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Effects of exhaust gas recirculation on combustion and emissions of a homogeneous charge compression ignition engine fuelled with primary reference fuels M Yao, B Zhang, Z Zheng, Z Chen and Y Xing Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering 2007 221: 197 DOI: 10.1243/09544070JAUTO102 The online version of this article can be found at: http://pid.sagepub.com/content/221/2/197

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Effects of exhaust gas recirculation on combustion and emissions of a homogeneous charge compression ignition engine fuelled with primary reference fuels M Yao*, B Zhang, Z Zheng, Z Chen, and Y Xing State Key Laboratory of Engines, Tianjin University, Tianjin, People’s Republic of China The manuscript was received on 24 May 2005 and was accepted after revision for publication on 11 October 2006. DOI: 10.1243/09544070JAUTO102

Abstract: The effects of exhaust gas recirculation (EGR) of primary reference fuel on combustion and emissions of a homogeneous charge compression ignition (HCCI) engine are investigated. The results show that EGR delays the ignition timing, slows down the combustion reaction rate, reduces the pressure in cylinder, and extends the HCCI operation region. The maximum indicated thermal efficiency of fuels with different octane numbers appears in the region of high EGR rate and large fuel : air equivalence ratio. In the region of high EGR rate, hydrocarbon emissions rise up sharply as the EGR rate increases. Carbon monoxide emissions increase with the increase in EGR rate. Nitrogen oxides emissions increase abruptly as knocking combustion occurs. Keywords: exhaust gas recirculation, octane number, homogeneous charge compression ignition

1 INTRODUCTION Homogeneous charge compression ignition (HCCI) incorporates the best features of both spark ignition (SI) and compression-ignition, direct-injection (CIDI) engines. It has the potential to realize high efficiency and achieve low emissions [1–7]. Like an SI engine, the charge is well mixed which minimizes particulate emissions, and similar to a CIDI engine it is compression ignited and has no throttling losses. This combination not only makes the efficiency of the HCCI engine as high as that of a CIDI engine, but also produces ultra-low oxides of nitrogen (NO ) and x particulate matter (PM) emissions. It is commonly accepted that the ignition and combustion of HCCI engines are controlled by chemical kinetics [8–10]. Temperature history (and to a lesser extent, pressure history) – together with the concentration of O , different fuel contents, and 2 combustion products – govern how combustion is initiated. As a consequence, ignition and combustion * Corresponding author: State Key Laboratory of Engines, Tianjin University, No. 92 Weijin Road, Tianjin 300072, People’s Republic of China. email: [email protected]

JAUTO102 © IMechE 2007

are strongly influenced by air : fuel ratio, inlet temperature, compression ratio, residual gases, and exhaust gas recirculation (EGR) [11]. HCCI combustion starts more or less simultaneously in the entire cylinder. To limit the rate of combustion under these conditions, the mixture must be highly diluted. EGR is an important technology in the control of ignition and combustion in an HCCI engine. In an HCCI engine, a highly diluted mixture is achieved by EGR, including exterior EGR and interior EGR. For high cetane rating fuel, the autoignition (AI) timing occurs too early and combustion rates become excessive; this results in HCCI only operating in light-load regions with a low indicated thermal efficiency (ITE). It demands the AI timing to be delayed until near top dead centre (TDC) and decreased combustion rates. Therefore, exterior EGR is used in the HCCI combustion of high cetane fuel. With the cooled EGR, the AI timing is limited to near the TDC and the combustion reaction rate is decreased [11–13]. For high octane number (ON) fuel, in-cylinder gas temperature must be high enough to initiate and sustain the chemical reactions leading to AI processes. Substantial charge dilution is necessary to control combustion rate. Both of these requirements can be realized by recycling the Proc. IMechE Vol. 221 Part D: J. Automobile Engineering


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M Yao, B Zhang, Z Zheng, Z Chen, and Y Xing

burnt residual gases. Therefore, the interior EGR, using variable valve timing (VVT) [14, 15] and/or variable valve actuation (VVA) [16] to change the amount of hot residual gases retained in the cylinder, is used in the combustion of high-ON fuel. There are several reasons for the described effect of EGR on the HCCI combustion process [17]. 1. Effect of heating charge – the burnt gases (not cooled EGR or residual burnt gases) are introduced into the combustion chamber to heat the fresh charge. This will make the temperature of the charge in a cylinder increase; the fuel would be ignited by compression. 2. Effect of concentration of O in a cylinder (also 2 known as the ‘dilution effect’) – the burnt gases partially replace air from the intake port, and O 2 concentration is decreased. On the other hand, the increase in temperature of the intake charge and pipe wall reduces the volumetric efficiency. This leads to a slow down in the combustion reaction rate and extends the combustion duration. 3. Effect of heat capacity – the total heat capacity of the in-cylinder charge will be higher as burnt gases increase, owing mainly to the higher specific heat capacity values of H O and CO . As a result, 2 2 it decreases the in-cylinder temperature, then delays the ignition timing and extends the combustion duration. 4. Effect of chemistry – some combustion products in burnt gases can participate in low-temperature and high-temperature reactions, and the active radicals have an obvious influence on the combustion reaction. Zhao et al. [18] presented some detailed work investigating the effects of EGR on controlled autoignition (CAI) combustion. They concluded that the charge heating effect is mainly responsible for the advanced AI timing due to hot burnt gases. It increases the heat release rate and shortens the combustion duration. The dilution effect does not affect AI timing, but it extends combustion duration. It slows down the heat release rate only when a large amount of burnt gases are present in a cylinder. The heat capacity effect is mainly responsible for the reduction in the heat release rate and, as with dilution, it extends the combustion duration. The chemical effect does not affect AI timing and heat release rate. It reduces the combustion duration slightly at high concentrations of burned gases. The stratified burned gases facilitate the CAI combustion due to the presence of a higher temperature region at the boundary between the hot burnt gases and combustible charge. Peng et al. [13] investigated the

