Exploring the use of natural gas and light alcohol fuels in compression ignition engines
Keywords: Engineering.
Mechanical Engineering.

There is a need to decrease the environmental impact of the heavy-duty transportation sector. This can be accomplished through improvements in Diesel engine efficiency and the use of alternative fuels with a lower carbon intensity than traditional Diesel fuel. Prior research demonstrated that Diesel engine efficiency can be improved by reducing heat transfer losses, and the resulting high in-cylinder temperatures can enable the use of low-cetane fuels with lower carbon intensity than Diesel fuel. If the alternative fuels produce low levels of soot emissions, they could enable operation at stoichiometric conditions to improve engine power density and enable the replacement of costly Diesel aftertreatment systems with a three-way catalyst. Prior modeling efforts utilized this low-heat rejection (LHR) engine architecture operating at grossly fuel-rich conditions in a mixed electrochemical-combustion energy system to achieve exergy efficiencies exceeding 60%. Both of these high-efficiency applications require that the engine does not produce prohibitively high levels of soot. This thesis explores natural gas and light alcohol fuels as potential low-carbon alternatives to Diesel fuel. The primary goal is to quantify the soot emissions from these fuels when used in a Diesel engine. The study uses a direct-injected, compression ignition (DI CI) single-cylinder engine with a thermal barrier coating applied to the piston face to reduce heat transfer and intake air heating to further increase the in-cylinder temperature. An equivalence ratio sweep is performed for each fuel. The range of equivalence ratios is 0.4-1.5 for natural gas and 0.6-1.8 for alcohol fuels. Three natural gas mixtures are used to understand the impact on soot emissions of realistic levels of ethane and propane found in natural gas distributed at refuelling stations across the US. Neat methane produces soot emissions below or near the US EPA regulatory soot limit up to stoichiometric operation. The addition of ethane and propane does increase soot emissions, but not significantly above the regulatory limit. At fuel-rich operating conditions, soot emissions are high but could be handled by a Diesel particulate filter. Ethane and propane have a similar impact on soot emissions, indicating a relationship between the number of distinct C-C bonds of a fuel and the soot formation process. The addition of higher hydrocarbons offers improved autoignition characteristics relative to methane alone. Similarities in gas-phase emissions for different mixtures indicate that the LHR DI CI engine can accommodate the variability in natural gas composition in the US. Thirteen alcohol mixtures are used to understand the impact of n-propanol and n-butanol on soot emissions when blended with ethanol and methanol. Prior and concurrent work demonstrated that both ethanol and methanol produce very low levels of soot up to an equivalence ratio of 2. Both n-propanol and n-butanol increased soot emissions relative to neat methanol and ethanol, though many of the mixtures remained close to the US EPA regulatory soot limit at stoichiometric. Soot emissions of all mixtures could likely be accommodated by a Diesel particulate filter up to an equivalence ratio of 1.8. Mixtures with methanol produced significantly lower soot than mixtures without it, indicating methanol could have a strong soot-mitigating quality as an additive. N-propanol and n-butanol have a similar impact on soot emissions, indicating a relationship between soot emissions and the number of distinct C-C bonds that are not part of a larger C-C-O group. Trends in soot emissions of all alcohol mixtures indicate the overall C:H and C:O ratios serve as key governing parameters for soot formation in the DI CI combustion process. Similarities in gas-phase emissions for different alcohol mixtures further support that the LHR DI CI engine is insensitive to fuel composition.