Influence of the Compression Ratio on the Performance and Emissions of a Mini HCCI Engine Fueled Ether with Diethyl 2007-01-4075

Power supply systems play a very important role in applications of everyday life. Mainly, for low power generation, there are two ways of producing energy: electrochemical batteries and small engines. In the last few years many improvements have been carried out in order to obtain lighter batteries with longer duration but unfortunately the energy density of 1 MJ/kg seems to be an asymptotic value. If the energy source is an organic fuel with an energy density of around 29 MJ/kg and a minimum overall efficiency of only 3.5%, this device can surpass the batteries. Nowadays the most efficient combustion process is HCCI combustion which is able to combine high energy conversion efficiency and low emission levels with a very low fuel consumption.

In this paper, an investigation has been carried out concerning the effects of the compression ratio on the performance and emissions of a mini, Vd = 4.11 [cm³], HCCI engine fueled with diethyl ether. Because of its high reactivity, autoignition of the mixture was achieved only using compression energy. The compression ratio was changed by altering the squish distance: 0.25, 0.50, 0.75, 1.00 and 1.25 [mm]. For each compression ratio, three sets of measurements were performed: 3000, 7000 and 12000 [rpm]. The study showed that diethyl ether was only slightly affected by quenching problems when the squish distance was 0.25 and 0.50 [mm] at 7000 [rpm]. It was also demonstrated that the performance improved when decreasing the compression ratio to an optimum point and subsequently dropped to zero when the highest spacer, 1.25 [mm], was used. Due to a very low combustion and thermodynamic efficiencies, the specific emissions of CO and HC were one order of magnitude higher than for a normal car/truck engine, whereas NOx emissions were comparable to those of a conventional diesel engine. Finally, the study rendered it possible to understand how much an HCCI engine fueled with diethyl ether could be scaled down since it was shown that this fuel was not very sensitive to quenching, with a squish distance of 0.25 [mm].