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post #3 of (permalink) Old 9th July 2008
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Originally Posted by VelSatisfied View Post
I was getting embroiled in a bit of a difference of opinion regarding the principles behind turbocharging, so rather than hijack someone else's thread, I'll explain what I mean in greater detail.

I believe that since the exhaust part of the turbo is spun by high pressure gases escaping (i.e. blowing), the induction part whereby fresh air is introduced to the engine is driven by low pressure (i.e. sucking).

One equalises the other, and the balance of nature is restored.

Now the thread I was originally adding to, related to how a high-flow air filter can assist with a turbocharger's efficiency - well, if you've ever driven a car with a dirty air filter - you will appreciate the relationship between airflow & power/economy (i.e. more of one = more of the other). This is because the spinning action of the induction side draws more air by creating a low pressure point, which nature is driven to minimise.

Granted, once the air has passed the filter and has entered the turbo , it will then start to be compressed, but just in the same way as per an normally aspirated engine. (i.e. part of the otto cycle - suck (there's that word again ), squeeze (this is where the pressure begins to increase), bang (pressure), blow (release of pressure).

As you can see, it is only the compression stage onwards where the air is under positive pressure.

If air wasn't drawn into the induction (induce means to lead/draw - not push), then all cars would need a supply of compressed air for combustion - there would be no driving force for the air to enter the filter.

Again, if proof were needed - check out the inlet manifold on your car, and compare it with the exhaust - the inlet one runs quite cool (strange, if it were pressurising air, it should be hot), whereas the exhaust manifold is where the heat is at (admittedly partially because of the spent hot gases being carried, but also because they are under pressure), which is why free-flowing exhaust systems can also improve engine efficiency.

Hope that helps to set my stall out.

No doubt some will differ with my postulations - but this is the place to do so.


Hey Paul, how's it going mate?

I posted on the thread you mentioned about the principals of turbo charging. All I can offer is my undestanding of the way it works. I by no means mean to say you're wrong or belittle you in any way. You've helped me in the past and I am greatful. Just here to offer my opinion

Here goes.

A turbo is split into two halves, A compresser and a turbine.

You are right in what you say, high pressure exhaust gas does spin the turbo , it does it by passing through the turbine, in a very similar way to how steam is used to spin a turbine in a power plant to create electricity. That is the only job the exhaust gas does, spin the turbine.
The turbine is linked to the compressor side via a shaft, the compressor does 2 jobs,sucks air in and compresses it. It is then forced it into the engine at a mucher higher positive pressure than a N/A engine.
I think normal pressure without modding is around 7 - 8 psi (is on an RS turbo anyway) with the option of going up to about 14psi or 1 bar without the need for a bigger turbo , a bigger turbo means a bigger compsessor. This means the air is already compressed and is under positve pressure before it reaches the cylinder where it is then compressed futher by the piston. The reason a turbo car uses lower compression pistons is because if you use standard compression pistons the air will heat up too much (as a result of being compressed) and will ignite the fuel (like a diesel) before the spark plugs gets a chance, resulting in pre-ingnition.

The way a N/A engine sucks air in is by the piston moving down on the induction stroke. It sucks in ambiant air from the atmosphere (0 psi if you will, I know it's not that but lets just keep it simple) and is only then compressed on the compression stroke by the piston itself. Just to clarify, the air passing through the inlet manifold is at ambiant, 0 psi, pressure until it enters the cylinder where it is compressed. In a tubro engine, compressed air passes through the inlet manifold at around 8 psi or more where it enters the cylinder and is compressed further.

You are right, the inlet manifold is cool compared to the exhaust, in an N/A car anyway.
This is because ambiant air is passing through it, the only reason the air is passing thorugh it is because of the suction action created by the piston as it's moving down on it's cycle.
In a turbo car, without the use of an intercooler the inlet manifold would get hot. Granted, not all turbo cars have intercoolers, but as a result they can't compress the air too much or it will get too hot, resulting in pre-ignition, but in theory if you removed the intercooler from a turbo car that is meant to have one fitted, it would get hot, very hot indeed. This is because when you compress air you generate heat. Basic rule as to why a diesel runs without spark plugs.
The intercooler is meant to cool the air, nothing to do with colder air contains more oxygen ( the colder more ogygenated air needs to be intorduced before it's compressed, hence the reason for a decent induction system made up of a Cone filter, I don't rate pannels as they are prone to heat soak), but to enable the turbo to run at higer psi and compress more air. More air compressed means more air forced into the engine resulting in more fuel being added meaning the faster you go.

Top thread by the way, always nice to have a good discussion like this.
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Last edited by Mikey; 9th July 2008 at 10:32 PM.
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