Numerical Investigation on the Effects of Water/Methanol Injection as Knock Suppressor to Increase the Fuel Efficiency of a Highly Downsized GDI Engine

A new generation of highly downsized SI engines with specific power output around or above 150 HP/liter is emerging in the sport car market sector. Technologies such as high-boosting, direct injection and downsizing are adopted to increase power density and reduce fuel consumption. To counterbalance the increased risks of pre-ignition, knock or mega-knock, currently made turbocharged SI engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter induce an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A possible solution to increase knock resistance is investigated in the paper by means of 3D-CFD analyses: water/methanol emulsion is port-fuel injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. The peak power engine operation of a currently made turbocharged GDI engine is investigated comparing the adopted fuel-only rich mixture with stoichiometric-to-lean mixtures, for which water/methanol mixture is added in the intake port under constant charge cooling in the combustion chamber and same air consumption level. In order to find the optimum fuel/emulsion balance analytic considerations are carried out. Different strategies are evaluated in terms of percentage of methanol-water emulsion rate, to assess the effects of different charge dilutions and mixture compositions on knock tendency and combustion efficiency. Thanks to the lower chemical reactivity of the diluted end gases and the faster burn rate allowed by the methanol addition, the water/methanol-injected engine allows the spark advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption.