The Usefulness of Negative Valve Overlap for Gasoline Partially Premixed Combustion, PPC

Partially premixed combustion has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in PPC mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions, even at higher loads. The problem is the ignitability at low load and idle operating conditions.

The objective is to investigate the usefulness of negative valve overlap on a light duty diesel engine running with gasoline partially premixed combustion at low load operating conditions. The idea is to use negative valve overlap to trap hot residual gases to elevate the global in-cylinder temperature to promote auto-ignition of the high octane number fuel. This is of practical interest at low engine speed and load operating conditions because it can be assumed that the available boost is limited. The problem with NVO at low load operating conditions is that the exhaust gas temperature is low. While an increase of NVO potentially increases the in-cylinder temperature at intake valve closing, increasing NVO also increases the EGR fraction which lowers the global in-cylinder temperature. The question is to what extent NVO can be used to extend the low load operating region. Investigations on the effect of the glow plug are also included.

The experimental engine is modeled with the engine simulation tool AVL Boost to retrieve information about trapped residual gas fraction and in-cylinder temperature with varying NVO and load at low engine speed and load operating conditions. Measured experimental data is used as input to the engine simulation model at all operating conditions. Measured model inputs include valve lift curves, in-cylinder pressure trace and calculated heat-release profiles.