Present-day implementations of premixed compression ignition low temperature (PCI) combustion in diesel engines use higher levels of exhaust gas recirculation (EGR) than conventional diesel combustion. Two common devices that can be used to achieve high levels of EGR are an intake throttle and a variable geometry turbocharger (VGT). Because the two techniques affect the engine air system in different ways, local combustion conditions differ between the two in spite of, in some cases, having similar burn patterns in the form of heat release. The following study has developed from this and other observations; observations which necessitate a deeper understanding of emissions formation within the PCI combustion regime.

This paper explains, through the use of fundamental phenomenological observations, differences in ignition delay and emission indices of particulate matter (EI-PM) and nitric oxides (EI-NOx) from PCI combustion attained via the two different techniques to flow EGR. PCI attained via the use of a VGT to flow EGR, i.e. unthrottled PCI, has significantly less EI-PM emissions than PCI attained via the use of a throttle to flow EGR, i.e. throttled PCI. To complete the analysis, emission indices of carbon monoxide (EI-CO) and hydrocarbons (EI-HC) and performance parameters are used to differentiate the application of the two techniques on the studied engine.