Experimental Investigation of Cycle-by-Cycle Variations in CAI/HCCI Combustion of Gasoline and Methanol Fuelled Engine

Abstract

The development of vehicles continues to be determined by increasingly stringent emissions standards including CO2 emissions and fuel consumption. To fulfill the simultaneous emission requirements for near zero pollutant and low CO2 levels, which are the challenges of future powertrains, many research studies are currently being carried out world over on new engine combustion process, such as Controlled Auto Ignition (CAI) for gasoline engines and Homogeneous Charge Compression Ignition (HCCI) for diesel engines. In HCCI combustion engine, ignition timing and combustion rates are dominated by physical and chemical properties of fuel/air/residual gas mixtures, boundary conditions including ambient temperature, pressure, and humidity and engine operating conditions such as load, speed etc. Because of large variability of these factors, wide cycle-to-cycle variations are observed in HCCI combustion engines, similarly small variations in ignition timing and combustion rates result in wide variation in engine performance and emissions. Also, cycle-to-cycle combustion variations result in objectionable engine noise and vibrations. As a result of wide cycle-to-cycle variations, HCCI combustion can be achieved in an engine for narrow range of lean and rich operating limits. This motivates the researchers to systematically investigate mechanism and control of cycle-to-cycle variations on HCCI engines. In this paper, the combustion stabilities and cycle-tocycle variations of a HCCI combustion engine fuelled with gasoline and methanol were investigated on a modified two-cylinder, four-stroke engine. In this investigation, port fuel injection technique is used for preparing homogeneous charge. The experiment is conducted with variable intake air temperature at different air-fuel ratios at constant engine speed. Incylinder pressure of 100 combustion cycles for each test condition was recorded. Consequently, cycle-to-cycle variations of the main combustion parameters and performance parameters were analyzed and evaluated. To evaluate the cycle-to-cycle variations of HCCI combustion parameters at various test conditions, coefficient of variation (COV) of each parameter was used. The results show that critical parameters, which can be used to define HCCI operating range, are maximum rate of pressure rise, and COV of indicated mean effective pressure (IMEP).