effects of air : fuel ratios and EGR rates on HCCI combustion of n-heptane. They concluded that the AI timing is very sensitive to EGR rates, but combustion durations primarily depend on the air : fuel ratios. EGR could extend the HCCI operation region to a high-load mode, and NO emissions are at near x zero (parts/million) levels in all attainable operating regions. On the other hand, fuel has a significant impact on HCCI combustion. The fuel requirements for HCCI operation are not the same as for different operating modes. For example, for a high load, it is desirable to use a fuel with a low cetane rating to delay the ignition timing near TDC; while for a light load, a fuel with a high cetane rating may be desirable. Christensen et al. [19–21] presented some detailed work investigating the heat release and emissions characteristics of several fuels including primary reference fuel (PRF), natural gas, mixtures of commercial diesel and gasoline, and ethanol. Olsson et al. [11] reported the results of research investigating the effects of cooled EGR on emissions and performance of a turbocharged HCCI engine in different operating regions. They presented insightful analyses of the effects of EGR on combustion, emissions, and performance of an HCCI engine. However, the engine was running under closed-loop control to achieve fixed combustion phasing by adjusting the ratio of the two fuels (ethanol and n-heptane) and the intake temperature. Thus, they had difficulty in separating the effects of fuel, EGR rate, and temperature explicitly. The current authors have previously investigated the effects of ON of the PRF on HCCI combustion characteristics, performance, and emissions. The results showed that, with the increase in the ON of the PRF, the ignition timing is delayed, the combustion rate decreased, and the cylinder pressure decreased. The HCCI combustion can be controlled and then the HCCI operating range can be extended by burning different ON fuels at different engine modes, so that the engine burns low-ON fuel in lightload mode and high-ON fuel in large-load mode. There is an optimum ON that achieves the highest ITE at different engine loads [22]. In this paper, the effects of EGR on HCCI operation of PRFs with different ONs were investigated. PRFs could be obtained by mixing iso-octane with ON 100 and normal heptane with ON 0. It is straightforward to get these different ON fuels. For the PRF, boiling point is much lower than that of diesel and gasoline, and so the difficulty of fuel vaporization can be eliminated. PRFs were delivered by port injection in order to get the premixed charge as homogeneous as possible. First, the HCCI operating regions of

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Combustion and emissions of an HCCI engine

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different PRFs, with regards to air : fuel ratio and EGR rates, were obtained. Secondly, the effects of EGR and ON on HCCI combustion characteristics, such as rate of heat release, AI timing, combustion duration, and combustion efficiency, were examined. Finally, the effects of EGR and ON on emissions (including NO , CO, and HC), in HCCI operating regions are X presented.

2 EXPERIMENTAL APPARATUS Experiments were conducted on a modified singlecylinder, water-cooled, direct-injection diesel engine with bore and stroke both of 115 mm, an v-shaped combustion chamber, a compression ratio of 17, rated power of 14.7 kW at 2200 r/min. Table 1 shows the engine specifications. In order to reduce the effect of intake air pulsation variation on the measurement of intake air flowrate, a large tank was added as a pressure stabilizer in the intake system. The flow meter was mounted at the inlet of the tank. The experimental set-up is illustrated in Fig. 1. An electronically controlled fuel injection system was used for the fueling. The electromagnetic valve was mounted close to the intake port. PRFs were obtained by mixing iso-octane with ON 100 and normal heptane with ON 0. PRFs with ONs of 0, 40, 60, and 80 were investigated. The ON of a fuel mixture of iso-octane and n-heptane can be calculated by ON=V ×100 iso

(1)

Table 1 Engine specifications Bore Stroke Displacement Compression ratio Intake valve opens Intake valve closes Exhaust valve opens Exhaust valve closes

115 mm 115 mm 1200 cm3 17 : 0 : 1 12° BTDC 45° ABDC 55° BBDC 14° ATDC

BTDC, before top dead centre; ABDC, after bottom dead centre; BBDC, before bottom dead centre; ATDC, after top dead centre.

