Spark Ignition Combustion of Direct Injected Alternative Fuels
Doctoral thesis, 2016
Several technologies and modifications exist to increase the fuel efficiency of passenger vehicles in general and spark ignition engines in particular. Combining a spark ignition engine with an electric drive system in hybrid powertrain and thermodynamic cycle improvements are two such approaches with the potential to greatly reduce fuel consumption. Thermodynamic efficiency can be improved by lean engine operation. Extension of the lean limit leading to unthrottled operation is possible with lean stratified combustion. Stratified combustion leads to increased soot formation in addition to NOx with standard catalyst after treatment. Ethanol is an alternative fuel for spark ignition engines which is renewable and potentially leads to a reduction in soot formation in lean stratified combustion. In addition to liquid fuels, spark ignition engines are able to operate with gaseous fuels.
The potential for reduction of spark ignition engine emissions during engine load transients using assistance from the electric powertrain was investigated. NOx emissions during engine load transients were reduced with simulated electric powertrain assistance. CO, hydrocarbon and soot emissions occurred at the beginning and end of engine load transients due to transient fuel rich conditions in the engine as a result of incorrect fuel metering. Electric assistance did not reduce soot emissions as they were unconnected to the engine load value during the transient.
Combustion, sources of soot formation and soot oxidation in lean, spark ignited, stratified combustion of E10 (10 % vol. ethanol, 90 % vol. gasoline) and E85 were investigated in an engine with optical access using pressure analysis, high speed imaging, OH * chemiluminescence and soot incandescence. Diffusion combustion of liquid fuel films on the surface of the piston, referred to as a pool fire, was a major source of soot formation for both E10 and E85 with a single injection due to the fuel spray impingement on the piston. The effect was magnified when the engine load was increased. For E10 soot formation also occurred in the gas phase. Pool fires were the sole source of soot formation with E85 as the fuel. Splitting the single injection into multiple injections reduced the magnitude of soot formation from pool fires for both E10and E85. Soot formation in the gas phase with E10, appeared to increase slightly with multiple injection for both engine loads. Multiple injections led to an increase in the rate and stability of combustion in all cases.
Gaseous fuels such as hydrogen and methane amongst others are another class of alternative fuels for spark ignited engines. The self similarity and the Turner model for the structure of transient jets during injection with a range of injection and ambient pressures was investigated with high speed schlieren imaging of helium jets. The Turner model was found to be an accurate approximation for the structure of the jet in all cases. The ratio of the jet width to the length was found to vary with the injection pressure to ambient pressure ratio indicating that the jet width to length ratio varied with different pressure ratio.
internal combustion engine