SUPERIOR DOWNSIZING
6.3 Approaches to autoignition combustion operation in gasoline engines
6.3.2 Gasoline autoignition combustion with internal exhaust gas recirculation (iEGR)
In the previous section, the fast thermal management approach utilises the exhaust thermal energy indirectly through heat exchangers and introduces the dilution effect of burnt gases by externally recirculating exhaust gas from the exhaust manifold through an EGR runner back into the intake manifold.
However, the dead volume of exhaust gases in the EGR system and the maldistribution of EGR between cylinders can cause cycle-to-cycle and cylinder-to-cylinder variations, as well as slow transient response. In addition, the heat loss through the EGR system is significant. A method of overcoming deficiencies of the external EGR approach is the use of ‘intern EGR’. The most significant research on the use of such internal exhaust gas recirculation to achieve autoignition combustion in a four-stroke direct injection gasoline engine was carried out by Fuerhapter et al. (2003, 2004). Figure 6.5 shows the schematic valve timing diagrams for different operation modes. In order to achieve autoignition combustion, one of the two exhaust valves was set to
reopen during the intake stroke. If the second opening of the exhaust valve occurs during the first half of the intake stroke, the hot burnt gases will be sucked back from the exhaust port into the cylinder and mixed with cold air from the intake port. The hot rebreathed exhaust gas will then experience a cooling-down period in the cylinder as the cylinder volume expands and heat is lost to the cylinder wall. In comparison, if the second opening of the exhaust valve takes place towards the end of the intake stroke after the fresh air has been admitted into the cylinder, the hot burnt gases will be sucked back into the cylinder shortly before the start of the compression process.
as a result, the rebreathed burnt gases will remain hot as they stay in the hot exhaust port rather than in the expanding cylinder volume. In addition, the late rebreathing approach allows the effective compression ratio to be altered by the same actuator for the reopening of the exhaust valve. In both cases, the rate of iEGR is primarily controlled by the reopening exhaust valve duration and timing. It should also be noted that a shorter intake valve opening period is needed to restrict the air flow into the cylinder so that a large amount of EGR can be trapped in the cylinder. It is also noted that the same low-lift intake valve profile is used for part-load SI operations outside the autoignition combustion range, with early intake valve closing time so that the engine is operating effectively unthrottled in a Miller cycle.
A 2.0 litre direct injection gasoline engine was modified to accomplish
Valve timing diagrams
Operation regimes in the engine map Exhaust valves Intake valves
1¥ Exhaust valve hydraulically Direct injection strategy
SI l £ 1.0
SI with int. EGR l = 1.0 CAI with int.
EGR l > 1.0 Engine speed SI stratified l > 1.0
BMEP
6.5 Valve timing diagram for autoignition combustion in the AVL-CSI engine (adapted from Fuerhapter et al., 2007).
autoignition combustion as well as spark ignition combustion. The intake camshaft was equipped with a VCT device and a two-stage valve lift shifting mechanism. a standard exhaust camshaft was used to actuate the two exhaust valves in the exhaust stroke. an electro-hydraulic valve actuation (EHVa) system was implemented to reopen one exhaust valve towards the end of the intake stroke. The EHVa system was able to adjust the opening timing and duration of the exhaust valve individually for each cylinder so that the combustion in each cylinder could be controlled independently and on a cycle-to-cycle basis. This was realised by implementing a closed-loop combustion control through an in-cylinder pressure sensor and a physically based model that was capable of predicting the autoignition combustion behaviour.
Figures 6.6 and 6.7 show the relative improvement in brake specific fuel consumption (BSFC) and Nox emissions with autoignition combustion compared to a purely SI operation without the flexible valve train system.
The range of autoignition operation is indicated by the two white lines and extends from 1000 rpm to 3000 rpm, which is likely to be the limit imposed by the EHVa system. The upper load limit is given by the limitation of the maximum cylinder pressure rise, which was defined as 3 bar/°CA, whereas misfiring due to reduced exhaust gas temperature prevents autoignition occurs at the lowest load and speed operations. Within the range of autoignition
BSFC improvement (%)
Spark ignition
Autoignition
1000 1500 2000 2500 3000
Engine speed (rpm)
BMEP (bar)
7
6
5
4
3
2
1
0
6.6 BSFC improvement achieved with the AVL-CSI engine (adapted from Fuerhapter et al., 2007).
combustion operation, Nox emissions are reduced by a factor of 10 to 100 compared with the values with spark-ignition combustion.
In order to understand the source of improvement, Fuerhapter et al. (2007) carried out an energy balance analysis of the different combustion modes. as shown in Fig. 6.8, at the selected operating point, the net indicated efficiency is increased from 29% in SI homogeneous operation to 37.5% in CaI mode, representing an improvement of about 30%. The energy split analysis shows that the pumping losses account for about two-thirds of the overall improvement of the net indicated efficiency of autoignition combustion and the remaining one-third is due to the reduced exhaust enthalpy and chemical energy losses caused by the high internal exhaust gas recirculation rates.