Investigation of mixture formation and combustion in an ethanol direct injection plus gasoline port injection (EDI+GPI) engine
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Ethanol direct injection plus gasoline port injection (EDI+GPI) is a new technology to utilise ethanol fuel in spark-ignition engines more effectively and efficiently than E10 or E85 fuels in the current market. It takes the advantages of ethanol’s high octane number and great enthalpy of vaporisation which allow higher compression ratio and consequently increase the thermal efficiency. Primary experimental investigation showed that the engine performance was improved by EDI+GPI. The thermal efficiency was increased, the NO emission was decreased and the spark timing could be advanced without engine knock. However, the CO and HC emissions were increased when EDI was applied. To understand the mechanisms behind the experimental results, the mixture formation and combustion processes of an EDI+GPI engine were investigated using CFD simulation, and constant volume chamber and engine experiments. To investigate the spray and evaporation characteristics of ethanol fuel and provide experimental data for CFD simulation, spray experiments were conducted in a constant volume chamber using high speed shadowgraphy imaging technique. The results showed that ethanol fuel evaporated slowly when fuel temperature was in the range of 275-325 K. However, the evaporation rate increased quickly when fuel temperature was higher than 350 K. The low evaporation rate of ethanol fuel in low temperature environment implied that EDI should be only applied in high temperature engine environment. When the excess temperature was smaller than 4 K, the spray behaved the same as the subcooled spray did. The spray collapsed when the excess temperature was 9 K. Flash-boiling did not occur until the excess temperature reached 14 K. Numerical simulation of the EDI+GPI engine showed that the overall cooling effect of EDI was enhanced with the increase of ethanol ratio from 0% to 58%, but not with further increase of ethanol ratio. When the ethanol ratio was greater than 58%, the fuel impingement became severe and a large number of liquid ethanol droplets were left in the combustion chamber during combustion, leading to local over-cooling in the near-wall region and over-lean mixture at the spark plug gap. As a consequence, the CO and HC emissions increased due to incomplete combustion. Compared with GPI only condition, the faster flame speed of ethanol fuel in EDI+GPI condition resulted in shorter combustion initiation duration and major combustion duration, leading to the increase of IMEP and thermal efficiency when the ethanol ratio was 0-58%. However, the combustion performance was deteriorated by over-cooling and fuel impingement when ethanol ratio was greater than 58%. Experimental results showed consistently that the combustion and emission performance of this engine could be the best in the ethanol ratio of 40-60% at the investigated engine condition (medium load, 4000 rpm and early EDI timing of 300 CAD BTDC). Numerical results showed that the best engine performance was resulted from effective charge cooling and combustion efficiency improved by avoiding the wall wetting, over-lean and local over-cooling issues. Numerical simulations were also carried out to investigate the effect of direct injection timing on the EDI+GPI. The results showed that when the EDI timing was retarded from 300 to 100 CAD BTDC, the mixture around the spark plug became leaner and the distribution of equivalence ratio became more uneven. Moreover, late EDI timing at 100 CAD BTDC resulted in severe fuel impingement and caused local over-cooling effect and over-rich mixture. Consequently, the combustion speed and temperature were decreased by retarded EDI timing, leading to the decreased NO emission and the increased HC and CO emissions. The fuel impingement and incomplete combustion of late EDI timing at 100 CAD BTDC could be addressed by reducing the ethanol ratio to an appropriate point. Experiments on the EDI+GPI engine were conducted to verify the idea of EDI heating on improving the engine performance, which was developed based on the understanding gained from the numerical investigation. Results showed that EDI heating effectively reduced the CO and HC emissions at the original engine’s spark timing of 15 CAD BTDC. Meanwhile, the NO emission was slightly increased, but still much smaller than that in GPI only condition. However, the IMEP and combustion speed were slightly reduced by EDI heating. To enhance the effect of EDI heating, experiments were conducted at varied spark timing. The results at the MBT timing (19 CAD BTDC) showed that the reduction of IMEP by EDI heating was less significant whilst the CO and HC emissions were effectively reduced. Therefore EDI heating was effective to address ethanol’s low evaporation rate and over-cooling effect issues in the development of EDI+GPI engine in terms of minimizing the emissions.
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