Why is it needed?
Abbreviation of EGR stands for exhaust gas recycle. This is special technique for internal combustion engines that uses petrol / gasoline and diesel to reduce nitrogen oxide NOx emissions by cooling engine combustion temperature.
The exhaust gas is mainly consisting of CO, N, H and etc. During acceleration internal combustion temperature raises up to 2500 deg C creating convenient condition for NOx formation. The principal reactions at near stoichiomentric fuel - air mixture governing the formation of NO from molecular nitrogen are:
N2 + O = NO + N
N + O2 = NO + O
N + OH = NO + H
Re-circulating exhaust gas through air intake displaces some amount of fresh air entering into the combustion chamber. As a consequence the amount of O2 in combustion chamber decreases subsequently leading lower engine temperature which prevents NO formation.
Types of EGR valves
First generation of EGR valves was just a hole between exhaust and intake manifolds. However, then it was discovered that recycling too much exhaust gas do effect engine efficiency or applying it at the wrong time creates problems with engine. Based on user feedbacks over the years engineers have designed hundreds of EGR valves to eliminate negative effect and properly control exhaust gas amount into combustion chamber.
Vacuum operated EGRs
This type of EGR valves was very first member of EGR family. Operating principle was very simple, a vacuum diaphragm opened and closed a valve which was controlled by temperature inputs from engine. You definitely do not need EGR valve to be open when your engine is cool as the main purpose of recycling exhaust gas is to decrease ignition chamber temperature which in this case is lower anyway.
This type of design of EGR has solved excessive exhaust gas in ignition chamber issue. However, there were still problems with design. As previously mentioned control system of vacuum operated EGRs used temperature inputs for opening and closing valve and this primitive control method has created problems when engine operated in idle mood. Normally, at low speed your combustion temperature is naturally lower and it is undesirable adding exhaust gas into your combustion chamber for cooling purpose. This problem was overcome by introducing positive back-pressure EGR valve.
The principle of operation was completely based on mechanical parts. The positive back pressure design has hallow valve stem allowing stem movement of valve as exhaust gas pressure changes. Back pressure in downstream of EGR rises when vehicle starts acceleration which pushes vacuum loaded spring allowing exhaust gas into ignition chamber. When pressure is lower spring pusses plug back and closes exhaust gas flow.
Differential pressure feedback EGRs
In modern cars EGR valve is controlled by engine computers. Embedded logic in engine computer sends open/close command to EGR comparing multiple inputs such as engine temperature, oxygen analyser and DP cross the EGR valve. Computer sends output signal to solenoid valve which regulates vacuum source.
Disadvantages of EGR
Because of exhaust gas CO containment exhaust gas injection pots may get blocked by coat particles resulting diagnostic issues with your car by lighting up check the engine sign. This mainly happened on old cars where only one injection port used for exhaust gas injection in air intake. Modern cars have got multiple exhaust injection ports connected to each cylinder for discharging gas into ignition chamber. If several ports plug up, it may cause excessive exhaust gas flowing into cylinder decreasing combustion efficiency which couldn’t be misdiagnosed. The only repair is to remove the intake and clear all passages.
An EGR valve that is stuck open: This will result excessive amount of exhaust gas in engine which will decrease efficiency of your engine and feeling of hesitation when you try to accelerate. In severe cases your engine will start stalling frequently.
A valve that is stuck closed: In such cases, NO production in your car engine will increase and this will lead to engine knocking.
Re-introducing exhaust gas may also coat the throttle valve body plate. This is very often happening in cars which resulting rough idle. In some cases dying can occur as well. The car computer may generate misfire code or light up check the engine sign.
How exhaust system works?
Exhaust system mainly consist of exhaust manifold, catalytic convertor, resonator, muffler. The hot exhaust gas along with sound waves generated at the end of exhaust stroke sent to exhaust manifold through the exhaust valve. Then sound wave and exhaust gas passes to catalytic converter though the pipe. As previously mentioned due to partial combustion exhaust gas consists of mixture of Carbon Monoxide, Unburned Hydrocarbons and nitrogen oxides which are harmful for environment.
Where it is located?