Fuel economy analysis of part-load variable camshaft timing strategies in two modern small-capacity spark ignition engines

The results of the investigation show that the two engines demonstrate consistent thermal efficiency response to valve timing changes in the low and mid part-load envelope, up to a load of 4 bar BMEP. At the lower engine loads investigated, reduced intake valve opening advance limits the hot burned gas internal recirculation, while increasingly retarded exhaust valve opening timing favours engine efficiency through greater effective expansion ratio. At mid load (4 bar BMEP), a degree of intake advance becomes beneficial, owing mostly to the associated intake de-throttling. In the upper part-load domain, for engine load of 5 bar BMEP and above, the differences between the two engines determine very different efficiency response to the valve timing setting. The lower compression ratio engine continues to benefit from advanced intake valve timing, with a moderate degree of exhaust timing retard, which minimises the exhaust blow-down losses. The higher compression ratio engine is knock-limited, forcing the valve timing strategy towards regions of lower intake advance and lower hot gas recirculation. The theoretical best valve timing strategy determined peak fuel economy improvements in excess of 8% for the port fuel injection engine; the peak improvement was 5% for the more efficient direct injection engine platform.