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Effects of charge temperature and exhaust gas re-circulation on combustion and emission characteristics of an acetylene fuelled HCCI engine
Fuel 89 (2010) 515–521
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Effects of charge temperature and exhaust gas re-circulation on combustion and emission characteristics of an acetylene fuelled HCCI engine S. Swami Nathan, J.M. Mallikarjuna, A. Ramesh * Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e
i n f o
Article history: Received 14 April 2008 Received in revised form 12 August 2009 Accepted 18 August 2009 Available online 4 September 2009 Keywords: HCCI engines Acetylene Alternative fuels Emission control EGR
a b s t r a c t In this work, experiments were conducted on a homogeneous charge compression ignition (HCCI) engine with acetylene as the sole fuel at different power outputs. Initially, the intake air was heated to different temperatures in order to determine the optimum level at every output. Charge temperatures needed were in the range of 40–110 °C from no load to a BMEP (Brake Mean Effective Pressure) of 4 bar. Subsequently, exhaust gas re-circulation (EGR) was done at the identified charge temperatures and brake thermal efficiency was found to improve. At high BMEPs, use of EGR led to knocking. Thus, fine control over charge temperature and EGR quantity is needed at these conditions. Nitric oxide and smoke levels were very low. However, HC levels were high at about 1700–2700 ppm. Brake thermal efficiencies were comparable to or even better than the compression ignition mode of operation. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Simultaneous reduction of smoke and NOx (oxides of nitrogen) emission is a challenge in diesel engines and normally the operating condition is a tradeoff between them. Though timed injection of fuel at high pressures with common rail systems coupled with EGR (exhaust gas re-circulation) has improved the performance of the diesel engine significantly, newer technologies have to be developed to make them cleaner for the future. One technique that has been gaining interest is homogeneous charge compression ignition (HCCI). In a HCCI engine, a premixed charge of fuel and air is compressed and allowed to self-ignite. This engine emits lesser NOx and smoke compared to conventional diesel engines. The main difficulty in HCCI operation is control of ignition, which in turn is governed by chemical kinetics. Fuel composition, equivalence ratio and thermodynamic state of the mixture play an important role in HCCI combustion. Onishi et al. first proposed the HCCI concept. They studied HCCI combustion experimentally in a two-stroke engine and achieved low cyclic variations from idling to 40% load [1]. Najt and Foster Abbreviations: ATAC, active thermo-atmospheric combustion; BMEP, Break Mean Effective Pressure (bar); BSU, Bosch smoke units; CA, crank angle (°); CI, compression ignited; CO, carbon monoxide; EGR, exhaust gas re-circulation; EGT, exhaust gas temperature (°C); FID, flame ionization detector; HC, hydrocarbon; HCCI, homogeneous charge compression ignition; NOx, nitrogen oxide; NDIR, NonDispersive Infrared; ppm, parts per million; TDC, top dead center. * Corresponding author. Tel.: +91 44 2257 4676; fax: +91 44 2257 4652. E-mail address: [email protected] (A. Ramesh). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.08.032
adopted this concept on four stroke engines and used a simplified kinetics scheme to correlate heat release in a Waukesha Cooperative Fuels Research (CFR) engine with modest compression ratio and high EGR rate [2]. Thring used a Labeco Cooperative Lubricant Research engine with a wedge shaped combustion chamber at a compression ratio of 8:1 [3]. External mixture preparation and heating of the gasoline–air mixture above 640 K were done. Further, EGR rates of 13–33% were used to map variations in permissible operating parameters. He was the first to call this type of operation as homogeneous charge compression ignition (HCCI). Ryan and Callahan prepared the charge (diesel–air) in the intake manifold and used a variable compression ratio engine for HCCI combustion using 47 Cetane diesel fuel. They found that compression ratio, EGR rate and equivalence ratio were adequate for controlling a HCCI engine [4]. Nakagome et al. used early injection of diesel fuel in a DI diesel engine to achieve HCCl combustion resulting in very low NOx emissions of about 20 ppm [5]. There are still many challenges facing HCCI engine operation particularly with diesel. They include very rapid (often early) heat release and high hydrocarbon and CO emissions. High HC emission is attributed to wall wetting, wall quenching and/or crevice volumes. Gray and Ryan measured particulate matter and found that they were due to the presence of long-chain hydrocarbons in the fuel [6]. The authors of this paper have conducted experiments and found high smoke emissions and inferior brake thermal efficiency due to wall wetting and poor volatility of diesel in a manifold injected diesel fuelled HCCI engine [7]. Diesel fuelled HCCI mode with conventional compression ratios leads to early heat re-
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lease on account of its low self-ignition temperature. On the other hand, a low compression ratio can lead to lack of combustion. To overcome these problems, some researchers have tried alternative fuels like LPG, natural gas (high octane), which may be suitable for HCCI operation [8,9]. Often the problem faced was high rate of pressure rise [10,11]. These led the authors of this paper to investigate the influence of a gaseous fuel, which has a reasonably low self-ignition temperature like acetylene for HCCI operation. The initial drive to use acetylene was academic. However, there is a chance that this fuel can be used at least in rural areas near the consumption sites when produced from carbide stones. Gas produced in this way is used for welding. Hilden and Stebar used acetylene as a SI (Spark Ignition) engine fuel. Their experience shows that it is not suitable for SI engine operation because of knocking and pre-ignition. The octane number of acetylene is reported as 50 [12]. Properties of acetylene are shown in Table 1. It has a high flame velocity, wide flammability limits, low self-ignition temperature and high calorific value. Since acetylene is a gaseous fuel, problems like wall wetting, vaporization and mixing do not arise. The self-ignition temperature of acetylene is slightly higher than that of diesel; therefore it can be used with high compression ratios. In addition, it will also help avoid very early combustion phasing at high outputs, which is experienced with diesel fuelled HCCI operation [13,14]. At low to medium outputs, air preheating helps control combustion in the HCCI mode [7]. Christensen et al. used preheating of intake air for achieving HCCI operation [15]. This strategy could potentially be implemented in an engine by adding a heat exchanger to preheat the intake air from the exhaust gas. However, this method may be difficult to implement in automobiles; therefore, in the present work, the thermal energy of EGR is utilized to preheat the air by direct mixing.
pumping in India. It is expected that the HCCI mode of operation can be easily adopted on such engines where operation is most of the time at constant speed and load. The engine was fitted with an intake charge electrical heating system that was developed and used to control the temperature of the intake charge. Acetylene was inducted into the intake manifold at a point after the electrical heater under a pressure slightly higher than that in the manifold. Intake charge temperature was measured by means of a K-type thermocouple. The cooling water outlet temperature was measured using a resistance temperature detector (RTD). Provisions were made to measure the flow rates of fuel, air and cooling water. A flame ionization detector (FID) for HC, Non-Destructive Infra Red (NDIR) detector for CO and chemiluminescence detector for NO were used. A Bosch smoke meter was used for the measurement of smoke levels. The engine setup was modified to work with EGR, which was circulated through an external piping, which connected the exhaust manifold to the intake manifold with a set of control valves as shown in Fig. 2. The amount of EGR was controlled manually by adjusting the exhaust throttle and the EGR throttle. This increases the exhaust back pressure and hence will influence engine performance. However, the results presented include the effects of increased exhaust back pressure and thus no additional corrections are needed. Provisions were made on both the intake and exhaust manifolds to measure the percentages of CO2 in the intake charge and exhaust gas respectively. The percentage of recirculated exhaust gas was calculated as given below in Eq. (1) [16]
EGRð%Þ ¼
ðCO2 Þintake ðCO2 Þambient 100 ðCO2 Þexhaust ðCO2 Þambient
ð1Þ
The intake charge temperature after mixing of the exhaust gas with it was measured by a thermocouple.
