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Potential of High Load Extension for Gasoline HCCI Engine Using Boosting and Exhaust Gas Recirculation Fan Xu,* Zhi Wang, Dongbo Yang, and Jianxin Wang State Key Laboratory of AutomotiVe Safety and Energy, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed NoVember 25, 2008. ReVised Manuscript ReceiVed February 15, 2009
Homogeneous charge compression ignition (HCCI) still faces challenges in high load extension. In this paper, HCCI high load operation range was extended and combustion phasing was controlled by boosting combined with internal EGR and external EGR in a gasoline HCCI engine. Internal EGR was obtained by negative valve overlap (NVO) and used to achieve gasoline HCCI at ambient temperature without intake heating. Combustion phasing was optimized by adjusting the external EGR inducted into the intake system. The experimental results show that both boosting and EGR are effective means for HCCI high load extension but with limitations of peak pressure (Pmax), maximum rate of pressure rise (Rmax), combustion efficiency (ηc), and NOx emissions. Under the acceptable Pmax, Rmax, ηc, and NOx levels of a production gasoline engine, the achievable maximum IMEP is in the following order: external EGR, boosting, boosting combined with EGR. With increasing boost pressure, NOx emissions are significantly reduced. But with acceptable Pmax, heavy boosting leads to an ultralean mixture, followed by uncompleted combustion, which results in CO and HC emissions. At a fixed NVO, there is an appropriate range of the external EGR rate for stable HCCI combustion without knocking or misfire. Therefore, the optimized path was achieved using various internal EGR and external EGR combined to extend HCCI high load operation range. In addition, it can be found that boost pressure, percentage of CO2 addition, and internal and external EGR rate have shown significant effects on combustion phasing of HCCI.
1. Introduction Homogenous charge compression ignition (HCCI) is a promising combustion concept in internal combustion engines. Engines applying this combustion concept have shown an improvement in fuel economy and emissions simultaneously. However, naturally aspirated HCCI engines have a much reduced high load compared to typical spark ignition (SI) engines. To make it practical for an on-road motor vehicle, applying boosting to gasoline HCCI operation is used as a method to extend the upper load of the engine. To achieve stable HCCI combustion, the rate of combustion has to be in control. Thus, charge must be diluted by excess air or by exhaust gas recirculation (EGR). The requirements for dilution lower the maximum power density of HCCI engines because when the air fuel ratio is reduced via increasing the amount of fuel delivered into the engine, violent combustion will occur. To increase the fuel delivery per cycle, larger amounts of excess air or exhaust gas have to be inducted into the engine. Boosting has proved to be an effective way to raise the power density of HCCI engines. The first study to clarify the effect of boosting on HCCI was carried out by Christensen et al. on a diesel type engine with intake air heating and an external air compressor.1 At 0.2 MPa boost pressure, 1.4 MPa indicated mean effective pressure (IMEP) was achieved on natural gas with excessively high maximum pressure rise rate (up to 3.3 MPa/crank angle degree * To whom correspondence should be addressed. E-mail: xuf03@ mails.tsinghua.edu.cn. Tel.: +86 10 62794876. (1) Christensen, M.; Johansson, B.; Amnéus, P.; Mauss, F. Supercharged Homogeneous Charge Compression Ignition. SAE Technical Paper 980787 (1998).
(°CA)). Then with the use of EGR, the maximum load achieved was up to 1.6 MPa IMEP at 0.15 MPa boost pressure on natural gas.2 By employing a turbocharger to this engine, 2.0 MPa IMEP was shown to be attainable with ethanol.3 In later work, the effects of supercharging on gasoline HCCI combustion were studied by Yap et al. on a single cylinder port fuel injection (PFI) engine.4 Negative valve overlap was utilized to provide the required amount of trapped residual gas for charge heating. At 0.14 MPa boost pressure, 0.76 MPa IMEP was achieved with gasoline. The effects of inlet valve timing on boosted HCCI have been studied in later work.5 The effects of combined internal and external EGR on HCCI were studied by Cairns and Blaxill on a four cylinder gasoline direct injection (GDI) engine.6 At 1500 rpm, the maximum brake mean effective pressure (BMEP) rose from 0.38 to 0.58 MPa with increasing the amount of external EGR from 0 to 8.5%. Then a small fixed geometry turbocharger was fitted on the same engine, and the effects of turbocharging on gasoline HCCI were (2) Christensen, M.; Johansson, B. Supercharged Homogeneous Charge Compression Ignition (HCCI) with Exhaust Gas Recirculation and Pilot Fuel. SAE Technical Paper 2001-01-1835 (2000). (3) Olsson, J.-O.; Tunestål, P.; Haraldsson, G.; Johansson, B. A Turbo Charged Dual Fuel HCCI Engine. SAE Technical Paper 2001-01-1896 (2001). (4) Yap, D.; Wyszynski, M. L.; Megaritis, A.; Xu, H. Applying boosting to gasoline HCCI operation with residual gas trapping. SAE Technical Paper 2005-01-2121 (2005). (5) Yap, D.; Megaritis, A.; Wyszynski, M. L.; Xu, H. Effect of inlet valve timing on boosted gasoline HCCI with residual gas trapping. SAE Technical Paper 2005-01-2136 (2005). (6) Cairns, A.; Blaxill, H. The Effects of Combined Internal and External Exhaust Gas Recirculation on Gasoline Controlled Auto-Ignition. SAE Technical Paper 2005-01-0133 (2005).
