A Numerical Study of Fuel Stratification, Heat Transfer Loss, Combustion, and Emissions Characteristics of a Heavy- Duty RCCI Engine Fueled by E85/Diesel
Reactivity-controlled compression ignition is a new advanced combustion strategy developed to reach cleaner and more efficient combustion by controlling fuel stratification inside the engine cylin-der and reducing heat loss. While its potential to produce high efficiency and low emissions and to reach higher loads than other Low-Temperature Combustion strategies (LTC) has been confirmed numerous times, its operating range is still limited to moderate loads. One potential solution to in-crease the operating range is using E85 fuel as the premixed fuel due to the potential of providing a longer combustion duration. This work will focus on developing a computational fluid dynamics (CFD) model for a reactivity-controlled compression ignition (RCCI) engine fueled by E85/diesel with a double step piston bowl geometry. The model is used to investigate the effects of four differ-ent design parameters, namely injection timing, boost pressure, initial temperature, and spray in-cluded angle, to identify their impact on all crucial parameters describing combustion i.e. the strati-fication level, heat loss, and emissions characteristics. It has been found that the start of injection affects the fuel stratification levels inside the cylinder, with the optimum location for efficiency lo-cated in the moderate stratified region. The boost pressure mainly influences the mean gas tem-perature, the start of combustion, combustion duration, and the recession time of the Heat Release Rate (HRR) curve. It is found that the boost pressure does not have an influence on the heat loss of the engine and the heat loss is more correlated to flame temperature than the average tempera-ture. It is also proven that the boost pressure could assist in the suppression of NOx, but when the intake pressure is too high, the thermal efficiency drops. Furthermore, the results show that the ini-tial temperature is preferred to be as low as possible but sufficiently high enough to burn all the in-troduced fuel. Intake temperature alters the HRR shape and combustion duration significantly. Last-ly, it is found that the combination of the spray included angle and piston bowl geometry can sub-stantially determine the way the flame is formed and its location. The study on the effect of spray angle provides essential insights on the origin of unburned hydrocarbon emission, HRR shape, and heat loss