2. Present work A single-cylinder, diesel engine was converted to operate in the HCCI mode with acetylene as the sole fuel. First, the intake charge temperature was controlled in the range of 40 and 110 °C using an electrical heater. The influence of intake charge temperature on performance and emission characteristics at different Brake Mean Effective Pressures (BMEPs) at a constant speed of 1500 rpm was studied. By this method, the best charge temperatures were identified for every BMEP in the operating zone. Then, hot exhaust gas was recirculated while the best charge temperature was maintained by suitably adjusting the heater. Performance, combustion and emission characteristics of the engine were studied. The results have been presented and compared appropriately with that of neat diesel operation. 3. Experimental setup Fig. 1 indicates the experimental setup developed and used for this work. Engine specifications are shown in Table 2. A single-cylinder, water-cooled, DI, diesel engine was used. This engine is widely used for decentralized stationary power generation and Table 1 Properties of acetylene. Molecular weight (g) Specific gravity @ 15.6 °C Flame speed (m/s) Stoichiometric air fuel ratio (molar basis) Autoignition temperature (°C) Vapor pressure @ 20 °C (kPa) Flammability range with air (%) Flammability range with oxygen (%) Calorific value (kJ/kg)
4. Experimental procedure First, the engine was started in the HCCI mode with acetylene by setting the intake charge temperature at 83 °C (the minimum needed temperature to start the engine) and the cooling water temperature at 50 °C. Once the engine was started, operating conditions were maintained at a particular load and readings were recorded. The engine was operated from a BMEP (Brake Mean Effective Pressure) of 0–4 bar. At every BMEP, the intake charge temperature was adjusted such that the engine operated without knock. The flow rate of acetylene was controlled to maintain the BMEP and speed. The intake air temperature was varied from 40 to 110 °C to find the best value at all BMEP conditions. EGR was done at the best operating point in a BMEP range of 1.5–3.5 bar. At every BMEP, different rates of EGR were tried. However, the engine could not be operated with EGR at high exhaust gas temperatures (high BMEPs) as a very fine control was needed over its flow rate at these conditions to prevent engine knock. All experiments were carried out at a constant coolant outlet temperature of 50 °C. 5. Results and discussion 5.1. Break thermal efficiency
24.06 0.906 6.097 11.82 330 4378 (2.5–80) (3–93) 50,000
In the following figures all the curves pertaining to acetylene are for the HCCI mode of operation, whereas the curves pertaining to diesel are for the standard CI mode of operation with diesel as the sole fuel. The results obtained with different charge temperatures when BMEP was varied are shown in Figs. 3–16. In the HCCI mode, charge temperatures higher than that for best brake thermal efficiency resulted in heavy knocking and hence could not be investigated. The optimal charge temperatures are given in Table 3.
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1. Acetylene cylinder 2. Pressure regulator 3. Flow measurement 4. Intake manifold 5-6.Heater system 7. Air flow measurement 8. Pressure transducer 9. Crank angle encoder 10. Data acquisition system 11. Personal computer 12. Cooling water inlet 13. Water flow measurement 14. Cooling water temperature measurement
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15. Cooling water outlet 16. Exhaust gas temperature measurement 17. Exhaust manifold 18. 18-21: Exhaust gas analyzers 19. Dynamometer controller 20. Eddy current dynamometer 21. Clutch controller 22. Electric motor 23. Electromagnetic clutch 24. Engine 25. Engine test bed
Fig. 1. Schematic of experimental setup.
Table 2 Engine specifications. Bore stroke Connecting rod length Compression ratio Rated output Displacement volume Injector NOP
80 110 mm 231 mm 16:1 3.7 kW @ 1500 rpm 553 cm3 220 bar
In the acetylene-HCCI operation, the values of brake thermal efficiency are comparable to or even better than that of the standard diesel-CI (Compression Ignition) mode (Fig. 3). In a previous work with manifold injection and early in-cylinder injection of diesel in a HCCI engine, the brake thermal efficiencies were far lower as a homogeneous mixture could not be formed. Further, vaporization and wall wetting were major problems [7]. On the other hand, acetylene forms a homogeneous mixture ensuring better HCCI engine operation. In addition, the reasonably low self-ignition tem-
CO2 measurement at intake manifold
Engine
Intake manifold EGR Throttle
Exhaust manifold Exhaust Throttle CO2 measurement at exhaust manifold Fig. 2. Layout of the EGR system.
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35
100% Acetylene Coolant temperature 50°C Charge temperature 80°C
90
28
Heat Release Rate (J/°CA)
Brake thermal efficiency (%)
Coolant temperature = 50°C
CI Mode 110°C - C2H2 100°C - C2H2 90°C - C2H2 80°C - C2H2 70°C - C2H2 60°C - C2H2 50°C - C2H2 40°C - C2H2
21
14
7
70
1.5 bar bmep 1.0 bar bmep
50
0.5 bar bmep 30
noload
10
0 0
1 2 3 4 Brake mean effective pressure (bar)
-10 330
5
Fig. 3. Variation of brake thermal efficiency with BMEP for acetylene-HCCI (without EGR).
340
350
360 370 380 Crank angle (°CA)
390
400
Fig. 6. Heat release rate at different BMEP and fixed charge temperature (without EGR).
Coolant temperature = 50°C
100% Acetylene Coolant temperature 50°C Load = 1.5 bar bmep
90
28
Heat Release Rate (J/°CA)
Brake thermal efficiency (%)
35
21
14
3.5 bar bmep 3 bar bmep 2.5 bar bmep 2 bar bmep 1.5 bar bmep
7
70
80°C 70°C
50
60°C 30
50°C
10
0 0
15
30 45 EGR (%)
60
Fig. 4. Variation of brake thermal efficiency with BMEP for acetylene-HCCI (with EGR).
100% Acetylene
-10 330
75
340
350
360 370 380 Crank angle (°CA)
390
400
Fig. 7. Heat release rate at different charge temperature and fixed BMEP (without EGR).
Coolant temperature 50°C 100% Acetylene 40°C - 4.0 bar bmep
95
50°C - 3.5 bar bmep
70
60°C - 2.5 bar bmep 70°C - 2.0 bar bmep
50
90°C - 1.5 bar bmep 100°C - 1.0 bar bmep
30
110°C - 0.5 bar bmep
10 -10 340
350
360
370 380 390 Crank angle (°CA)
400
410
420
Heat Release Rate (J/°CA)
Heat Release Rate (J/°CA)
90
Coolant temperature 50°C Charge temperature 90°C
80 65
46% EGR
50
35% EGR
35
28% EGR
20
0% EGR
5 -10 330
340
350
360 370 380 Crank angle (°CA)
390
400
Fig. 5. Heat release rate at different BMEP and best charge temperature (without EGR).
Fig. 8. Heat release rate at different EGR rate and 1.5 bar BMEP.
perature and wide flammability limits of acetylene (Table 1) are found to be very helpful in achieving stable combustion in the HCCI mode. From Fig. 3, it is observed that the best charge temperature needed reduces as the BMEP increases. This is because of the rise in the engine temperature and the fact that the inducted mixture becomes richer as the BMEP is elevated. When the inducted mix-
ture is rich, the self-ignition temperature reduces and the combustion rate increases. The highest BMEP that could be achieved with acetylene in the HCCI mode is about 4 bar, whereas it was 4.2 bar with diesel HCCI operation [7]. Fig. 4 shows the variation of brake thermal efficiency with the percentage of exhaust gas re-circulation (EGR) at the best intake
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100% Acetylene
2700 2200 HC emissions (ppm)
Heat Release Rate (J/°CA)
Coolant temperature = 50°C
Coolant temperature 50°C Chrage temperature 50°C load = 3.0 bar bmep
90 70
0% EGR 2% EGR
50
4% EGR 30
6% EGR 8% EGR
1700 CI Mode 100°C - C2H2 80°C - C2H2 60°C - C2H2 40°C - C2H2
1200 700
10 -10 350
110°C - C2H2 90°C - C2H2 70°C - C2H2 50°C - C2H2
200 360
370 380 Crank angle (°CA)
390
1
2
3
4
5
Brake mean effective pressure (bar) Fig. 12. Variation of HC emissions with BMEP and charge temperatures (without EGR).
