Abstract
Research and/or Engineering Questions/Objective
Gasoline compression ignition (GCI) engines achieve high brake efficiency and simultaneous reduction of smoke and nitrogen oxides (NOx) emissions via partially premixed charge (PPC) combustion using a low cetane number fuel for extended ignition delay. Compared to kinetics-controlled homogenous charge compression ignition (HCCI) and its variants, GCI engines are more practical because the combustion phasing is closely coupled with the fuel injection timing. In this study, ethanol produced from biomass has been selected as a GCI fuel, considering its lower cetane number, evaporative cooling and oxygen contents than gasoline, all of which could further improve the GCI combustion.
Methodology
The ethanol-fuelled GCI was investigated in a single-cylinder automotive-size diesel engine connected to an EC dynamometer running at fixed speed of 2000 rpm. The focus is how key fuel injection conditions such as the common-rail pressure and double injection strategies impact GCI combustion. For each operating condition, the in-cylinder pressure traces were recorded using a piezo-electric pressure transducer, which was used to calculate the indicated mean effective pressure (IMEP), apparent heat release rate, and combustion phasing. The brake MEP (BMEP) was also calculated from a brake torque reading in the EC dynamometer, which was then used to obtain fuel conversion (or brake) efficiency, brake specific fuel consumption (BSFC), and friction MEP (FMEP). Simultaneously, engine-out emissions of smoke (opacity), NOx, uHC, and CO were measured.
Results
The results show that the friction loss plays a significant role in GCI combustion such that the increased FMEP with increasing common-rail pressure leads to the increased BSFC and decreased brake efficiency. The engine-out emissions also show an increasing trend with increasing common-rail pressure due to the over-penetration of fuel sprays and wall wetting, which suggests that the common-rail pressure for GCI applications should remain low for both high efficiency and low emissions. Regarding the double-injection strategy, it is found that the higher proportion in the first injection results in higher in-cylinder pressure and apparent heat release rate, which contributes to the increased IMEP, BMEP, and brake efficiency. The BSFC is also improved by 16% even if lower calorific value of ethanol is considered. The engine-out emissions of smoke, uHC, and CO show a decreasing trend with increasing first-injection fraction; however, the NOx emissions start to increase if higher than 50% first-injection fraction is applied.
Limitations of this study
Care should be taken to interpret the results due to the limitations given by a single-cylinder engine. It is well known that single-cylinder engines have higher FMEP than multi-cylinder engines due to less inertia. Also, we used a heavy flywheel to compensate the loss of inertia due to the lack of other pistons. Therefore, the impact of reduced FMEP at low common-rail pressure might have been exaggerated as the FMEP was very high and thus had a larger room for the improvement.
What does the paper offer that is new in the field including in comparison to other work by the authors?
While GCI or PPC combustion has been widely studied using various gasoline fuels, little is known about ethanol-fuelled GCI combustion. In particular, detailed analysis of fuel injection conditions provides new knowledge about ethanol-fuelled GCI combustion and a clear direction for parameter settings.
Conclusions
Compared to the diesel reference case, the optimised ethanol-fuelled GCI combustion at low 50-MPa common-rail pressure and 50:50 double injection conditions achieves 50% higher fuel conversion efficiency, 5% lower BSFC and 27% lower NOx emissions while smoke emissions are kept at a negligible level.
Keywords : Ethanol, Gasoline Compression Ignition (GCI), Partially Premixed Charge (PPC), Common-rail System