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Partially Premixed Combustion (PPC) is used to meet the increasing demands of emission legislation and to improve fuel efficiency. PPC with gasoline fuels have the advantage of a longer premixed duration of fuel/air mixture which prevents soot formation at higher loads. The objective of this paper is to investigate the degree of stratification for low load (towards idle) engine conditions using different injection strategies and negative valve overlap (NVO). The question is, how homogenous or stratified is the partially premixed combustion (PPC) for a given setting of NVO and fuel injection strategy. In this work PRF 55 has been used as PPC fuel. The experimental engine is a light duty (LD) diesel engine that has been modified to single cylinder operation to provide optical access into the combustion chamber, equipped with a fully variable valve train system. Hot residual gases were trapped by using NVO to dilute the cylinder mixture.

Laser-Rayleigh imaging has been employed to measure the relative fuel concentration in the gaseous jet region of DME sprays. The measurements were performed in an optically accessible diesel truck engine equipped with a common rail injection system. A one-hole nozzle was used to guarantee that the recorded pressure history was associated with the heat release in the imaged spray. To compensate for the low compression ratio in the modified engine the inlet air was preheated. Spray development was studied for two levels of preheating, from the start of injection to the point where all fuel was consumed. The results indicate that there is a strong correlation between the amount of unburned fuel present in the cylinder and the rate of heat release at a given time. The combustion can not be described as purely premixed or purely mixing-controlled at any time, but always has an element of both. After all fuel appears to have vanished there is still an extended period of heat release.

Dimethyl Ether (DME) has proved to be a promising fuel for diesel engines. It virtually eliminates particulate emissions and reduces the formation of nitrogenous oxides, without negatively affecting engine efficiency. Obtaining a deeper understanding of the mechanisms behind these properties is thus highly desirable. Various authors have suggested that the low NO emissions associated with DME are an effect of the mixing conditions, which are thought to differ from those of diesel sprays. To examine this, laser-Rayleigh imaging was employed for quantitative measurement of the local equivalence ratios in DME sprays. The quantitative images were analyzed using a statistical approach, in which probability distributions of ϕ-values for burning and for non-reacting sprays were compared. It was concluded that the diffusion flame is established in the stoichiometeric or slightly lean regions of the spray.