Diesel Engine Fuel-
Diesel engines use diesel oil. In a diesel engine, only the air is sucked inside the cylinder and compressed to a high pressure, the compression ratio being 11:1 to 22:1. At the end of the compression stroke, diesel fuel is injected inside the cylinder by an injector, which burns due to the heat of compression; and the power stroke follows.
Diesel fuel is light with a low viscosity and cetane number. Diesel fuel must have the following requirements to operate satisfactorily :
- It must ignite readily and burn evenly.
- It must have a certain amount of lubricating qualities to operate in the pumps and valves.
- It must have a low viscosity so that it can flow through small holes and be readily atomized.
- It should not have too large a percentage of foreign matter that might injure the engine parts.
Factors matters for Pollution-
To better understand diesel emissions, it will be a quick sketch of the combustion process. Through the discussion, we can identify where the basic chemical elements originate and how pollutants are formed during combustion.
In all combustion engines, including gasoline, diesel fuel turbine steak, etc, power is produced from the heat released when fuel is mixed with air and burned.
The fuel supplied to combustion engines consists primarily of two elements hydrogen and carbon. Air consists primarily of nitrogen and oxygen.
If it were possible to achieve perfect combustion, all the hydrogen in the fuel would combine with the oxygen in the air and form water vapor. Likewise, all the carbon in the fuel would combine with the oxygen in the air and form carbon dioxide. Both water vapor and carbon dioxide occur naturally in the atmosphere and are not, for all practical purposes, considered pollutants. Any excess air would be emitted as free nitrogen and free oxygen. In fact, it would have a zero-pollution engine.
No commercial engine can achieve perfect combustion and thereby emit no pollutants, in addition to water vapor and carbon monoxide, unburned hydrocarbons, nitrogen oxides, and sulfur oxides. Fuel contains sulfur compounds which can form sulfur oxides, but the levels are low and can be controlled by the fuel manufacturer. Carbon particles may be present in the exhaust in the form of smoke.
There are three main pollutant formations that can dilute with air-
- Carbon monoxide- Carbon monoxide is formed rather than carbon dioxide when there is a lack of oxygen or air to support the complete combustion of the fuel.
- Nitrogen oxides- The oxygen and nitrogen in the air can combine with each other to form various nitrogen oxides whenever air is heated to high temperatures such as encountered in the fuel-burning process.
- Carbon particles- Carbon particles or smoke may be formed when the combustion temperature is high enough to decompose the fuel, but there is not sufficient oxygen available in the immediate area to burn the carbon. This is a particular problem for diesel and gas turbines because of the way the fuel and air are mixed.