The development of the compression-ignition (CI) engine, also known as Diesel engine was mainly due to the work of Dr. Rudolf Diesel, who got a patent of his engine in 1892. Today the CI engine is a very important prime mover, being used in buses, trucks, locomotives, tractors, pumping sets, and other stationary industrial applications, small and medium electric power generation and marine propulsion.
The first CI engine was started in August 10, 1893. The principle that compressed air was capable of igniting a fuel, in this particular case gasoline, was thus demonstrated. However, the combustion was sporadic, due to the defective fuel injection system. The injection system and the engine construction were improved only four years later, the efficiency of the engine was 30.2%. After another four years, 1901, and with the contribution from Jonas Hesselman working with the design of the combustion space, the efficiency for the CI engine was raised to 36%.Because of the utilization of higher compression ratio 12:1 to 22:1 the forces coming on the various parts of the engine are greater and therefore heavier parts are necessary [7]. Also because of heterogeneous mixture, lean mixture (large air-fuel ratio) is used. Both the factors result in a heavier engine. The smoke and odor are the result of the nature of diesel combustion phenomenon, i.e. incomplete combustion of a heterogeneous mixture, and droplet combustion, compression-ignition engines, because of their varied applications, are manufactured in a large range of sizes, speeds, and power output.
The emission researches of diesel engines are going on mostly for the regulated emissions such as NOx, PM and others due to severe regulation standards of that components.Emission standard specify the maximum amount of pollutants allowed in the exhaust gases discharged from a diesel engine.

Evolution of the EGR systems the first EGR valves appeared in 1973 on GM cars. Bolted to the intake manifold next to the carburetor, it has ports to the intake and exhaust manifolds. It has a diaphragm that pulls open a valve stem, which allows exhaust to enter the intake manifold when ported vacuum is applied to it. Ported vacuum increases with throttle opening. A thermal vacuum switch prevents vacuum from reaching the EGR during cold engine starts. This system had many problems. It would often open too soon or too much, which caused a hesitation on acceleration as massive amounts of recirculated exhaust hit the combustion chamber. Many people simply disconnected it when it began to cause problems because they did not understand its importance or design. By 1975, if you unplugged an EGR valve, you'd have a driveability complaint of engine ping. Manufacturers and technicians of that era experimented with vacuum orifice restrictors and vacuum delay valves to try to find a happy medium between clean air and performance.
Closed loop systems By 1981, closed loop computer controls were in place. EGR flow was now more carefully controlled with dual diaphragm and back-pressure EGR valves. Modulating the vacuum to the EGR valve's pull, open diaphragm controlled the flow of recirculated ex- haust. Called by various names such as amplifiers, transducers and modulators, both remote and integral vacuum modulated devices were used. The flow of vacuum was further controlled by solenoids that blocked the vacuum ports until certain criteria were met such as engine temperature, rpm and manifold absolute pressure (MAP).