Fig. 1 Experimental set-up

where V is the iso-octane volume fraction of the iso PRF. Table 2 shows the fuel specifications of n-heptane and iso-octane. Compared with diesel and gasoline, n-heptane and iso-octane have much lower boiling points. Therefore, a homogeneous mixture of fuel with air can be formed during the compression stroke. In order to separate the effects of air : fuel ratio (l), fuel ON, and EGR dilution explicitly, it is necessary to eliminate the influence of different EGR rates on the temperature of the charge. The exhausts were cooled through a heat exchanger before they were mixed with fresh intake air. The charge temperature was measured at the entry of the intake manifold, downstream of the mixing of EGR and the charge of fresh air and upstream of fuel injection; it was controlled at 30–40 °C. The percentage EGR is defined as the percentage of the total intake mixture by volume, which shows the volume of controlled EGR. An accurate measurement of EGR rate is required to control EGR adequately, but this task is difficult to achieve using currently available technology. There are two common measurements of EGR rate: measuring the concentration of CO in the intake or recording 2 the output gas and air :fuel ratio. The first method was used in this investigation, which means that

Table 2 Fuel specifications

Cetane number Octane number T (°C) Boil Density (kg/m3) (Air : fuel)


Normal heptane

Iso-octane

Diesel fuel

Gasoline unleaded

— 0 98.4 686 15.18

— 100 118 700 15.13

54 — 280 (95%) 814 14.5

— 98(RON) 195 750 14.6



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the EGR rate was calculated by the concentration of CO in the intake and output gas by volume. The 2 formula is EGR%=

(CO %) 2 inlet ×100% (CO %) 2 exhaust

(2)

Before the experiment, it was necessary to run the engine through a warm-up procedure in diesel operation. When the coolant temperature reached approximately 80 °C and the oil temperature reached approximately 70 °C, the engine was switched to HCCI operation and the test began. A pressure transducer, mounted on the cylinder head and connected to a Labview (Hong Kong) data acquisition system, recorded the cylinder pressure. The rates of heat release and in-cylinder temperatures were calculated using a zero-dimensional model. A vortex flow meter was used to measure the air flowrate, its error was 1.0% (of full scale, 150 m3/h). A volumetric fuel meter was used to measure the fuel consumption, its error was 1% (of actual reading). The HORIBA (Kyoto, Japan) MEXA-7100 DEGR automobile emission analysis system was used to measure the NO , CO, and total hydrogen carbon x (THC) emissions. Important parameters in the experiments were: engine speed, 1400 r/min; coolant temperature, 80 °C; oil temperature, 70 °C; inlet temperature (mixed with EGR), 30–40 °C.

3 RESULTS AND DISCUSSION In order to get detailed information of the effects of EGR on the combustion, performance, and emissions of PRFs with different ONs, both the experimental results in all testing regions and the iso-lines of those results in HCCI operation regions are described in this paper. The graphs, for example Fig. 6, indicate the testing points or calculated results in all testing regions with the x-coordinate as the air excess ratio (l) and the y-coordinate indicating the different parameters (for example, the y-coordinate in Fig. 6 shows combustion efficiency calculated by the component of exhaust gases). In these graphs, each curve covers all the testing points or calculated results in all testing regions of diverse EGR rates and shows the effect of EGR on different parameters. The iso-line graphs, for example Fig. 7, indicate the effects of EGR rate (as x-coordinate) and air excess ratio (as y-coordinate) on different parameters in HCCI operation regions, with operation regions where iso-lines indicate different parameters in

different figures (the combustion efficiency iso-lines in Fig. 7 correspond to data in Fig. 6). Based on the experimental data or calculated results (for example those shown in Fig. 6), iso-lines (for example those shown in Fig. 7) were obtained by the correlation method in the ORIGIN (www.originlab.com) data and curve management software [23]. 3.1 Operation region definition As indicated in the discussions above, one of the purposes of this study was to investigate the effect of EGR on the HCCI operation region. In order to do this, the criteria for defining the operating range of HCCI combustion had to be established. The HCCI operation region is limited by misfire and knocking, the operation boundaries are associated with these factors (misfire and knocking). The first boundary defines the lower limit for HCCI combustion. At light loads, fuel flowrate is decreased; the net heat release is also decreased. It is believed that the resulting gradual reduction of the average combustion temperature leads to more unburnt charge – characterized by high CO and unburnt HC emissions, and by an increase in cycle-to-cycle variation. Cycle-to-cycle variations of the combustion process in an engine can be monitored by a cylinder pressure sensor. Fluctuations in both maximum cylinder pressure and the indicated mean effective pressure (IMEP) were used as a measure of the cycle-to-cycle variations and were expressed as COV and COV . The COV pmax IMEP IMEP was used in this investigation, and the COV for IMEP 100 consecutive engine cycles was calculated as the standard deviation (s ) divided by the mean value IMEP (IMEP) as a percentage [24]. COV

