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Environment protection issues regarding CO2 emissions as well as customers requirements for fun-to-drive and fuel economy explain the strong increase of Diesel engine on European market share in all passenger car segments. To comply future purposes of emission regulations, particularly dramatic decrease in NOx emissions, technology need to keep upgrading; the reduction of the volumetric compression ratio (VCR) is one of the most promising research ways to allow a simultaneous increase in power at full load and NOx / PM trade-off improvement at part load. This study describes the combustion effects of the reduction of compression ratio and quantifies improvements obtained at full load and part load running conditions on a HSDI Common Rail engine out performance (power, fuel consumption, emissions and noise). Potential and limitations of a reduced compression ratio from 18:1 to 14:1 are underlined.

A study has been initiated to understand the influence of coolant temperature on HC emissions employing a dual system cooling the cylinder head independently from the engine block. Especially, we have studied its influence on post-oxidation, fuel absorption-desorption, crevice volumes and fuel-air mixture preparation. The results show that the cylinder head temperature has more influence on HC emissions than the block temperature. It was also found that mixture preparation, absorption/desorption and crevice volumes with commercial gasoline is greatly improved by the cooling temperature. The post-oxidation process is also reduced for a decrease of the coolant temperature from 90°C to 35°C.

In-cylinder absorption and desorption of fuel by lubricating oil films is assumed to be among the main sources of unburned hydrocarbon emissions from spark ignition engines. As this phenomenon is mainly conditioned by the solubility of fuel in lubricant, differences in hydrocarbon emission levels were measured running an automotive engine, with various couples of singular constituent fuels and lubricants. In this way it was found that lowering the solubility by a factor of 40 caused an average HC emission reduction of 30%. It was found too that with commercial unleaded fuels the lubricant constitution had little influence on the emissions. As this fact was difficult to explain, and because the gasoline solubility values are not available, a single cylinder engine with an extended piston was specially designed to run with or without oil between the cylinder liner and the piston rings. The experimental method has been validated using an insoluble gaseous fuel (propane).