PERFORMANCES IMPROVEMENT OF A S. I. CNG BI-FUEL ENGINE BY MEANS OF DOUBLE-FUEL INJECTION

Natural gas represents today a promising alternative to conventional fuels for road vehicles propulsion, since it is characterized by a relatively low cost, better geopolitical distribution than oil, and lower environmental impact. This explains the current spreading of Compressed Natural Gas (CNG) fuelled S.I. engine, above all in the bi-fuel version, i.e. capable to run either with gasoline or with natural gas. This characteristic, on the one hand, permits the vehicle to go even when natural gas is not available, on the other hand requires the engine to be designed to run safely with gasoline, i.e. with compression ratio lower than what natural gas would allow. Moreover the electronic control units are programmed to adopt rich mixture and poor spark advance when running with gasoline at medium-high loads, in order to prevent the engine from dangerous knocking phenomena: this causes an increase in fuel consumption and pollutant emissions. Starting from these considerations, the authors decided to investigate on the benefit attainable by means of a double-fuel injection, i.e. the injection of a certain amount of natural gas during the gasoline operation in order to increase the knocking resistance of the mixture and to run the engine with “overall stoichiometric” mixture even at full load, thus improving both engine efficiency and its environmental impact. To this purpose, the authors carried out an experimental campaign on the engine test bed, equipped with a fully instrumented series production bi-fuel spark ignition engine; the gasoline injection was managed by means of a real-time controlled ECU, while the simultaneous injection of natural gas was performed by means of IGBT transistors properly designed for fuel injection or spark timing control connected to a counter/timing PCI board. The results obtained fuelling the engine with both fuels in stoichiometric proportion with air show, with respect to the pure gasoline operation, considerable increase in fuel economy without remarkable power losses, while, with respect to the pure natural gas operation, only power improvements have been achieved: these advantages may lead the way to the adoption of the double-fuel injection in bi-fuel-engines. INTRODUCTION As is known gaseous fuels, such as Liquefied Petroleum Gas (LPG) and Natural Gas (NG), thank to their good mixing capabilities, allow complete and cleaner combustion than normal gasoline, resulting in lower pollutant emissions and particulate matter. Moreover the use of natural gas, mainly constituted by methane, whose molecule has the highest hydrogen/carbon ratio, leads also to lower CO2 equivalent emissions. Some of the automobile producers already put on the market “bifuel” engines, which may be fed either with standard gasoline or with natural gas. These engines, endowed of two separate injection systems, are originally designed for gasoline operation, hence they do not fully exploit the good qualities of methane, such as its high knocking resistance [1], which would allow higher compression ratios. Moreover, when running with gasoline at mediumhigh loads, the engine is often operated with rich mixture and low spark advance in order to prevent from dangerous knocking phenomena: this produces both high hydrocarbon and carbon monoxide emissions (also due to the low catalyst efficiency caused by the rich mixture) and high fuel consumption. As example Figure 1 reports mixture strength (in terms of lambda values), HC and CO raw emissions acquired on the test bed running at full load a FIAT bi-fuel engine (whose characteristics are reported in Table 1) fuelled with gasoline: as can be seen the strong mixture enrichment operated by the series production ECU causes high HC and CO levels (for a comparison, the dashed lines represent the probable pollutant concentrations which would be measured with stoichiometric mixture). Figure 2 instead reports indicated and effective efficiencies (i.e. evaluated on the base of the IMEP and BMEP respectively) measured at WOT (Wide Open Throttle, i.e. full load) running the bi-fuel engine either with gasoline or with natural gas (best spark timing for both cases); due to the relatively low compression ratio, the engine may be fed with stoichiometric air-natural gas mixtures even at full load without knocking to occur, while, during gasoline operation, in order to prevent knocking, the mixtures adopted are those reported in Figure 1: this is the main cause of the strong difference in the measured efficiencies. SAE Technical Paper 2009-24-0058 Copyright © 2009 SAE International DOI: 10.4271/2009-24-0058 0.84 0.85 0.86 0.87 0.88 0.89 0.9