An exhaust gas recirculation valve, or EGR valve, helps a gasoline engine reduce harmful nitrogen oxide emissions. It does this by sending a controlled amount of exhaust gas back into the intake manifold instead of sending all of it out the tailpipe. This matters because nitrogen oxides form most strongly when combustion temperatures are very high.
By lowering peak combustion temperature, the EGR system helps the engine run cleaner during many driving conditions.
Exhaust gas contains very little oxygen, so mixing it with fresh air and fuel makes the combustion charge less reactive. The engine control unit opens the EGR valve when conditions are right, such as steady cruising or light acceleration, and closes it at idle, cold start, or full throttle. Modern systems may use electronic valves, position sensors, and feedback from oxygen or pressure sensors to control the flow accurately.
If the EGR valve sticks open or closed, the vehicle may develop rough idle, pinging, poor fuel economy, or increased emissions.
Key Facts
- EGR stands for exhaust gas recirculation.
- The EGR valve connects the exhaust manifold to the intake manifold through a controlled passage.
- Recirculated exhaust gas lowers peak combustion temperature, which reduces NOx formation.
- NOx formation increases rapidly when combustion temperature is very high, often above about 2500 degrees Fahrenheit.
- EGR flow rate can be estimated as EGR percent = EGR flow / total intake flow x 100%.
- The EGR valve is usually closed during cold start, idle, and wide open throttle, and open during warm steady cruising.
Vocabulary
- EGR valve
- A controlled valve that allows some exhaust gas to flow back into the engine intake to reduce combustion temperature and emissions.
- NOx
- Nitrogen oxides are air pollutants formed when nitrogen and oxygen react at high combustion temperatures.
- Intake manifold
- The passage system that distributes incoming air, and sometimes fuel mixture, to the engine cylinders.
- Exhaust manifold
- The passage system that collects burned gases from the engine cylinders and sends them toward the exhaust system.
- Combustion temperature
- The temperature reached inside the cylinder as the air fuel mixture burns and releases energy.
Common Mistakes to Avoid
- Thinking EGR adds more oxygen to the engine, which is wrong because exhaust gas is mostly inert and contains little usable oxygen.
- Assuming the EGR valve is open all the time, which is wrong because it is usually closed at idle, cold start, and full throttle to protect drivability and power.
- Confusing EGR with the catalytic converter, which is wrong because EGR prevents some NOx from forming while the catalytic converter treats exhaust after combustion.
- Ignoring carbon buildup in EGR passages, which is wrong because blocked passages can stop recirculation even if the valve itself still moves.
Practice Questions
- 1 An engine has a total intake flow of 120 grams per second and an EGR flow of 12 grams per second. What is the EGR percent?
- 2 During steady cruising, an engine takes in 90 grams per second of fresh air and 10 grams per second of recirculated exhaust gas. What fraction of the total intake flow is exhaust gas?
- 3 A car idles roughly and stalls when warm, and a scan tool shows the EGR valve is stuck open. Explain why an open EGR valve can cause poor idle but may be useful during steady cruising.