The effects of charge dilution on low-temperature diesel combustion and emissions were investigated in a smallbore
single-cylinder diesel engine over a wide range of injection timing. The fresh air was diluted with additional N2 and CO2, simulating 0 to 65% exhaust gas
recirculation in an engine. Diluting the intake charge lowers the flame temperature T due to the reactant being replaced by inert gases with increased heat
capacity. In addition, charge dilution is anticipated to influence the local charge equivalence ratio prior to ignition due to the lower O2 concentration and longer
ignition delay periods. By influencing both and T, charge dilution impacts the path representing the progress of the combustion process in the -T plane,
and offers the potential of avoiding both soot and NOx formation.
In-cylinder pressure measurements, exhaust-gas emissions, and imaging of combustion luminosity were performed to clarify the path of the combustion process
and the effects of charge dilution and injection timing on combustion and fuel conversion efficiency. Based on the
findings, a postulated combustion process in the -T plane is presented for different dilution levels and injection timings. Although the ignition delay increased
with high dilution and early injection, the heat release analysis indicated that a large portion of the combustion
and emissions formation processes was still dominated by the mixing-controlled phase rather than the premixed phase. Because of the incomplete premixing, and the need to mix a greater volume of charge with unburned or partially-burned fuel to complete combustion, the diluted
mixtures increased CO emissions. Injecting the fuel at earlier timings to extend the ignition delay helped alleviate this problem, but did not eliminate it. Fuel
conversion efficiencies calculated for each dilution level and start of injection provide guidance as to the
appropriate combustion phasing and practical levels of charge dilution for this low-temperature diesel
combustion regime.