U. Wisc, GM team extends operation range for gasoline direct-injection compression-ignition using triple-pulse injection
Researchers from the University of Wisconsin-Madison and General Motors have extended the operation range for a light-duty diesel engine operating on gasoline using extended controllability of the injection process via a triple-pulse injection strategy. (Earlier post.)
In a paper presented at SAE 2012 World Congress, the team reported the use of a triple-pulse injection strategy to extend gasoline direct-injection compression-ignition (GDICI) combustion in a single-cylinder engine to full load (16 bar IMEP, 2500 rpm) while maintaining results achieved earlier with a double-pulse injection strategy (~0.1 g/kg-f of NOx and PM and ~ 173 g/kWh of indicated specific fuel consumption, ISFC).
In that previous work, the team had revealed high sensitivity of GDICI operation to changes in EGR ratio, initial gas temperature and boost pressure, and found the existence of an optimal injection pressure to maximize the extension of the operation map for a given engine load condition.
Although operable ranges in the GDICI engine studied were identified, excessive pressure rise rates still remained as the toughest constraint to meet, which tends to narrow the operable range. This limitation led to a further investigation of possible strategies to attenuate and control PRR, and thus to extend the GDICI operation range using the current advanced injection system technologies.
Triple pulse injection strategy has been employed recently in several experimental studies on diesel engine combustion with gasoline-like fuels mainly to mitigate the combustion rate that would be problematic and mid to high engine load...In the present study, an investigation of high speed GDICI engine combustion using a triple-pulse injection strategy in the LTC [low temperature combustion] regime is presented.—Ra et al.
In their investigation, the team first used numerical simulations to identify characteristics of GDICI with triple injections, including the sensitivity to engine parameters. Subsequent to the modeling, they performed engine experiments to compare results with the predicted characteristics. Based on the work, they drew the following conclusions:
Triple-injection extends GDICI significantly to higher loads, while meeting combustion efficiency, noise and emission level targets.
Combustion stability and maximum PRR can be controlled by the second pulse, while the third pulse can control engine operation load.
Operation ranges are more confined by the soot lines in the operation maps than the combustion stability lines near the late injection timing boundary. “This is a characteristic difference of triple-pulse injection from double-pulse operation.”
Controllability of maximum PRR was obtained by triple-pulse injection at the expense of slight increases in ISFC and reduced IMEP.
The operation ranges are very sensitive to EGR ratio, initial gas temperature and injection pressure. Decreasing EGR ratio tends to narrow the operating range of the third pulse timing. Increasing initial gas temperature and decreasing EGR rati have similar effects on the operation range variation, byt the CS-line is more affected by initial temperature variation.
Increasing injection pressure reduces soot emissions significantly. The soot-line can be retarded to etend the overall operation map.
In future work, they will explore the effects of a triple-pulse injection strategy at lower engine loads.
Ra, Y., Loeper, P., Andrie, M., Krieger, R. et al. (2012) Gasoline DICI Engine Operation in the LTC Regime Using Triple-Pulse Injection, SAE Int. J. Engines doi: 10.4271/2012-01-1131