combustion operation will then be presented.
6.4.1 Operation of gasoline engines with autoignition combustion
The power output of a gasoline engine operating with autoignition combustion is fundamentally determined by the amount of combustible mixture presented in the cylinder. In spark-ignition combustion, a premixed flame front travels through the near-stoichiometric unburnt fuel and air mixture. Departure from the stoichiometric mixture will hinder the flame propagation and quickly extinguish the flame front. As the load on the engine is reduced, less fuel is needed to meet the power requirement. In order to keep the fuel and air mixture near the stoichiometric air/fuel ratio, air flow is throttled in current gasoline engine operations. Therefore, the opening of the intake throttle is directly linked to the power output of the gasoline engine.
In the case of autoignition combustion with residual gas trapping, the power output of the engine is directly controlled by the valve timings. Figure 6.13(a) shows brake mean effective pressure (BMEP) contours as a function of EVC (exhaust valve closing) and IVo (intake valve opening) at 1500 rpm and l = 1.0 in a four-cylinder four-stroke gasoline engine (Li et al., 2001;
Zhao et al., 2002). The engine was equipped with a pair of low-lift camshafts and two independent VCT devices that allowed the intake and exhaust valve timings to be altered by 40° crank angles. As shown in Fig. 6.13, when the EVC is retarded from 115 Ca BTDC to 75 Ca BTDC the engine output increases from 1.45 bar BMEP to 3.65 bar BMEP. This can be understood
by referring to the plots in Figs 6.14 and 6.15. It can be seen in Fig. 6.14 that as the exhaust valve closes earlier the residual gas fraction increases.
Figure 6.15 shows that there was a linear correlation between the residual fraction and the engine’s performance, independent of the engine speed. The higher the residual fraction, the lower the torque became. as the engine was operated at WoT during autoignition combustion, the mass in the cylinder was more or less the same and only the mixture concentration changed. The more residuals were trapped, the less air/fuel mixture the engine could breathe in, and hence the lower torque could be generated. Therefore, changing the residual fraction by adjusting the valve phasing is an effective means of controlling the engine load during the CaI combustion operation, resulting in throttleless engine operation and hence reduced pumping losses.
Figure 6.11 also shows that for each engine speed there were upper and lower limits of BMEP and IMEP outputs. The upper limit was a consequence of the restrictions of the gas exchange process imposed by the special camshafts. The lower end of the torque output was limited by misfires. The range of torque output was dependent upon the engine speed. at lower engine speed, the breathing capability of the engine was improved and hence a higher maximum torque output could be obtained. as the engine speed went up, the maximum fresh charge that the engine could take in was reduced, as
65 70 75 80 85 90 95 100 105 110 IVO (ATDC)
EVC (BTDC)
115 110
105
100
95 90
85
80
75
1.45
1.80 1.83
1.83 2.20
2.49 2.70
3.65 3.52 3.40 3.16 3.00
2.20 2.49 2.70
2.49 2.70
3.00 3.16 3.16
3.52 3.40 3.52
6.13 Contours of brake mean effective pressure (bar).
less time was available for the gas exchange process to take place through the low lift valves, leading to a lower maximum torque output.
In addition to controlling the engine’s output, the valve timings are found to affect the engine’s fuel consumption and exhaust emissions. as shown in Figs 6.16 and 6.17, at the same BMEP value of 3.4 bar, both the fuel
68 68
68
64
64 64
57 57
55 55
48
65 70 75 80 85 90 95 100
IVO (ATDC)
EVC (BTDC)
115
110
105
100
95
90
85
80
75
6.14 Contours of concentration of residual gas concentration.
1000 rpm 1500 rpm 2000 rpm 2500 rpm 3000 rpm
30 40 50 60 70 80 90
Trapped residuals (%)
BMEP (bar)
6 5 4 3 2 1 0
6.15 Relationship between BMEP values and the percentage of trapped residual gas.
370 370
EVC (BTDC)
65 70 75 80 85 90 95 100 105 110 IVO (ATDC)
115
110
105
100
95
90
85
80
75
350 350
320 350
310 320
300 294 290
290 200
294 300 300 310
320 310
285
6.16 Contours of brake specific fuel consumption (g/kWh).
11.1 9.0 11.4
11.1
9.0 9.0
6.9 6.9
6.9
5.8
5.3 5.8
5.3
5.8
5.3 4.2 4.8
4.5 4.8
EVC (BTDC)
65 70 75 80 85 90 95 100 105 110 IVO (ATDC)
115
110
105
100
95
90
85
80
75
6.17 Contours of brake specific hydrocarbon emissions (g/kWh).
consumption and HC emissions increase with the retarded intake opening as the effective compression ratio is decreased and combustion becomes less complete. Furthermore, in-cylinder pressure measurements and the subsequent heat release analyses indicate that the autoignition and heat release processes are also affected by the valve timings. as shown in Fig. 6.18, autoignition occurs earliest at the intermediate EVC and IVo timings, corresponding to the middle range of the autoignition combustion region. This is because at the same EVC timing or load point, retarded IVo reduces the effective compression ratio, giving a lower compression temperature and retarded autoignition. Earlier opening of the inlet valve induces more backflow of the hot residual gases into the intake port immediately after the opening of the inlet valve due to higher in-cylinder pressure than that in the intake port. This portion of residual gas will lose heat to the intake port and the in-cylinder gas mixture will start the compression process at a lower charge temperature, resulting in delayed autoignition combustion. For the same IVo setting in Fig. 6.18, the change of combustion timing with EVC timing is associated with the variation in engine output. as EVC is advanced, the concentration of residual gas increases at reduced load. Combustion takes place with a more diluted mixture and hence lower combustion and burnt gas temperature. The trapped residual gas from the previous cycle will therefore
EVC (BTDC)
65 70 75 80 85 90 95 100 105 110 IVO (ATDC)
115
110
105
100
95
90
85
80
75
359.0 359.0
360.0 358.0 359.0 360.0
358.0
360.0 359.0 360.0 357.0
357.5
358.0 361.0
359.0 357.5
358.0 358.0
359.0
6.18 Contour of the position of 10% mass fraction burned.
be at a lower temperature, retarding the autoignition process. on the other hand, the increase in load with retarded EVC leads to a higher residual gas temperature but the quantity of residual gas will be reduced. Beyond a certain load and EVC setting, the total thermal energy of residual gas, which is proportional to its temperature and mass, will start to drop when the effect of mass becomes dominant. This will cause the autoignition to be delayed at the upper load region of CaI operations. Therefore, the start of