Ask A Scientist© Environmental Earth Science Archive

name         Stefan M.
status       student
age          16

Question -   What does a blow-off valve do in a car?
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Well, several eons ago when I was sixteen, I recall being blown off by a
number of girls while in a car.  However, there was no valve involved, just
the phrase "...not if you were the last guy on Earth" or words to that
effect.

The positive crankcase ventilation (or PCV) valve is sometime referred to as
the blow-by valve because the PCV system scavenges the gases that "blow by"
the piston rings and end up in the crankcase.  Cars built prior to the late
1950's had a "road tube" which would simply exhaust these gases to the
atmosphere.  However, under some conditions these gases could build up and
reduce performance and even damage the engine, as well as pollute the
atmosphere.  So, the PCV system was developed to use engine intake vacuum to
suck these gases out of the crankcase and recycle them through the engine.
The PCV valve regulates the amount of vacuum applied to the crankcase and is
a common maintenance item because the gases that flow through it tend to
leave deposits that clog it.

Less commonly, turbocharged engines have a wastegate that exhausts excess
pressure to prevent overcharging the engine.  This is also referred to
sometimes as a blow-off valve.  Those who hot-rod turbocharged engines like
to replace the stock wastegates with ones that provides higher peak
pressure.  This provides instant horsepower but may also provide instant
engine destruction when taken to excess.  (My brother once had a Mitsubishi
Starion modified in this way that whistled like a party favor when the valve
actuated.  I remember this noise clearly because I generally had my eyes
closed and was braced for impact when hearing it.)

Andy Johnson
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The blow-off valve should not be confused with a waste-gate.  The waste-gate is a 
device that by-passes the turbine wheel of the turbocharger, limiting the 
shaft-speed of the turbocharger.  Therefore, limiting the boost 
(pressure) that the compressor generates and keeping the turbocharger 
from over-speeding.

A blow-off valve is mounted in the intake plumbing between the 
turbocharger compressor and the throttle plate.  The blow-off valve is a 
second safety measure against the turbocharger over-boosting and damaging 
the engine.

The blow-off valve is more commonly used to keep the turbocharger spinning 
when the throttle plate is suddenly closed.  When the turbocharger is 
generating maximum boost pressure at full throttle and then the throttle 
is suddenly closed, compressed air coming from the compressor slams 
against the throttle, generating extremely high pressures that travel 
backwards to the compressor stopping the compressor from spinning.  When 
the throttle plate is again opened, the engine must spool the turbocharger 
shaft again.  The effects of this high pressure can also be very damaging 
to the turbocharger.

Brian Wright
Washington University in St. Louis Formula SAE Racing
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