Fuel is injected by the fuel-injection system into the engine cylinder toward the end of the compression stroke, just before the desired start of combustion. The liquid fuel usually injected at high velocity as one or more jets through small orifices or nozzles in the injector tip, atomizes into small drops and penetrates into the combustion chamber. The fuel vaporizes and mixes with the high-temperature high-pressure cylinder air [8]. Since the air temperature and pressure are above the fuel's ignition point, spontaneous ignition of portion of the already-mixed fuel and air occurs after a delay period of a few crank angle degrees. The cylinder pressure increases as combustion of the fuel-air mixture occurs. The consequent compression of the unburned portion of the charge shortens the delay before ignition for the fuel and air, which has mixed to within combustible limits, which then burns rapidly. It also reduces the evaporation time of the remaining liquid fuel. Injection continues until the desired amount of fuel has entered the cylinder. Atomization, vaporization, fuel-air mixing, and combustion continue until essentially all the fuel passed through each process. In addition. mixing of the air remaining in the cylinder with burning and already burned gases continues throughout the combustion and expansion processes.
The ignition delay is defined as the time (or crank angle) interval between the art of injection and the start of combustion. The start of injection is usually taken, as the time when the injector needle lifts off its seat (determined by a needle-lift indicator) the start of combustion is more difficult to determine precisely [9]. It is best identified from the change in slope of the heat release rate. Depending on the character of the three combustion processes, the pressure data alone may indicate when pressure change due to combustion first occurs; in DI engines under normal conditions ignition is well defined, but in IDI engines the ignition point is harder to identify. Flame luminosity detectors are also used to determine the first appearance of the flame.

The chemical component of the ignition delay is controlled by the pre -combustion reactions of the fuel. Though ignition occurs in vapor phase regions, oxidation reactions can precede in the liquid phase as well between the fuel molecules and the oxygen dissolved in the fuel droplets. In addition, cracking of large hydrocarbon molecules to smaller molecules is occurring. These chemical processes depend on the combustion of the fuel and the cylinder charge temperature and pressure, as well as the physical processes described above which govern the distribution of fuel throughout the air charge.
Since the ignition characteristics of the fuel affect the ignition delay, this property of a fuel is very important in determine diesel-operating characteristics such as fuel conversion efficiency, smoothness of operation, misfire, smoke emissions, noise, and ease of starting. The ignition quality of a fuel is defined by bits cetane number. For low cetane fuels with too long an ignition delay, most of the fuel is injected before ignition occurs, which results in very rapid burning rates once combustion starts with high rates of pressure rise and high peak pressures [10]. Under extreme conditions, when auto ignition of most of the injected fuel occurs, this produces an audible knocking sound, often referred to as "diesel knock". For fuels with very low cetane numbers, with an exceptionally long delay, ignition may occur sufficiently late in the expansion process for the burning, process to be quenched, resulting in combustion, reduced power output, and poor fuel conversion efficiency [11]. For higher cetane number fuels, with shorter ignition delays, ignition occurs before most of the fuel is injected. The rates heat release and pressure rise are then controlled primarily by the rate of injection and fuel-air mixing, and smoother engine operation results.

The diesel combustion reaction consists of hydrocarbon chains being oxidized in an explosive reaction to form carbon dioxide (CO2) and water (H2O). However, the reaction is not hundred percent efficient and the constituent are not pure. The air used to supply the oxygen (O2) contains about 80% nitrogen and diesel fuel contains small percentage of sulfur [12]. The result is that trace amount of other chemicals are found in the reaction. All of the trace constituents are of concentration to the environment or can pose a health risk in higher concentration.
Oxides of nitrogen (NOx) are a combination of nitric oxides (NO) and nitrogen dioxide ((NO2). The air supplied for combustion contains about 77% of nitrogen. At lower temperature, the nitrogen is inert. However, at temperatures higher than 1100°C reacts with oxygen (O2). Therefore high temperature and availability if oxygen is two main reasons for the formation of NOx. When the proper amount of oxygen (O2) is available with highest local peak of combustion temperature then highest amount of NOx is found in diesel exhaust.

Organic and inorganic compounds of higher molecular weights are exhausted in the form of small size particles of the odor of 0.02 to 0.06µ bul. Hundreds of separate organic compounds can be formed when the combustion reaction is not complete. These organic fractions (VOF). When the concentration of other organic compound in the exhaust rises, they can pose severe health risk.

It is product of incomplete combustion due to insufficient amount of air in the air fuel mixture. It is not formed in large quantities due to excess amount of oxygen available during combustion, and generally not the concern [13].
The two main reasons for the formation of NOx are high temperature and available of oxygen. Engine design and the model of vehicle operation affect the NOx concentration in exhaust. A pre-combustion chamber engine produces less NOx than a direct injection engine due to lower peak temperature. The maximum NOx is formed at ratios between 14:1 and 16:1. At lean and rich air-fuel mixtures, the NOx concentration is comparatively low. At high fuel-air ratio the additional fuel tends to cool the charge, so the localized peak temperatures are lowered resulting in drop in NOx concentration .
Injected system and time also significantly affect the NOx formation. Also, the variation in fuel characteristics such as cetane number, viscosity, modulus of elasticity and rate of burning etc. all contribute to differences in NOx levels obtained from different levels [ 14,15, & 16].
It is a product of incomplete combustion due to insufficient amount of air in the air-fuel mixture. It is not formed in large quantities due to excess amount of oxygen available during combustion and generally not the concerned

The main harmful pollutants that emit in diesel exhaust are:
1. Oxides of nitrogen (NOx)
2. Particulate matter (PM)
3. Carbon monoxide (CO
4. Carbon dioxide (CO2)
5. Unburned hydrocarbon (UBHC)
6. Odor