The use of modern biofuels in mobile applications has an enormous potential to reduce greenhouse gases as well as engine pollutant emissions, such as soot or nitrogen oxides. This beneficial effect is directly related to the molecular structure of the biofuel as a product of an optimized production process. To understand the influence and emission reduction potential of the large variety of different fuel properties, this study aims to identify desirable fuel characteristics and define optimized biofuel components. In a first step, a literature survey is carried out, focusing on the impact of the cetane number, boiling characteristics, and aromatic and oxygen contents on the diesel combustion process. The incorporated investigations that analyze the combustion behavior, engine efficiency, and emission performance underline the potential of tailoring fuels to desired properties. From this foundation, a modelbased analysis of desired fuel properties was conducted, using a large database with 32 different fuels (single molecules and fuel mixtures). With multiple correlation methods, different fuel properties can be used to predict the emission performance of the engine. The following fuel optimization based on emission performance and engine efficiency results in ideal fuel properties for diesel engine combustion. As it turns out, a blend of 2-methyltetrahydrofurane (2-MTHF) (which can be derived from cellulose) blended with di-n-butylether complies with the desired fuel properties, which were defined before. In combination with an improved homogeneous low-temperature combustion process and an increased ignition delay, a nearly soot-free diesel combustion over a wide load range is realized. The oxygenated fuel enables increased exhaust gas recirculation (EGR) rates while maintaining the high engine efficiency of the diesel process.

The effect of back pressure on the light-off of a modern spark ignition engine three-way catalyst has been assessed by measuring the hydrocarbon conversion efficiency in a hot flow bench and in the cold-idle period in an engine. In the flow bench experiment, a small amount of propane/air mixture is used as a surrogate for the hydrocarbon mixture. The conversion efficiency is found to be only a function of temperature. The efficiency is independent of pressure, space velocity, and the equivalence ratio of the hydrocarbon mixture for λ ≥ 1. In the engine test, while the engine-out exhaust gas temperature is higher at a higher back pressure, there is little difference between the gas temperatures at the catalyst entrance for different back pressures at retarded spark timing. This observation is attributed to the larger amount of exhaust hydrocarbon conversion oxidation between the engine exit and the catalyst entrance with the lower back pressure. The heat release from this oxidation compensates for the lower engine-out exhaust temperature at the lower back pressure. The catalyst temperature increases modestly and light-off time shortens correspondingly at the higher back pressure. This observation is attributed solely to the increase in mass flow rate (and thus exhaust sensible enthalpy flow rate) of the engine needed to overcome the additional pumping loss due to the throttling of the exhaust. These results have been confirmed with a simple one-dimensional catalyst model.

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