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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.

A heavy duty diesel engine operating case producing no engine-out smoke was studied using combined simultaneous optical diagnostics. The case was close to a typical low load modern diesel operating point without EGR. Parallels were drawn to the conceptual model by Dec and results from high-pressure combustion vessels. Optical results revealed that no soot was present in the upstream part of the jet cross-section. Soot was only observed in the recirculation zones close to the bowl perimeter. This indicated very slow soot formation and was explained by a significantly higher air entrainment rate than in Dec's study. The local fuel-air equivalence ratio, Φ, at the lift-off length was estimated to be 40% of the value in Dec's study. The lower Φ in the jet produced a different Φ -T-history, explaining the soot results. The increased air entrainment rate was mainly due to smaller nozzle holes and increased TDC density.

Concentrations of hydroxyl radicals and formaldehyde were calculated using homogeneous (HRM) and stochastic reactor models (SRM), and the result was compared to LIF-measurements from an optically accessed iso-octane / n-heptane fuelled homogeneous charge compression ignition (HCCI) engine. The comparison was at first conducted from averaged total concentrations / signal strengths over the entire combustion volume, which showed a good qualitative agreement between experiments and calculations. Time- and the calculation inlet temperature resolved concentrations of formaldehyde and hydroxyl radicals obtained through HRM are presented. Probability density plots (PDPs) through SRM calculations and LIF-measurements are presented and compared, showing a very good agreement considering their delicate and sensitive nature.

Simultaneous OH- and formaldehyde planar-LIF measurements have been performed in an optical engine using two laser sources working on 283 and 355 nm, respectively. The measurements were performed in a light duty Diesel engine, using n-heptane as fuel, converted to single-cylinder operation and modified for optical access. It was also equipped with a direct injection common rail system as well as an EGR system. The engine was operated in both HCCI mode, using a single fuel injection, and UNIBUS (Uniform Bulky Combustion System) mode, using two injections of fuel with one of the injections at 50 CAD before TDC and the other one just before TDC. The OH and formaldehyde LIF images were compared with the heat-release calculated from the pressure-traces. Analyses of the emissions, for example NOx and HC, were also performed for the different operating conditions.

Simultaneous formaldehyde and OH PLIF have been applied in a direct-injected HCCI engine. The engine is a 0.5 l single-cylinder optical engine equipped with EGR system. PLIF measurements were performed with the engine run with two different fuels of low and high volatility, respectively. Different ratios of EGR were also examined. The aim of the study was to investigate how fuels with different volatility and EGR affect the HCCI combustion and measurements were performed for early and late injection timings. Measurements are presented for different injection timings showing formaldehyde and OH from start of injection until late in the expansion stroke. Also, formaldehyde distributions obtained from after the low temperature regime and before the high temperature regime are studied for different tuning of the start of injection from 300CAD to 20CAD before top dead center.

Simultaneous OH- and formaldehyde planar-LIF measurements have been performed in an optical engine using two laser sources working on 283 and 355 nm, respectively. The engine used for the measurements was a car Diesel engine converted to single-cylinder operation and modified for optical access. The fuel, n-heptane, was injected by a direct injection common rail system and the engine was also fitted with an EGR system. The engine was operated in both HCCI mode and Diesel mode. Due to the low load, the Diesel mode resulted in low-temperature Diesel combustion and because of limitations in maximum pressure and maximum rate of pressure increase of the optical engine, the Diesel mode was run at a higher EGR percentage than the HCCI mode to slow down the combustion. A third mode, pilot combustion, was also investigated. This pilot combustion is created by an injection at 30 CAD before TDC followed by a second injection just before TDC.

Simultaneous OH- and formaldehyde LIF measurements have been performed in an HCCI engine using two laser sources working on 283 and 355 nm, respectively. Two ICCD camera systems, equipped with long-pass filters, were used to collect the LIF signals. The simultaneous images of OH and formaldehyde were compared with heat-release calculated from the pressure-trace matching the cycle for the LIF measurements. The measurements were performed on a 0.5 l single-cylinder optical engine equipped with port-fuel injection system. A blend of iso-octane and n-heptane was used as fuel and the compression ratio was set to 12:1. The width of the laser sheet was 40 mm and hence covered approximately half of the cylinder bore. At some 20 CAD BTDC low temperature reactions is present and formaldehyde is formed. The formaldehyde signal is then rather constant until the main heat-release starts just before TDC, where the signal decreases rapidly to low values.

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.