Vedharaj Sivasankaralingam

The experimental in-cylinder combustion process was compared with the numerical simualtion for naphtha fuel under conventional compression ignition (CI) and partially premixed combustion (PPC) conditions. The start of injection timing (SOI) with the single injection strategy was changed from late of −10 CAD aTDC to early of −40 CAD aTDC. The three-dimensional full cycle engine combustion simulation was performed coupling with gas phase chemical kinetics by the CFD code CONVERGE™. The flame index was used for evaluating the combustion evolution of premixed flame and diffusion flame. The results show that the flame index could be used as an indicator for in-cylinder homogeneity evaluation. Hydroperoxyl shows a similar distribution with the premixed combustion. Formaldehyde could be used as an indicator for low temperature combustion.

Gasoline compression ignition (GCI) is widely studied for the benefits of simultaneous reduction in nitrogen oxide (NO_X) and soot emissions without compromising the engine efficiency. Despite this advantage, the operational range for GCI is not widely expanded, as the auto-ignition of fuel at low load condition is difficult. The present study aims to extend the low load operational limit for GCI using negative valve overlap (NVO) strategy. The engine used for the current experimentation is a single cylinder diesel engine that runs at an idle speed of 800 rpm with a compression ratio of 17.3. The engine is operated at homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) combustion modes with the corresponding start of injection (SOI) at −180 CAD (aTDC) and −30 CAD (aTDC), respectively. In the presented work, intake air temperature is used as control parameter to maintain combustion stability at idle and low load condition, while the intake air pressure is maintained at 1 bar (ambient). The engine is equipped with variable valve cam phasers that can phase both inlet and exhaust valves from the original timing. For the maximum cam phasing range (56 CAD) at a valve lift of 0.3 mm, the maximum allowable positive valve overlap was 20 CAD. In the present study, the exhaust cam is phased to 26 CAD and 6 CAD and the corresponding NVO is noted to be 10 CAD and 30 CAD, respectively. With exhaust cam phasing adjustment, the exhaust valve is closed early to retain hot residual gases inside the cylinder. As such, the in-cylinder temperature is increased and a reduction in the required intake air temperature to control combustion phasing is possible. For a constant combustion phasing of 3 CAD (aTDC), a minimum load of indicated mean effective pressure (IMEP_net) = 1 bar is attained for gasoline (RON = 91) at HCCI and PPC modes. The coefficient of variance was observed to below 5% at these idle and low load conditions. At the minimum load point, the intake air temperature required dropped by 20°C and 15°C for NVO = 30 CAD at HCCI and PPC modes, respectively, when compared to NVO = −20 CAD and NVO = 10 CAD. Similarly, for the load range of IMEP_net = 1 to 3 bar, decrease in temperature requirement is noted for negative valve overlap cases and the translational table in terms of d (Tin)/d (NVO) is attained. However, the low load limit was extended with negative valve overlap at the expense of decreased net indicated thermal efficiency due to heat losses and reduction in gas exchange efficiency. Ultra low soot concentration and NO_X emission were noted at HCCI condition.

The present study intends to explore the effect of the addition of fuel additives with camphor oil (CMO) on the characteristics of a twin-cylinder compression ignition (CI) engine. The lower viscosity and boiling point of CMO when compared to diesel could improve the fuel atomization, evaporation, and air/fuel mixing process. However, the lower cetane index of CMO limits its use as a drop in fuel for diesel in CI engine. In general, NO_X emission increases for less viscous and low cetane (LVLC) fuels due to pronounced premixed combustion phase. To improve the ignition characteristics and decrease NO_X emissions, fuel additives such as diglyme (DGE)—a cetane enhancer, cumene (CU)—an antioxidant, and eugenol (EU) and acetone (A)—bio-additives, are added 10% by volume with CMO. The engine used for the experimentation is a twin-cylinder tractor engine that runs at a constant speed of 1500 rpm. The engine was operated with diesel initially to attain warm-up condition, which facilitates the operation of neat CMO. At full load condition, brake thermal efficiency (BTE) for CMO is higher (29.6%) than that of diesel (28.1%), while NO_X emission is increased by 9.4%. With DGE10 (10% DGE + 90% CMO), the ignition characteristics of CMO are improved and BTE is increased to 31.7% at full load condition. With EU10 (10% EU + 90% CMO) and A10 (10% A + 90% CMO), NO_X emission is decreased by 24.6 and 17.8% when compared to diesel, while BTE is comparable to diesel. While HC and CO emission decreased for DGE10 and CU10, they increased for EU10 and A10 when compared to baseline diesel and CMO.