An electronically controlled Caterpillar single-cylinder oil test engine (SCOTE) was used to study diesel combustion. The SCOTE retains the port, combustion chamber, and injection geometry of the production six cylinder, 373 kW (500 hp) 3406E heavy-duty truck engine. The engine was equipped with an electronic unit injector and an electronically controlled common rail injector that is capable of multiple injections.An emissions investigation was carried out using a six-mode cycle simulation of the EPA Federal Transient Test Procedure. The results show that the SCOTE meets current EPA mandated emissions levels, despite the higher internal friction imposed by the single-cylinder configuration. NOx versus particulate trade-off curves were generated over a range of injection timings for each mode and results of heat release calculations were examined, giving insight into combustion phenomena in current “state of the art” heavy-duty diesel engines.Next, a study of the effects of varying boost pressure levels was conducted. For fixed brake specific NOx levels, with low-pressure (90 MPa) single injections, particulate was found to reduce monotonically as the boost pressure was increased. Interestingly, with low pressure double injections and with high pressure (>90 MPa) single injections, particulate was found to decrease at first and then to increase as the boost pressure was increased beyond an optimum level. A minimum value for particulate with respect to boost level was found for all cases, including the low-pressure single injections, when a correction for the six-cylinder turbocharger efficiency was applied. Computer modeling confirms that this is due to a reduction in the spray penetration and mixing that occurs as the engine gas density is increased. BSFC was generally reduced with increasing boost pressure. These results suggest that variable geometry turbochargers or other enhanced boosting methods will aid in the reduction of emissions and fuel consumption from heavy-duty truck engines.