The Flexible Diesel Engine

Some advantages may be gained from permitting variation in diesel engines of parameters that are normally fixed, such as mechanical compression ratio. Such an engine could be described as a flexible engine. This paper describes the results of computer modeling work carried out on a hypothetical engine that could be in production by the year 2000 for heavy-duty truck application. The engine-a six-cylinder, in-line, turbocharged, fourstroke engine with air-to-air aftercooling-was modeled using the TRANSENG computer program. It had a swept volume of 8.5 liters and produced 224 kW (300 hp) at 2000 rpm. Modeling work was carried out with a variable geometry compressor, a low-speed optimized compressor, variable compression ratio, and variable valve timing using the Miller cycle. The variable geometry compressor allowed an increase in BMEP of 5 percent and a decrease in fuel consumption of the same amount at rated power. Variable compression ratio showed no improvement, since the optimum compression ratio was close to 14:1, the baseline value. The use of variable valve timing to achieve the Miller cycle showed an increase in rated BMEP of 11 percent and a decrease in BSFC of the same amount.힎There was a simultaneous reduction in total heat transfer to the cylinder liners, piston crown, and cylinder head of 23 percent. These improvements made the Miller cycle a very attractive option.