Combustion And Emission Characteristics Of A Direct Injection Spark Ignition Engine With Exhaust Gas Recirculation

Downsized direct injection spark ignition (DISI) engines exhibit a huge potentialin higher efficiency and lower emissions, while facing the problems of knock andparticle matter emissions. The rise of engine compression ratio or turbochargerpressure ratio is strongly limited by knock, which becomes a barrier for furtherimprovement of engine performance and thermal efficiency. With more concern onair pollution, particle matter emissions have become an important issue for DISIengines since more stringent standards have been proposed.In this study, exhaust gas recirculation (EGR) was employed to mitigate knockand reduce particle matter emissions in a downsized DISI engine. Since EGR willslow down the flame propagation speed, which is not beneficial to a high combustionefficiency, higher tumble ratio and alcohol/gasoline blends were employed in thisstudy. Thus, the advantages of EGR can be maintained, while the flame propagationspeed can still be promoted.The effects of tumble ratio combined with EGR on fuel economy, combustionand emissions were experimentally investigated in a DISI engine at part loads. A lowpressure loop EGR system and a variable tumble valve in the intake ports wereemployed, by which a wide range of EGR rate and tumble ratio could be achieved.The results show that both increased EGR and enhanced tumble offer benefits in fueleconomy for spark ignition engines at part loads, and a significant improvement infuel economy about13.1-19.5%can be achieved with the combination of EGR andenhanced tumble. The reduced pumping loss from EGR addition and improvedcombustion efficiency from enhanced tumble ratio contribute to the improvement offuel economy. With the increased EGR from0to10%,10-90%mass fraction burned(MFB) combustion durations are significantly prolonged. The cases with high tumbleratio show shorter0-10%and10-90%MFB combustion durations than those with lowtumble ratio for all the test EGR rates, and this trend becomes increasingly apparent athigher EGR rates. The strengthened tumble flow has a significantly positive effect onflame propagation. The increased tumble ratio results in enhanced turbulent flowwithin the cylinder, and consequently the reduction of combustion duration at all thetest conditions. The combination of enhanced tumble with introduction of EGR iscapable of realizing a low level of coefficient of variations of indicated mean effectivepressure (CoVIMEP). The combustion stability is improved and the EGR tolerancerange is extended. With increased EGR, nitrogen oxides (NOx) emissions are dramatically reduced. However, the enhanced tumble increases NOx emissions due tohigher combustion temperature. The utilization of EGR combined with enhancedtumble is an effective method for achieving low NOx emissions in gasoline engines.HC emissions slightly increase both with increased EGR and enhanced tumble. Withincreased EGR, the effect of incomplete combustion overcomes the cooling effect ofEGR on CO emissions, leading to increased CO emissions. The enhanced tumbleshows a positive effect on the reduction of CO emissions.The fuel economy, combustion, knock and emission characteristics of a DISIengine fuled with ethanol/gasoline and n-butanol/gasoline blends with addition ofEGR were also studied. The results indicate that, with20%EGR, the brake specificfuel consumption (BSFC) for all the test fuels decreases by about4.9-6.1%comparedto that of pure gasoline. Moreover, the thermal efficiency can be apparently improvedwith the addition of EGR at high load conditions due to the reduced combustiontemperature and heat transfer loss. The differences in gasoline equivalent BSFCamong all test fuels are much less (2.9%) compared to those of BSFC. The chargecooling ability (LH/LHV) of all test fuels are arranged as follows: E20> E10>n-B20> n-B10> Gasoline. Both EGR and alcohol addition show evident chargecooling effect, and EGR has much more significant effect than the evaporation of testfuels. The0-10%MFB combustion duration increases with increased EGR andLH/LHV value of fuels, and the former one has a more significant effect. There is aclose relation between the10-90%MFB combustion duration and the laminar burningvelocity. A higher laminar burning velocity leads to a shorter10-90%MFBcombustion duration. As the EGR rate is increased from0%to20%, the laminarburning velocity decreases. As a result, the10-90%MFB combustion duration isprolonged by25.5-38.9%for alcohol/gasoline blends, and64.1%for gasoline. Theextension of the10-90%MFB combustion duration for blends is much less than thatof gasoline. With the addition of alcohol, the10-90%MFB combustion durationdecreases. This can be attributed to the different laminar burning velocities of the testfuels. The laminar burning velocity can be promoted with the addition of ethanol orn-butanol. Thus, the problem of slow burn rate with EGR can be partially solved bythe utilization of ethanol or n-butanol. With increased EGR rate, the combustionstability becomes worse, even misfire cycles occur. With the addition of ethanol orn-butanol to gasoline, the distributions of maximum pressure in250consecutivecycles for E20and n-B20become more concentrated than that of gasoline, indicatingthe improved combustion stability. For the caess with the same alcohol content, thelaminar burning velocity of ethanol/gasoline blends is higher than that ofn-butanol/gasoline blends, so the improvement for ethanol/gasoline blends is moreremarkable. The anti-knock ability mainly depends on the octane number andLH/LHV of fuels. Moreover, EGR plays an important role in the knock mitigation. Either ethanol/gasoline blends or addition of EGR is beneficial to improve theanti-knock ability. However, n-butanol/gasoline blends show degraded anti-knockability. Combined with EGR, it is possible to overcome the negative effect ofn-butanol on knock.Under full load conditions, EGR has a positive effect on particle number (PN)emissions in the test DISI engine. With increased EGR rate, the total numberconcentration remarkably decreases, in which the proportion of nucleation modeincreases, while the proportion of accumulation mode decreases. Alcohol/gasolineblends have a superior performance on PN emissions relative to neat gasoline. Forgasoline, PN emissions are dominantly controlled by the accumulation mode. Withincreased alcohol content, the total number concentration remarkably decreases,especially for the PN from the accumulation mode.