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Turbo Fundamentals:- 4 / 4.

 
The story so far:
We have determined what a turbo is, how the exhaust turbine functions,
what the inlet compressor is,
and hinted a little at what selecting a turbo requires.
 
By: Dennis Grant dg50@chrysler.com
June 1997
Subject: Turbo Fundamentals.
PAGE 4/4
 
The Wastegate
Intercooler & BOV
 
Page 1
FREE Energy?
 
Page 2
Turbine Theory
 
Page 3
Compressor Side
 
Page 4
InterCooler Wastegate & BOV
  So, previously we left off with high pressure air leaving the compressor outlet. Unfortunately, physics has worked against us this time, and the act of doing work to our inlet air to compress it has raised its temperature.

This is bad.

Not only are we reducing density, we're increasing the possibility of the great bugaboo - detonation. Remember, the onset of detonation is usually the limiting factor on the amount of power a given engine can produce, and that increased intake temperature (as measured at the intake valve) increases the chance of detonation.

So we have to cool the air back down again, without losing any pressure.

That's the job of the intercooler, basically a "air radiator" placed in the flow stream between the turbo compressor outlet and the intake manifold. There's really not much else to say about them, except:

1) The more you can cool the air flow, the better. This _normally_ means the bigger the intercooler, the better. (There are some smaller coolers that are better designed than the lower-end "big" coolers though, so size does not necessarily indicate effectiveness.

2) The cooler must be placed in a location where ambient air can flow through it. This means that your cooler must have an intake path and an _exhaust_ path. Mounting a cooler flush against a plate does no good!

3) There's always a pressure drop across a cooler. How much depends on the cooler design.

Wastegates

A turbo is a positive-feedback device. The more boost you make, the more exhaust you make, which makes more exhaust, which makes more boost... in a vicious circle. So we have to have some way of limiting boost.

What we _really_ want is a way of keeping the turbine operating at a constant speed (see yesterday's post) so that we can maximise the compressor efficiency - remember that turbines like to run at a single speed. However, as measuring turbo RPM is not practical, and as boost level is directly related to turbo speed, keeping the boost constant is the waste gate's job.

The waste gate is just a valve that opens when we have exceeded our desired boost level, and allows exhaust to flow around the turbine, instead of through it. This lowers the pressure differential across the turbine, less work is done, and the turbo slows down.

The only "gotcha" with the waste gate is that it must be able to flow enough gas to let the turbo slow down. If it can't, then you get "boost creep" where boost levels slowly grow as the car remains under b&127;ost. Bad.

BOV

Everybody likes BOV's because of the nifty sneeze sound they make. However, a BOV is an evil device. It's taking your precious boost and venting it to someplace else. Bad!

Unfortunately, it's a necessary evil, and we have to live with it. Here's why: You're under boost, the turbo is fully spooled, and life is good - then you shift. That means your foot comes off the gas - and the throttle plate slams shut. Suddenly, instead of flowing in a continuous stream through the engine, the intake air smacks into the closed throttle plate. The turbo, which is still spinning and producing boost because if it's rotational inertia keeps producing pressure, and the intake stream, caught in between a rock and a hard place, jumps in pressure. In fact, you get a high-pressure shockwave that travels from the throttle plate back to the compressor vanes, that once it gets there, is a little like poking a stick into the spokes of a bike wheel.

The repeated shock is hard on the compressor vanes and the shaft bearings, and in any case acts like a brake, slowing the turbo, and requiring it to be spooled up again.

The BOV sits in between the turbo and the throttle plate, and if it detects the shockwave created by a shift, vents it elsewhere - either to atmosphere, or back to the inlet side of the turbo.

So, we lost boost pressure, but we kept the turbo spooled... tough to say without a dyno if that was a fair trade on a race vehicle. On a street vehicle, it was definitely a good idea, because we spared our expensive turbo a mechanical shock.

For information on general Turbo Modifications :- Click here

For information BEFORE any Turbo Modifications :- Click here

 
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 This page was last updated on 19-Mar-2009 18:34.

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