IMEP

s = IMEP ×100% IMEP

(3)

It has been found that vehicle driveability problems usually occur when COV exceeds about 10 per IMEP cent [24]. Consequently, the lower limit of HCCI operation could be defined as when COV IMEP exceeded 10 per cent. When the fuelling rate is increased (lower l), the HCCI combustion rates also increase and intensify, and gradually cause unacceptable noise and potential engine damage, and eventually lead to unacceptable levels of NO emissions. Therefore, knocking comx bustion can be defined as being at the upper limit of HCCI combustion. In this investigation, the upper limit of HCCI combustion was defined as being when the rate of pressure rise in a cylinder exceeded 1.0 MPa per degree crank angle (°CA) (dP/dQ =1.0 MPa/°CA) for each individual cycle. max





The recorded pressure in the cylinder determined the values of both COV and dP/dQ . Therefore, IMEP max the HCCI operation region is the area in which values of COV are less than 10 per cent and the values of IMEP dP/dQ are less than 1.0 MPa/°CA. This definition max is applied for all of the PRFs in this study. 3.2 Effect of EGR on HCCI operation region Figure 2 shows the successful HCCI operation region and the IMEP values for different PRF fuels with ONs of 0, 40, 60, and 80. The x-axis represents the EGR rate, and the y-axis represents the overall air : fuel ratio (l) of the cylinder charge. As shown in Fig. 2, the upper trace is the boundary of partial combustion, the bottom trace is the boundary of knocking, the symbols on the graphs are the experimental data and the iso-lines are IMEPs of the HCCI operation region. Figure 2 shows that IMEP is affected mainly by l. It increases from a high l value to a low l. From Fig. 2, it can be seen that EGR can extend HCCI operation ranges. An increase in EGR rate increases both the maximum and minimum limits of HCCI operation.

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When the EGR rate is too high, misfire occurs at the right of the panels in Fig. 2. A slight increase or decrease in the concentration of the mixture would cause knocking or misfiring in-cylinder. With high EGR rates and large load (l is small), the change scale of the EGR rate becomes narrower. The figure also shows that PRFs with different ONs can tolerate different EGR rates. The higher the ON, the lower the tolerance there is to EGR dilution. n-Heptane (ON of 0) can tolerate a very high EGR rate, up to 75 per cent by volume, but the ON 80 fuel can only tolerate 45 per cent EGR rate. By comparing fuels of four ONs, it was found that the maximum IMEP increases with the increase in ON. The maximum IMEP of ON 80 fuel is 0.58 MPa while that of ON 40 fuel is 0.47 MPa. However, the IMEP of the lower limit also increases with the increase of ON. In particular, for the fuel of ON 80, the lower limit is 0.29 MPa, and the range of air : fuel ratios and EGR rates is narrow. It can be concluded that the PRF of ON 60 covers the largest operation region. From this viewpoint, the fuel with an ON of 60 fits HCCI best under these test conditions.

Fig. 2 Successful HCCI operation region and the IMEP values of PRFs with different octane numbers JAUTO102 © IMechE 2007



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Fig. 3 Effects of EGR and ON of the PRF on cylinder pressure history and heat release rate

3.3 Effect of EGR on HCCI combustion characteristics Figure 3 shows the effects of EGR on cylinder pressure and the rate of heat release for the PRFs with ONs of 0, 40, 60, and 80, engine speed of 1400 r/min, and an IMEP of 0.34 MPa. It shows that HCCI combustion is characterized by a two-stage heat release process: heat release of the low-temperature reaction (LTR) and that of the high-temperature reaction (HTR). Between the LTR and the HTR, there is almost no heat released, this is the negative temperature coefficient (NTC) period. Data indicate that LTR, HTR, and NTC are affected by EGR rate and ON. The effect of EGR on the timing of the LTR is slight, but the effect on the amount of heat released during the LTR is strong. With an increase in EGR rate, the timing of the HTR is delayed, the NTC period is prolonged, the rate of heat release is decreased, and this results in a decrease in the maximum cylinder pressure. This tendency becomes more and more distinct as the EGR rate increases. For example, in the

case of n-heptane (with ON 0), compared with the condition of no EGR, with 70 per cent EGR the timing of the HTR is delayed nearly 15° CA, and the timing of maximum cylinder pressure is delayed 8.5° CA. There are three reasons for this phenomenon, all related to the increase in EGR rate. First, as the heat capability of the charge in-cylinder increases, the

Fig. 4 Definition of AI timing and combustion duration (MCIT and MCD)





temperature in-cylinder decreases. The increase in specific heat capability causes the maximum temperature in the cylinder to fall. Secondly, when the concentration of O is reduced in a cylinder, the 2

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combustion reaction rate slows down, the combustion temperature in the cylinder decreases and the combustion duration is prolonged. Thirdly, some combustible materials in exhaust gas – such as

