Water Injection Benefits in a 3-cylinder Downsized SI Engine 2019-01-0034
With progressing electrification of automotive powertrain and the demand to meet increasingly stringent emission regulations, a combination of electric motor and downsized turbocharged Spark Ignited Engines has been recognized a viable solution. Maximum Power and reduction of tailpipe CO2 need to come from optimizing the IC Engine. However, the increased BMEP (Brake Mean Effective Pressure) and drive for higher compression ratio/thermal efficiency increases the propensity of knocking in Downsized Engines. Engine knock occurs when the unburnt charge auto-ignites before the propagation flame reaches the unburnt charge.
Currently, knock is mitigated by retarding the ignition timing. Under high power demand, ignition retard is limited by the exhaust components temperature limit. This is compensated by mixture enrichment – injecting more fuel – which in turn reduces combustion temperature at the cost of increased fuel consumption and Carbon Monoxide (CO) emissions.
The potential of Water injection to alleviate these problems has been identified over the last decade – following the trail of same technology used in Aircraft combustion engines. Water Injection suppresses combustion knocking by decreasing the local in-cylinder temperature. During liquid to vapor phase transformation, water requires energy – latent heat of vaporization and the injected water absorbs most of this energy from the mixture, subsequently decreasing its temperature. Addition of adequate water can result in obtaining stoichiometric Air/Fuel Ratio engine operation henceforth leading to both reduced fuel consumption and CO emissions.
For this study, a 4-stroke, 1.5 liter, 3-cylinder turbocharged engine with direct fuel injection and port water injection has been operated on 91, 95 and 98 RON Gasoline fuel. An experimental investigation to the effect of water injection on knock mitigation, combustion phasing improvement, required AFR and exhaust gas temperature control has been performed. Full load curves are presented and analyzed for different fuels and different water injection strategies.