An exhaust gas recirculation (EGR) system feeds a measured slice of the exhaust back into the engine. Use the buttons to look inside it: hot gas is chilled in a tube bundle, then a solenoid valve dials exactly how much is let through to the intake.
An exhaust gas recirculation (EGR) system takes a small, carefully controlled part of the exhaust and sends it back into the engine's intake. Mixing burnt gas with the fresh charge lowers the peak temperature of combustion, and because nitrogen oxides form mainly when combustion runs hot, that is what cuts them.
Older systems fed the exhaust back hot. A cooled EGR system puts a heat exchanger — the cooler — in the path first. The gas passes through a bundle of tubes while engine coolant flows around the outside of those tubes, so the gas arrives at the valve much cooler and denser.
From the outside an EGR assembly is just a metal block with a couple of hoses. Cutting it open shows two jobs working together: a cooler that drags heat out through a tube bundle and a solenoid valve that measures the flow. One is a heat exchanger, the other is a fast-acting tap.
The idea that holds it together is control. The cooler only conditions the gas; it is the pulsed valve that decides how much of it is allowed to reach the intake at any moment.
The gas that leaves the engine is still very hot. It flows to the EGR cooler through the inlet, carrying the heat that the system is about to remove.
Inside the cooler the gas runs through several small tubes. Around the outside of those tubes is engine coolant, so the two are separated by thin metal with a large surface area between them.
Cold coolant enters at one end and leaves hot at the other, travelling opposite to the gas. Counter-flow keeps a temperature difference all along the cooler, so the gas keeps shedding heat right up to the exit.
With the heat drawn off, the exhaust is denser than it was. It rises into the valve body, where the flow through to the intake is about to be measured.
The engine computer pulses a solenoid coil. Each pulse pulls a tapered pintle off its seat and opens the passage, so exhaust can pass through into the intake manifold. How long each pulse lasts sets the average opening.
When the pulse stops there is no magnetic pull, so a return spring presses the pintle firmly back onto its seat and seals the passage. Pulsing open and shut this way lets the computer set the recirculation rate very precisely.
The measured exhaust joins the fresh air in the intake manifold and is carried into the cylinders. There it dilutes the charge and lowers combustion temperature — the whole point of the system.
It controls how much exhaust gas is fed back into the engine's intake. Recirculating a measured amount of exhaust lowers peak combustion temperature and cuts nitrogen-oxide emissions.
Cooling the recirculated gas makes it denser, so more of it can be packed into each cylinder. That takes more heat out of combustion and lets the engine run at a higher EGR rate without losing power.
Hot exhaust flows through a bundle of tubes while engine coolant flows around the outside of the tubes in the opposite direction. That counter-flow arrangement pulls heat out of the gas before it reaches the valve.
A solenoid actuator is pulsed by the engine computer. It lifts a tapered pintle to open the passage; a return spring presses the pintle back onto its seat when the pulse stops.
Pulsing lets the computer set the average flow very finely. By varying how long each pulse lasts, it can command exactly the recirculation rate the running conditions call for.
No. It is used mainly at light and part load when the engine is warm. At full throttle or while the engine is cold, the valve usually stays closed so it does not hurt power or driveability.