Combustion system development and analysis of a downsized highly turbocharged PFI small engine

Abstract

This paper provides some insight into the future direction for developing smaller capacity downsized engines, which will be needed to meet tight CO2 targets and the world’s future powertrain requirements. This paper focuses on the combustion system development and combustion analysis results for a downsized 0.43 liter highly turbocharged engine. The inline two cylinder engine used in experiments was specifically designed and constructed to enable 25 bar BMEP. Producing this specific output is one way forward for future passenger vehicle powertrains, enabling in excess of 50% swept capacity reduction whilst maintaining comparable vehicle performance.

Previous experiments and analysis have found that the extent to which larger engines can be downsized while still maintaining equal performance is combustion limited. Hence, small engine combustion is explored over a number of parametric studies, including a range of manifold absolute pressures up to 270 kPa, engine speeds exceeding 10,000 rev/min and compression ratios ranging from 9 to 13. Experimental results indicate that small engine combustion hurdles can be overcome to reliably extend the specific output to 25 bar BMEP. This is believed to be the highest recorded specific output for a non-intercooled small spark ignition PFI engine operating on pump gasoline. However, the boosted combustion effects illustrate that the thermal efficiency is highly dependent on the combustion efficiency, which deteriorates rapidly if uncontrolled combustion, specifically knock in the end-gas region is encountered. However, with this combustion system design strategy, potential drive cycle fuel consumption improvements in excess of 20% are still achievable.