Fig. 9. Heat release rate at different EGR rate and 3 bar BMEP.
100% Acetylene; EGR
0
400
Coolant temperature 50°C
Coolant temperature = 50°C 3000
1.5 bar bmep
Heat Release Rate (J/°CA)
90
2.0 bar bmep
2700 HC emissions (ppm)
70
2.5 bar bmep 3.0 bar bmep
50
3.5 bar bmep 30 10 -10 350
360
370 380 Crank angle (°CA)
390
2400
2100
1800
1500
400
0
Fig. 10. Heat release rate at best brake thermal efficiency points with corresponding charge temperatures and BMEP (with EGR).
15
30 45 EGR (%)
60
75
Fig. 13. Variation of HC emissions with EGR rate and BMEP (with EGR).
0.5
Coolant temperature = 50°C 10
Coolant temperature = 50 C
CO emission (% vol)
CI Mode Maximum rate of pressure rise (bar/°CA)
3.5 bar bmep 3 bar bmep 2.5 bar bmep 2 bar bmep 1.5 bar bmep
0% EGR - Acetylene HCCI
8
Best EGR - Acetylene HCCI 6
4
2
0.4 0.3
CI Mode
110°C - C2H2
100°C - C2H2
90°C - C2H2
80°C - C2H2
70°C - C2H2
60°C - C2H2
50°C - C2H2
40°C - C2H2
0.2 0.1 0 0
0 0
1
2
3
4
5
Brake mean effective pressure (bar)
1
2 3 BMEP (bar)
4
5
Fig. 14. Variation of CO emissions with BMEP and charge temperatures (without EGR).
Fig. 11. Variation of maximum rate of pressure rise with BMEP.
charge temperature corresponding to every BMEP as given in Table 4. During tests, the percentage of EGR was increased until there was a drop in the brake thermal efficiency. Fig. 4 indicates that the brake thermal efficiency increases with the amount of EGR up to a certain level. At high BMEPs (above 3.5 bar), it was not possible to use EGR with the selected charge temperatures due to knocking. At low BMEPs like 1.5 and 2 bar, high EGR rates, even
of the order of 50% could be tolerated without knocking. In this work, the effect of EGR was not investigated below a 1.5 bar BMEP. It may be noted that in the calculation of brake thermal efficiency above, the energy input to the heater has not been taken into account. It was possible to operate the engine without the electrical heater at BMEPs of 1.5 and 2.5 bar at the best charge temperatures through the use of EGR alone. The range of BMEPs of 2.5–3.5 bar, it was not possible to operate with EGR alone. This is because the ex-
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is evident that the engine can be operated without any electrical input through the use of EGR with proper temperature control over most of the operating range. Only at BMEPs below 1.5 bar, electrical heating will be needed and this will lower the overall efficiency significantly. At such very low BMEP conditions, one may consider conventional modes of operation.
Coolant temperature = 50 C 0.36 0.31
CO(% vol.)
0.26 0.21
5.2. Heat release rate
0.16
3.5 bar bmep 3 bar bmep 2.5 bar bmep 2 bar bmep 1.5 bar bmep
0.11 0.06 0.01 0
15
30 45 EGR (%)
60
75
Fig. 15. Variation of CO emissions with EGR rate and BMEP (with EGR).
Coolant temperature = 50°C
Brake thermal efficiency (%)
30 24 18 12
CI Mode 0% EGR - Acetylene HCCI
6
Best EGR - Acetylene HCCI
0 0
1
2
3
4
5
Brake mean effective pressure (bar) Fig. 16. Comparison on brake thermal efficiency for acetylene-HCCI and diesel-CI mode.
Table 3 Optimum charge temperatures in the HCCI mode with acetylene. Charge temperature (°C)
BMEP (bar)
110 100 90 70 60 50 40
0.5 1.0 1.5 2.0 2.5 3.5 4.0
Table 4 Optimum percentage of EGR. Charge temperature (°C)
BMEP (bar)
EGR (%)
90 70 60 50 50
1.5 2.0 2.5 3.0 3.5
46 25 15 8 5
haust gas temperature was high and thus it was difficult to control the charge temperature through EGR. However, it is expected that the engine can be operated with EGR alone i.e., without the heater even at BMEPs above 2.5 bar if the temperature of the recirculated exhaust and its flow rate could be finely controlled. In this case, the temperature of the recirculated exhaust has to be reduced. Hence, it
Heat release rates (HRR) at the best brake thermal efficiency points (combination of BMEP and charge temperature) are shown in Fig. 5 for the condition where no-EGR was employed. It was observed at BMEPs of 0.5 and 1.0 bar that the start of combustion was almost the same even though the charge temperature was decreased when the BMEP was raised. This is because of the increase in the equivalence ratio and engine temperature with rise in BMEP. Similar trends existed for BMEPs ranging from 1.5 to 4 bar. As the charge temperature is to be lowered to about 40–50 °C at high BMEPs (to avoid knock), it is seen that, the peak heat release rates were low because combustion occurs in the expansion stroke. Therefore, there is scope to achieve better thermal efficiencies at high BMEPs, if combustion can be controlled properly. When the start of combustion is almost the same, it was observed that, the equivalence ratio significantly affects the heat release rate. Fig. 6 shows that the heat release rate increased and the start of combustion got advanced (with combustion TDC) as the BMEP was increased at fixed charge temperature. The influence of charge temperature on HRR is shown in Fig. 7 at 1.5 bar BMEP, wherein a wide range could be tried. The start of combustion was advanced and the heat release rate was increased when the charge temperature was raised. Fig. 8 shows the HRR for the acetylene-HCCI mode at different EGR rates at a BMEP of 1.5 bar and a constant charge temperature of 90 °C. In this case, by the use of EGR, the amount of electrical heating was reduced. It was observed that, after 46% of EGR, the charge temperature went above 90 °C even without electrical heating resulting in too rapid combustion, thereby making the engine difficult to operate. As the percentage of EGR was increased from 0 to 35, the peak HRR was found to reduce and the start of combustion was delayed. However, at a high EGR rate (about 46%), combustion rate was increased resulting in improved brake thermal efficiency. Fig. 9 indicates similar trends at a BMEP of 3 bar also. The HRR drops with 2% EGR, but rises as the EGR rate is increased further at a constant charge temperature. It is also seen that with high EGR rates, the start of combustion is not affected much, but the rate of heat release is affected significantly. The rise in the combustion rate with EGR could be due to the influence of some of the components in the exhaust gas. It may be recalled that under these conditions of high EGR, the exhaust gas contains significant amounts of hydrocarbons as seen in Fig. 13. Fig. 10 shows the best HRR with EGR addition at different BMEPs. There is no clear trend as the charge temperature and EGR rate are both different in these curves as shown in Table 4. Fig. 11 shows the variation of maximum rate of pressure rise (MRPR) with BMEP for the best operating points with no-EGR and best EGR conditions. The average MRPR was about 4 bar/crank angle degree for the no-EGR condition. However, with EGR, it is always lower. However, in the CI mode of operation, it is about 2–4 bar/crank angle degree in the BMEP range of 0.5–4.3 bar. 5.3. Exhaust emissions The HC levels generally shoot up with EGR near an equivalence ratio of about 0.28. Fig. 12 shows the trend of HC emissions for the acetylene-HCCI and the standard CI modes. The acetylene-HCCI mode led to considerably more hydrocarbon emissions. High HC
