Novel strategies to overcome the limitations of a low compression ratio light duty diesel engine

Abstract

Low compression ratio (LCR) approach in diesel engines can reduce the oxides of nitrogen (NOx) and soot emissions simultaneously owing to lower temperatures and longer fuel-air premixing time. The present work investigates the effects of lowering the geometric compression ratio (CR) from 18:1 to 14:1 in a naturally aspirated (NA) single cylinder common rail direct injection (CRDI) diesel engine. Based on the investigations done across the entire speed and load range, significant benefits were observed in the NOx and soot emissions. However, lowering the compression ratio had adverse effects on brake specific fuel consumption (BSFC), unburned hydrocarbon (HC) and carbon monoxide (CO) emissions, especially at low-load and high-speed operating points. To overcome these limitations, novel strategies including split-cooling system (SCS) and secondary exhaust valve opening (SEVO) are proposed in the present work. While the fuel injection parameters optimization specific to LCR could help to improve the BSFC, HC and CO emissions penalty to a reasonable extent, the SCS concept can provide further benefits by reducing the heat loss to coolant and improving the engine component temperatures. Increasing the residual gas fraction using the optimized SEVO concept could further improve the charge temperature leading to a further reduction in the BSFC, HC and CO emissions. The net benefits of the optimized LCR approach are quantified for the modified Indian drive cycle (MIDC) using a one-dimensional simulation tool. The results obtained show a signification reduction of 22% and 74% in NOx and soot emissions respectively as compared to the base 18 CR engine results. Moreover, the penalty in HC and CO emissions could be contained to a large extent resulting in only a slight penalty of 23% and 20% respectively. Furthermore, the higher BSFC with the LCR approach could be successfully addressed and the final values were found to be better than the stock compression ratio by 1.5%. Overall, the strategies proposed in the present work are found to be beneficial to develop modern diesel engines in compliance with the future emission regulations which demand extreme control on NOx and soot emissions.

Enhancing the NOx and soot emissions reduction benefits of a low compression ratio light duty diesel engine by optimization of piston bowl design

“…It is to be noted that the LCR approach leads to a significant increase in ignition delay leading to highly retarded combustion phasing, 17,18 especially at low-load operation at 5 N-m operating point. As a result, a significant increase in BSFC, HC and CO emissions could be observed, which warrants additional measures, including optimizing injection settings and engine hardware systems.…”

“…As a result, a significant increase in BSFC, HC and CO emissions could be observed, which warrants additional measures, including optimizing injection settings and engine hardware systems. 17,63 However, no significant challenges were observed in combustion stability as the experiments were conducted in engine warmed-up conditions, maintaining coolant and oil temperatures at 90°C.…”

“…The potential benefits of the LCR approach in a single-cylinder diesel engine have been well established. 17,18 The LCR approach can reduce the bulk gas temperature at the end of the compression stroke, limiting local peak temperatures to reduce NO x significantly. Similarly, increased ignition delay and enhanced fuel-air mixing can help limit local regions with high equivalence ratios and temperatures to reduce soot emissions.…”

A novel strategy of extremely delayed intake valve opening to improve the cold-start characteristics of a low compression ratio diesel engine

“…32,33 Hence, the present work explores the cold-start challenges associated with a reduced compression ratio of 14:1 in a naturally aspirated single-cylinder diesel engine. The challenges associated with reducing the compression ratio from 18:1 to 14:1 on brake specific fuel consumption and emissions and the strategies to overcome those challenges are discussed in detail in a previous publication by Vikraman et al 9 The present work is a continuation of the previous wherein the cold-start challenges are addressed. In the present work, a novel intake valve timing is proposed to improve the cold-startability of a low compression ratio light-duty diesel engine.…”

“…[5][6][7][8] In this context, the benefits of adopting the low compression ratio (LCR) approach in diesel engines for simultaneous reduction of NO x and soot emissions are well established. 9 However, diesel engines' inherent limitation in terms of cold-startability restricts the use of the LCR approach in several ways. Since the diesel engines operate on the compression ignition (CI) principle, 10,11 startability in cold conditions is often challenging if the threshold in-cylinder temperatures are not reached at the time of fuel injection.…”