Experimental investigations of anhydrous and wet ethanol combustion in a spark iginition engine

Abstract

The demand for higher spark ignition engine efficiency has been pushed by stricter CO2 legislation worldwide. Greenhouse gases mitigation and reduction on the use of fossil fuels is a global concern. For these reasons, the use of renewable and low carbon liquid fuels, such as ethanol, have considerably grown in the last decades. Due to the azeotropic ethanol-water mixture nature, the ethanol energy production cost considerably increases to achieve purity levels above 95%. Thus, the use of higher water-in-ethanol contents (so called wet ethanol) may improve the ethanol life cycle energy balance and result in a cheaper fuel. At the same time, new engine technologies, such as direct injection and variable valve actuation, have propitiated the spark ignition engine to reach higher efficiency levels than in the past. Even then, the low part load spark ignition engine efficiency is still a problem. This research investigates the application of ethanol, wet ethanol and gasoline in a naturally aspirated single cylinder research engine equipped with an electro-hydraulic fully variable valve actuation system. Experimental thermodynamic engine tests were carried out in the four-stroke spark ignition operation mode at several engine loads and stoichiometric combustion. The effects of direct fuel injection and port fuel injection on engine operation were investigated. Initial studies with anhydrous ethanol were carried out to find the most promising valve strategies to be used when applying wet ethanol. The conventional throttled spark ignition valve strategy was investigated. The effect of the positive valve overlap period on the engine operation parameters and emissions were described. Unthrottled spark ignition operation with either early and late intake valve closure (EIVC and LIVC, respectively) load control methods were compared to understand the potentials of each strategy to increase SI engine efficiency. A comprehensive study on the effects of the intake valve lift for the EIVC load control method was carried out. Residual gas trapping methods as exhaust rebreathing and negative valve overlap were tested using the early intake valve closure load control method. Spark assisted compression ignition was achieved at some operating conditions. Another study comparing the use of anhydrous ethanol, wet ethanol and gasoline was developed for three different valve strategies: conventional throttle SI valve strategy, variable positive valve overlap through intake profile phasing, and negative valve overlap with EIVC load control. In each presented study the gas exchange process, combustion, engine-out emissions and engine performance were discussed. The best valve events strategy for wet ethanol spark ignition operation is presented and the use of PFI and DI injection methods commented. It was possible to achieve similar engine indicated efficiency when using wet ethanol with 15% of water volumetric content to that achieved with commercial gasoline when using advanced valve events strategies. Comparing the wet ethanol baseline case to its best valve events and injection strategies scenario, 11.6% (from 2.0 to 4.5 bar IMEP loads) and 3.9% (from 6.0 to 9.0 bar IMEP loads) average relative efficiency gains could be achieved. Keywords: Spark