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Soot formation and oxidation are complex and competing processes during diesel combustion. The balance between the two processes and their history determines engine-out soot values. Besides the efforts to lower soot formation with measures to influence the flame lift-off distance for example or to use HCCI-combustion, enhancement of late soot oxidation is of equal importance for low-λ diffusion-controlled low emissions combustion with EGR. The purpose of this study is to investigate soot oxidation in a heavy duty diesel engine by statistical analysis of engine data and in-cylinder endoscopic high speed photography together with CFD simulations with a main focus on large scale in-cylinder gas motion. Results from CFD simulations using a detailed soot model were used to reveal details about the soot oxidation.

This work has focused on studying the in-cylinder pressure fluctuations caused by rapid HCCI combustion and determine what they consist of. Inhomogeneous autoignition sets up pressure waves traversing the combustion chamber. These pressure waves induce high gas velocities which causes increased heat transfer to the walls or in worst case engine damage. In order to study the pressure fluctuations a number of pressure transducers were mounted in the combustion chamber. The multi transducer arrangement was such that six transducers were placed circumferentially, one placed near the centre and one at a slight offset in the combustion chamber. The fitting of six transducers circumferentially was enabled by a spacer design and the two top mounted transducers were fitted in a modified cylinder head. During testing a disc shaped combustion chamber was used. The results of the tests conducted were that the in-cylinder pressure experienced during rapid HCCI-combustion is inhomogeneous.

The present study uses a numerical model to investigate the effects of flow turbulence on premixed iso-octane HCCI engine combustion. Different levels of in-cylinder turbulence are generated by using different piston geometries, namely a disc-shape versus a square-shape bowl. The numerical model is based on the KIVA code which is modified to use CHEMKIN as the chemistry solver. A detailed reaction mechanism is used to simulate the fuel chemistry. It is found that turbulence has significant effects on HCCI combustion. In the current engine setup, the main effect of turbulence is to affect the wall heat transfer, and hence to change the mixture temperature which, in turn, influences the ignition timing and combustion duration. The model also predicts that the combustion duration in the square bowl case is longer than that in the disc piston case which agrees with the measurements.

Detailed chemical kinetics was implemented in the KIVA-3V multidimensional CFD code to study the combustion process in Homogeneous Charge Compression Ignition (HCCI) engines. The CHEMKIN code was implemented such that the chemistry and flow solutions were coupled. Detailed reaction mechanisms were used to simulate the fuel chemistry of ignition and combustion. Effects of turbulent mixing on the reaction rates were also considered. The model was validated using the experimental data from two modified heavy-duty diesel engines, including a Volvo engine and a Caterpillar engine operated at the HCCI mode. The results show that good levels of agreement were obtained using the present KIVA/CHEMKIN model for a wide range of engine conditions, including various fuels, injection systems, engine speeds, and EGR levels. Ignition timings were predicted well without the need to adjust any kinetic constants.

A new ignition and combustion model has been developed and tested for use in premixed spark-ignition engines. The ignition model is referred to as the Discrete Particle Ignition Kernel (DPIK) model, and it uses Lagrangian markers to track the flame-front growth. The model includes the effects of electrode heat transfer on the early flame kernel growth process, and it is used in conjunction with a characteristic-time-scale combustion model once the ignition kernel has grown to a size where the effects of turbulence on the flame must be considered. A new term which accounts for the effect of air-fuel ratio, was added to the combustion model for modeling combustion in very lean and very rich mixtures. The flame kernel size predicted by the DPIK model was compared with measurements of Maly and Vogel. Furthermore, predictions of the electrode heat transfer were compared with data of Kravchik and Heywood. In both comparisons the model predictions were in good agreement with the experiments.

An experimental and theoretical study of the effect of thermal barriers and catalytic coatings in a Homogeneous Charge Compression Ignition (HCCI) engine has been conducted. The main intent of the study was to investigate if a thermal barrier or catalytic coating of the wall would support the oxidation of the near-wall unburned hydrocarbons. In addition, the effect of these coatings on thermal efficiency due to changed heat transfer characteristics was investigated. The experimental setup was based on a partially coated combustion chamber. The upper part of the cylinder liner, the piston top including the top land, the valves and the cylinder head were all coated. As a thermal barrier, a coating based on plasma-sprayed Al2O3 was used. The catalytic coating was based on plasma-sprayed ZrO2 doped with Platinum. The two coatings tested were of varying thickness' of 0.15, 0.25 and 0.6 mm. The compression ratio was set to 16.75:1.

The potential of a Homogeneous Charge Compression Ignition (HCCI) engine with variable compression ratio has been experimentally investigated. The experiments were carried out in a single cylinder engine, equipped with a modified cylinder head. Altering the position of a secondary piston in the cylinder head enabled a change of the compression ratio. The secondary piston was controlled by a hydraulic system, which was operated from the control room. Dual port injection systems were used, which made it possible to change the ratio of two different fuels with the engine running. By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain any octane rating between 0 and 100. By using an electrical heater for the inlet air, it was possible to adjust the inlet air temperature to a selected value.

Gas-phase in-cylinder mixing was examined by two different methods. The first method for observing mixing involved planar Mie scattering measurements of the instantaneous number density of silicon oil droplets which were introduced to the in-cylinder flow. The local value of the number density was assumed to be representative of the local gas concentration. Because the objective was to observe the rate in which gas concentration gradients change, to provide gradients in number density, droplets were admitted into the engine through only one of the two intake ports. Air only flowed through the other port. Three different techniques were used in analyzing the droplet images to determine the spatially dependent particle number density. Direct counting, a filtering technique, and autocorrelation were used and compared. Further, numerical experiments were performed with the autocorrelation method to check its effectiveness for determination of particle number density.

The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the reciprocating engine. Here, a homogeneous charge is used as in a spark ignited engine, but the charge is compressed to auto-ignition as in a diesel. The main difference compared with the Spark Ignition (SI) engine is the lack of flame propagation and hence the independence from turbulence. Compared with the diesel engine, HCCI has a homogeneous charge and hence no problems associated with soot and NOX formation. Earlier research on HCCI showed high efficiency and very low amounts of NOX, but HC and CO were higher than in SI mode. It was not possible to achieve high IMEP values with HCCI, the limit being 5 bar. Supercharging is one way to dramatically increase IMEP. The influence of supercharging on HCCI was therefore experimentally investigated. Three different fuels were used during the experiments: iso-octane, ethanol and natural gas.