Experimental studies of performance and emissions in a 2/4-stroke engine

Abstract

Direct Injection (DI) gasoline engines are staging a come-back because of its potential for improved fuel economy through principally the engine down-sizing by boosting, stratified charge combustion and Controlled Auto Ignition (CAI) at part load operations. The problem with the Spark Ignition (SI) engine is its inherent low part-load efficiency. This arises due to the pumping loses that occur when the throttle closes or partially opens. One way of decreasing the pumping losses is to operate the engine lean or by adding residual gases. It is not possible to operate the engine unthrottled with a very lean or diluted mixture at low loads due to misfire. However, the load can also be controlled by changing the valve timing. This reduce the pumping loses and hence increase the efficiency. Due to the limited time available for complete fuel evaporation and the mixing of fuel and air mixture, locally fuel rich mixture or even liquid fuel can be present during the combustion process. This causes a significant increase in Particulate Matter (PM) emissions from direct injection gasoline engines compared to the conventional port fuel injection gasoline engines, which are of major concern because of its health implications. In the meantime, depleting reserves of fossil fuels and the increasing environmental pollution caused by burning of fossil fuels, have paved the way for fuel diversification. Cleaner and renewable fuel is being introduced worldwide. The use of ethanol as an alternative transportation fuel shows promise for several reasons. While ethanol can be produced from several types of biomass, it offers properties such as high octane number, higher oxygen content and high heat of evaporation, which make it a most attractive alternative fuel, in particular for the direct injection gasoline engine. In this research, a single cylinder camless engine equipped with an electro-hydraulic valve train system has been used to study and compare different engine operation modes in the SI and CAI combustion. The fuel consumption, gaseous and particulate emissions of gasoline and its mixture with ethanol (E15 and E85) were measured and analysed at the same engine operating condition. The heat release analysis and performance characteristics of CAI and SI combustion were carried out by the in-cylinder pressure measurement. The effect of load and valve timings on the gaseous and Particulate Matter (PM) emissions was investigated for both 4-stroke SI and CAI combustion. Within the achieved CAI operational ranges, particle emissions were found to be dominated by smaller particles (<50nm). Hotter charge and better mixing are the main parameters affecting the soot particles in the exhaust irrespective of the combustion modes and valve timings. At part-load conditions investigated, it was found that the CAI combustion produced the lowest NOx emissions of 0.4g/KWh in all fuel blends and lower fuel consumption 223g/KWh with improved combustion efficiency of 94.7% in ethanol fuel E15 and E85. The positive valve overlap was found to produce lowest fuel consumption of 222.8 g/KWh in all fuel blend and respond better to ethanol fuel in E15 and E85 with improved indicated efficiency of 40.5% compared to the other modes investigated. The early intake valve throttled SI operation led to a moderate improvement in the fuel consumption of 243.5g/KWh over the throttled SI operation but it was characterised by the slowest combustion and highest CO (33.5g/KWh) and HC (16.8g/KWh) emissions . Less and smaller particles numbers were detected for Early Intake Valve Closure (EIVC) from the combustion of E0 and E15 (4.0E+07#/cm3 less than 50nm in diameter) fuel blends. The particulate emission results showed that soot was the dominant particles in the exhaust, which could be reduced by leaner mixture combustion .