As it becomes more and more difficult to find "easy" oil, various alternative fuels are introduced to the markets. These fuels have chemical properties that are different from the traditional gasoline and diesel fuels so that engine efficiency and other engine behaviors may be affected To improve engine efficiency and to identify which alternative fuel is the cleanest fuel solution, it is necessary to compile information about the ignition delay, which governs auto-ignition in spark-ignition (SI), compression-ignition (CI) and homogeneous charge compression-ignition (HCCI) engines. In this study, we measured ignition delay on the Rapid Compression Machine (RCM). RCM is a single-stroke device, which compresses uniform mixtures to engine-like condition. We can interpret from the pressure the detailed heat release process. A comprehensive ignition delay database of toluene/n-heptane mixtures and gasoline/ethanol mixtures was established The data allow us to calculate the auto-ignition behavior in engines. Depending on application the correct choice of alternative fuels may be made.

In the first part of this work, ignition of methane-air mixtures under excess air dilution is studied. When excess air is used in SI engine operation, thermal efficiency is increased due to increase in compression ratio together with reduced pumping and heat loses. However, stable operation with excess air is challenging due to poor flammability of the resulting diluted mixture. Hence in order to achieve stable and complete combustion a turbulent jet ignition (TJI) system is used to improve combustion of lean methane-air mixtures. Various nozzle designs and operating strategies for a TJI system were tested in a rapid compression machine. 10-90% burn duration measurements were useful in assessing the performance of the nozzle designs while the 0-10% burn durations indicated if optimal air-fuel ratio is achieved within the pre-chamber at the time of ignition. The results indicated that distributed-jets TJI system offered faster and stable combustion while the concentrated-jets TJI system offered better dilution tolerance.Knock in a SI engine occurs due to autoignition of the end gas mixture and typically occurs in the negative temperature coefficient (NTC) region of the fuel-air mixture. Dilution of intake charge with cold exhaust recirculation gases (EGR) reduces combustion temperatures and decreases mixture reactivity thereby reducing knocking tendency. This enables optimal spark timings to be used, thereby increasing efficiency of SI engines which would otherwise be knock limited. Effect of cold EGR dilution is studied in the RCM by measuring the autoignition delay times of gasoline and gasoline surrogate mixtures diluted with varying levels of CO2. The autoignition experiments in the RCM were performed using a novel direct test chamber (DTC) charge preparation approach. The DTC approach enabled mixture preparation directly within the combustion chamber and eliminated the need for mixing tanks. Effect of CO2 dilution in retarding the autoignition delay times was more pronounced in the NTC region, while it was weaker in the low temperature and high temperature regions. The retarding effect was found to be dependent on both the octane number and the fuel composition of the gasoline being studied.Finally, the effect of substituting ethanol(biofuel) in gasoline surrogates for up to 40% by volume is studied. Ethanol is an octane booster, but it blends antagonistically with aromatics such as toluene and synergistically with alkanes with respect to the resulting octane number of the blends. In order to study this blending effect, two gasoline surrogates containing only alkanes (PRF), and alkanes with large amounts of toluene (TRF) are blended with varying levels of ethanol. The ignition delay times of the resulting mixtures are measured in a rapid compression machine and kinetic analysis was carried out using numerical simulations. The kinetic analysis revealed that ethanol controlled the final stages of ignition for the PRF blends when more than 10% by volume of ethanol is present. However, in the TRF blends, toluene controlled the ignition until mole fractions of ethanol became higher than the toluene indicating the reason for the antagonistic blending nature. It was found that the RON values of the resulting blends matched the trend of the ignition delay times recorded at 740K and 21 bar compressed conditions. This enables qualitative assessment of the RON numbers for new biofuel blends by measuring their ignition delay times in the RCM.

As alternatives to traditional petroleum-based fuels are increasingly sought after, the National Jet Fuel Combustion Program (NJFCP) was established to streamline the evaluation and certification of these fuels. The current mandate is for the replacements of traditional fuels to be equally safe and to provide better environmental performance [1]. These so-called "drop-in" jet fuels refer to hydrocarbon fuels that deliver identical combustion performance and are produced from non-petroleum sources [2]. Following the mandate delivered by the NJFCP for alternative fuels, this study aims to improve the traditionally phenomenological understanding of combustion performance by making connections between fuel properties and the chemical composition of fuels. The ignition delay time is an important measure of the combustion performance of fuels, as it is an integrated measure of the fuels' physical and chemical properties, such as volatility, diffusivity, and chemical reactivity. Consequently, it is a very useful validation target in chemical kinetic modeling and has implications in practical aviation phenomena such as, among others, lean blowout, cold-start ignition and altitude relight. Shock tubes are well-suited for ignition delay time measurements, as they provide a well-defined time zero and a quasi-constant temperature and pressure test region behind the reflected shocks. All experiments in this thesis were performed on the Stanford Flexible Application Shock Tube (FAST). Reactive gas mixtures were prepared with equivalence ratios of 1 ± 0.05, and mixed in the shock tube driven section to avoid fuel loss attributed to non-idealities in the jet fuel vapor. Changes in the fuel mole fraction during mixing and ignition were monitored using laser absorption diagnosis at 3.39 μm. The ignition delay time is defined in this study by the onset of emission from electronically excited OH radicals at 306 nm. Ignition delay times were measured in the temperature range of 1200-1500 K and at 4 atm pressure for five distillate jet fuels from refineries around the US (termed geographical fuels), and for six synthetic jet fuels with varying cetane numbers ranging from 30-55 (termed CN fuels). The ignition delay times for A1-3 and C1-9 jet fuels were also measured at 1300 K and at 4 atm. The dependence of combustion properties on fuel chemical composition were investigated using the ignition delay times for these fuels. In particular, the key role that the degree of branching in the jet fuel molecular structure plays in the combustion kinetics and performance is discussed.