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levels are always a problem with the HCCI mode because of the lean premixed charge that is used, which can lead to partial or no combustion at certain locations. In general, increasing the charge temperature reduces HC emissions; but can lead to poor brake thermal efficiency particularly at high BMEPs due to improperly phased rapid combustion. Increase in BMEP at a given charge temperature reduces HC levels due to improved combustion. Fig. 13 shows HC emissions with different EGR rates. Zero percentage of EGR indicates operation at the best charge temperature mentioned in (Table 3) with only electrical heating. It was found that, with no-EGR, the HC level raised first and then fell as BMEP was increased. This trend is due the fact that the best charge temperatures are different with different BMEPs. It may also be noted that, HC emission at a given BMEP rose when the EGR rate went up even though the brake thermal efficiency was improved. On the whole, an increase in EGR rate elevated HC levels. At lower BMEPs, rise in HC emission was about 100–200 ppm up to 30% EGR. It was very high at higher EGR rates as oxygen was replaced by exhaust. The values of NO emission in the HCCI mode were extremely low with a maximum of about 20 ppm. This was due to homogeneous lean combustion, which resulted in low combustion gas temperatures. In the case of diesel-CI operation, even though the overall mixture was lean, the heterogeneous nature of combustion and the sudden autoignition of the mixture at the initial stages led to high local temperatures and hence high emissions of NO (80– 500 ppm). There was a slight increase in NO levels with increase percentage of EGR. Smoke levels, as expected, were very low in the acetylene-HCCI mode. The homogeneous nature of combustion with acetylene was the reason. The maximum smoke value in the acetylene-HCCI mode was only about 0.1 BSU, whereas in the standard diesel-CI mode, it was about 2.5 BSU. Normally, acetylene is a precursor for soot formation. However, the low levels of soot in the acetylene-HCCI mode prove that in the low temperature atmosphere with sufficient oxygen, acetylene is not subjected to pyrolysis. The acetylene-HCCI mode exhibits very low levels of CO emissions than that of the CI and diesel HCCI modes at high BMEPs as shown in Figs. 14 and 15. Complete combustion and absence of rich pockets is the reason for low CO emissions. However, at low BMEPs, the CO levels are high. In fact, the CO level is higher at higher charge temperatures like 90–110 °C as compared to 80 °C. It may be noted here that the engine was near knocking conditions when the intake charge temperature has increased above 80 °C. EGR increases the CO levels. But still CO emissions are less than 0.11% for 30% EGR for all BMEP conditions (1.5–3.5 bar). Higher EGR levels increase CO very sharply because of controlled and lowered combustion temperature by means of high dilution. Fig. 16 compares the thermal efficiencies in the standard dieselCI mode and the HCCI mode with acetylene as the sole fuel. In the case of acetylene, operation with the best charge temperature at every BMEP and operation with the best EGR rate at the best charge temperature (obtained without EGR) are plotted. It is seen that the brake thermal efficiency is always higher than the diesel mode and with EGR there is a further benefit. 6. Conclusions Experimental results of this study indicate the possibility to operate a HCCI engine with reasonably high thermal efficiencies in a wide range of BMEPs with acetylene as the sole fuel. The thermal efficiencies are comparable to the base diesel engine and a slight increase in brake thermal efficiency is observed with optimized EGR operation.
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The intake charge temperature and amount of EGR have to be controlled based on the output of the engine. At high outputs, operation is very sensitive to charge temperature and EGR rates. Here, a very precise control is required to obtain high brake thermal efficiencies. It is found that, at the BMEP of 1.5 and 2.0 bar, 46% and 16% EGR are required respectively to run the engine without external charge heating. At high BMEPs hot EGR leads to knock. Thus proper control over the temperature and amount of recirculated exhaust is needed at these conditions. It is found that, nitric oxide levels are always lower than about 20 ppm and smoke levels are below 0.1 BSU. However, the average HC level is about 2000 ppm in the HCCI mode with acetylene as against only about 300 ppm with diesel in the CI mode. It was possible to operate the engine without the electrical heater at low BMEPs (<2.5 bar) through the use of hot EGR. It is expected that the engine can be operated without the heater even at BMEPs above 2.5 bar if the temperature of the recirculated exhaust could be cooled and its flow rate could be finely controlled. Acknowledgments The authors wish to thank, Nagarajan K., Michael John Bose M., Subramanian M.K., Syed Mudhasir N., Sudheesh K. and Babu S. for their help in preparing the experimental setup. Their contributions are greatly appreciated. References [1] Onishi S, Hong Jo S, Shoda K, Do Jo P, Kato S. Active thermo-atmosphere combustion (ATAC) – a new combustion process for internal combustion engines. SAE paper no. 790840; 1979. [2] Najt PM, Foster DE. Compression-ignited homogeneous charge combustion, SAE paper no. 830264; 1983. [3] Thring RH. Homogeneous charge compression ignition engines. SAE paper no. 892068; 1989. [4] Ryan TW, Callahan TJ. Homogeneous charge compression ignition (HCCI) of diesel fuel, SAE paper no. 961160; 1996. [5] Keiichi Nakagome, Naoki Shimazaki, Keiichi Niimura, Shinji Kobayashi, Combustion and emission characteristics of premixed lean diesel combustion engine. SAE paper no. 970898; 1997. [6] Gray AW, Ryan III TW. Homogeneous charge compression ignition (HCCI) of diesel fuel. SAE paper no. 971676; 1997. [7] Swami Nathan SJ, Mallikarjuna M, Ramesh A. Manifold HCCI combustion: an experimental study about the effect of coolant temperatures and charge temperatures. In: International conference on IC engines and combustion ICONICE 2007, F-131, Hyderabad, India. [8] Iida N. Alternative fuels and homogeneous charge compression ignition combustion technology. SAE paper no. 972071; 1997. [9] Christensen M, Hultqvist A, Johansson B. Demonstrating the multi-fuel capability of a homogeneous charge compression ignition engine with variable compression ratio. SAE paper no. 1999-01-3679; 1999. [10] Iida M, Hayashi D, Foster E, Martin JK. Characteristics of homogeneous charge compression ignition (HCCI) engine operation for variations in compression ratio, speed, and intake temperature while using n-butane as a fuel. ASME 2003;125:472–8. [11] Kitae Yeom, Jinyoung Jang, Choongsik Bae. Homogeneous charge compression ignition of LPG and gasoline using variable valve timing in an engine. Fuel 2007;86:494–503. [12] Hilden DL, Stebar RF. Evaluation of acetylene as a spark ignition engine fuel. Int J Energy Res 1979;3:59–71 [January–March]. [13] Swami Nathan S, Mallikarjuna JM, Ramesh A. The effect of mixture preparation in a diesel HCCI using early in-cylinder injection during the suction stroke. Int J Automot Technol 2007;8:543–53. [14] Kima Myung Yoon, Lee Chang Sik. Effect of a narrow fuel spray angle and a dual injection configuration on the improvement of exhaust emissions in a HCCI diesel engine. Fuel 2007;86:2871–80. [15] Christensen M, Johansson B, Amnéus P, Mauss F. Supercharged homogeneous charge compression ignition. SAE paper no. 980787; 1998. [16] Maiboom Alain, Tauzia Xavier, Hetet Jean-Francois. Experimental study of various effects of exhaust gas recirculation (EGR) on combustion and emissions of an automotive direct injection diesel engine. Energy 2008;33:22–34.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Effects of charge temperature and exhaust gas re-circulation on combustion and emission characteristics of an acetylene fuelled HCCI engine S. Swami Nathan, J.M. Mallikarjuna, A. Ramesh * Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e
i n f o
Article history: Received 14 April 2008 Received in revised form 12 August 2009 Accepted 18 August 2009 Available online 4 September 2009 Keywords: HCCI engines Acetylene Alternative fuels Emission control EGR
