Abstract
Widely used in stationary applications, Compressed Natural Gas (CNG) as fuel for high performance internal combustion engines is currently showing increasingly interest in mobile applications. In order to address the CO2 emissions issue and to diversify the energy for transportation, CNG is considered as one of the most promising alternative fuels. The first developments consisted in minor adaptations of gasoline engines to enable the use of CNG as fuel, without exploiting its whole potential. Indeed, CNG is not only an environmentally friendly fuel, but also a highly efficient one: much effort is now carried out in engine development to use its interesting features such as its high knocking resistance (octane higher than 120).
This paper presents the development of a dedicated natural gas engine based on the Renault F4Rt: a 2.0L spark ignited, turbocharged engine. Main objectives of this work were to optimise the engine's technical characteristics in order to maximize its performances and efficiency with CNG fuel, with a limited performance reduction when using gasoline fuel, and to evaluate the final configuration with other alternative fuels such as E85 and LPG.
A preliminary design office study and calculations were first carried out to conceive and optimise the piston shape, to select an adapted valve train associated to an intake Variable Valve Timing system, to adjust the turbocharging loop and to design the gas injection circuit. This initial work led to a first engine technical configuration that was evaluated on test bench in order to validate these choices. Different compression ratios, tumble levels and valve timings were tested with the aim to optimise the operation of the engine with CNG and gasoline.
The performances obtained with CNG in the final configuration with a 11.5:1 compression ratio were excellent. Low-end torque was drastically improved with a maximum value of 400 N.m achieved at 2000 rpm and kept constant up to 3000 rpm, while the maximum power output was 150 kW at 5500 rpm for a stoichiometric air-fuel ratio. With gasoline fuel, higher performance values than the stock engine were also obtained thanks to the specific combustion chamber design. The addition of a low pressure cooled EGR loop, having a positive influence on knock sensitivity, showed an interest in limiting the enrichment with gasoline and thus reducing the fuel consumptions up to 20 % at full load.
Alternative fuels were also tested to demonstrate the potential of this configuration as a multifuel engine. Outstanding results were reached with E85: 440 N.m at 2000 rpm and 160 kW between 4500 and 5500 rpm in continuous stoichiometric conditions.
At last, an innovative multi-fuel injection concept called CIGALTM (Concomitant Injection of Gas And Liquid fuels) was investigated. Simultaneous injections showed a synergistic effect that allowed higher performances than with one of both fuels to be obtained and open up wide perspectives.
Keywords: CNG, turbocharged, spark ignition, EGR, alternative fuel, E85, LPG, simultaneous injection