Study On Crankcase Emissions And Their Formation Of Electric-Controlled Diesel Engine

Crankcase emissions have been included in the emissions regulations since 2007 and are receiving more and more attentions. Currently, there are still a lot of diesel engines, especially non-road diesel engine with open crankcase ventilation systems. Previous studies focused on emissions reduction in exhaust manifold. Therefore, with the decrease of emissions in exhaust manifold, the proportion of crankcase emissions in total emissions increases. It is noteworthy that the particles in crankcase emissions are mainly generated by engine oil which has larger molecular weight and meanwhile the temperature is not high enough for it to be eliminated by aftertreatment. So the crankcase gas and particle emissions should not be ignored. In this dissertation, experimental and simulation methods are used to investigate the crankcase gas/particulate emissions and their formation in detail.Oil break-up in crankcase and effect of engine speed are experimentally studied visibly. The results show: the number of oil arcs increases with engine speed. As time goes on, the oil arc length shows a proportional rise and the swept area increases slowly first and then fast. The center points of oil arc nearly located on a line with 45 degrees to X-axis and they are distribute gradually far away from the crankshaft rotation axis as engine speed increases. The cycle-to-cycle variation also becomes large. As time goes on, the average speed fluctuation of oil particles increases and the separation of them tends more obviously. At a high speed, the oil arc separates into oil droplets.The tree-based adaptive local mesh refinement technology coupled with VOF(Volume Of Fluid) method is used to simulate oil-air interaction and the oil aeration rate are experimentally studied under different conditions. The oil droplet is compressed into a quasi-oval. Three phenomena, namely droplet coalescence, droplet marginal splashing and droplet splashing, occur at different initial impinging speeds. A new volume method with temperature compensation is indispensable. The oil aeration rate and oil temperature decrease steeply first and then gently to zero and ambient temperature, individually. There is a certain effect of oil aeration rate on the parameters which influence the oil break-up process.Firstly, the influence of diesel engine operating condition on particle number/mass, volatile/oxidation characteristics and PAHs are experimentally investigated. The results show that: With increasing engine load, crankcase particle size distribution pattern changes from unimodal to bimodal and the nuclei mode particles decrease while accumulation mode particles increase. Both of them decrease with engine speed. With delayed injection, the nuclei ones decrease and the accumulation ones first decrease then increase. The proportion of VOC(Volatile Organic Compounds) in crankcase particle are more than 95% and varies little with operating conditions. The specific emissions of both crankcase/tailpipe PAHs decrease with engine load in which the one with 3- and 4-cycle account for larger proportion. The equivalent toxicity of both crankcase and tailpipe particulates are close.Then, with CO as the marker, influences of engine operating condition and injection timing on in-cylinder combustion, tailpipe and crankcase HC emissions are studied. Crankcase gas temperature varies from52℃ to 68℃ while the crankcase flow rate increases with speed and load. Crankcase CO emission is one to two orders of magnitude lower than that of tailpipe with one order for HC and two order for NOx. The flow rate ratio of crankcase to tailpipe decreases with the load and injection timing delay but the change quantity decreases with the increase of speed. The HC emissions ratio of crankcase to tailpipe varies between 5 and 35%.Finally,the injection, combustion, engine speed, tailpipe/crankcase HC emissions and the impact of injection timing and set rail pressure are associatively investigated during cold start process. From 2nd cycle, fuel injection starts. Tailpipe HC emissions rise quickly to peak after a delay, and then decreases to a stable value while crankcase HC emissions grow rapidly to stable a value right after cold start. It is more difficult to start-up under low ambient temperature than high temperature. The former tailpipe HC emissions are higher while the crankcase HC emissions rarely change.With delayed injection, maximum and stable values of tailpipe/crankcase HC decrease and the ones of tailpipe HC are very large with injection at 0°CA. Both of tailpipe and crankcase HC emissions reduce as set rail pressure increases. With pilot injection, the in-cylinder pressure peak in accelerating stage is lower than that without pilot injection, and changes less with pilot injection interval, but firstly decreases then increases with pilot injection quantity. The tailpipe HC emissions keep increasing during idle stage, and amplitude increases with pilot injection interval, while the stable crankcase HC emissions are higher than those of single injection.