A Study on the LPG dual fuel combustion characteristics of an indirect injection compression ignition engine

Abstract

This investigation which aimed to identify the effect of biodiesel as the pilot injection in dual fuelled engine can be divided into 3 parts. Firstly, combustion characteristics of liquefied petroleum gas (LPG) premixed charge diesel dual fuelled engine was studied. Next, the investigation continued with the pilot injection changed to palm biodiesel (PME). Lastly, it finished with varied injection timing for neat liquid as well as dual fuelling to fulfill a comparison. Test bench experiments (steady state) were conducted with a 4-cylinder IDI CI engine, at selected high probability operating points corresponding to the ECE15+EUDC cycle, covering the range [10-70] Nm@ [1250-2750] rev/min. The engine ran at overall lean mixture. The LPG-air premixed mixture was maintained at four fixed values by an electronic controlling system. The acquired data included basic parameters. pressure history of fuel line and combustion chambers, and accessed combustion chamber visualization. The comparative analysis deal with: energy conversion efficiencies, specific total energy consumption, liquid fuel substitution, net release, combustion chamber phenomena including spray, combustion, flame probability distribution, flame temperature, and soot concentration (two color method). A new approach to determine the start of injection (SOI) based on fuel line pressure data was also proposed. Both diesel and PME provided smooth, knock-free dual operation, at all planned test points and LPG-air premixed charge, with the energy conversion efficiency deterioration increased at lower speeds and higher LPG ratios. Achieved substitution was about 26%-27% at high speed, load (lower with LPG-PME). lgnition delays which retarded within 0.6 ํ CA in LPG-diesel might increase up to 1.2 ํ CA with LPG-PME, especially at low speed, increased LPG. The start of combustion which retarded in LPG-diesel, mostly dur to retarded start of injection brought by pump effect as diesel was reduced, was found to advance in LPG-PME due to their shorter ignition delays and advanced SOI caused by the higher bulk modulus and viscosity of the PME. Both LPG-diesel and LPG-PME produced faster combustion compared to the neat liquid fuel, leading to reduced exhaust gas temperatures and the centers of heat release area's moving of towards TDC. Though the moving was much more (up to 2.8 ํ CA) in LPG-PME cases, their combustion duration was longer compared to LPG-diesel due to the lower heating value and volatility of the PME. While the coefficient of variation of IMEP was comparaple, the combustion noise of LPG-PME was slightly higher. With LPG-PME, the probability distribution and the area of high flame temperature was smaller, due to the PME properties: lower adiabatic flame temperature, and heavier. This was also thought due to the limit of the two color method when applied for gaseous and oxgenated fuel combustion. Concentration of soot in flame was observed to be lower with higher LPG and was much lower in LPG-PME cases. The 1.2-degree-advanced injection timing gave better LPG-diesel combustion while the OEM setting was suitable for LPG-biodiesel combustion.