Characteristics And Suppression Strategies Of Knock And Particle Emissions For Downsized Boosted Spark-ignition Direct-injection Engines

Knock combustion and particle emissions are not only two important technicalbariers at high load conditions in boosted spark ignition direct injection (SIDI) gasolineengines, but also two important factors which might limit the development of nextgeneration SIDI engines. Knock is a kind of abnormal combustion which has direct effecton the engine downsizing level, and the potential of increasing geometric compressionratio. Particle emissions, especially for particle number emissions in boosted SIDIgasoline engine, is a relative new problem, and more and more stringent emissionregulations are driving greater challenge for the development of boosted SIDI engine.Under this circumstance, the objective of this thesis is to develop new effective strategiesto suppress knock and particle number emissions for downsized boosted SIDI gasolineengines. Based on the evaluation on how engine control parameters affect knock andparticle number emissions, two concepts of "cooled external EGR (exhaust gasrecirculation) for exhaust dilution combustion " and "multiple injection for stratifiedstoichiometric combustion " are proposed to solve knock and particle number emissions,and the suppression mechanism are investigated.The experiments on the engine operating parameters indicate that low temperaturecombustion, lean or rich combustion, retarded injection, and inhomogeneous mixturecombustion can suppress knock combustion. For particle number emissions, however, lowtemperature combustion, lean combustion and homogeneous mixture combustion arebeneficial for lower particle number emissions. Increasing injection pressure properly andoptimizing injection timing are important strategies for low particle number emissions inboosted SIDI engines.The experiments and theory calculation are conducted in terms of knock predictionmodel with consideration of cooled EGR, potential of increasing geometric compressionratio and improving fuel economy using cooled EGR, and particle number suppression by cooled EGR. Results demonstrate that Douaud-Eyzat model, which has been widely usedin knock prodiction, provides good knock prediction performance for the cases of EGRrate less than5%, however, the knock onset error between the model prediction and theexperimental result becomes more and more significant as the EGR rate further increases.The proposed new knock model not only provides good knock onset prediction at thecases of no EGR, but also provides good knock onset prediction over a wide EGR rangefrom0to25%. As compared to naturally aspirated engines, the boosted gasoline engine istypically designed at a lower geometric compression ratio (CR) due to the increased knocktendency, limiting the potential of improving fuel economy. To better evaluate thepotential of increasing geometric compression ratio by suppressing knock whenimplementing cooled EGR, combing effects of increased CR and cooled EGR to furtherimprove fuel economy within acceptable knock tolerance has been investigated using a2.0L downsized boosted SIDI engine over a wide range of engine operating conditions from1000r/min to3000r/min at low to high loads. To clarify the mechanism of thiscomplicated effects, the first law of thermodynamics analysis was conducted with theinputs from GT-Power engine simulation. Experiment results indicate that cooled EGRprovides more brake thermal efficiency improvement than increasing geometric CR from9.3to10.9. The combined effects of18-25%cooled EGR and10.9CR lead to2.1-3.5%improvements in the brake thermal efficiency (6-9%improvements in the fuel economy)compared to the baseline (9.3CR without EGR). Among several effects contributing to thefuel economy improvement at low and high load conditions, the theoretical thermalefficiency improvement is primary which is caused by increased ratio of specific heats andincreased geometric CR. The reduction in heat transfer loss due to lower combustiontemperature with EGR is the secondary effect. At full load conditions, the improvement incombustion efficiency by fuel enrichment elimination is the dominant factor for theobserved thermal efficiency improvement. The experiment results also illustrate thatcooled EGR is an effective strategy to suppress particle emissions at high load conditionsin boosted SIDI engine; the reduction in soot emissions up to50%can be obtained withcooled EGR. The theory analysis based on3D CFD calculation indicates the importantfactor for the reduction in soot emission by cooled EGR is the lowered temperature withinsoot formation regions, especially for piston surface.Multiple injection for stratified stoichiometric combustion is proposed to suppress knock and particle number emissions. Double injection (one injection during the intakestroke, another injection during the compression stroke) is one of effective strategies, butit also leads to the deterioration in combustion efficiency and particle number emissions.To address this problem, the experiments illustrate that the triple injection strategy (thefirst injection is conducted during intake stroke, the second and third injection arecompleted during compression stroke) can avoid the reduction in combustion efficiencywhile keeping the knock suppression advantage, but the potential of improving particlenumber emissions is limited. Experiments results demonstrate that triple injection, fuel isinjected during intake stroke, can reduce particle number emission by up to80%ascompared to single injection strategy. Based on3D CFD calculation, the mechanism ofinjection timing and injection ratio in multiple injection，which is conducted during intakestroke, on particle number emissions are explored. Results indicate that reducing the fuelmass at the first injection and increasing injection times can effectively reduce thespray-piston impingement and improve the level of fuel-air mixing, hence reducingparticle number emissions. Based on the investigations above, quadruple injection strategy(one injection during intake stroke, remaining injection during compression stroke) isproposed to suppress knock and particle emissions while keeping high combustionefficiency. Experiments indicate that quadruple injection can suppress knock and reducingparticle number emission by up to45%without the sacrifice of combustion efficiency andfuel economy.