KAIST team explores mode transition between low-temperature and conventional combustion in a light-duty diesel
A team at the Korea Advanced Institute of Science and Technology (KAIST) reports on the effect of operating parameters such as rate of exhaust gas recirculation (EGR) change, residual gas, EGR path length, fuel injection pressure and engine speed on the mode transition between low-temperature combustion (LTC) and conventional combustion in a light-duty diesel engine.
The researchers also developed injection control strategies for a smooth combustion mode transition. A paper on their work, which was supported by the Ministry of Knowledge Economy (MKE) of Korea, appears in the International Journal of Engine Research.
LTC is a promising approach to reduce emissions of both NOx and soot to acceptable levels for future emission regulations. However, LTC can also result in increased fuel consumption and emissions of carbon monoxide (CO) and hydrocarbons (HCs) because of the low combustion temperature. A low combustion temperature also limits the operating range at low to medium load, the authors note.
Multiple injection strategies under LTC conditions have shown potential to reduce CO and HC emissions. Further, the increase in intake pressure provided by systems such as turbochargers and superchargers also reduces CO and HC emissions as well as the specific fuel consumption without any deterioration of NOx and soot emissions in LTC. In addition to these benefits, the operating range of LTC could also be extended by intake charge boosting.
However, the extended operating range of LTC is still too narrow to cover the entire operating range of diesel engines. This feature is an obstacle for its practical application for vehicles. Therefore, research on the mode transition between LTC and conventional combustion is required. It is based on the concepts to derive benefits from LTC at low to medium load and switch to conventional combustion at higher load. Most research on combustion mode transition has focused on gasoline engines.
...most studies [on diesel engines] performed up to now have not dealt with the effects of various operating parameters on the combustion mode transition. A parametric study on the combustion mode transition in diesel engines is considered indispensable for better understanding of combustion mode transition and hence the practical application of LTC for vehicles.—Han et al.
In their work, the KAIST researchers first studied characteristics of mode transition between LTC and conventional combustion with various operating parameters by changing the EGR rate in a light-duty diesel engine. In the second part of the study, they proposed the transient injection strategies for smooth combustion mode transition.
They worked with a four-stroke, five-cylinder, direct-injection diesel engine modified to operate in single-cylinder mode. Four cylinders (1–4) were operated by a programmable electronic control unit (ECU) in the conventional diesel combustion mode at low load for stable engine operation. The mode transition was performed in cylinder 5 by independently controlling the fuel injection parameters and EGR rate.
Among their findings:
A rapid decrease in IMEP (indicated mean effective pressure) and increase in MPRR (maximum pressure rise rates) occurred due to improper injection conditions as the mode transition proceeded from LTC to conventional combustion by decreasing the EGR rate. On the other hand, IMEP and MPRR in the case of mode transition from conventional combustion to LTC by increasing the EGR rate were changed slowly without rapid change due to the thermal effect of hot residual gas from conventional combustion.
Faster mode transition could be achieved by the use of shorter EGR paths due to the reduction of EGR delay, which depends on the time needed for the gas to travel through the EGR pipe.
In the case of EGR rate control by means of controlling the exhaust back pressure, the charge dilution effect of EGR was greater due to the larger amount of residual gas resulting from the higher exhaust back pressure. However, this effect diminished as mode transition proceeded from LTC to conventional combustion due to reduced exhaust back pressure.
Higher fuel injection pressure resulted in lower HC emission due to improved atomization of spray and air–fuel mixing. However, the effect of fuel injection pressure on the IMEP variation trend was negligible despite the different combustion pro- cesses that resulted from different mixing conditions corresponding to fuel injection pressure.
With faster engine speed, EGR delay was reduced due to the higher flow velocity, while the number of mode transition cycles increased. However, the absolute time for mode transition was shorter at faster engine speeds.
Rapid changes of IMEP and MPRR that occurred when the combustion mode switched from LTC to conventional combustion when only changing the EGR rate could be solved by cycle-by-cycle injection modulation. As a consequence of gradually retarding injection timing, gradually decreasing injection duration and adding a pilot injection, MPRR was significantly reduced, while IMEP maintained stable.
As a result of cycle-by-cycle injection modulation, the differences in IMEP and MPRR before and after mode transition, which have to be small enough for desirable combustion mode transition, were reduced by up to 100% and 225%, respectively, compared to baseline conditions. In addition, the response lags of IMEP and MPRR were also reduced by up to 100% and 42%, respectively.
Sangwook Han, Choongsik Bae, and Seibum B. Choi (2012) Effects of operating parameters on mode transition between low temperature combustion and conventional combustion in a light duty diesel engine. International Journal of Engine Research doi: 10.1177/1468087412454249