Novel strategies to improve the potential benefits and overcome the limitations of a Low Compression Ratio Light-duty Diesel Engine

The current emission legislations across the world demand extreme reduction in oxides of nitrogen (NOx) and soot emissions which can pose significant challenges to the prospects of diesel engines. Since reducing both these emissions need expensive and complicated exhaust after-treatment systems and controls, alternate solutions need to be explored. In this regard, lowering the engine geometric compression ratio provides the benefit of simultaneous reduction in NOx and soot emissions. However, several challenges need to be addressed to consider low compression ratio (LCR) as a feasible approach.
In the present work, the geometric compression ratio (CR) of a naturally aspirated single-cylinder light-duty diesel engine was reduced from 18:1 to 14:1 to convert it into LCR mode. Based on the experimental investigations done across the entire operating map of the engine, significant benefits were observed with the LCR approach in the NOx and soot emissions due to lower charge temperatures and increased pre-mixing time. However, it has adverse effects on brake specific fuel consumption (BSFC), unburned hydrocarbon (HC) and carbon monoxide (CO) emissions, and cold-startability. Based on extensive experimental investigations, the challenges of the LCR approach could be addressed by adopting different methodologies, including optimizing fuel injection parameters, split cooling system (SCS), secondary exhaust valve opening (SEVO), intake air boosting, engine downspeeding and valve timing modifications. In the present seminar, optimization of fuel injection parameters specific to LCR engine and valve timing modifications to improve cold-startability will be discussed. Based on extensive investigations, it was observed that the main injection timing and injection pressure play a significant role in improving the BSFC and reducing exhaust emissions of the LCR engine. Advancing the main injection timing and reducing the injection pressure was beneficial at low-load operating points to reduce HC, CO and BSFC. However, at high load operating points, retarding the injection timing and increasing the injection pressure could reduce the NOx emissions. The injection strategies are combined carefully to result in a significant reduction in a regulatory drive cycle. Further, transient emission characteristics of a light commercial vehicle powered by the LCR engine were investigated in a chassis dynamometer corresponding to the regulatory modified Indian drive cycle (MIDC). Based on the investigations, the net impact of reducing the compression ratio and optimizing the fuel injection parameters was quantified. Further, the cold-startability challenges of the LCR approach were addressed by adopting a novel concept of extremely delayed intake valve open (IVO) timing, and the benefits were validated in a cold chamber.