a b s t r a c t In this work, experiments were conducted on a homogeneous charge compression ignition (HCCI) engine with acetylene as the sole fuel at different power outputs. Initially, the intake air was heated to different temperatures in order to determine the optimum level at every output. Charge temperatures needed were in the range of 40–110 °C from no load to a BMEP (Brake Mean Effective Pressure) of 4 bar. Subsequently, exhaust gas re-circulation (EGR) was done at the identified charge temperatures and brake thermal efficiency was found to improve. At high BMEPs, use of EGR led to knocking. Thus, fine control over charge temperature and EGR quantity is needed at these conditions. Nitric oxide and smoke levels were very low. However, HC levels were high at about 1700–2700 ppm. Brake thermal efficiencies were comparable to or even better than the compression ignition mode of operation. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Simultaneous reduction of smoke and NOx (oxides of nitrogen) emission is a challenge in diesel engines and normally the operating condition is a tradeoff between them. Though timed injection of fuel at high pressures with common rail systems coupled with EGR (exhaust gas re-circulation) has improved the performance of the diesel engine significantly, newer technologies have to be developed to make them cleaner for the future. One technique that has been gaining interest is homogeneous charge compression ignition (HCCI). In a HCCI engine, a premixed charge of fuel and air is compressed and allowed to self-ignite. This engine emits lesser NOx and smoke compared to conventional diesel engines. The main difficulty in HCCI operation is control of ignition, which in turn is governed by chemical kinetics. Fuel composition, equivalence ratio and thermodynamic state of the mixture play an important role in HCCI combustion. Onishi et al. first proposed the HCCI concept. They studied HCCI combustion experimentally in a two-stroke engine and achieved low cyclic variations from idling to 40% load [1]. Najt and Foster Abbreviations: ATAC, active thermo-atmospheric combustion; BMEP, Break Mean Effective Pressure (bar); BSU, Bosch smoke units; CA, crank angle (°); CI, compression ignited; CO, carbon monoxide; EGR, exhaust gas re-circulation; EGT, exhaust gas temperature (°C); FID, flame ionization detector; HC, hydrocarbon; HCCI, homogeneous charge compression ignition; NOx, nitrogen oxide; NDIR, NonDispersive Infrared; ppm, parts per million; TDC, top dead center. * Corresponding author. Tel.: +91 44 2257 4676; fax: +91 44 2257 4652. E-mail address: [email protected] (A. Ramesh). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.08.032
adopted this concept on four stroke engines and used a simplified kinetics scheme to correlate heat release in a Waukesha Cooperative Fuels Research (CFR) engine with modest compression ratio and high EGR rate [2]. Thring used a Labeco Cooperative Lubricant Research engine with a wedge shaped combustion chamber at a compression ratio of 8:1 [3]. External mixture preparation and heating of the gasoline–air mixture above 640 K were done. Further, EGR rates of 13–33% were used to map variations in permissible operating parameters. He was the first to call this type of operation as homogeneous charge compression ignition (HCCI). Ryan and Callahan prepared the charge (diesel–air) in the intake manifold and used a variable compression ratio engine for HCCI combustion using 47 Cetane diesel fuel. They found that compression ratio, EGR rate and equivalence ratio were adequate for controlling a HCCI engine [4]. Nakagome et al. used early injection of diesel fuel in a DI diesel engine to achieve HCCl combustion resulting in very low NOx emissions of about 20 ppm [5]. There are still many challenges facing HCCI engine operation particularly with diesel. They include very rapid (often early) heat release and high hydrocarbon and CO emissions. High HC emission is attributed to wall wetting, wall quenching and/or crevice volumes. Gray and Ryan measured particulate matter and found that they were due to the presence of long-chain hydrocarbons in the fuel [6]. The authors of this paper have conducted experiments and found high smoke emissions and inferior brake thermal efficiency due to wall wetting and poor volatility of diesel in a manifold injected diesel fuelled HCCI engine [7]. Diesel fuelled HCCI mode with conventional compression ratios leads to early heat re-
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lease on account of its low self-ignition temperature. On the other hand, a low compression ratio can lead to lack of combustion. To overcome these problems, some researchers have tried alternative fuels like LPG, natural gas (high octane), which may be suitable for HCCI operation [8,9]. Often the problem faced was high rate of pressure rise [10,11]. These led the authors of this paper to investigate the influence of a gaseous fuel, which has a reasonably low self-ignition temperature like acetylene for HCCI operation. The initial drive to use acetylene was academic. However, there is a chance that this fuel can be used at least in rural areas near the consumption sites when produced from carbide stones. Gas produced in this way is used for welding. Hilden and Stebar used acetylene as a SI (Spark Ignition) engine fuel. Their experience shows that it is not suitable for SI engine operation because of knocking and pre-ignition. The octane number of acetylene is reported as 50 [12]. Properties of acetylene are shown in Table 1. It has a high flame velocity, wide flammability limits, low self-ignition temperature and high calorific value. Since acetylene is a gaseous fuel, problems like wall wetting, vaporization and mixing do not arise. The self-ignition temperature of acetylene is slightly higher than that of diesel; therefore it can be used with high compression ratios. In addition, it will also help avoid very early combustion phasing at high outputs, which is experienced with diesel fuelled HCCI operation [13,14]. At low to medium outputs, air preheating helps control combustion in the HCCI mode [7]. Christensen et al. used preheating of intake air for achieving HCCI operation [15]. This strategy could potentially be implemented in an engine by adding a heat exchanger to preheat the intake air from the exhaust gas. However, this method may be difficult to implement in automobiles; therefore, in the present work, the thermal energy of EGR is utilized to preheat the air by direct mixing.
pumping in India. It is expected that the HCCI mode of operation can be easily adopted on such engines where operation is most of the time at constant speed and load. The engine was fitted with an intake charge electrical heating system that was developed and used to control the temperature of the intake charge. Acetylene was inducted into the intake manifold at a point after the electrical heater under a pressure slightly higher than that in the manifold. Intake charge temperature was measured by means of a K-type thermocouple. The cooling water outlet temperature was measured using a resistance temperature detector (RTD). Provisions were made to measure the flow rates of fuel, air and cooling water. A flame ionization detector (FID) for HC, Non-Destructive Infra Red (NDIR) detector for CO and chemiluminescence detector for NO were used. A Bosch smoke meter was used for the measurement of smoke levels. The engine setup was modified to work with EGR, which was circulated through an external piping, which connected the exhaust manifold to the intake manifold with a set of control valves as shown in Fig. 2. The amount of EGR was controlled manually by adjusting the exhaust throttle and the EGR throttle. This increases the exhaust back pressure and hence will influence engine performance. However, the results presented include the effects of increased exhaust back pressure and thus no additional corrections are needed. Provisions were made on both the intake and exhaust manifolds to measure the percentages of CO2 in the intake charge and exhaust gas respectively. The percentage of recirculated exhaust gas was calculated as given below in Eq. (1) [16]
EGRð%Þ ¼
ðCO2 Þintake ðCO2 Þambient 100 ðCO2 Þexhaust ðCO2 Þambient
ð1Þ
The intake charge temperature after mixing of the exhaust gas with it was measured by a thermocouple.
