Alcohol fuels have inherent properties that make them suitable candidates to
replace conventional fossil fuels in internal combustion engines by reducing the
formation of harmful emissions such as lifecycle carbon dioxide
(CO2), nitrogen oxides (NOX), and particulate matter (PM).
There is an increasing amount of work to use fuels such as ethanol or methanol
in mixing-controlled compression ignition (MCCI) as a replacement for diesel
fuel. However, employing these fuels in a strictly MCCI strategy results in an
evaporative cooling penalty that lowers indicated fuel efficiency. This work
proposes the use of an advanced compression ignition (ACI) strategy with a high
autoignition resistant fuel, where a fraction of the fuel is premixed and
autoignited in conjunction with a fraction of fuel that is burned in a
mixing-controlled manner to achieve diesel-like efficiencies with significant
emission reductions.
A computational model for MCCI with diesel and wet ethanol in an opposed piston
two-stroke (OP-2S) engine was validated against experimental data. Diesel and
wet ethanol MCCI were then compared at a similar operating condition, where it
was seen that wet ethanol provided a significant reduction in NOX
emissions but resulted in a lower indicated efficiency. A triple injection
strategy to enable ACI is then proposed by redesigning one of the injectors to
enable compression stroke injections. The injector included angle, the injection
split fractions, and injection timings of the triple injection strategy were
varied to understand the impact of each on combustion performance and emissions.
An optimal triple injection strategy based on simulation data was approximated
and then simulated to compare to conventional diesel (i.e., MCCI with diesel)
and MCCI with wet ethanol. ACI demonstrated a 3.5% point efficiency improvement
over MCCI with wet ethanol, resulting in an efficiency that was the same as
conventional diesel while still demonstrating nearly a 4 times reduction in
NOX emissions.