10.1021/ef801016j CCC: $40.75 2009 American Chemical Society Published on Web 03/19/2009
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Figure 1. Experiment setup.
studied in the following work.7 At 3000 rpm, a noticeable increase in load was achieved, from 0.25 MPa BMEP to 0.46 MPa BMEP. The effects of external EGR and light boosting on emissions and efficiency of SI and HCCI were studied by Gaynor et al.8 Experiments and simulation were done on supercharged HCCI engines by Gharahbaghi et al.9 However, boosted HCCI is still facing challenges in high pressure rise rate,4 excessively high peak in-cylinder pressure,10 and exorbitant cost on high boost. The work documented here concerns extending HCCI operation to high load by boosting, external EGR and CO2 addition. The experimental study can be divided into three parts. First, the effects of boosting were tested, and the experimental results showed the HCCI high-load capability increased significantly. But both the maximum rate of pressure rise (Rmax) and the peak pressure (Pmax) were too high. Then the effects of external EGR combined with internal EGR were investigated under naturally aspired conditions. It can be seen that the maximum IMEP was considerably increased via introducing external EGR, while the maximum rate of pressure rise and the peak pressure were kept at low levels. Finally, the combination of boosting and EGR was experimented. The results showed that the HCCI high load operation can be achievable under light boosting conditions with the induction of CO2 addition. 2. Experimental Apparatus The experiments were performed in a two-cylinder, two-valve per cylinder, GDI research engine, with HCCI operated on one (7) Cairns, A.; Blaxill, H. Lean Boost and External Exhaust Gas Recirculation for High Load Controlled Auto-Ignition. SAE Technical Paper 2005-01-3744 (2005). (8) Gaynor, J. A.; Fleck, R.; Kee, R. J.; Kenny, R. G.; Cathcart, G. A Study of Efficiency and Emission for a 4-Stroke SI and a CAI Engine with EEGR and Light Boost. SAE Technical Paper 2006-32-0042 (2006). (9) Gharahbaghi, S.; Wilson, T. S.; Xu, H.; Cryan, S.; Richardson, S. H.; Wyszynski, M. L.; Misztal, J. Modelling and Experimental Investigations of Supercharged HCCI Engines. SAE Technical Paper 2006-01-0634 (2006). (10) Olsson, J.-O.; Tunestål, P.; Johansson, B. Boostng for High Load HCCI. SAE Technical Paper 2004-01-0940 (2004).
Table 1. Basic Engine Specification bore/stroke compression ratio injector injection pressure throttle opening intake temperature
95 mm/115 mm 13 high pressure swirl injection 5 MPa WOT 20 ( 1 °C
cylinder only. From Figure 1, it can be seen that the boosting simulation system, external EGR system, and CO2 induction system were fitted to the cylinder operating in HCCI mode. As this work concerns high load extension, boosting, external EGR, and CO2 addition are the main methods to achieve high load. The HCCI engine was started using SI mode. After warming, it was then switched to HCCI mode. The prototype engine has been described in previous papers.11,12 Some general characteristics of the unit are listed in Table 1. A Kistler 6125B pressure transducer was mounted flush with the wall of the combustion chamber, connected to the AVL indicom 621 data acquisition system via a Kistler 5011A charge amplifier. A crankshaft encoder was fitted to provide the CMD and trigger. The in-cylinder pressure versus crank angle was recorded for a representative number of consecutive engine cycles at each measurement point. Both emissions and A/F during HCCI operation were measured via an AVL CEB II Emission analysis bench. The experiment setup is depicted in Figure 1. Compressed air from an air compressor was used for forced induction. Intake boost pressure is given as gauge value. It was adjusted via a pressure reducing valve and measured in stable condition at the air box. To avoid overhigh exhaust pressure, the exhaust was not throttled. The external EGR system was also fitted on the testing engine. And the testing on external EGR was carried out under naturally aspirated conditions. The exhaust gas from the cylinder operating in HCCI mode was recirculated into an air box via an EGR valve and was mixed with intake air there. The burned gas was cooled (11) Tian, G.; Wang, Z.; Wang, J.; Shuai, S.; An, X. HCCI Combustion Control by Injection Strategy with Negative Valve Overlap in a GDI Engine. SAE Technical Paper 2006-01-0415 (2006). (12) Tian, G.; Wang, Z.; Ge, Q.; Wang, J.; Shuai, S. Mode Swith of SI-HCCI Combustion on a GDI Engine. SAE Technical Paper 2007-010195 (2007).