2. Present work A single-cylinder, diesel engine was converted to operate in the HCCI mode with acetylene as the sole fuel. First, the intake charge temperature was controlled in the range of 40 and 110 °C using an electrical heater. The influence of intake charge temperature on performance and emission characteristics at different Brake Mean Effective Pressures (BMEPs) at a constant speed of 1500 rpm was studied. By this method, the best charge temperatures were identified for every BMEP in the operating zone. Then, hot exhaust gas was recirculated while the best charge temperature was maintained by suitably adjusting the heater. Performance, combustion and emission characteristics of the engine were studied. The results have been presented and compared appropriately with that of neat diesel operation. 3. Experimental setup Fig. 1 indicates the experimental setup developed and used for this work. Engine specifications are shown in Table 2. A single-cylinder, water-cooled, DI, diesel engine was used. This engine is widely used for decentralized stationary power generation and Table 1 Properties of acetylene. Molecular weight (g) Specific gravity @ 15.6 °C Flame speed (m/s) Stoichiometric air fuel ratio (molar basis) Autoignition temperature (°C) Vapor pressure @ 20 °C (kPa) Flammability range with air (%) Flammability range with oxygen (%) Calorific value (kJ/kg)
4. Experimental procedure First, the engine was started in the HCCI mode with acetylene by setting the intake charge temperature at 83 °C (the minimum needed temperature to start the engine) and the cooling water temperature at 50 °C. Once the engine was started, operating conditions were maintained at a particular load and readings were recorded. The engine was operated from a BMEP (Brake Mean Effective Pressure) of 0–4 bar. At every BMEP, the intake charge temperature was adjusted such that the engine operated without knock. The flow rate of acetylene was controlled to maintain the BMEP and speed. The intake air temperature was varied from 40 to 110 °C to find the best value at all BMEP conditions. EGR was done at the best operating point in a BMEP range of 1.5–3.5 bar. At every BMEP, different rates of EGR were tried. However, the engine could not be operated with EGR at high exhaust gas temperatures (high BMEPs) as a very fine control was needed over its flow rate at these conditions to prevent engine knock. All experiments were carried out at a constant coolant outlet temperature of 50 °C. 5. Results and discussion 5.1. Break thermal efficiency
24.06 0.906 6.097 11.82 330 4378 (2.5–80) (3–93) 50,000
In the following figures all the curves pertaining to acetylene are for the HCCI mode of operation, whereas the curves pertaining to diesel are for the standard CI mode of operation with diesel as the sole fuel. The results obtained with different charge temperatures when BMEP was varied are shown in Figs. 3–16. In the HCCI mode, charge temperatures higher than that for best brake thermal efficiency resulted in heavy knocking and hence could not be investigated. The optimal charge temperatures are given in Table 3.
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1. Acetylene cylinder 2. Pressure regulator 3. Flow measurement 4. Intake manifold 5-6.Heater system 7. Air flow measurement 8. Pressure transducer 9. Crank angle encoder 10. Data acquisition system 11. Personal computer 12. Cooling water inlet 13. Water flow measurement 14. Cooling water temperature measurement
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15. Cooling water outlet 16. Exhaust gas temperature measurement 17. Exhaust manifold 18. 18-21: Exhaust gas analyzers 19. Dynamometer controller 20. Eddy current dynamometer 21. Clutch controller 22. Electric motor 23. Electromagnetic clutch 24. Engine 25. Engine test bed
Fig. 1. Schematic of experimental setup.
Table 2 Engine specifications. Bore stroke Connecting rod length Compression ratio Rated output Displacement volume Injector NOP
80 110 mm 231 mm 16:1 3.7 kW @ 1500 rpm 553 cm3 220 bar
In the acetylene-HCCI operation, the values of brake thermal efficiency are comparable to or even better than that of the standard diesel-CI (Compression Ignition) mode (Fig. 3). In a previous work with manifold injection and early in-cylinder injection of diesel in a HCCI engine, the brake thermal efficiencies were far lower as a homogeneous mixture could not be formed. Further, vaporization and wall wetting were major problems [7]. On the other hand, acetylene forms a homogeneous mixture ensuring better HCCI engine operation. In addition, the reasonably low self-ignition tem-
CO2 measurement at intake manifold
Engine
Intake manifold EGR Throttle
Exhaust manifold Exhaust Throttle CO2 measurement at exhaust manifold Fig. 2. Layout of the EGR system.
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35
100% Acetylene Coolant temperature 50°C Charge temperature 80°C
90
28
Heat Release Rate (J/°CA)
Brake thermal efficiency (%)
Coolant temperature = 50°C
CI Mode 110°C - C2H2 100°C - C2H2 90°C - C2H2 80°C - C2H2 70°C - C2H2 60°C - C2H2 50°C - C2H2 40°C - C2H2
21
14
7
70
1.5 bar bmep 1.0 bar bmep
50
0.5 bar bmep 30
noload
10
0 0
1 2 3 4 Brake mean effective pressure (bar)
-10 330
5
Fig. 3. Variation of brake thermal efficiency with BMEP for acetylene-HCCI (without EGR).
340
350
360 370 380 Crank angle (°CA)
390
400
Fig. 6. Heat release rate at different BMEP and fixed charge temperature (without EGR).
Coolant temperature = 50°C
100% Acetylene Coolant temperature 50°C Load = 1.5 bar bmep
90
28
Heat Release Rate (J/°CA)
Brake thermal efficiency (%)
35
21
14
3.5 bar bmep 3 bar bmep 2.5 bar bmep 2 bar bmep 1.5 bar bmep
7
70
80°C 70°C
50
60°C 30
50°C
10
0 0
15
30 45 EGR (%)
60
Fig. 4. Variation of brake thermal efficiency with BMEP for acetylene-HCCI (with EGR).
100% Acetylene
-10 330
75
340
350
360 370 380 Crank angle (°CA)
390
400
Fig. 7. Heat release rate at different charge temperature and fixed BMEP (without EGR).
Coolant temperature 50°C 100% Acetylene 40°C - 4.0 bar bmep
95
50°C - 3.5 bar bmep
70
60°C - 2.5 bar bmep 70°C - 2.0 bar bmep
50
90°C - 1.5 bar bmep 100°C - 1.0 bar bmep
30
110°C - 0.5 bar bmep
10 -10 340
350
360
370 380 390 Crank angle (°CA)
400
410
420
Heat Release Rate (J/°CA)
Heat Release Rate (J/°CA)
90
Coolant temperature 50°C Charge temperature 90°C
80 65
46% EGR
50
35% EGR
35
28% EGR
20
0% EGR
5 -10 330
340
350
360 370 380 Crank angle (°CA)
390
400
Fig. 5. Heat release rate at different BMEP and best charge temperature (without EGR).
Fig. 8. Heat release rate at different EGR rate and 1.5 bar BMEP.
perature and wide flammability limits of acetylene (Table 1) are found to be very helpful in achieving stable combustion in the HCCI mode. From Fig. 3, it is observed that the best charge temperature needed reduces as the BMEP increases. This is because of the rise in the engine temperature and the fact that the inducted mixture becomes richer as the BMEP is elevated. When the inducted mix-
ture is rich, the self-ignition temperature reduces and the combustion rate increases. The highest BMEP that could be achieved with acetylene in the HCCI mode is about 4 bar, whereas it was 4.2 bar with diesel HCCI operation [7]. Fig. 4 shows the variation of brake thermal efficiency with the percentage of exhaust gas re-circulation (EGR) at the best intake
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100% Acetylene
2700 2200 HC emissions (ppm)
Heat Release Rate (J/°CA)
Coolant temperature = 50°C
Coolant temperature 50°C Chrage temperature 50°C load = 3.0 bar bmep
90 70
0% EGR 2% EGR
50
4% EGR 30
6% EGR 8% EGR
1700 CI Mode 100°C - C2H2 80°C - C2H2 60°C - C2H2 40°C - C2H2
1200 700
10 -10 350
110°C - C2H2 90°C - C2H2 70°C - C2H2 50°C - C2H2
200 360
370 380 Crank angle (°CA)
390
1
2
3
4
5
Brake mean effective pressure (bar) Fig. 12. Variation of HC emissions with BMEP and charge temperatures (without EGR).
