The Influence of Boost Pressure on Emissions and Fuel Consumption of a Heavy-Duty Single-Cylinder D.I. Diesel Engine

Abstract

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 sixmode 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. INTRODUCTION The capability of direct-injection (DI) diesel engines to produce efficient power is well known. However, diesel engines exhibit a propensity for high nitrogen oxide (NOx) and particulate matter (particulate) emissions, and most strategies to reduce either NOx or particulate emissions cause an increase in the other emission. Since 1990, progress in engine technology has allowed a significant reduction in all forms of emissions from diesel engines. However, demands for further emissions reduction, quieter operation, and improved performance still provide serious challenges. Advanced fuel injection technology can help reduce NOx and particulate emissions. More precise control of the rate, pressure, and timing of fuel injection, while using a high-pressure injection system, not only permits simultaneous reductions in NOx and particulate formation during combustion, but can also reduce engine noise. The electronic unit injectors (EUIs) found on most commercially available heavy-duty engines are capable of precise highpressure injection [1]. Another technology used to gain more control over combustion processes in diesel engines is the use of a variable boost pressure system. A variable boost system allows flexible control, and thus optimization, of boost pressure for different load and speed conditions. In addition to the original power and efficiency goals of variable boost systems, these systems have proven to improve emissions and transient response as well [2]. Thus, variable boost systems are expected to be an important component for future low emissions heavy-duty diesel engines [1]. When a fixed geometry, un-wastegated turbocharger is used to provide boost for an engine, a compromise must be made. Midrange torque and efficiency must be sacrificed in order to not over-boost at high loads and speeds. Wastegates allow some exhaust gas to bypass the turbocharger turbine at high loads and speeds, thus reducing