typical 4 stroke naturally aspirated engines have peak pressures around 5 – 7MPa but calculating more specific estimates is difficult for the 1GM10 type model as the swirl chamber complicates matters, reducing the maximum peak pressure from a more typical diesel engine. It must also be acknowledged that being a diesel engine means that modelling needs to account for effects of: o o o o o o
unsteady liquid-fuel jet phenomena-atomization, liquid jet and droplet motion, fuel vaporization, air entrainment, fuel-air mixing, ignition chemistry
Appendix *** - Proposed Full Engine Flow Measurement Specifications Figure ***.1 - Concept Design Features and Requirements
Priority (High, Medium, Low) High High Medium High High Low Low Medium Medium Low Low Medium High High High High Medium High High Medium
Feature Minimal interference with air flow stream Minimal interference with fuel flow stream Accounts for high pressure pulsation of single cylinder Measures air flow rates up to 0.01075 kg/s Measures fuel flow rates up to 0.69 cm3/s and 0.58 g/s Accounts for external temperature variations due to seawater cooling Accounts for fuel quality variations Provides data in format suitable for output to Arduino microprocessor Non-destructive interface with engine Accounts for external air quality variations Ease of calibration Ease of understanding processes for teaching Geometry compatible with typical small marine diesels Suitable for use with diesel engines Suitable for use with naturally aspirated engines Suitable for use with single cylinder engines Suitable for use with pre-swirl chamber engines Suitable for up to maximum engine speed 3800 min-1 Suitable for fuel injection pressures up to 16.7±0.5 MPa Precision of fuel injection timing minimum of ±1°
Appendix ** - Properties of Engine Flow Instrumentation Proposals Airflow Measurement - Properties of Bosch HFM-8 Hot Film Air Flow Meters Figure ***.1 - Table of Properties for Feature Output Signal New Part Tolerance Lifetime Tolerance Pulsation Error Power Consumption of Basic Sensor Optional Add-ons Available Supply Voltage
Values Digital using the SENT Protocol or FAS or LIN ±1.5% ±3.5% ±6% < 20mA Temperature, Humidity 5V/12V
The Bosch series of air mass flow sensors are designed for effective use within engine flow analysis. The HFM-8 is one of the highest quality hot-wire or hot-film sensors available and so without further costing or budgetary information, can be chosen as an ideal design proposal.
Fuel Flow Measurement - UK Flowtechnik Oval Gear Positive Displacement Flowmeters Figure ***.2 - Table of Properties for UK Flowtechnik OM004A Oval Gear Positive Displacement Flowmeter Feature Material Nominal Size Flow Range Accuracy @ 3cp Repeatability Temperature Range Maximum Pressure Output Pulse Resolution – with Reed Switch Output Pulse Resolution – with Hall Effect (NPN) Reed Switch Output Hall Effect Output (NPN)
Values [21] Aluminium 4mm 0.5 ~ 36 litres/hour ±1% o.r Typically ±0.03% -20°C to +120°C 15 bar 2890 pulses/litre 2890 pulses/litre 30 Vdc x 200mA max 3 wire open collector, 5 ~ 24 Vdc max x 20mA max
The specifications in Figure ***.2 gives approximately 2.01 pulses/second at maximum fuel flow rate and 1.53 pulses/second at nominal engine speed fuel flow rate, and overall complies successfully with the specifications, with a cost based upon approximate referenced quotes from indirect sources (not taken directly from the manufacturer) of around £200 per unit, and it is generally reviewed as high quality value for cost. However, the same manufacturer, UK Flowtechnik, also have an EM004 range designed specifically for fuel measurement and that they claim “can tackle the problem of pulsation in the low pressure lines caused by the injector pumps…By employing special rotors we can remove the inaccurate readings which can be generated by this problem, giving very precise and accurate results.” The material is for EM004 range is 316 stainless steel but otherwise the specification is the same. The decision between them would require a more in depth cost-benefit analysis, and thus more detailed costing information.
Pressure Measurement - Properties of Kistler Pressure Sensors Figure ***.3 - Table of Properties for Kistler Piezoelectric Pressure Sensor Type 603B Feature Sensitivity Natural Frequency Diameter Temperature Range Calibrated partial ranges Acceleration Sensitivity Temperature Coefficient of Sensitivity Shock Resistance Capacitance
Values [21] 5 [pC/bar] >300kHz 5.5mm -196°C to +200°C 0-10 bar and 0-20 bar