Fig. 5 Effects of EGR and ON of the PRF on MCIT and MCD

Fig. 6 Effects of EGR and ON of the PRF on combustion efficiency at all test points JAUTO102 © IMechE 2007



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unburnt HC, H , and CO – would take part in the 2 LTR and HTR in the new cycle, the process of combustion reaction. When EGR rate is high, some combustion products present in the exhaust gas would influence and shorten the combustion duration slightly. Although values (such as 5 per cent and 10 per cent) of mass fraction burnt are usually defined as AI timing, it is believed that the heating value of the LTR is related to both ON and EGR rate, as shown in Fig. 3. On the other hand, most heat is released during HTR. If the total HCCI combustion process is considered in its entirety, the HTR can be defined as the main combustion period, the main combustion timing (MCIT) can be treated as AI timing, and the main combustion duration (MCD) can be treated as combustion duration. Therefore, in this investigation, ignition timing is defined as MCIT in order to compare different EGR rates and fuel ONs, while the combustion duration is defined as MCD. Figure 4 shows the definitions of MCIT and MCD. Figure 5 illustrates the effect of EGR on the MCIT and the MCD at an engine speed of 1400 r/min and an IMEP value of 0.34 MPa. The trace is the same as that

mentioned above. With the increase in EGR rate, the MCIT is delayed gradually and the MCD is prolonged. The higher the EGR rate, the more obvious this tendency is. In other words, with the increase in EGR rate, AI timing is delayed, heat release rate is decreased, and combustion duration is prolonged. 3.4 Effect of EGR on combustion efficiency Combustion efficiency is the ratio between accumulated heat release and the heat supplied by the total fuel. However, the total heat released in the cylinder is hard to measure accurately in practice. Generally, combustion efficiency (g ) is calculated from the c exhaust composition using [24] (m ˙ +m ˙ )W xQ a f i i HVi (4) m ˙ Q f HVf where x are the mass fractions of CO, H , or HC i 2 respectively, Q are the lower heating values of HVi these components, m ˙ is the mass flow, and subscripts f and a denote fuel and air respectively. Figure 6 shows the effects of EGR on combustion efficiency at all test points and Fig. 7 shows the g =1− c

Fig. 7 Effects of EGR and ON of the PRF on combustion efficiency in the HCCI operation region Proc. IMechE Vol. 221 Part D: J. Automobile Engineering




effects in the HCCI operation region. When the value of l is the same, the combustion efficiency decreases as the EGR rate increases. When the EGR rate is low, the effect on the combustion efficiency is weak; when the EGR rate is increased, the effect becomes stronger. As shown in Figs 6 and 7, when the EGR rate is high, the combustion efficiency decreases. As the ON of the PRF increases, the combustion efficiency decreases at the same EGR rate. This is due to the contrary effect of EGR on combustion efficiency. On the one hand, unburnt fuel could oxidize in the next cycle, which increases the combustion efficiency; compared with the experimental result in reference [22], combustion efficiency showed some increase with EGR introduction. On the other hand, after increasing the EGR rate, the combustion temperature in-cylinder is reduced, this leads to a decrease in combustion efficiency. When the EGR rate is small, its effect on the combustion temperature is not strong. Because these two effects are basically counteracted, the effect of EGR on combustion efficiency is small. As the EGR rate increases, the combustion efficiency decreases in that the

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decrease of combustion temperature has a major effect on the decrease of the combustion efficiency. As the ON of the fuel increases, the combustion temperature in-cylinder decreases and the effect of EGR on combustion temperature becomes more obvious. When comparing Fig. 6 to Fig. 7, it can be seen that the combustion efficiency is very low outside the HCCI operation region. Figure 7 illustrates that the maximum combustion efficiency appears in the area of high EGR rate (next to the highest EGR rate) and high mixture concentration (low l), which is next to the knocking boundary. It can be found that the combustion efficiencies of the lower limit and upper limit decrease with an increase in fuel ON. 3.5 Effect of EGR on the ITE of the HCCI engine ITE is defined as the work delivered to the piston over the compression and expansion stroke (W ) to i the heat supplied by the fuel W i g= i m ˙ Q f HVf

(5)

Fig. 8 Effects of EGR and ON of the PRF on ITE at all test points JAUTO102 © IMechE 2007



