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The first stage of ignition in saturated hydrocarbon fuels is characterized as low temperature heat release (LTHR) or cool flame combustion. LTHR takes place as a series of isomerization reactions at temperatures from 600K to 900K, and is often detectable in HCCI, rapid compression machines, and early injection low temperature combustion (LTC). The experimental investigation presented attempts to determine the behavior of LTHR in late injection low temperature combustion in a medium duty diesel as fuel varies and the influence of such behavior on LTC torque and emissions.

A key element to achieving vehicle emission certification for most light-duty vehicles using spark-ignition engine technology is prompt catalyst warming. Emission mitigation largely does not occur while the catalyst is below its “light-off temperature”, which takes a certain time to achieve when the engine starts from a cold condition. If the catalyst takes too long to light-off, the vehicle could fail its emission certification; it is necessary to minimize the catalyst warm up period to mitigate emissions as quickly as possible. One technique used to minimize catalyst warm up is to calibrate the engine in such a way that it delivers high temperature exhaust. At idle or low speed/low-load conditions, this can be done by retarding spark timing with a corresponding increase in fuel flow rate and / or leaning the mixture. Both approaches, however, encounter limits as combustion stability degrades and / or nitrogen oxide emissions rise excessively.

The often-observed differences in nitrogen oxides, or NOx, emissions between biodiesel and petroleum diesel fuels in diesel engines remain intense topics of research. In several instances, biodiesel-fuelled engines have higher NOx emissions than petroleum-fuelled engines; a situation often referred to as the "biodiesel NOx penalty." The literature is rich with investigations that reveal many fundamental mechanisms which contribute to (in varying and often inverse ways) the manifestation of differences in NOx emissions; these mechanisms include, for example, differences in ignition delay, changes to in-cylinder radiation heat transfer, and unequal heating values between the fuels. In addition to fundamental mechanisms, however, are the effects of "system-response" issues.