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The present paper is Part III of an investigation on the influences of the late intake valve closing (LIVC) and the early intake valve closing (EIVC) on the engine fuel consumptions at different loads and speeds. The investigation was conducted with two 1.5L turbo-charged gasoline direct injection (TGDI) engines, one with a low-lift intake cam (the Miller engine) and the other with a high-lift intake cam (the Atkinson engine). This paper focuses on the influence of the intake-valve-closing timing on the fuel economy with and without exhaust gas recirculation (EGR). It was found that the Miller engine had a lower friction than the Atkinson engine; however, the impact of the difference in engine frictions on the fuel economy was mainly for low-speed operations. Across the engine speed range, the Miller engine had longer combustion durations than the Atkinson engine as a result of the impact of EIVC on the cylinder charge motion.

In order to improve low speed torques, turbocharged gasoline direct injection (TGDI) engines often employ scavenging with a help of variable valve timing (VVT) controlled by the cam phasers. Scavenging improves the compressor performance at low flows and boosts low-speed-end torques of the engines. Characteristics of the engine combustion in the scavenging zone were studied with a highly-boosted 1.5L TGDI engine experimentally. It was found that the scavenging zone was associated with the highest blowby rates on the engine map. The blowby recirculation was with heavy oil loading, causing considerable hydrocarbon fouling on the intake ports as well as on the stem and the back of the intake valves after the engine was operated in this zone for a certain period of time. The low-speed pre-ignition (LSPI) events observed in the engine tests fell mainly in the scavenging zone.

Turbocharged gasoline direct injection (TGDI) engines can achieve a very high level of brake mean effective pressure and thus the engines can be downsized. The biggest challenge in developing highly-boosted TGDI engines may be how to mitigate the pre-ignition (PI) triggered severe engine knocks at high loads and low engine speeds. Since magnitudes of cylinder pressure fluctuations during aforementioned engine knocks reach those for peak firing pressures in normal combustion, they are characterized as super knocks. It is widely believed that the root cause for super knocks is the oil particles entering the engine cylinder, which pre-ignite the cylinder mixture in late of the compression stroke. It is neither possible nor practical to completely eliminate the oil particles from the engine cylinder; a reasonable approach to mitigate super knocks is to weaken the conditions favoring super knocks.