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where W is the rate of working on the piston calcui lated by the pressure data in-cylinder over the operating cycle of the engine, m ˙ is the fuel flowrate, f and Q is the lower heating value of the fuel. HVf Figure 8 shows the effects of EGR on ITE at all test points and Fig. 9 shows the effects in the HCCI operation region. Comparing Fig. 8 to Fig. 9, it can be seen that the highest ITE increases with the increase of EGR rate in the HCCI operation region. The l value with the highest ITE decreases with the increase in EGR rate. However, outside the HCCI operation region, the introduction of an unusually high EGR rate results in a decrease in ITE because of the ultra-low combustion efficiency. Figure 9 also shows that the maximum ITE of various ON fuels is located in the area of high EGR rate and high concentration of mixture (low l), which is next to the knocking boundary. The effects of EGR rate and the mixture concentration on the maximum ITE vary according to the different fuel ONs. In the HCCI operation region, when the mixture concentration is low, the effect of EGR on ITE is weak, with the boundary related to the ON of the fuel. For lowON fuels, IMEP increases slightly as the EGR rate

increases. For high-ON fuels, IMEP decreases a little as the EGR rate increases. With the increase in mixture concentration, the influence of EGR on ITE is enhanced. This means that as the EGR rate increases, the ITE increases to an optimum area and then falls back. There are two dominant factors, i.e. combustion efficiency and ignition timing, that affect the ITE. Although increasing the combustion efficiency is beneficial for improving ITE, as can be seen in Figs 7 and 9, the trend of combustion efficiency is different from the trend of ITE. When the l value is decreased or the fuel ON is reduced, the combustion efficiency increases, but the AI timing advances. If the ignition timing is located on the optimum area, the increase in combustion efficiency results in increasing ITE (the effect of AI timing on ITE is not obvious in this area). However, the operation range of unnecessarily early AI causes a decrease in the ITE. At this time, the increase in EGR rate could delay the ignition timing and reduce the combustion reaction rate (delaying AI timing is good for raising the ITE). However, with the introduction of an unusually high EGR rate, too much delay in AI would lead to a decrease in the ITE.

Fig. 9 Effects of EGR and ON of the PRF on ITE in the HCCI operation region Proc. IMechE Vol. 221 Part D: J. Automobile Engineering




As a result, there is an optimum area for the highest ITE in the HCCI operation region; this is located in the area of high mixture concentration (low l) and the highest EGR rate, which is next to the knocking boundary. Figure 9 also shows that the PRF with an ON of 60 has the highest ITE, up to 45 per cent, and the area of high ITE covers the largest range of l values and EGR rates (it also covers the largest range of IMEP). Therefore, the PFR with an ON of 60 fits HCCI best under these test conditions. However, PRF 60 might not be the best choice of fuel if engine speed is changed.

3.6 Effect of EGR on emission 3.6.1 HC emissions HC emissions are the consequence of incomplete combustion of fuel. HC emissions are mainly from the quenching area near the walls of the engine com-

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bustion chamber and from the crevices, such as the region between the piston crown and the cylinder wall. In addition, if the ignition timing is not right the exhaust stroke will produce unburnt HC. EGR has two contrary effects on HC emission: one is that the intake of some unburnt HC with exhausted gas into the next cycle leads to a decrease in HC emissions; the other is that a decrease in combustion temperature in-cylinder leads to an increase in HC emissions. Figure 10 illustrates the effects of EGR rate and l on HC emissions at all test points. It shows that HC emissions at first decrease and then increase with increasing l. At the high EGR rate, Figure 11 shows the effects in the HCCI operation region. Figure 10 shows that HC emissions at first decrease and then increase with the increase in l. When EGR rate is high, HC emissions increase sharply, especially outside the HCCI operation region. The figure also shows that the effect of EGR on HC emissions relates to the EGR rate, l, and ON of the PRF. For n-heptane

Fig. 10 Effects of EGR and ON of the PRF on HC emissions at all test points JAUTO102 © IMechE 2007



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(ON 0) at a low EGR rate, HC emissions have a minimum value around l of 4.5, HC emissions could increase with the increase or decrease of l, and the effect of EGR on HC emissions is insensitive in this area. However, the effect of EGR is more sensitive when the EGR rate is above 50 per cent. For the other three PRF, HC emissions are increased with the increase of l or the increase in EGR rate. As the EGR rate increases, HC emissions increase accordingly. The higher the EGR rate is, the more obvious this tendency is. The reason for this is that the average combustion temperature decreases as l or EGR rate increases. The low temperature leads to incomplete combustion, resulting in higher HC emissions. Figure 11 shows that the effect of EGR on HC emissions gets more sensitive with the increase in EGR rate in the HCCI operation region. In addition, with the increase in PRF ON, the effect of EGR on HC emissions becomes more sensitive. The area of maximum HC emissions corresponding to fuels of various ONs appears in the high EGR rate area, which is next to the ‘misfire’ boundary. With the same EGR rate introduction, the HC emissions increase as the ON of the fuel increases.

