University of Wisconsin and GM team investigating Gasoline Direct Injection Compression Ignition in light-duty diesel engines
A team from the University of Wisconsin and General Motors has found that high-speed gasoline direct injection compression ignition (GDICI) operation in the low temperature combustion (LTC) regime in a light-duty diesel engine is feasible. GDICI is one of the approaches under investigation further to reduce NOx and PM emissions from diesel engines. However, the majority of research effort focused on the utilization of GDICI combustion has so far mostly targeted heavy-duty engines, the team noted in a new SAE paper (2011-01-1182) presented at the World Congress last month.
Other researchers, Ra et al. note in their paper, have explored the effects of using conventional gasoline in compression ignition engine, with some of the findings including that with injection timings near TDC, the larger ignition delay of gasoline enabled the engine to run at significantly higher loads with lower smoke with no detriment to NOx, CO, UHC and fuel consumption.
Others have shown that the use of gasoline in a diesel engine operated at light and medium load conditions with a single injection strategy can reduce both NOx and soot, and that there is an optimal octane number fuel to minimize the emissions.
One of the crucial characteristics of fuels considered for use in compression ignition engines is their autoignitability. This depends on the chemical composition of the fuel, as well as on the evolution of the thermal state and the compositions of the charge mixture. Cetane number (CN) or Research octane number (RON) are widely used as indicators if ignitability or resistance to autoignition. They are measured under pre-defined conditions and thus can essentially indicate the effects of fuel composition on the propensity to autoignite.
Typically, practical diesel fuels have CN grater than 40 and are so prone to autoignition that they can autoignite before the fuel is sufficiently mixed with air. On the contrary, gasoline fuels have CN lower than 30 (or RON higher than 60). Since a well mixed charge condition before ignition is desirable in low emission CI combustion, fuel characteristics such as those of gasoline that provide longer ignition delay to allow fuel to mix with air are expected to be beneficial. In order to enhance the mixing with air, both high volatility and diffusivity are additional key properties of desirable fuels.—Ra et al.
In their new study, Ra et al. first used numerical simulations (employing a multi-dimensional CFD code, featuring improved sub-models and the Chemkin library) to identify characteristics of GDICI including sensitivity to operating parameters. They used a skeletal reaction mechanism for primary reference fuel oxidation with 41 species and 130 reactions to calculate the chemical kinetics of combustion.
In the modelling, they used a small-bore light-duty diesel with a compression ratio of 16.5, and split injections through a 7-hole injector with an included spray angle of 155 °. To investigate the effects of parametric variation, they varied initial gas temperature, boost level, EGR ratio and injection pressure from baseline values.
Following the modelling, they carried out engine experiments under the conditions suggested by the numerical results. Here, they used a single-cylinder version of a GM 1.9L, 4-cylinder light-duty diesel engine. A reentrant bowl design with lower compression ratio that the current Euro-4 compliant production configuration was used, along with a slightly longer connecting rod length. The Bosch common rail injection system utilized a 7-hole injector with 155 ° spray angle.
Based on their results, they concluded:
GDICI operation of light-duty diesels under full-load conditions is feasible, thereby extending low-emissions engine concepts to gasoline-fueled high-speed engine operation.
Due to the high volatility and low cetane index of gasoline combined with the reduction of combustion temperature through utilization of EGR, both PM and NOx emissions could be reduced to levels of about 0.1 g/kg-f while achieving ISFC at about 180 g/kWh.
There was an optimal injection pressure to maximize the extension of the operation map for a given engine load condition. Numerical simulations revealed that in-cylinder mixture stratification in the fuel vapor and gas temperature distribution were substantially affected y the injection pressure and both under and over mixing of fuel and air resulted in significant incomplete combustion of the in-cylinder mixtures.
The operation maps were found to be very sensitive to EGR ratio, initial gas temperature and boost pressure, especially for operations at high split ratios.
Decreasing EGR ratio retards the injection timing range of the operation map.
Increasing initial gas temperature and decreasing EGR ratio have similar effects on the operation map variation.
Decreasing boost pressure advances the injection timings of the operation map.
They also concluded that the good agreement between the experiments and the model predictions confirmed the validity of numerical simulations to be used as a guiding tool for design of experiments under conditions that have high sensitivity to operating parameters.
The team recommends further investigation using a more systematic and through approach such as genetic algorithm optimization to find the global optimal operating conditions including engine speed, injector geometry, compression ratio, and injection strategy, with the additional constraints of UHC and CO emissions.
Youngchul Ra et al. (2011) Study of High Speed Gasoline Direct Injection Compression Ignition (GDICI) Engine OPeration in the LTC Regime (SAE 2011-01-1182)
Ra, Y., Jun, J.E., and Reitz, R.D. (2009) Numerical Parametric Study of Diesel Engine Operation with Gasoline. Combust. Sci and Tech. 181: 350-378 doi: 10.1080/00102200802504665