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The influence of the small amounts (1-3%) of the additive di-tertiary butyl peroxide (DTBP) on the combustion event of Homogeneous Charge Compression Ignition (HCCI) engines was investigated using engine experiments, numerical modeling, and carbon-14 isotope tracing. DTBP was added to neat ethanol and diethyl ether (DEE) in ethanol fuel blends for a range of combustion timings and engine loads. The addition of DTBP to the fuel advanced combustion timing in each instance, with the DEE-in-ethanol mixture advancing more than the ethanol alone. A numerical model reproduced the experimental results. Carbon-14 isotope tracing showed that more ethanol burns to completion in DEE-in-ethanol blends with a DTBP additive when compared to results for DEE-in-ethanol without the additive. However, the addition of DTBP did not elongate the heat release in either case.

We have conducted a detailed numerical analysis of HCCI engine operation at low loads to investigate the sources of HC and CO emissions and the associated combustion inefficiencies. Engine performance and emissions are evaluated as fueling is reduced from typical HCCI conditions, with an equivalence ratio ϕ = 0.26 to very low loads (ϕ = 0.04). Calculations are conducted using a segregated multi-zone methodology and a detailed chemical kinetic mechanism for iso-octane with 859 chemical species. The computational results agree very well with recent experimental results. Pressure traces, heat release rates, burn duration, combustion efficiency and emissions of hydrocarbon, oxygenated hydrocarbon, and carbon monoxide are generally well predicted for the whole range of equivalence ratios. The computational model also shows where the pollutants originate within the combustion chamber, thereby explaining the changes in the HC and CO emissions as a function of equivalence ratio.

A multi-zone model has been developed that accurately predicts HCCI combustion and emissions. The multi-zone methodology is based on the observation that turbulence does not play a direct role on HCCI combustion. Instead, chemical kinetics dominates the process, with hotter zones reacting first, and then colder zones reacting in rapid succession. Here, the multi-zone model has been applied to analyze the effect of piston crevice geometry on HCCI combustion and emissions. Three different pistons of varying crevice size were analyzed. Crevice sizes were 0.26, 1.3 and 2.1 mm, while a constant compression ratio was maintained (17:1). The results show that the multi-zone model can predict pressure traces and heat release rates with good accuracy. Combustion efficiency is also predicted with good accuracy for all cases, with a maximum difference of 5% between experimental and numerical results.

This research work has focused on measuring the effect of fuel/air mixing on performance and emissions for a homogeneous charge compression ignition engine running on propane. A laser instrument with a high-velocity extractive probe was used to obtain time-resolved measurements of the fuel concentration both at the intake manifold and from the cylinder for different levels of fuel-air mixing. Cylinder pressure and emissions measurements have been performed at these mixing levels. From the cylinder pressure measurements, the IMEP and peak cylinder pressure were found. The fuel-air mixing level was changed by adding the fuel into the intake system at different distances from the intake valve (40 cm and 120 cm away). It was found that at the intake manifold, the fuel and air were better mixed for the 120 cm fuel addition location than for the 40 cm location.

We have developed a methodology for predicting combustion and emissions in a Homogeneous Charge Compression Ignition (HCCI) Engine. The methodology judiciously uses a fluid mechanics code followed by a chemical kinetics code to achieve great reduction in the computational requirements; to a level that can be handled with current computers. In previous papers, our sequential, multi-zone methodology has been applied to HCCI combustion of short-chain hydrocarbons (natural gas and propane). Applying the same procedure to long-chain hydrocarbons (iso-octane) results in unacceptably long computational time. In this paper, we show how the computational time can be made acceptable by developing a segregated solver. This reduces the run time of a ten-zone problem by an order of magnitude and thus makes it much more practical to make combustion studies of long-chain hydrocarbons.