Search Results

This research investigates the influences of the injection timing, injection pressure, and compression ratio on the combustion and exhaust emissions in a single cylinder 1.0 L DI diesel engine operating with ultra-high EGR. Longer ignition delays due to either advancing or retarding the injection timing reduced the smoke emissions, but advancing the injection timing has the advantages of maintaining the thermal efficiency and preventing misfiring. Smokeless combustion is realized with an intake oxygen content of only 9-10% regardless of the injection pressure. Reduction in the compression ratio is effective to reduce the in-cylinder temperature and increase the ignition delay as well as to expand the smokeless combustion range in terms of EGR and IMEP. However, the thermal efficiency deteriorates with excessively low compression ratios.

In a DI diesel engine, THC emissions increase significantly with lower compression ratios, a low coolant temperature, or during the transient state. During the transient after a load increase, THC emissions are increased significantly to very high concentrations from just after the start of the load increase until around the 10th cycle, then rapidly decreased until the 20th cycle, before gradually decreasing to a steady state value after 1000 cycles. In the fully-warmed steady state operation with a compression ratio of 16 and diesel fuel, THC is reasonably low, but THC increases with lower coolant temperatures or during the transient period just after increasing the load. This THC increase is due to the formation of over-lean mixture with the longer ignition delay and also due to the fuel adhering to the combustion chamber walls. A low distillation temperature fuel such as normal heptane can eliminate the THC increase.

The thermal cracking and polyaromatic hydrocarbon (PAH) formation processes of dimethyl ether (DME), ethanol, and ethane were investigated with chemical kinetics to determine the soot formation mechanism of oxygenated fuels. The modeling analyzed three processes, an isothermal constant pressure condition, a temperature rising condition under a constant pressure, and an unsteady condition approximating diesel combustion. With the same mole number of oxygen atoms, the DME rich mixtures form much carbon monoxide and methane and very little non-methane HC and PAH, in comparison with ethanol or ethane mixtures. This suggests that the existence of the C-C bond promotes the formation of PAH and soot.

The fundamental parameters related to engine combustion and performances, such as, heating value, theoretical air-fuel ratio, adiabatic flame temperature, carbon dioxide (CO2), and nitric oxide (NO) emissions, specific heat and engine thermal efficiency were investigated with computations for a wide range of oxygenated fuels. The computed results showed that almost all of the above combustion-related parameters are closely related to oxygen content in the fuels regardless of the kinds or chemical structures of oxygenated fuels. An interesting finding was that with the increase in oxygen content in the fuels NO emission decreased linearly, and the engine thermal efficiency was almost unchanged below oxygen content of 30 wt-% but gradually decreased above 30 wt-%.

Direct injection of various ignition suppressors, including water, methanol, ethanol, 1-propanol, hydrogen, and methane, was implemented to control ignition timing and expand the operating range in an HCCI engine with induced DME as the main fuel. Ultra-low NOx and smoke-less combustion was realized over a wide operating range. The reaction suppressors reduced the rate of low-temperature oxidation and consequently delayed the onset of high-temperature oxidation. Analysis of the chemical kinetics showed a reduction of OH radical in the premixed charge with the suppressors. Among the ignition suppressors, alcohols had a greater impact on OH radical reduction resulting in stronger ignition suppression. Although water injection caused a greater lowering of the temperature, which also suppressed ignition, the strong chemical effect of radical reduction with methanol injection resulted in the larger impact on suppression of oxidation reaction rates.

Two laboratory engines, one direct, injection and one indirect injection, were operated for a range of speeds, loads, injection timings, fuels, and steady and transient conditions. Rate of combustion data were derived and analyzed using a double Wiebe's function approximation. It is shown that three of the six function parameters are constant for a wide range of conditions and that the other three can be expressed as linear functions of the amount of fuel injected during ignition lag. Engine noise, smoke, and thermal efficiency correlate with the parameters describing the amount of premixed combustion and diffusive combustion duration. These characteristics may be optimized by reducing the quantity of premixed combustion while maintaining the duration of diffusive combustion to less than 60°CA.