The objective of this research is a detailed investigation of unburned hydrocarbon (UHC) in a highly-dilute diesel low temperature combustion (LTC) regime. This research concentrates on understanding the mechanisms that control the formation of UHC via experiments and simulations in a 0.48L signal-cylinder light duty engine operating at 2000 r/min and 5.5 bar IMEP with multiple injections. A multi-gas FTIR along with other gas and smoke emissions instruments are used to measure exhaust UHC species and other emissions. Controlled experiments in the single-cylinder engine are then combined with three computational tools, namely heat release analysis of measured cylinder pressure, analysis of spray trajectory with a phenomenological spray model using in-cylinder thermodynamics [1], and KIVA-3V Chemkin CFD computations recently tested at LTC conditions [2]. This study looks at the effect of inlet oxygen concentration, fuel spray targeting, injection event timing, injector sac volume, rail pressure, and boost pressure which are each explored within a defined operation range in LTC. This research compliments simultaneous research which concentrates on understanding the benefits of multiple injections on engine performance and emissions operating in the LTC regime [3].
The results of this research show that total UHC can be divided into light (<6 carbon/molecule) UHC and heavy (≥6 carbon/molecule) UHC emissions. The light UHC closely track with CO having minimum exhaust concentrations on the order of 250 ppm and occur when CO is at its 0.5% minimum. Spray targeting, percent inlet oxygen, and boost pressure variation produce the largest light UHC changes, where as injection pressure has minimal effect. Liquid fuel injected outside the piston bowl, which can occur with early injection, is linked to the heavy UHC components where a 3 times increase is observed relative to later injections where the liquid stays within the bowl. Injector sac-volume experiments suggest the sac volume is a relatively constant source of heavy UHC and can contribute as high as 15% of the total UHC. Based on these results, a simple three-part UHC model is suggested and hypotheses are discussed for the over-rich dominant sources of the unburned hydrocarbons.