The carbon monoxide and hydrocarbon emissions of diesel are relatively low compared to those of gasoline engines.
Gallons of diesel intake greater amounts of air than is needed to support complete combustion For good combustion, about 7 kg of air is needed for each pound of fuel to be burned. Gallons of diesel will supply 10 to over 15 kg of air for each pound of fuel, whereas gasoline engines use air-to-fuel ratios of 20: I or less. The surplus air results in the formation of carbon dioxide rather than carbon monoxide.
Unburned hydrocarbons are not emitted as a result of “valve overlap” because the fuel is not injected until after all valves are closed. Diesel fuel spray can contact the liner walls. but diesel has a smaller clearance volume and hotter wall temperature which reduces the HC emission levels. This is because diesel uses much higher compression ratios.
If fuel is not properly mixed with air and is allowed to collect in one spot, i.e_. on the injector tip, valves, between the piston and liner walls, or in local areas within the combustion chamber space it will form smoke. The local temperature will usually be sufficient to crack the heavy fuel hydrocarbon compounds into a variety of lighter hydrocarbons. The hydrogen will immediately react with the available oxygen to form water. The remaining carbon will be emitted out of the exhaust as smoke.
Oxide of nitrogen-
Oxides of nitrogen are formed when the products of combustion are heated above 3000°F. The oxygen and nitrogen combine to form basically nitric oxide. The higher the temperature and the more air-exposed, the higher the concentration of nitric oxide produced. These compounds are unstable, and if allowed to cool slowly, would decompose. However, the combustion gases of the internal combustion engine cool rapidly, before this decomposition can take place. After the nitric oxide enters the atmosphere, it combines with the oxygen in the air to form more stable compounds such as nitrogen dioxide.
The cetane number of diesel fuel refers to the ease with which the fuel ignites. The best diesel fuel is that which ignites readily and burns steadily and rapidly. It is rated by means of a scale called cetane rating. The cetane number assigned to diesel fuel is a measure of its ignition qualities compared to those of standard fuel, normal cetane (C16 H34). A diesel fuel having a high cetane number ignites at a relatively low temperature. The lower the cetane number, the more likely the fuel is to knock.
The cetane number (or cetane value) is defined as the percentage by volume of normal cetane in a mixture of normal cetane and alpha-methyl-naphthalene which has the same tendency to knock as the diesel oil under test. Normal cetane has a good ignition quality and a short ignition lag, but alpha- methyl-naphthalene has poor ignition quality and a long ignition lag.
What is Diesel Smoke–
There are three general types of smoke: White, blue, and black.
White smoke is caused by liquid droplets of water or fuel being present in the exhaust. This may occur when the engine’s operating temperatures are too low to support complete combustion.
Blue smoke is usually the result of the engine burning lubricating oil caused by engine wear or a malfunction.
Black smoke can occur in diesel for several reasons, such as :
- Low air-to-fuel ratio, where the engine’s air intake is inadequate to burn the fuel being metered.
- Poor air and fuel mixing, where the fuel and air are in proper proportions but not being mixed together.
- Improper injection timing, where even a small amount of fuel is entering the combustion chamber at the wrong time.
Smoke opacity-Smoke visibility is measured by various means. The method used and preferred by the diesel industry is opacity.
This consists of measuring the amount of light that can penetrate the plume of smoke. If all the light is obscured from reaching the photocell, the opacity is 100%. Conversely, if all the light gets through the exhaust gases, the opacity is 0%.
Diesel Knock and Prevention–
The high-pressure waves set up in the diesel engine cylinder during the power stroke cause diesel knock, just like in a petrol engine. The piston head receives an uneven heavy load that sounds like a hammer blow, known as knocking.
The diesel knocks may be prevented by the following methods :
- By accelerating the combustion. It can be done by adding %of ethyl nitrate in diesel fuel.
- By raising the compression ratio to produce a higher temperature to ignite the fuel spontaneously.
- By increasing the turbulence of the compressed air promotes a homogenous mixture of injected diesel fuel in the cylinder.
- By increasing the inlet pressure of the air.
- By raising the temperature of the coolant, intake air, cylinder head, and combustion chamber.
- By raising the injection pressure which encourages the atomization of the fuel.