as discussed in the previous sections, it will not be possible to operate a gasoline engine in the autoignition combustion mode over the entire engine speed and load range. a practical engine has to be developed that can operate in and switch between flame propagation and autoignition combustion operations.
The first demonstration of switched operation between SI and autoignition combustion was carried out in a four-stroke single-cylinder research engine and a five-cylinder engine both equipped with the electromechanical camless valve actuation system (Koopmans et al., 2003). When it is necessary to change from SI to CaI engine operation, the residual mass has to be increased and the air excess ratio and fuel amount often also have to change. as shown in Fig. 6.23, the spark ignition operation was operated with a typical exhaust valve lift profile but the intake valve lift profile was set to a much narrower duration in order to achieve unthrottled SI operation. When change to CaI was required, the exhaust valves were closed much earlier to trap sufficient
burnt gas and the intake valve opening was shifted to the mid-intake stroke.
Although the change in the valve lift profiles could be achieved within one cycle, the first few cycles of CAI operation were characterised by earlier, quicker and faster combustion than the later cycles, due to the hotter exhaust gas from the previous SI cycle and hotter boundary conditions created by the hotter SI combustion operations. This was accompanied by a jump in torque as the thermal efficiency of CAI combustion was higher. Koopmans et al. (2003) suggested that optimisation of valve timings and fuel adjustment were needed to achieve improved SI to CaI switching operations. Switching from CaI to SI operation did not have many complications as SI combustion was such less sensitive to the ambient conditions.
a much more practical approach is to achieve both SI and CaI operations in a gasoline engine with a cam profile switching (CPS) mechanism in conjunction with twin independent variable cam timing (VCT) devices. The CPS device allows operation with two different valve lift modes, whilst the valve timing is altered by the VCT devices. The components of the CPS mechanism are typically inner and outer co-axial bucket tappets and three cam lobes per engine valve (Fig. 6.24). The inner tappet operates directly on the engine valve via a hydraulic lash adjuster and is controlled by the central cam lobe. The outer tappet is driven by a pair of outer cam lobes of identical profiles and has its own set of lost motion springs within the tappet to absorb lost motion and to ensure that it remains in correct contact with its cams at all times. To achieve profile switching (from the outer to the inner cam profile) the valves within a chosen cylinder are switched by releasing
HCCI
Valve lift
SI
0 90 180 270 360 450 540 630 720 Crank angle (degrees)
6.23 Valve lift profiles adopted for SI and HCCI operations by the electromechanical camless system.
the locking pins and allowing the inner and outer tappets to slide relative to each other. The engine valve is now controlled purely by the inner tappet.
If the inner lobe has been manufactured with a small lift relative to the outer lobes, the engine valve will be lifted with a small lift. During normal engine operation the locking pins are held in the locked position by engine oil pressure. When a profile switch is required to the lower lift value, the oil pressure supplied to the tappets is reduced to a lower level by the use of a simple pressure-modulating solenoid valve, thus allowing a light spring to move the locking pins to the unlocked position.
one example of such work was reported by Cairns and Blaxill (2007).
They applied two-stage cam profile switching to the transition between SI and CaI combustion in a four-cylinder direct injection research engine. They found that in order to avoid individual cylinder misfires, it was necessary to employ sequential CPS strategies. Furthermore, they stated that to improve the response of the switching tappets, the oil circuit dead volumes needed to be minimised and the inlet and exhaust tappet oil supply pressures were decreased to the minimum value possible for robust SI operation (~1 bar gauge). Following these modifications, it was possible to achieve transition without misfire on the four-cylinder direct injection gasoline engine.
In a similar study carried out on a 3.0 litre V6 DI gasoline engine with twin CPS and twin VCT devices in the author’s laboratory (Kalian and Zhao, 2008), it was found that the control strategy had to be tuned in order to realise a successful transition from the SI mode to the CaI mode. First,
Inner coaxial bucket tappet
Hydraulic lash adjuster
Primary profile
cam lobe Secondary profile
cam lobes
Outer coaxial bucket tappet
6.24 Cam profile switching device with coaxial bucket tappet.
Locking pins
the control program was modified so that two different sets of parameters, namely ignition timing, injection timing and valve timing, could be pre-set, one for the high-lift SI mode and one for the low-lift CaI mode. Hence, when a transition was undertaken, the parameters would change instantly.
However, the values of these parameters did not correspond to their steady- state operation and would need careful calibration through the closed-loop control. In addition, the throttle setting and speed of its opening and closing may need to be adjusted to avoid misfire due to excessive fluctuation in the air inducted into each cylinder.
Recently, a more flexible and practical approach to achieving SI and CaI combustion in a production-type four-stroke gasoline engine has been researched and developed by the author and his colleagues in SKLE (Zhang et al., 2007) by making use of mechanically variable valve lift systems in production engines which are capable of continuously varying the valve lift from less than 1 mm to the full lift of 9 mm (Fig. 6.10). Examples of successful CaI–SI and SI–CaI transitions are shown in Fig. 6.25. Figure 6.25(a) shows the sequence of cycles during the SI to CaI operation. In the first two cycles, the engine was operated in the CAI combustion mode with a low exhaust valve lift and early EVC. The switching process started as the exhaust valve lift was increased and the EVC was retarded in order to reduce the level of residual gases. During the switching cycles (from cycle 3 to cycle 5), spark-assisted autoignition took place, the spark-ignition combustion process became more dominant and the heat release rate decreased. In order to maintain the engine output and efficiency, the effective compression ratio was adjusted by altering the IVC timing. as a result, the CaI–SI switching process lasted about three cycles. Similarly, by simultaneously controlling the residual gas concentration through adjusting the exhaust valve lift profile and heat release process through spark timing and IVC, swift and smooth switching was achieved from SI to CaI mode operation, as shown in Fig.
6.25(b). If IMEP fluctuations (dIMEP) are employed to indicate the stability of the transition process, experimental results show that dIMEP in the 4VVaS engine can be reduced to 0.2 bar in the CaI–SI transition and to less than 0.3 bar during the SI–CaI transition process through the optimised control strategies, which represents perhaps one of the best results reported in the literature.