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Table 2. Test Conditions HCCI valve timing (°CA)
EVO BBDC EVC ATDC IVO BTDC IVC ABDC
Case 1
Case 2
Case 3
40 -61.4 -56.4 34.8
65 -75 -75 65 42
60 -70 -70 60 38
IEGR rate (%)
to environmental temperature by an EGR cooler. The external EGR rate was defined as the ratio between the CO2 fraction of the intake mixture and the CO2 fraction of the burned gas. It can be used to represent the percentage of burned gas recirculated into the intake system. The intake CO2 fraction was measured at the intake port downstream from the air box via an AVL 4000 Light gas analyzer, where the fresh air and recycled gas had been fully mixed. While boosting the engine by an external air compressor, the intake pressure was higher than the exhaust pressure. So it was hard to recirculate the exhaust gas into the intake port. To investigate the effects of external EGR combined with boosting, CO2 addition was used as a simulative method to study the effects of external EGR. It is known that N2, CO2, H2O, and O2 are the main components of the exhaust gas under HCCI lean-burn. And it also consists of small amounts of CO and HC. Among these, CO2 is a triatomic gas which has a lower adiabatic index (κ). This is one of the major differences between intake fresh air and exhaust gas. To simulate the effects of cooled external EGR, CO2 was used in the experiment. In this work, the CO2 gas was introduced into the inlet system after the air compressor from a liquid CO2 gas cylinder, and the concentration of inducted CO2 was also adjusted via a pressure reducing valve. During the experiments, the percentage of CO2 addition was controlled via adjusting the CO2 fraction of the mixture consisting of compressed air and inducted CO2. And the fraction of external CO2 was also measured via an AVL 4000 Light gas analyzer at the intake port. Negative valve overlap was used to trap a certain amount of residual gas. The timing of intake and exhaust valves was then changed manually. The valve timing used in the experiments is described in Table 2. Case 1 was used during the tests of boosting and boosting combined with CO2 addition. Cases 2 and 3 were tested to examine the effects of reducing the amount of trapped residuals to extend HCCI high load combined with the introduction of external EGR, under naturally aspirated conditions. And the IEGR rate of Cases 2 and 3 was calculated using 1-D engine cycle code, AVL Boost. These values represent the approximation of IEGR rate in each case. The valve timings are chosen to achieve HCCI combustion without intake heating. The internal EGR rate of each of the three cases is around 40%, and the cams used in the boost test had a low lift profile, which would help to trap sufficient internal exhaust under boost conditions.
3. Results and Analysis 3.1. Boosting Effects. To investigate boosting, the results presented in this section were achieved without external EGR. Figure 2 illustrates the maximum engine load achieved under various boost pressures at 800, 1200, and 1600 rpm, respectively. It can be seen that a considerable increase in IMEP can be achieved by increasing the boost pressure at 800, 1200, and 1600 rpm. At 800 rpm and 0.1 MPa boost pressure, the maximum IMEP achieved was 0.74 MPa. As the engine speed increased, there was less time for heat transfer from the combustion chamber. As a result, the wall temperature of the combustion chamber was increased, which raised knocking tendency and decreased maximum load. During these experiments the upper load of the engine (the amount of fuel delivered to the engine) was limited by NOx emission (NOx < 0.5 g/(kW · h)), knocking (Rmax < 1.0 MPa/°CA), and high peak incylinder pressure (Pmax < 9.0 MPa). The air fuel ratios of the points depicted in Figure 2 ranged from 17 to 35.
Figure 2. Maximum engine load for various boost pressures at different engine speeds.
Figure 3. Effects of boost pressure on engine performance.
