Optimization of Diesel Combustion System for Reducing PM to Meet Tier4-Final Emission Regulation without Diesel Particulate Filter

摘要

A 2.4 L commercial diesel engine was developed for light-duty commercial and off-road applications such as skid-loader, forklift and construction equipments. This engine complies with US Tier 4-final regulation, without PM after-treatment device by virtue of improved combustion strategy which is named as ULPC (Ultra-Low Particulate Combustion). This paper explains how ULPC works to reduce particulate matter (PM) based on the optimization of combustion system - piston bowl and nozzle specification. A baseline, re-entrant shape piston bowl, is useful to improve mixing of fuel and air in the main combustion chamber due to squish jet flow, especially in high-speed diesel engines. However, operating speed of commercial engines is almost half slower than that of passenger car engines and the merits by squish flow become deteriorated. It has another limit to reduce PM due to the fact that most combustion only occurs in main combustion chamber resulting in insufficient use of air in the upper region of the piston. For these reasons, a new piston bowl shape for ULPC has proposed to have a large diameter of piston entrance to use the air in upper region of main combustion bowl. Under ULPC bowl, combustion takes place in the two regions of piston, combustion bowl and upper region of the bowl, resulting from the split of fuel spray. By optimizing spray targeting (in terms of spray angle and bowl design), the amount of fuel split is well matched to the amount of air inside the bowl and upper region of the bowl that leads to maximized local air-fuel ratio and reduced PM. This ULPC strategy is realized by optimized bowl design via computational analysis and engine bench tests. Results of 3-D combustion simulation at rated power operating points explain that the ULPC piston has higher potential to reduce PM by faster soot oxidation through the improved mixing of fuel and air in the combustion chamber, especially during expansion stroke. Two test engines (baseline and ULPC piston bowl installed) were calibrated to find out the best PM emission on the engine dynamometer. Test results depicts that the ULPC piston has improved NO_x-PM trade off curve at the all range of engine operation. The result of C1-8 NRSC (Non-Road Steady Cycle test) bench test proves that optimized ULPC strategy reduces over 60% PM as the same NO_x level that leads for this engine to the Tier-4 Final emission compliance without DPF.