Fuel Property Impact on a Premixed Diesel Combustion Mode.

Abstract

New premixed diesel combustion strategies, with their low engine-out PM and NOx emissions, are highly attractive for production implementation given increasingly strict emissions regulations. Accordingly, premixed diesel combustion strategies must operate effectively on commercially available diesel fuel, whose critical properties vary substantially. It is therefore critical to understand how premixed diesel combustion strategies respond to variations in fuel properties, especially cetane number the primary quantification of ignition behavior.

This research study sought to understand the connection between diesel fuel properties, in particular cetane number, and the combustion and emissions behavior of premixed diesel combustion. Four primary test fuels with cetane numbers varying over the range expected in the field (42-53) were used, along with a secondary matrix of fuels to characterize the behavior of a nitrate cetane improver. Fuel effects were quantified across a range of EGR levels, injection pressures, and engine loads to identify secondary parameter interactions.

Gaseous emissions, particularly NOx emissions, were found to be dependent solely on combustion phasing and EGR rate for the primary test fuels at the studied condition. Fuel cetane number shifts the combustion phasing (increasing cetane number advances phasing) but is only one of many different parameters which shift combustion. The effect of varying cetane number can be counteracted by varying injection timing to yield matched combustion phasing.

The presence of 2-ethylhexyl nitrate (2-EHN) cetane improver within the fuel introduces a new fuel-borne NOx formation mechanism to the combustion process, which significantly increases NOx emissions in a premixed diesel combustion mode. The increase in NOx emissions stems from NOx formed by the decomposition of the 2-EHN additive.

The trends and magnitudes of soot, CO, and HC emissions remain constant for all tested fuels across a range of engine loads. The high load limit of the tested premixed diesel combustion mode is primarily limited by equivalence ratio, with excessive soot, CO, and HC emissions resulting as the overall equivalence ratio approaches stoichiometric. The light load limit is limited by high CO and HC emissions and the ability of a diesel oxidation catalyst to reduce these emissions to acceptable levels.