Strategies for Improving Efficiency and Emissions in Heavy-Duty Diesel Engines

Abstract

This study presents an experimental investigation of the novel combination of a “wave" bowl piston with thermal barrier coatings (TBCs) and Miller cycle intake valve strategies. All experiments were carried out with a single cylinder research engine with fully-flexible valve timing capabilities. The results indicate the wave bowl geometry enables this combination to improve fuel consumption, steady-state engine-out NOₓ emissions, and particulate matter (PM) emissions, improving the NOₓ-PM tradeoff that compromises diesel engine efficiency. These benefits were achieved at the expense of elevated turbocharger efficiency requirements.

Three TBCs of varying composition and thickness were tested at seven operating conditions of varying speed and load. TBC performance was highly dependent on volumetric efficiency (VE), as cases with reduced VE increased the heat transfer gradient between the combustion gasses and the combustion chamber. The insulative properties of each TBC determined the impact of the aforementioned change in the heat transfer gradient, with the most pronounced effects on fuel conversion efficiency, up to a 0.6% increase, observed at medium and high load operation. The soot oxidation impacts of the wave piston were diminished at higher engine speeds, with the lowest PM emission increases for the TBC cases with reduced VE observed at the low speed conditions.

Early Intake Valve Closing (EIVC) and Late Intake Valve Closing (LIVC) Miller cycle strategies were compared to a conventional intake valve profile at a low speed-medium load condition under constant engine-out NOₓ emissions. The reduction in effective compression ratio from using Miller cycle was symmetric around bottom dead center, while EIVC profiles were more effective at reducing VE than LIVC profiles. The implementation of an overall turbocharger efficiency metric clarified the source of discrepancies found in the current body of work on Miller cycle, as studies reporting fuel consumption penalties were typically underutilizing boost capabilities and those reporting significant efficiency improvements were exceeding boost capabilities. Miller cycle profiles yielded 0.5% BSFC and 30% PM emission increases at the baseline turbocharger efficiency. Those penalties were nullified with an 8% relative increase in turbocharger efficiency.

The combined TBC-Miller cycle study compared extreme Miller cycle strategies to a conventional intake valve profile at a low speed-medium load operating point under high boost conditions. Comparisons were made under fixed cylinder composition, engine-out NOx emissions, and turbocharger efficiency constraints. Miller cycle at fixed cylinder composition demonstrated that LIVC strategies effectively reduced heat transfer losses, elevated exhaust losses, and reduced engine-out NOₓ emissions by up to 35%. Extreme LIVC timings increased fuel consumption by 3% because increased exhaust losses exceeded the reduced heat transfer losses. Miller cycle strategies enabled increased charge dilution at the fixed NOₓ constraint, improving fuel consumption by 1.3% over the baseline without compromising exhaust temperatures. This study produced the novel insight that varying EGR rates for NOₓ control suppresses the benefits of the inherent low NOₓ operation of Miller cycle applications. At an equivalent turbocharger efficiency representative of high boost operation, Miller cycle reduced NOₓ emissions by 31% and elevated exhaust temperatures relative to the conventional IVC case without compromising fuel consumption. The TBC piston's insulative properties shifted the inflection point of the heat transfer and exhaust loss tradeoff such that the optimum IVC timing is more extreme than with the uncoated piston for the fixed NOₓ emissions and turbocharger efficiency cases.