The main goal of researches in the field of automotive engineering is to obtain a
large-scale implementation of low- or zero-emissions vehicles in order to
substantially reduce air pollution in urban areas. A fundamental step toward
this green transition is represented by the improvement of current internal
combustion (IC) engines in terms of fuel economy and pollutant emissions. The
spark ignition (SI) engines of modern light-duty vehicles are supercharged,
down-sized, and equipped with direct injection. Gaseous fuels, such as liquefied
petroleum gas (LPG) or natural gas (NG), proved to be a valid alternative to
gasoline in order to reduce pollutant emissions and increase fuel economy. In
previous works the authors investigated the simultaneous combustion, in an SI
engine, of gasoline and a gaseous fuel (referred to as Double-Fuel operation,
DF) both in the naturally aspirated and supercharged version; a significant
increment of engine efficiency and a great reduction of pollutant emissions were
obtained with respect to pure gasoline operation, with almost unchanged
performance.
This article is a development of the previous work and shows the results of a
detailed heat release analysis, performed on the DF supercharged engine fueled
with mixtures of gasoline and NG in order to highlight the effects of engine
speed, charging pressure, and fuel mixture composition (the proportion between
gasoline and NG) on the combustion speed.
It was found that both gasoline content in the DF mixture and supercharging
pressure contribute to increase the combustion speed, which, in some cases,
produced engine-indicated efficiency increments up to 5%. The wide set of
experimental data presented in this article allows us to better understand the
combustion behavior of gasoline-NG fuel mixtures and can be also used to
calibrate combustion submodels integrated into engine numerical simulations.
