Medium-duty Diesel Research Articles

Effect of Exhaust Gas Recirculation on Biodiesel Blend Level Estimation in Diesel Engines

Biodiesel is an alternative fuel derived from vegetable oils, animal fats, or other sources, and it can be made into biodiesel blends by mixing with conventional diesel. To achieve optimal engine combustion as well as minimal emissions with biodiesel blends, on-board blend level estimation system is one of the prerequisites. The paper investigates a blend level estimation system by evaluating exhaust oxygen fraction differences between diesel and biodiesel. In the system, a wideband oxygen sensor is utilized to measure the oxygen concentration. Then, research on the oxygen-content based biodiesel blend level estimation system is extended by taking the exhaust gas recirculation (EGR) into account. Since estimation accuracy under the lean conditions for this kind of system has been especially problematic, the effect of EGR on system estimation uncertainty is discussed. The theoretical formulation shows that oxygen consumption factor is an intrinsic indicator of fuels, which is not affected by the EGR level. However, the “shifting effect” caused by introducing EGR can help moving the estimation points into a range where not only the oxygen sensor but also the whole estimation system exhibits lower measurement uncertainty. Validations are provided by both simulation results based on a high-fidelity GT-Power engine model and experimental results on a medium-duty turbo-charged diesel engine platform.

Two-Stage Ignition as an Indicator of Low-Temperature Diesel Combustion

Two-stage ignition is characterized by an initial cool-flame reaction followed by typical hot ignition. In traditional combustion conditions the ignition is fast such that the cool flame is not observed. By controlling initial conditions (pressure, temperature, and composition) in a rapid compression machine, for example, the creation and duration of the cool-flame event is predictable. This study focuses on linking the results from the rapid compression machine to those of low-temperature combustion behavior in a medium-duty compression ignition diesel engine. A correlation between cool-flame duration, nitric oxide concentration, and engine control settings allows the postulation that a sufficiently long cool-flame reaction results in a combustion event that can be classified as low-temperature combustion. A potential method for identifying low-temperature combustion events using only the rate of heat release profile is theorized which utilizes the cool-flame reaction duration as a metric.

Influence and potential of flexible injection rate shaping for medium and heavy duty diesel engine combustion processes

Modern fuel injection systems for medium and heavy duty diesel engines combine the potential of very high injection pressures and flexible injection rate shaping. Against this background, the Commercial Vehicle Division of DaimlerChrysler AG, in close collaboration with DaimlerChrysler Research, performed principle tests to assess the influence of a flexible injection process, focusing on the effects of injection rate shaping. Besides pressure indexing, optical diagnostic methods and simulation techniques provided valuable insights in this study. Within this project, four different injection systems with different potentials of flexibility were investigated. New insights, such as the influence of needle opening and closing behaviour on nitric oxide emissions and the influence of injection rate shaping on soot emissions have been elaborated.