One high-efficiency combustion concept under investigation is gasoline compression ignition (GCI)—the use of gasoline-like fuels to deliver very low NOx and PM emissions as well as high efficiency in a diesel compression ignition engine. (Earlier post.) A challenge to be overcome with this approach is the higher resistance to autoignition of gasoline fuels.
A team from the University of Wisconsin-Madison’s Engine Research Center now reports in a paper in the journal Fuel on the effects of biodiesel-gasoline blends compared to neat gasoline using a partially premixed, split-injection GCI combustion strategy.
Recent studies have shown that gasoline and other fuels with similarly low to mid range cetane numbers and higher volatility are beneficial for LTC [low-temperature combustion]. One way of achieving these fuel properties would be to blend biodiesel fuel with gasoline, although in-tank vapor flammability must be a factor in choosing proportions of these components.
...Low load operation is a challenge for gasoline compression-ignition (GCI) combustion due to the high octane number of neat gasoline and its resistance to auto-ignition at the low mixture temperatures typical of low-load operation, thus requiring high intake air temperatures. The focus of this work is to reduce the intake temperature requirements of GCI engines at low load conditions by blending biodiesel with gasoline. This study also investigates the effects that this blended fuel has on combustion phasing, unburned hydrocarbon (UHC), CO, NOx, and soot emissions.—Adams et al.
For the study, the team used a single-cylinder engine based on a four-cylinder production GM 1.9L direct-injection light-duty diesel engine. They used a re-entrant bowl design with lower compression ratio than the production configuration, along with a slightly longer connecting rod length required for the Labeco CLR crankcase.
They tested three blends of gasoline and soy methyl ester biodiesel (biodieseline) tested, possessing 0%, 5% and 10% biodiesel by mass (G, BG5 and BG10, respectively). The commercially available gasoline base had an octane number of 87. The BG5 and BG10 blends had estimated cetane numbers of 17.96 and 19.54, respectively.
The combustion strategy featured an early pilot injection at -350 atdc containing the majority of the fuel being injected (70-80%), followed by a main injection between -40 atdc and -30 atdc. Three operating conditions were used to investigate the effects of biodiesel blending. Stable combustion was achieved for all three fuels at 3 bar IMEP and 5.5 bar IMEP conditions.
Among their findings were:
Successful light-load operation was obtained at 1500 rpm with100 kPa abs inlet pressure at 130 °C intake temperature (G); 110 °C intake temperature (BG5); and 95 °C intake temperature (BG10). These were accomplished with a 70/30 injection split ratio with a 400 bar injection rail pressure. Thus, the BG5 and BG10 blends lowered the intake temperature requirements compared to gasoline operation by 20 °C and 35 °C respectively.
The combustion efficiency of the neat gasoline was between 1% and 3% lower than that of the two blended fuels.
Biodiesel content at the 5% and 10% levels significantly reduced ignition delay and therefore advanced the phasing of combustion compared with operation on neat gasoline. The team concluded that the reduced ignition delays resulted from the increased cetane numbers of the blended fuels leading to reduced intake temperature requirements.
The varied ignition delay times among the three fuels were inferred to have a significant impact on mixture strength and therefore on UHC emissions. CO oxidation was enhanced by increased bulk gas temperatures. NOx emissions increased as start of second injection command was retarded.
Neat gasoline had higher spatial averaged bulk gas temperatures resulting in higher NOx emissions as well as lower UHC and CO. Oxidation of CO was not enhanced by increased oxygen content of the biodiesel blends.
The higher cetane number of the blended fuels may present challenges at high loads. Introducing EGR at higher loads will help control pressure rise rates as well as NOx emissions, the team suggested. A triple-pulse injection strategy could also work, they added.
Cory A. Adams, Paul Loeper, Roger Krieger, Michael J. Andrie, David E. Foster (2013) Effects of biodiesel–gasoline blends on gasoline direct-injection compression ignition (GCI) combustion, Fuel, Volume 111, Pages 784-790 doi: 10.1016/j.fuel.2013.04.074
Kalghatgi, G., Risberg, P., and Ångström, H. (2007) Partially Pre-Mixed Auto-Ignition of Gasoline to Attain Low Smoke and Low NOx at High Load in a Compression Ignition Engine and Comparison with a Diesel Fuel. (SAE Technical Paper 2007-01-0006) doi: 10.4271/2007-01-0006