Uncertainty of fuel reserves, environmental crisis, and health concerns arise from transport demands and reliance on fossil fuels.
Downsized gasoline turbocharged direct injection (GTDI) engines have been developed and applied to most modern gasoline vehicles, delivering superior efficiency in high-load operation, reduced friction, and weight. But fuel enrichment and late combustion phasing to mitigate knocking combustion have hindered the efficiency benefits at higher loads with high boost. Furthermore, the wide valve-overlap with a three-cylinder setup for the maximum scavenging efficiency produces bursts of short-circuit (SC) air to cause underestimation of the equivalence ratio by the oxygen sensor, resulting in higher tailpipe nitrogen oxides (NOx) emissions with three-way catalyst (TWC) exhaust aftertreatment.
Reducing the valve overlap to limit short-circuiting and enrichment will recover the combustion efficiency and the engine equivalence ratio (ER), but at the cost of high knock onset.
This article will present and analyze the synergy of valve-overlap reduction and external exhaust gas recirculation (EGR) at boosted loads of a three-cylinder GTDI engine. The strategy aims to realize the efficiency gains of reduced valve overlap by replacing the fuel enrichment and late combustion phasing strategies.
The combined use of 10% external EGR and overlap reduction gained 3% of brake-specific fuel consumption (BSFC) improvement, which is more than that of EGR dilution alone with regular wide valve-overlap, due to the ineffectiveness of EGR to lower the efficiency at the tested case. Test analysis uncovered that external EGR lowers the reactivity of residual gas, adding knock mitigation properties to high residual gas fractions (RGF) operation at high loads. Besides, the reduction of the overlap through the dual variable valve timing (VVT) device also changed the effective compression and expansion ratio. In addition to efficiency improvements, bursts of short-circuiting were eradicated to reduce tailpipe NOx.
However, the reduction in volumetric efficiency is expected to reduce the torque limit of the engine for a given turbocharger setup. Combustion instabilities were also reported due to the increased fraction of trapped combustion products.
