Components of Combustion Process
Combustion in diesel engines is very complex and until the 1990s, its detailed mechanisms were not well understood. 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 application of laser-sheet imaging to the conventional diesel combustion process in the 1990s was key to greatly increasing the understanding of this process.
This paper will review the most established combustion model for the conventional diesel engine. 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 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.
The basic premise of diesel combustion is its unique way of releasing the chemical energy stored in the fuel. To perform this process, oxygen must be made available to the fuel in a specific manner to facilitate combustion. One of the most important aspects of this process is the mixing of fuel and air, which is often referred to as mixture preparation.
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 [Heywood 1988]. The liquid fuel is usually injected at high velocity as one or more jets through small orifices or nozzles in the injector tip. It atomizes into small droplets and penetrates into the combustion chamber. The atomized fuel absorbs heat from the surrounding heated compressed air, vaporizes, and mixes with the surrounding high-temperature high-pressure air. As the piston continues to move closer to top dead center (TDC), the mixture (mostly air) temperature reaches the fuel’s ignition temperature. Rapid ignition of some premixed fuel and air occurs after the ignition delay period. 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. Increased pressure resulting from the premixed combustion compresses and heats the unburned portion of the charge and shortens the delay before its ignition. It also increases the evaporation rate of the remaining fuel. Atomization, vaporization, fuel vapor-air mixing, and combustion continue until all the injected fuel has combusted.
Diesel combustion is characterized by lean overall A/F ratio. The lowest average A/F ratio is often found at peak torque conditions. To avoid excessive smoke formation, A/F ratio at peak torque is usually maintained above 25:1, well above the stoichiometric (chemically correct) equivalence ratio of about 14.4:1. In turbocharged diesel engines the A/F ratio at idle may exceed 160:1. Therefore, excess air present in the cylinder after the fuel has combusted continues to mix with burning and already burned gases throughout the combustion and expansion processes. At the opening of the exhaust valve, excess air along with the combustion products are exhausted, which explains the oxidizing nature of diesel exhaust. Although combustion occurs after vaporized fuel mixes with air, forms a locally rich but combustible mixture, and the proper ignition temperature is reached, the overall A/F ratio is lean. In other words, the majority of the air inducted into the cylinder of a diesel engine is compressed and heated, but never engages in the combustion process. Oxygen in the excess air helps oxidize gaseous hydrocarbons and carbon monoxide, reducing them to extremely small concentrations in the exhaust gas.
The following factors play a primary role in the diesel combustion process:
- The inducted charge air, its temperature, and its kinetic energy in several dimensions.
- The injected fuel’s atomization, spray penetration, temperature, and chemical characteristics.
While these two factors are most important, there are other parameters that may dramatically influence them and therefore play a secondary, but still important role in the combustion process. For instance:
- Intake port design, which has a strong influence on charge air motion (especially as it enters the cylinder) and ultimately the mixing rate in the combustion chamber. The intake port design may also influence charge air temperature. This may be accomplished by heat transfer from the water jacket to the charge air through the intake port surface area.
- Intake valve size, which controls the total mass of air inducted into the cylinder in a finite amount of time.
- Compression ratio, which influences fuel vaporization and consequently mixing rate and combustion quality.
- Injection pressure, which controls the injection duration for a given nozzle hole size.
- Nozzle hole geometry (length/diameter), which controls the spray penetration as well as atomization.
- Spray geometry, which directly impacts combustion quality through air utilization. For instance, a larger spray cone angle may place the fuel on top of the piston, and outside the combustion bowl in open chamber DI diesel engines. This condition would lead to excessive smoke (incomplete combustion) because of depriving the fuel of access to the air available in the combustion bowl (chamber). Wide cone angles may also cause the fuel to be sprayed on the cylinder walls, rather than inside the combustion bowl where it is required. Fuel sprayed on the cylinder wall will eventually be scraped downward to the oil sump where it will shorten the lube oil life. As the spray angle is one of the variables that impacts the rate of mixing of air into the fuel jet near the outlet of the injector, it can have a significant impact on the overall combustion process.
- Valve configuration, which controls the injector position. Two-valve systems force an inclined injector position, which implies uneven spray arrangement that leads to compromised fuel/air mixing. On the other hand, four-valve designs allow for vertical injector installation, symmetric fuel spray arrangement and equal access to the available air by each of the fuel sprays.
- Top piston ring position, which controls the dead space between the piston top land (area between top piston ring groove and the top of the piston crown), and the cylinder liner. This dead space/volume traps air that is compressed during the compression stroke and expands without ever engaging in the combustion process.