3.6.2 CO emissions CO formation is one of the principal reaction steps in the HC combustion mechanism. In contrast to HC emissions, the formation of CO is controlled by chemical kinetics and it is mainly from the quenching area in the wall of the cylinder. In these areas the LTR and the blue flame reaction take place, but the hot flame reaction can not be realized because of the low charge temperature, which leads to incomplete oxidation of CO into CO . This can happen either by 2 cooling from the wall or by bulk quenching during expansion [11]. Figure 12 indicates the effects of EGR rate and ON of the PRF on CO emissions at all test points. Figure 13 shows the effects in the HCCI operation region. It can be found that CO emissions increase with an increase in l and/or EGR rate. Figure 12 shows that CO emissions are controlled primarily by the l value, CO emissions increase rapidly with an increase in l. With an increase in EGR rate, CO emissions rise gradually. However, this is different to HC emissions in that high CO emissions do not correspond to high EGR rates. Figure 14 illustrates the relationships between CO emissions and the

Fig. 11 Effects of EGR and ON of the PRF on HC emissions in the HCCI operation region Proc. IMechE Vol. 221 Part D: J. Automobile Engineering




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Fig. 12 Effects of EGR and ON of the PRF on CO emissions at all test points

maximum mean temperature in the cylinder. It shows that CO emissions have good coherence to the maximum mean temperature in the cylinder. As the temperature in the cylinder increases, CO emissions decrease monotonically. The lowest CO emissions are located at the knocking limit. This is because HCCI is a lean homogeneous charge combustion. The combustion temperature and its distribution are the major factors that dominate the CO emissions. When the EGR rate increases, the combustion reaction rate slows, the mean temperature in the cylinder decreases, and the combustion reaction becomes more incomplete. This is because more and more mid-product CO cannot be oxidized completely into CO because of the decrease in temperature. 2 Figure 13 also shows that as the ON increases, CO emissions increase at the boundary of the knocking limit, but decrease at the boundary of the misfire limit. The reason for this is that the combustion temperature falls when ON increases. At the boundary of the knocking limit, the relatively high combustion JAUTO102 © IMechE 2007

temperature of low-ON fuel results in decreasing CO emissions. However, at the boundary of the misfire limit, the relatively low combustion temperature of high-ON fuel means that more fuel did not participate in the combustion reaction, resulting in CO emissions decreasing. This exhibits good agreement with the CO formation mechanism. That is to say, CO emissions are products of chemical kinetics. 3.6.3 NO emissions x NO formation strongly depends on temperature and x fuel : air equivalence ratio. NO formation rate peaks x at the lean of stoichiometric composition due to the combined influence of temperature and oxygen availability, and it decreases rapidly as the mixture becomes leaner or richer. One of the most appealling attributes of HCCI is the leaner homogeneous combustion. This means that the temperature is expected to be nearly the same in the entire combustion chamber. Therefore, HCCI allows combustion to occur Proc. IMechE Vol. 221 Part D: J. Automobile Engineering


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Fig. 13 Effects of EGR and ON of the PRF on CO emissions in the HCCI operation region

Fig. 14 Relationship between CO emissions and the maximum mean temperature in the cylinder

at a much lower temperature. NO emissions from x the engine can be dramatically reduced. Figure 15 shows the effect of EGR and ON of the PRF on NO x emissions at all test points, and Fig. 16 shows the effects in the HCCI operation region. Figures 15 and 16 show that NO emissions are very small in an x

HCCI operating region, with the maximum value of only 5 parts/million. In an HCCI operating region the effect of EGR on NO emission is insensitive. This is x because the combustion temperature is very low in the HCCI operation region. However, when knocking combustion appears in the engine, the cylinder temperature rises sharply (the highest mean temperature in the cylinder is above 2200 K) and the NO x emissions increase accordingly. Figure 15 shows that NO emissions increase abruptly as l decreases to a x definite value where knocking combustion occurs. After the knocking limit, NO emissions increase x rapidly with the decrease in l. At this time, the increase in EGR rate also could reduce NO emissions. x On the other hand, EGR affects the l limit value, and the l of the knocking limit decreases with the increase in EGR rate, extending HCCI operation to large IMEPs. Therefore, the abrupt increase in NO emissions (although the absolute value of NO x x emissions is small) is another characteristic of knocking combustion of HCCI. It also provides evidence to establish whether knocking occurs in HCCI.





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Fig. 15 Effects of EGR and ON of the PRF on NO emissions at all test points x

4 CONCLUSIONS The results of this study lead to the following conclusions. 1. EGR could extend the HCCI operation region into larger IMEPs for various ON fuels. The PRF with an ON of 60 covers the largest operating region at an engine speed of 1400 r/min. 2. When the EGR rate increases, the ignition timing delays, the combustion rate slows down, the combustion duration becomes longer, and the maximum pressure and the maximum mean temperature in a cylinder decrease. However, an unnecessarily high EGR rate would cause misfire. The higher the ON is, the lower the tolerance to EGR dilution. 3. The maximum combustion thermal efficiency appears in the area of high EGR rate (next to the highest EGR rate) and lower l located next to the knocking boundary. 4. The maximum ITE appears in the area of the highest EGR rate and low l, which is next to the JAUTO102 © IMechE 2007

knocking boundary. The PRF with an ON of 60 had the highest ITE under the test conditions. 5. With a higher EGR rate, the effect of EGR on HC emissions is very sensitive. The maximum HC emissions area of fuels of various ONs appears in the high EGR rate area, which is next to the misfire boundary. 6. As the EGR rate increases, CO emissions increase. CO emissions show a good correlation to the maximum mean temperature in a cylinder. 7. In the HCCI operation region, the effect of EGR on NO emissions is insensitive. The abrupt increase x of NO emissions is another characteristic of x knocking combustion in HCCI.

