Towards an Innovative Combination of Natural Gas and Liquid Fuel Injection in Spark Ignition Engines

In order to address the CO2 emissions issue, and to diversify the energy for transportation, CNG (Compressed Natural Gas) is considered as one of the most promising alternative fuels given its high octane number. However, gaseous injection decreases volumetric efficiency, impacting directly the maximal torque through a reduction of the cylinder fill-up. To overcome this drawback, both independent natural gas and gasoline indirect injection systems with dedicated engine control were fitted on a RENAULT 2.0L turbocharged SI (Spark Ignition) engine and were adapted for simultaneous operation. The main objective of this innovative combination of gas and liquid fuel injections is to increase the volumetric efficiency without losing the high knocking resistance of methane. This paper describes how the CIGAL (TM) (Concomitant Injection of Gas And Liquid fuels) concept can generate a synergy effect between both fuels, which allows combustion efficiency to be increased significantly on turbocharged SI engines. Indeed, the gas and gasoline resultant mixture can be injected in stoichiometric conditions even at full load with spark timing set close to MBT (Maximum Brake Torque), thus leading to excellent performances, compliance with stringent regulations, good specific consumptions and reduced CO2 emissions. This injection strategy can also be extended to other liquid fuels, such as ethanol, and gaseous fuels like LPG (Liquid Petroleum Gas): the purpose remains having the best distribution between gas and liquid injections in order to maximize their advantages. Like for methane and gasoline, a strong synergy was observed with LPG and gasoline concomitant injection, leading to lower specific consumptions compared to those obtained in single fuel injection. This study demonstrates that the concept of concomitant injection offers direct benefits on the autonomy management of gas and gasoline powered vehicles. It also opens very interesting perspectives for meeting future emission regulations using only a 3-way catalyst since stoichiometry is maintained over the whole engine operating range.