Experimental Investigation of Diluents Components on Performance and Emissions of a High Compression Ratio Methanol SI Engine

Abstract

Increasing compression ratio and using lean burn are two effective techniques for improving engine performance. Methanol has a wide range of sources and is a kind of suitable fuel for a high-compression ratio spark-ignition lean burn engine. Lean burn mainly has a dilution effect, thermal effect and chemical effect. To clarify the influences of different effects and provide guidance for improving composition of dilution gases and applications of this technology, this paper chose Ar, N 2 and CO 2 as diluents. A spark-ignition methanol engine modified from a diesel engine with a compression ratio of 17.5 was used for the experiments. The results obtained by using methanol spark ignition combustion indicated that at engine speed of 1400 rpm and 25% load, NOx dropped by up to 77.5%, 100% and 100% by Ar, CO 2 and N 2 . Gases with higher specific heat ratio and lower heat capacity represented by Ar exhibited the least adverse effect on combustion and showed a downward break-specific fuel consumption (BSFC) trend. Gas with high specific heat capacity represented by CO 2 can decrease NOx and total hydro carbons (THC) emissions at the same time, but the BSFC of CO 2 showed the worst trend, followed by N 2 . Gas affecting the combustion process like CO 2 had chemical effect.Energies 2019, 12, 3366 2 of 18 on a SI engine. Dinesh Kumar Soni et al. [8] changed the blending ratio of methanol in diesel and found a 30% methanol blending ratio achieved a maximum reduction of NOx, CO and HC emissions (27%, 58% and 65%, respectively) compared to original diesel fuel on a CI engine. Erik [9] et al. showed that the emission of soot is non-existent and formaldehyde can be avoided on an engine running in partially premixed combustion (PPC) mode at medium loads, however CO and NOx emissions are not significantly improved. But in general, methanol is widely used in an SI engine for its high octane rating [10].The increasing compression ratio (CR) is a promising approach matched with enhancing thermal efficiency and reducing emissions of the engine. G. Di Blasio [11] et al. demonstrated CR is an important design parameter, which is closely related to the methane unburnt (MHC) on methane-diesel light-duty engine. Srivastava [12] et al. found increasing CR showed benefit on decreasing mean effective pressure cyclic variation (COV IMEP ) and break-specific fuel consumption (BSFC), a higher CR can also expand the lean flammability limit of λ from 1.62 to 1.76 which provides favorable conditions for lean burn. Ibrahim et al. [13] have found that when exhaust gas was employed for dilution on a natural gas SI engine, the fuel consumption rate could be reduced by about 10% with CR increased from 8 to 10. However, knock is a major obstacle for further increasing CR in the SI engine, and running engine with fuel has high octane number is an effective solution, as methanol exactly meets this condition [14]. M. Bahattin [15] et al. increased CR from 6 to 10 by fuelling with methanol then discovered the brake thermal efficiency of engine fueled with m...

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