ACKNOWLEDGEMENT The authors are grateful to the Ministry of Science and Technology (MOST) of China through project 2001CB2009201. Proc. IMechE Vol. 221 Part D: J. Automobile Engineering


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Fig. 16 Effects of EGR and ON of the PRF on NO emissions in the HCCI operation region x

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7 Kimura, S., Aoki, O., Kitahara, Y., and Aiyoshizawa, E. Ultra-clean combustion technology combining a low-temperature and premixed combustion concept for meeting future emission standards. SAE paper 2001-01-0200, 2001. 8 Christensen, M., Johansson, B., Amneus, P., and Mauss, F. Supercharged homogeneous charge compression ignition. SAE paper 980787, 1998. 9 Dec, J. A computational study of the effects of low fuel loading and EGR on heat release rates and combustion limits of HCCI engines. SAE paper 2002-01-1309, 2002. 10 Aceves, S., Martinez-Frias, J., Flowers, D., Smith, J., Dibble, R., Wright, J., and Hessel, R. A decoupled model of detailed fluid mechanics followed by detailed chemical kinetics for prediction of iso-octane HCCI combustion. SAE paper 2001-01-3612, 2001. 11 Olsson, J. O., Tunestal, P., Ulfvik, J., and Johansson, B. The effect of cooled EGR on emissions and performance of a turbocharged HCCI engine. SAE paper 2003-01-0743, 2003.





12 Kimura, S., Aoki, O., and Kitahara, Y. Ultraclean combustion technology combining a lowtemperature and premixed combustion concept for meeting future emission standards. SAE paper 2001-01-0200, 2001. 13 Peng, Z., Zhao, H., and Ladommatos, N. Effects of air/fuel ratios and EGR rates on HCCI combustion of n-heptane, a diesel type fuel. SAE paper 200301-0747, 2003. 14 Oakley, A., Zhao, H., and Ladommatos, N. Dilution effects on the controlled auto-ignition (CAI) combustion of hydrocarbon and alcohol fuels. SAE paper 2001-01-3606, 2001. 15 Chen, R., Milovanovic, N., Turner, J., and Blundell, D. The thermal effect of internal exhaust gas recirculation on controlled auto-ignition. SAE paper 2003-01-0751, 2003. 16 Furhapter, A., Piock, W. F., and Fraidl, G. K. CSIcontrolled auto ignition – the best solution for the fuel consumption versus emission trade-off. SAE paper 2003-01-0754, 2003. 17 Satoshi, S. M., Kawabata, Y., and Sakurai, T. Operating characteristics of a natural gas-fired homogeneous charge compression ignition engine (performance improvement using EGR). SAE paper 2001-01-1034, 2001. 18 Zhao, H., Peng, Z., Williams, J., and Ladommatos, N. Understanding the effects of recycled burnt gases on the controlled auto-ignition (CAI) combustion in four-stroke gasoline engines. SAE paper 2001-013607, 2001. 19 Christensen, M., Johansson, B., and Einewall, P. Homogeneous charge compression ignition (HCCI) using isooctane, ethanol, and natural gas – A comparison with spark ignition operation. SAE paper 972874, 1997. 20 Christensen, M. and Johansson, B. Influence of mixture quality on homogeneous charge compression ignition. SAE paper 982454, 1998. 21 Christensen, M., Hultqvist, A., and Johansson, B. Demonstrating the multi fuel capability of a homogeneous charge compression ignition engine with variable compression ratio. SAE paper 1999-013679, 1999.


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22 Yao, M., Zheng, Z., Zhang, B., and Chen, Z. The effect of PRF fuel octane number on HCCI operation. SAE paper 2004-01-2992, 2004. 23 http://www.originlab.com 24 Heywood, J. B. Internal combustion engine fundamentals, 1988 (McGraw-Hill Book Company, New York).

APPENDIX Abbreviations ABDC AI ATDC BBDC BTDC CA CAI CIDI COV IMEP EGR HCCI HTR IMEP ITE LTR MCD MCIT NTC ON PRF RON TDC THC VVA VVT

after bottom dead centre auto-ignition after top dead centre before bottom dead centre before top dead centre crank angle controlled auto-ignition compression ignition, direct injection cycle-to-cycle variation of indicated mean effective pressure exhaust gas recirculation homogeneous charge compression ignition high-temperature reaction indicated mean effective pressure indicated thermal efficiency low-temperature reaction main combustion duration main combustion ignition timing negative temperature coefficient octane number primary reference fuel research octane number top dead centre total hydrocarbon variable valve actuation variable valve timing



Engineering Engineers, Part D: Journal of Automobile ...

Engineering Engineers, Part D: Journal of Automobile ...

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