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发帖数: 3081 | 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? |
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