...the spray and combustion of the MPCI mode are alternatively occurred as “spray-combustion-spray combustion” near the TDC [top dead center], rather than “spray-spray-combustion” sequence as traditional PPCI gasoline engines. Since the combustion process of the MPCI mode is mainly controlled by the multiple gasoline injections, the SOC [start of combustion] and pressure rise rate (PRR) can be controlled easily and effectively through injection timing, injection split ratio and the number of injections.

It is expected that each premixed combustion process in the MPCI engine can be realized in the center of combustion chamber with pure air filling the gap between the combustion area and the cylinder wall, resulting in not only the low PM and NO_x emissions, but also the low THC and CO emissions while [retaining] the high thermal efficiency

—Wang et al.

The interlaced spray and combustion events in MPCI can be repeated three times or more, the team said, but ideally each spray and followed combustion are fully separated without overlap.

The Tsinghua team says that the biggest advantage of MPCI mode is that it decouples the pressure rise process from the process of pollutants formation—i.e., the maximum pressure rise rate can be tuned by optimizing the injection time and dwell, as well as the split ratio and the injection timing, while NO_x, PM, THC and CO formation can be regulated in each combustion process respectively.

As a result, the MPCI mode overcomes the difficulties of the high MPRR when more homogeneous charge is formed to reduce NO_x and PM emissions in HCCI or PPCI mode, and of the high NO_x and PM emissions when more stratified charge formed to alleviate the high pressure rise rate in the conventional diesel combustion mode.

Although the long combustion in the MPCI mode will worsen the fuel efficiency due to its multiple spray and combustion events to control heat release rate, the injection timing and dwell can be optimized to obtain a [high enough] thermal efficiency.

...The main goal of the gasoline MPCI mode is to separate each spray from the followed combustion to achieve a premixed combustion one by one...Although the followed second, third or more combustion take place in a high temperature and fuel rich atmosphere, the NO_x and soot emissions can be low after optimization of injection timing and split ratio.

—Wang et al.

The team carried out an experimental study on a 0.7-liter single cylinder compression ignition engine retrofitted from a four-cylinder light-duty diesel with a compression ratio of 18.5. For fuels, the team used 94.4 RON gasoline (widely used in the Beijing market) and #0 diesel (cetane number 52.6, also common in Beijing).

Although in theory, gasoline MPCI can span engine loads from low to high, in practice, it was hard to split the fuel into two injection and combustion sequences at 5.5 bar IMEP. With two injection-combustion sequences, stable auto-ignition is not reached following the first spray, the team said. At 8 bar IMEP, it was easier to realize the two sequences, and the team carried out their testing at that level with 40%, 50% and 60% injection split ratios by mass. Diesel testing was carried out at a 50% split ratio.

All tests were carried out at 1400 rpm, with common rail pressure of 800 bar. Among their reported findings were:

• They achieved a stable and moderate two-stage MPCI mode under 8 bar IMEP and 1400 rpm at 70 °C intake temperature without EGR or intake boosting using 94.4 RON gasoline with injection split ratios of 40%, 50% and 60%.

• Gasoline MPCI gives lower soot and CO emissions than diesel double injection mode. THC emissions are at the same level of diesel, while the NO_xemission is slightly higher than that of diesel fuel.

• Soot and CO emissions of the 60% split ratio are lower than that of the 40% split ratio, but with the penalty of slightly higher THC emissions. This implies that more fuel in the first injection is encouraged as long as the MPRR and combustion noise are within an acceptable range.

• Indicated thermal efficiency was around 40%.

• The indicated specific fuel consumption of gasoline MPCI mode is lower than that of conventional single-stage diffusion combustion of a diesel. Results vary with the variance of the timing of the first and second injections, but dropped slightly below 240 g/kWh for MPCI at its best results, compared to a low of about 280 g/kWh for diesel at its best results.

  The authors suggested that using low octane gasoline without intake heating or applying intake boosting can effectively improve the fuel efficiency of gasoline MPCI, while maintaining low emissions and MPRR at the same time.

Resources

• Hong-Qiang Wang, Shi-Jin Shuai, Zhi Wang and Jian-Xin Wang (2012) High Efficiency and Low Pollutants Combustion: Gasoline Multiple Premixed Compression Ignition (MPCI). (SAE 2012-01-0382)