What does the pcv valve and egr valve do?

Answer 1

PCV, stands for positive crankcase ventilation. Since the piston rings cannot stop every bit of combustion gases from escaping past them into the crankcase(oil pan) area, there needs to be some way to safely vent that pressure, so that it wont build up and blow the seals out of the engine. The PCV valve does this, by applying a controlled amount of vacuum to give the combustion gases a way out and keep them from building up. EGR stands for exaust gas recirculation. The EGR valve opens at the appropriate time, usually on moderate acceleration to allow exaust gas to flow back into the combustion process through the intake manifold. It is a common myth that the reason is to "re-burn" exaust gas to use any remaining unburned fuel and improve gas mileage. Anybody who gives that explanation as to why they have an EGR valve, has no idea what he/she is talking about and probably should not be working on cars. Here is the real reason there is an EGR valve: Under certain conditions, esp when the engine is hot, fuel may actually burn too quickly, and thus not burn completely. In fact, spark knock or "ping" is caused by "pre-ignition detonation," the fuel actually ignites before the spark plug sparks. Pre-ignition detonation will cause an incomplete fuel burn, loss of power, bad gas mileage, and could even damage the engine internally. Under certain conditions you may want to slow the burn of the fuel, so that it is a more complete burn, and is at the right time. That is why you introduce exaust gases into the combustion process, to slow the fuel combustion just a little, so you can get a more complete burn, improve fuel efficiency, and reduce pre-ignition detonation. Here is a side point about pre-ignition detonation. Octane actually slows the burn process as well. That is how high octane fuels work, and why they reduce ping. The higher the octane, the slower and more completely the fuel burns. But with high octane fuels, you may want to consider where you are driving. If you are planning a trip to, or live in the mountains where the air is thinner, and there is less oxygen, you probably don't need a higher octane. The reason is that, with less oxygen, the combustion will be slowed in the engine already. Why slow it down any more? I hope this information helps.