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, four-stroke 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. There 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.