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Partially premixed combustion (PPC) is a low-temperature combustion concept, which is between conventional diesel compression ignition (CI) and homogeneous charge compression ignition (HCCI). In PPC mode, the start of injection timing (SOI) is earlier than that of CI and later than that of HCCI and stratified in-cylinder fuel/air mixture can be formed to control the auto-ignition by the fuel injection timing. Gasoline fuel is beneficial for PPC mode because of its superior resistance to auto-ignition, which can enhance fuel-air charge mixing process with longer ignition delay time. The scope of this study is to investigate in-cylinder auto-ignition, combustion evolution, combustion stratification, and engine-out emissions at PPC operating mode under lean and low load engine conditions with different injection timings. Primary reference fuel PRF77, was selected as the low-octane test fuel.

Partially premixed combustion (PPC) is an operating mode that lies between the conventional compression ignition (CI) mode and homogeneous charge compression ignition (HCCI) mode. The combustion in this mixed mode is complex as it is neither diffusion-controlled (CI mode) nor governed solely by chemical kinetics (HCCI mode). In this study, CFD simulations were performed to evaluate flame index, which distinguishes between zones having a premixed flame and non-premixed flame. Experiments performed in the optical engine supplied data to validate the model. In order to realize PPC, the start of injection (SOI) was fixed at −40 CAD (aTDC) so that a required ignition delay is created to premix air/fuel mixture. The reference operating point was selected to be with 3 bar IMEP and 1200 rpm. Naphtha with a RON of 77 and its corresponding PRF surrogate were tested. The simulations captured the general trends observed in the experiments well.

The objective of this study was to investigate the effect of aromatic on combustion stratification and particulate emissions for PRF60. Experiments were performed in an optical CI engine at a speed of 1200 rpm for TPRF0 (100% v/v PRF60), TPRF20 (20% v/v toluene + 80% PRF60) and TPRF40 (40% v/v toluene + 60% PRF60). TPRF mixtures were prepared in such a way that the RON of all test blends was same (RON = 60). Single injection strategy with a fuel injection pressure of 800 bar was adopted for all test fuels. Start of injection (SOI) was changed from early to late fuel injection timings, representing various modes of combustion viz HCCI, PPC and CDC. High-speed video of the in-cylinder combustion process was captured and one-dimensional stratification analysis was performed from the intensity of images. Particle size, distribution and concentration were measured and linked with the in-cylinder combustion images.

The literature study on PPC in optical engine reveals investigations on OH chemiluminescence and combustion stratification. So far, mostly PRF fuel is studied and it is worthwhile to examine the effect of fuel properties on PPC. Therefore, in this work, fuel having different octane rating and physical properties are selected and PPC is studied in an optical engine. The fuels considered in this study are diesel, heavy naphtha, light naphtha and their corresponding surrogates such as heptane, PRF50 and PRF65 respectively. Without EGR (Intake O2 = 21%), these fuels are tested at an engine speed of 1200 rpm, fuel injection pressure of 800 bar and pressure at TDC = 35 bar. SOI is changed from late to early fuel injection timings to study PPC and the shift in combustion regime from CI to PPC is explored for all fuels. An increased understanding on the effect of fuel octane number, physical properties and chemical composition on combustion and emission formation is obtained.