Numerical Study on the Effects of Pilot Diesel Quantity Coupling EGR in a High Pressure Direct Injected Natural Gas Engine

ABSTRACT

The natural gas high pressure direct injection (HPDI) engines have attracted attention due to their high combustion stability and thermal efficiency comparable to diesel. However, the nitrogen oxides (NOx) emissions from HPDI engines remain high. In this study, the coupling effects of the exhaust gas recirculation (EGR) and pilot diesel quantity were investigated on a four stroke HPDI engine by simulation. Three pilot diesel quantities and four EGR rates were involved, and the combustion and emission characteristics of the engine were compared and discussed. The results show that with increasing EGR rate, the maximum combustion pressure (Pmax) decreases with different pilot diesel quantity, and the crank angle corresponding to the maximum combustion pressure (CPmax), the crank angle corresponding to the maximum heat release rate (CHRRmax) and the crank angle corresponding to the 50% cumulative heat release (CA50) are all pushed back. NOx emissions decrease gradually with the increase of EGR rate, but soot, unburned hydrocarbon (HC) and carbon monoxide (CO) emissions and methane escape all increase and the indicated thermal efficiency (ITE) decreases. The traditional “trade-off” relationship between NOx and soot emissions still exists on HPDI engine and becomes more obvious as the quantity of pilot diesel increases. NOx and soot emissions can be well balanced at moderate EGR rate. Increasing the quantity of pilot diesel can partially offset the hysteresis effect of EGR on the combustion process. And when the EGR rate is controlled within 30%, increasing the quantity of pilot diesel is beneficial to reduce HC and CO emissions and methane escape. By increasing the quantity of pilot diesel coupled with a moderate level of EGR rate, it is possible to significantly reduce NOx emissions while maintaining the ITE, methane escape and unburned diesel, HC and CO emissions at the same level as without EGR. But soot emissions will increase substantially.

KEYWORDS:

• High pressure direct injection
• natural gas
• pilot diesel quantity
• EGR
• emissions

Highlights

1. Coupling effects of EGR and pilot diesel quantity were studied on a HPDI engine.

2. Increasing pilot diesel can partially offset the hysteresis of EGR on combustion.

3. Increased pilot diesel and moderate EGR reduce NOx and maintain other performance.

Nomenclature

HPDIhigh pressure direct injection

DIdirect injection

PPCIpartial premixed compression ignition

CFDcomputational fluid dynamics

EGRexhaust gas recirculation

TDCtop dead center

AMRadaptive mesh refinement

Pmaxmaximum combustion pressure

Cpmaxcrank angle corresponding to Pmax

HRRheat release rate

CHRRmaxcrank angle corresponding to the maximum HRR

ISFCindicated specific fuel consumption rate

ITEindicative thermal efficiency

CAcrank angle

CA50crank angle corresponding to 50% cumulative heat release

CDcombustion duration

NOxNitrogen oxides

HChydrocarbon

COcarbon monoxide

CH4methane

D5/D10/D15pilot diesel accounts for 5%/10%/15% of the total energy

Acknowledgments

The authors would like to acknowledge the financial supports from the National Natural Science Foundation of China (No. 51909154), High-tech Ship Scientific Research Project of the Ministry of Industry and Information Technology (No. CB02N20), Shanghai Engineering Research Center of Ship Intelligent Maintenance and Energy Efficiency (No. 20DZ2252300) and Shanghai High-level Local University Innovation Team (Maritime safety & technical support).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [51909154]; Shanghai Engineering Research Center of Ship Intelligent Maintenance and Energy Efficiency [20DZ2252300]; High-tech Ship Scientific Research Project of the Ministry of Industry and Information Technology [CB02N20].