Fig. 9. Heat release rate at different EGR rate and 3 bar BMEP.
100% Acetylene; EGR
0
400
Coolant temperature 50°C
Coolant temperature = 50°C 3000
1.5 bar bmep
Heat Release Rate (J/°CA)
90
2.0 bar bmep
2700 HC emissions (ppm)
70
2.5 bar bmep 3.0 bar bmep
50
3.5 bar bmep 30 10 -10 350
360
370 380 Crank angle (°CA)
390
2400
2100
1800
1500
400
0
Fig. 10. Heat release rate at best brake thermal efficiency points with corresponding charge temperatures and BMEP (with EGR).
15
30 45 EGR (%)
60
75
Fig. 13. Variation of HC emissions with EGR rate and BMEP (with EGR).
0.5
Coolant temperature = 50°C 10
Coolant temperature = 50 C
CO emission (% vol)
CI Mode Maximum rate of pressure rise (bar/°CA)
3.5 bar bmep 3 bar bmep 2.5 bar bmep 2 bar bmep 1.5 bar bmep
0% EGR - Acetylene HCCI
8
Best EGR - Acetylene HCCI 6
4
2
0.4 0.3
CI Mode
110°C - C2H2
100°C - C2H2
90°C - C2H2
80°C - C2H2
70°C - C2H2
60°C - C2H2
50°C - C2H2
40°C - C2H2
0.2 0.1 0 0
0 0
1
2
3
4
5
Brake mean effective pressure (bar)
1
2 3 BMEP (bar)
4
5
Fig. 14. Variation of CO emissions with BMEP and charge temperatures (without EGR).
Fig. 11. Variation of maximum rate of pressure rise with BMEP.
charge temperature corresponding to every BMEP as given in Table 4. During tests, the percentage of EGR was increased until there was a drop in the brake thermal efficiency. Fig. 4 indicates that the brake thermal efficiency increases with the amount of EGR up to a certain level. At high BMEPs (above 3.5 bar), it was not possible to use EGR with the selected charge temperatures due to knocking. At low BMEPs like 1.5 and 2 bar, high EGR rates, even
of the order of 50% could be tolerated without knocking. In this work, the effect of EGR was not investigated below a 1.5 bar BMEP. It may be noted that in the calculation of brake thermal efficiency above, the energy input to the heater has not been taken into account. It was possible to operate the engine without the electrical heater at BMEPs of 1.5 and 2.5 bar at the best charge temperatures through the use of EGR alone. The range of BMEPs of 2.5–3.5 bar, it was not possible to operate with EGR alone. This is because the ex-
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is evident that the engine can be operated without any electrical input through the use of EGR with proper temperature control over most of the operating range. Only at BMEPs below 1.5 bar, electrical heating will be needed and this will lower the overall efficiency significantly. At such very low BMEP conditions, one may consider conventional modes of operation.
Coolant temperature = 50 C 0.36 0.31
CO(% vol.)
0.26 0.21
5.2. Heat release rate
0.16
3.5 bar bmep 3 bar bmep 2.5 bar bmep 2 bar bmep 1.5 bar bmep
0.11 0.06 0.01 0
15
30 45 EGR (%)
60
75
Fig. 15. Variation of CO emissions with EGR rate and BMEP (with EGR).
Coolant temperature = 50°C
Brake thermal efficiency (%)
30 24 18 12
CI Mode 0% EGR - Acetylene HCCI
6
Best EGR - Acetylene HCCI
0 0
1
2
3
4
5
Brake mean effective pressure (bar) Fig. 16. Comparison on brake thermal efficiency for acetylene-HCCI and diesel-CI mode.
Table 3 Optimum charge temperatures in the HCCI mode with acetylene. Charge temperature (°C)
BMEP (bar)
110 100 90 70 60 50 40
0.5 1.0 1.5 2.0 2.5 3.5 4.0
Table 4 Optimum percentage of EGR. Charge temperature (°C)
BMEP (bar)
EGR (%)
90 70 60 50 50
1.5 2.0 2.5 3.0 3.5
46 25 15 8 5
haust gas temperature was high and thus it was difficult to control the charge temperature through EGR. However, it is expected that the engine can be operated with EGR alone i.e., without the heater even at BMEPs above 2.5 bar if the temperature of the recirculated exhaust and its flow rate could be finely controlled. In this case, the temperature of the recirculated exhaust has to be reduced. Hence, it
Heat release rates (HRR) at the best brake thermal efficiency points (combination of BMEP and charge temperature) are shown in Fig. 5 for the condition where no-EGR was employed. It was observed at BMEPs of 0.5 and 1.0 bar that the start of combustion was almost the same even though the charge temperature was decreased when the BMEP was raised. This is because of the increase in the equivalence ratio and engine temperature with rise in BMEP. Similar trends existed for BMEPs ranging from 1.5 to 4 bar. As the charge temperature is to be lowered to about 40–50 °C at high BMEPs (to avoid knock), it is seen that, the peak heat release rates were low because combustion occurs in the expansion stroke. Therefore, there is scope to achieve better thermal efficiencies at high BMEPs, if combustion can be controlled properly. When the start of combustion is almost the same, it was observed that, the equivalence ratio significantly affects the heat release rate. Fig. 6 shows that the heat release rate increased and the start of combustion got advanced (with combustion TDC) as the BMEP was increased at fixed charge temperature. The influence of charge temperature on HRR is shown in Fig. 7 at 1.5 bar BMEP, wherein a wide range could be tried. The start of combustion was advanced and the heat release rate was increased when the charge temperature was raised. Fig. 8 shows the HRR for the acetylene-HCCI mode at different EGR rates at a BMEP of 1.5 bar and a constant charge temperature of 90 °C. In this case, by the use of EGR, the amount of electrical heating was reduced. It was observed that, after 46% of EGR, the charge temperature went above 90 °C even without electrical heating resulting in too rapid combustion, thereby making the engine difficult to operate. As the percentage of EGR was increased from 0 to 35, the peak HRR was found to reduce and the start of combustion was delayed. However, at a high EGR rate (about 46%), combustion rate was increased resulting in improved brake thermal efficiency. Fig. 9 indicates similar trends at a BMEP of 3 bar also. The HRR drops with 2% EGR, but rises as the EGR rate is increased further at a constant charge temperature. It is also seen that with high EGR rates, the start of combustion is not affected much, but the rate of heat release is affected significantly. The rise in the combustion rate with EGR could be due to the influence of some of the components in the exhaust gas. It may be recalled that under these conditions of high EGR, the exhaust gas contains significant amounts of hydrocarbons as seen in Fig. 13. Fig. 10 shows the best HRR with EGR addition at different BMEPs. There is no clear trend as the charge temperature and EGR rate are both different in these curves as shown in Table 4. Fig. 11 shows the variation of maximum rate of pressure rise (MRPR) with BMEP for the best operating points with no-EGR and best EGR conditions. The average MRPR was about 4 bar/crank angle degree for the no-EGR condition. However, with EGR, it is always lower. However, in the CI mode of operation, it is about 2–4 bar/crank angle degree in the BMEP range of 0.5–4.3 bar. 5.3. Exhaust emissions The HC levels generally shoot up with EGR near an equivalence ratio of about 0.28. Fig. 12 shows the trend of HC emissions for the acetylene-HCCI and the standard CI modes. The acetylene-HCCI mode led to considerably more hydrocarbon emissions. High HC
