The effect of ethanol direct injection on knock mitigation in a gasoline port injection engine

In the development of downsizing and high efficiency spark ignition (SI) engine, overcoming the knock is crucial. One method to suppress knock is to use with high octane number fuels as additives. However, in the current pre-blended fuel such as E10, the blending ratio cannot be varied with the engine operation conditions. The novel ethanol direct injection plus gasoline port injection (EDI + GPI) provides the opportunity to solve the fuel blending problem. Most importantly, EDI + GPI has great potential in anti-knocking. By directly injecting ethanol into the combustion chamber, blending ratio can be adjusted according to the engine conditions and the engine's anti-knocking ability will be strengthened by not only the ethanol's high octane rating but also its great latent heat. In the work, the effect of EDI on suppressing the knock caused by advancing spark timing and increasing inlet air pressure were experimentally investigated on a 250 cc single cylinder motorcycle engine equipped with an EDI + GPI system. Experimental results showed that compared with GPI and gasoline direct injection (GDI), EDI + GPI effectively mitigated engine knock and permitted more advanced spark timing. In the ethanol energy ratio (EER) range from 15% to 35%, every 2% or 3% increment of EER permitted about 2 CAD advance of knock limited spark advance (KLSA). EDI + GPI also showed benefits to combustion, indicated thermal efficiency and emissions (HC and CO) due to the advanced spark timing and ethanol's oxygen content and fast laminar flame speed. In conditions simulating turbocharging, compressed air was used to increase the inlet air pressure from 1.0 to 1.4 atmospheric pressure with the EER increased to handle the raised knock tendency. Indicated thermal efficiency was increased with the increase of inlet air pressure. Exhaust gas emissions in terms of ISCO, ISNO and ISHC increased with the increased inlet pressure and major combustion duration decreased with the increase of inlet pressure. Overall, the results demonstrated the potential of EDI + GPI in anti-knocking and consequently increasing the thermal efficiency in small engines.