Figure 3 shows the comparison between operating points under various boost pressures at 800 rpm, with same fuel mass. It can be seen that there was a considerable reduction in Rmax from 1.0 MPa/°CA at 0 MPa boost pressure to 0.51 MPa/°CA at 0.1 MPa boost pressure, while the Pmax was raised at high boost pressures. As shown in Figure 3, the Pmax increased from 5.8 at 0 MPa boost pressure to 8.4 at 0.1 MPa boost pressure. Figure 4 is the calculated HRR and in-cylinder temperatures of the operating points in Figure 3. It can be found that there was an extension of the duration of combustion (DOC) as the boost pressure increased. Also with an increase in boost pressure from 0 to 0.05 MPa, the start of combustion (SOC) retarded, but after that the start of combustion advanced again as the boost pressure increased from 0.05 to 0.1 MPa. This can explained by the following: (1) As the boost pressure increased, the oxygen concentration was increased, which resulted in advance autoignition. (2) At the same fuel mass, as the boost pressure increased, the mixture became leaner, which resulted in a higher value of κ. Thus, the temperature of mixture around top dead center (TDC) was increased, which advanced autoignition. (3) However, leaner mixture would lead to lowered temperature of the residual gas trapped by NVO, which would retard autoignition. (4) Also, as the mixture became leaner, the temperature of the wall (Twall) was lower, resulting in a larger amount of heat transfer, which would retard autoignition as well. Some of the effects lead to advanced autoignition, and some lead to retarded autoignition. As a result, the start of combustion was first retarded and then advanced again. It can also be seen
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Figure 4. HRR, in-cylinder pressure, and in-cylinder temperature around TDC.
Figure 6. CO, HC emissions and combustion efficiencies: (a) CO emissions versus boost pressure and IMEP, (b) HC emissions versus boost pressure and IMEP, and (c) combustion efficiencies versus boost pressure and IMEP.
Figure 5. Values of A/F and NOx emissions: (a) A/F versus boost pressure and IMEP and (b) NOx emissions versus boost pressure and IMEP.
that the combustion temperature decreased significantly from 1900 K at 0 MPa boost pressure to 1250 K at 0.1 MPa boost pressure. This is due to the enhancement of charge dilution effect via larger amounts of excess air. As a result, the NOx emissions were sharply reduced with increasing boost pressure, which will be demonstrated later. Shown in Figures 5-7 are the experimental results achieved under various boosted HCCI conditions
As seen in Figure 5a, the A/F of the upper load at high boost pressure was larger than that at low boost pressure. Shown in Figure 5b are corresponding NOx emissions of the engine. It can be seen that the NOx emissions were relatively low throughout the operating range. But there were relatively large amounts of NOx emissions (over 0.5 g/(kW · h)) at high load under naturally aspirated conditions, which were reduced rapidly as the boost pressure increased. The higher A/F at high boost pressure led to lower peak gas temperature, thus resulting in lower NOx emissions. It can also be acquired that even a moderate increase in boost pressure would largely suppress the formation of NOx. CO emissions (Figure 6a) tended to be higher at lower IMEP. Also HC emissions (Figure 6b) were higher at lower engine load. To further analyze the emission characteristics, combustion
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Figure 8. Maximum IMEP achieved under different external EGR rates.
Figure 7. Rmax and Pmax: (a) Rmax versus boost pressure and A/F and (b) Pmax versus boost pressure and A/F.
efficiencies were calculated and are shown in Figure 6c.13 It can be seen that the combustion efficiencies also tended to be low at high boost pressures and high air fuel ratios (low load), which coincide well with the results of CO and HC emission. It can be acquired that the HCCI combustion deteriorated as the mixture became too lean, which leads to an increase in the CO and HC emissions, while for a given air fuel ratio there was a rise in the combustion efficiency as the boost pressure increased, which resulted in reduced CO and HC emissions. Observing Figure 7, both Rmax and Pmax were too high at rich air fuel ratios. The Rmax can easily exceed 1.0 MPa/°CA at high load. It can be seen that at a fixed boost pressure, Rmax was increased as the air fuel ratio became richer, while at a fixed air fuel ratio, it was raised as the boost pressure increased. At 0.74 MPa IMEP, the peak in-cylinder pressure reached its maximum value of 11.4 MPa, which was prohibitively high. Also, it tended to be elevated with the increase of boost pressure at a fixed air fuel ratio. 3.2. External EGR Effects. Figure 8 illustrates the maximum IMEP under different EGR rates. The highest load was restricted by NOx emission (
