Investigation of Alternative Fuels and Advanced Engine Technology: Improving Engine Efficiency and Reducing Emissions
- Author(s): Rapp, Vi Hai
- Advisor(s): Dibble, Robert W.
- et al.
The internal combustion engine has vastly improved over the past 100 years. With global warming and pollution being a rising concern, engineers are working towards improving efficiency and emissions of engines. The spark-ignited engine (or gasoline engine) offers improvement in emissions with a sacrifice in thermal efficiency. The compression ignition engine (Diesel engine) increases the thermal efficiency, due to operation at higher compression ratios, but emits high amounts of particulate matter and oxides of nitrogen (NOx). Although improvements in fuel refinement have decreased the amount of engine pollutants, the output of pollutants for both spark-ignited and Diesel engines is still too great.
This dissertation explores two advanced engine concepts with alternative fuels for improving thermal efficiency and reducing emissions in automobiles. The first concept investigated is a spark-ignited internal combustion engine operating using hydrogen, oxygen, and argon. Basic engine theory predicts such an engine will see a considerable improvement in engine efficiency (theoretically ~75%, and in practice ~50% including heat transfer and friction losses) over standard engines. These gains in thermal efficiency are due to argon's monatomic structure, which yields a high specific heat ratio (γ = 1.67 compared to γ < 1.4 for air). The water produced by the combustion of hydrogen can be extracted in the exhaust by a condenser, allowing the recycling of nearly pure argon in a closed loop system. Therefore, argon re-fueling is not required.
Achieving efficiencies above 50% with a hydrogen-oxygen-argon engine, however, is difficult due to engine knock limiting spark advance. In an effort to obtain the highest efficiency of this engine concept, experiments were conducted using single and dual spark-ignition for high argon concentrations. Results showed dual spark-ignition slightly increased indicated thermal efficiency, but was still limited by engine-knock. A three-zone model showed that argon as a working fluid increases in-cylinder temperatures, unburned gas temperatures, and laminar flame speed. The model suggests that specific heat ratio affects end gas temperatures more than increasing flame speed.
The second engine concept investigates variables and fuel trends for predicting ignition in homogenous charge compression ignition (HCCI) engines. Octane number, a metric for fuel performance in gasoline engines, and cetane number, a metric for fuel performance in Diesel engines, do not accurately predict fuel performance in HCCI engines. To develop a metric for predicting fuel performance in HCCI engines, correlations between ignition of fuels in an HCCI engine and varying engine parameters are investigated. A relationship between fuel chemistry and ignition in HCCI engines is also explored. Results show that previous methods for predicting ignition do not correlate well with experimental data and auto-ignition is highly sensitive to fuel chemistry.
A single-zone well-mixed-reactor model is used to investigate three different mechanisms for predicting auto-ignition in the HCCI engine. All three mechanisms accurately predicted the auto-ignition order of fuels containing isooctane and n-heptane, but did not predict auto-ignition of blends containing toluene and ethanol. Blends of toluene and n-heptane were further investigated using the model to identify potential problems with the toluene mechanisms. The model results showed increasing the amount of toluene linearly by volume did not result in a linear advance in auto-ignition.