The lowest average A/F ratio is often found at peak torque conditions. Atomization, vaporization, fuel vapor-air mixing, and combustion continue until all the injected fuel has combusted.ĭiesel combustion is characterized by lean overall A/F ratio. It also increases the evaporation rate of the remaining fuel. Increased pressure resulting from the premixed combustion compresses and heats the unburned portion of the charge and shortens the delay before its ignition. This rapid ignition is considered the start of combustion (also the end of the ignition delay period) and is marked by a sharp cylinder pressure increase as combustion of the fuel-air mixture takes place. Rapid ignition of some premixed fuel and air occurs after the ignition delay period. As the piston continues to move closer to top dead center (TDC), the mixture (mostly air) temperature reaches the fuel’s ignition temperature. The atomized fuel absorbs heat from the surrounding heated compressed air, vaporizes, and mixes with the surrounding high-temperature high-pressure air. It atomizes into small droplets and penetrates into the combustion chamber. The liquid fuel is usually injected at high velocity as one or more jets through small orifices or nozzles in the injector tip. In diesel engines, fuel is often injected into the engine cylinder near the end of the compression stroke, just a few crank angle degrees before top dead center. One of the most important aspects of this process is the mixing of fuel and air, which is often referred to as mixture preparation. To perform this process, oxygen must be made available to the fuel in a specific manner to facilitate combustion. The basic premise of diesel combustion is its unique way of releasing the chemical energy stored in the fuel. This is different from combustion strategies that attempt to significantly increase the proportion of premixed burning that occurs-such as various flavours of low temperature combustion. This “conventional” diesel combustion is primarily mixing controlled with perhaps some premixed combustion that can occur due to mixing of fuel and air prior to ignition. This paper will review the most established combustion model for the conventional diesel engine. The application of laser-sheet imaging to the conventional diesel combustion process in the 1990s was key to greatly increasing the understanding of this process. For decades its complexity seemed to defy researchers’ attempts to unlock its many secrets despite the availability of modern tools such as high speed photography used in “transparent” engines, computational power of contemporary computers, and the many mathematical models designed to mimic combustion in diesel engines. The balance of fuel that had not participated in premixed combustion is consumed in the rate-controlled combustion phase.Ĭombustion in diesel engines is very complex and until the 1990s, its detailed mechanisms were not well understood. As the piston continues to move closer to top dead center, the mixture temperature reaches the fuel’s ignition temperature, causing ignition of some premixed quantity of fuel and air. During a phase known as ignition delay, the fuel spray atomizes into small droplets, vaporizes, and mixes with air. Abstract: In diesel engines, fuel is injected into the engine cylinder near the end of the compression stroke.