Exploring the effects of CR, EGR and spark timing on performance and emissions of HD natural gas engines
Researchers at Hunan University have investigated the effects of compression ratio (CR), EGR and spark timing strategies on performance, combustion and NOx emissions of a heavy-duty natural gas engine fueled with 99% methane content. A paper on their work is published in the journal Fuel.
… natural gas … is one of the most important alternative fuels in engines in terms of performance, safety, price and emission characteristics. However, there are still some problems [that] need to be solved in natural gas engine.
The key problem is that the natural gas has lower flame propagation speed, resulting in longer combustion duration in engines. Therefore, the peak cylinder pressure (PCP) stays away from the top dead center (TDC), which lowers the heat-work conversion efficiency and limits engine power. However, natural gas has a high octane number and shows better knocking resistance. Therefore, ICE fueled with natural gas can operate at high compression ratio and enable to adjust spark timing to improve BTE. But increasing the compression ratio will increase NOx emissions. Therefore, how to reduce NOx emissions in natural gas engine under high compression ratio becomes one of the difficult problems needed to solve.
… increasing the improvement on the economic performance and combustion performance of a heavy-duty natural gas engine with 99% methane content and higher CR is systematically investigated in this paper.—Li et al.
The original engine in the study was a heavy-duty natural gas engine with a CR of 11.6. For the study, the researchers modified the CR by replacing pistons with different bowl volumes.
The original engine does not have an external EGR system. Therefore, the researchers introduced a separate ECU to control the amount of CO2 in the intake system to simulate external EGR.
The ECU controlled the mass flow of CO2 to simulate external EGR in several steps. For example, when the engine speed was 1000 rpm, the EGR rates varied from 0% to 30% in 29 steps; at 1400 rpm, the EGR rates varied from 0% to 26% in 15 steps. The maximum EGR rate reached up to approximately 30% and the spark timing swept from 10 to 40 °CA before TDC (BTDC).
The main conclusions from the study were:
Employing a higher CR can improve the fuel economy efficiency, but generate higher NOx emissions.
With the increase of EGR rate, the combustion process slows down, which increases 10–90% combustion duration, delays 50% combustion position, and reduces the rate of heat release (ROHR). Ultimately, it brings about lower peak cylinder pressure (PCP), combustion temperature and effective expansion ratio (EER). In addition, the NOx emissions reduce significantly due to the increase of EGR rate and the maximum decreasing content of EER is 3.88% which happened at 1800 rpm.
At lower engine speeds, the EGR strategy slows down the combustion, which decreases the maximum combustion temperature and thereby reduces the NOx emissions. And at higher speeds, the EGR strategy has strong influences on the engine performance. Therefore, using EGR strategy is a good way to control the NOx emissions.
By adjusting the optimal spark timing, the NOx emissions could be further reduced.
Yangyang Li, Peng Wang, Shuqian Wang, Jingping Liu, Yunkun Xie, Wenjun Li (2019) “Quantitative investigation of the effects of CR, EGR and spark timing strategies on performance, combustion and NOx emissions characteristics of a heavy-duty natural gas engine fueled with 99% methane content,” Fuel, Volume 255 doi: 10.1016/j.fuel.2019.115803.