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levels are always a problem with the HCCI mode because of the lean premixed charge that is used, which can lead to partial or no combustion at certain locations. In general, increasing the charge temperature reduces HC emissions; but can lead to poor brake thermal efficiency particularly at high BMEPs due to improperly phased rapid combustion. Increase in BMEP at a given charge temperature reduces HC levels due to improved combustion. Fig. 13 shows HC emissions with different EGR rates. Zero percentage of EGR indicates operation at the best charge temperature mentioned in (Table 3) with only electrical heating. It was found that, with no-EGR, the HC level raised first and then fell as BMEP was increased. This trend is due the fact that the best charge temperatures are different with different BMEPs. It may also be noted that, HC emission at a given BMEP rose when the EGR rate went up even though the brake thermal efficiency was improved. On the whole, an increase in EGR rate elevated HC levels. At lower BMEPs, rise in HC emission was about 100–200 ppm up to 30% EGR. It was very high at higher EGR rates as oxygen was replaced by exhaust. The values of NO emission in the HCCI mode were extremely low with a maximum of about 20 ppm. This was due to homogeneous lean combustion, which resulted in low combustion gas temperatures. In the case of diesel-CI operation, even though the overall mixture was lean, the heterogeneous nature of combustion and the sudden autoignition of the mixture at the initial stages led to high local temperatures and hence high emissions of NO (80– 500 ppm). There was a slight increase in NO levels with increase percentage of EGR. Smoke levels, as expected, were very low in the acetylene-HCCI mode. The homogeneous nature of combustion with acetylene was the reason. The maximum smoke value in the acetylene-HCCI mode was only about 0.1 BSU, whereas in the standard diesel-CI mode, it was about 2.5 BSU. Normally, acetylene is a precursor for soot formation. However, the low levels of soot in the acetylene-HCCI mode prove that in the low temperature atmosphere with sufficient oxygen, acetylene is not subjected to pyrolysis. The acetylene-HCCI mode exhibits very low levels of CO emissions than that of the CI and diesel HCCI modes at high BMEPs as shown in Figs. 14 and 15. Complete combustion and absence of rich pockets is the reason for low CO emissions. However, at low BMEPs, the CO levels are high. In fact, the CO level is higher at higher charge temperatures like 90–110 °C as compared to 80 °C. It may be noted here that the engine was near knocking conditions when the intake charge temperature has increased above 80 °C. EGR increases the CO levels. But still CO emissions are less than 0.11% for 30% EGR for all BMEP conditions (1.5–3.5 bar). Higher EGR levels increase CO very sharply because of controlled and lowered combustion temperature by means of high dilution. Fig. 16 compares the thermal efficiencies in the standard dieselCI mode and the HCCI mode with acetylene as the sole fuel. In the case of acetylene, operation with the best charge temperature at every BMEP and operation with the best EGR rate at the best charge temperature (obtained without EGR) are plotted. It is seen that the brake thermal efficiency is always higher than the diesel mode and with EGR there is a further benefit. 6. Conclusions Experimental results of this study indicate the possibility to operate a HCCI engine with reasonably high thermal efficiencies in a wide range of BMEPs with acetylene as the sole fuel. The thermal efficiencies are comparable to the base diesel engine and a slight increase in brake thermal efficiency is observed with optimized EGR operation.
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The intake charge temperature and amount of EGR have to be controlled based on the output of the engine. At high outputs, operation is very sensitive to charge temperature and EGR rates. Here, a very precise control is required to obtain high brake thermal efficiencies. It is found that, at the BMEP of 1.5 and 2.0 bar, 46% and 16% EGR are required respectively to run the engine without external charge heating. At high BMEPs hot EGR leads to knock. Thus proper control over the temperature and amount of recirculated exhaust is needed at these conditions. It is found that, nitric oxide levels are always lower than about 20 ppm and smoke levels are below 0.1 BSU. However, the average HC level is about 2000 ppm in the HCCI mode with acetylene as against only about 300 ppm with diesel in the CI mode. It was possible to operate the engine without the electrical heater at low BMEPs (<2.5 bar) through the use of hot EGR. It is expected that the engine can be operated without the heater even at BMEPs above 2.5 bar if the temperature of the recirculated exhaust could be cooled and its flow rate could be finely controlled. Acknowledgments The authors wish to thank, Nagarajan K., Michael John Bose M., Subramanian M.K., Syed Mudhasir N., Sudheesh K. and Babu S. for their help in preparing the experimental setup. Their contributions are greatly appreciated. References [1] Onishi S, Hong Jo S, Shoda K, Do Jo P, Kato S. Active thermo-atmosphere combustion (ATAC) – a new combustion process for internal combustion engines. SAE paper no. 790840; 1979. [2] Najt PM, Foster DE. Compression-ignited homogeneous charge combustion, SAE paper no. 830264; 1983. [3] Thring RH. Homogeneous charge compression ignition engines. SAE paper no. 892068; 1989. [4] Ryan TW, Callahan TJ. Homogeneous charge compression ignition (HCCI) of diesel fuel, SAE paper no. 961160; 1996. [5] Keiichi Nakagome, Naoki Shimazaki, Keiichi Niimura, Shinji Kobayashi, Combustion and emission characteristics of premixed lean diesel combustion engine. SAE paper no. 970898; 1997. [6] Gray AW, Ryan III TW. Homogeneous charge compression ignition (HCCI) of diesel fuel. SAE paper no. 971676; 1997. [7] Swami Nathan SJ, Mallikarjuna M, Ramesh A. Manifold HCCI combustion: an experimental study about the effect of coolant temperatures and charge temperatures. In: International conference on IC engines and combustion ICONICE 2007, F-131, Hyderabad, India. [8] Iida N. Alternative fuels and homogeneous charge compression ignition combustion technology. SAE paper no. 972071; 1997. [9] Christensen M, Hultqvist A, Johansson B. Demonstrating the multi-fuel capability of a homogeneous charge compression ignition engine with variable compression ratio. SAE paper no. 1999-01-3679; 1999. [10] Iida M, Hayashi D, Foster E, Martin JK. Characteristics of homogeneous charge compression ignition (HCCI) engine operation for variations in compression ratio, speed, and intake temperature while using n-butane as a fuel. ASME 2003;125:472–8. [11] Kitae Yeom, Jinyoung Jang, Choongsik Bae. Homogeneous charge compression ignition of LPG and gasoline using variable valve timing in an engine. Fuel 2007;86:494–503. [12] Hilden DL, Stebar RF. Evaluation of acetylene as a spark ignition engine fuel. Int J Energy Res 1979;3:59–71 [January–March]. [13] Swami Nathan S, Mallikarjuna JM, Ramesh A. The effect of mixture preparation in a diesel HCCI using early in-cylinder injection during the suction stroke. Int J Automot Technol 2007;8:543–53. [14] Kima Myung Yoon, Lee Chang Sik. Effect of a narrow fuel spray angle and a dual injection configuration on the improvement of exhaust emissions in a HCCI diesel engine. Fuel 2007;86:2871–80. [15] Christensen M, Johansson B, Amnéus P, Mauss F. Supercharged homogeneous charge compression ignition. SAE paper no. 980787; 1998. [16] Maiboom Alain, Tauzia Xavier, Hetet Jean-Francois. Experimental study of various effects of exhaust gas recirculation (EGR) on combustion and emissions of an automotive direct injection diesel engine. Energy 2008;33:22–34.