This is good read. Glad we are finally getting to why one engine has an edge over the other.

 

Where does that pressure come from?Does the Turbo pressurize the gas?How is it not?

 

I'll put this simply, take a turbo on a pedestal hooked up the the IC tubes and DP just like you'd see on a Diesel. Now, put the same Pressure at ambient room temp against that turbo, with the same CFM as the exhaust gases that are produced with these trucks. You will build the exact same boost levels at room temp, as you would with 1000* exhaust gas.

Turbos don't spin off heat, they spin off air pressure.

 

Correct, kind of. In a thermal turbine (w/ a compressible fluid) it will come from flow of the fluid, with pressure differential usually being part of the process.

However, in this process, the pressure and flow COMES from heat.
The turbo's energy doesn't come from "restricting flow" - sure it may provide a restriction in the exhaust, but that is purely how it extracts energy, which in this case comes from a thermal source.



The argument originally was that Turbo's don't run off wasted heat sent out the exhaust. They DO!!!!


Its no different to a coal power station. There it is super-heated steam which creates a pressure, which turns a turbine (kinetic) which drives a generator (electrical).
The change from thermal to pressure energy is merely part of the process, it is not the input, nor the output energy source.

 

Wow there are some crazy discussions going on here.
In essence I believe everyone is correct pretty near.

One side of the argument is that a turbocharger runs off heat, which is and is not true. Theres nothing the turbo does with 1000 deg heated air it couldn't do with -40 air pumped through it.

Now, that being said, air leaving the engine is expanded from combustion, and will shrink as it cools off travelling through pipes / equipment.

The turbo makes it's "free power" by recovering wasted heated exhaust gasses. There's nothing about the "heat" that makes any power, without airflow a turbo would do nothing.

When the pistons are being pushed down, and then the exhaust is released, THIS is the energy being captured by the turbo, the high pressure gas that could not be used to drive the piston down any further. (because of BDC).
If you had an engine running where the pressures in the cylinder at BDC were near atmospheric at BDC (As in every amount of fuel burning power was transmitted to the crankshaft) then the air leaving the exhaust would be near atmoshperic, thus there would be no lost energy EXCEPT heat in the exhaust.

In our "real" engines though, the pressure and velocity from the exhaust stream are so far above atmospheric pressures that a turbocharger can use this to efficiently drive a compressor wheel to drive extra air into the engine .

 

To clarify,

A turbocharger does not "run" off wasted heat.

It runs off wasted combustion.

The extra pressure from the engine that could not be transmitted to the crankshaft.

So I guess yes, it does run off wasted heat, but not in the way that some people are imagining.

 

Then why are turbos mounted so closely to the exhaust ports?

 

So it can spool faster. The farther away from the engine the more "lag" before it spools.

 

i believe heat does play a role in spinning a turbo. The molecules in hot air are moving faster than cold air. The energy value in hot air is higher than cold also. [i. a balloon filled with hot air rises while a balloon filled with cold air falls]

 

Partially correct, there is a balance with regards to volume in the exhaust headers and up-pipes. On side they need to be big enough to flow a set air flow at WOT, and on the other not so big as to prevent sufficient pressure from being built. Initally when coming from a stop it takes X cu in of air to flow before pressure can be built and the turbo to spool. Not really a longer pipe, but pipe with greater volume will create lag. That's part of why most diesel owners do not replace headers unless their high HP build requires it. Again, nothing to do with heat, and everything to do with building pressure against the turbo.

The new 6.7L Powerstroke, may have the least amount of this volume I've seen in a V8 diesel because they moved the exhaust headers and up-pipes to the engine valley. Reduced distance and volume, without sacrificing flow rate.

 

It's 1 of the reasons. William is alluding to the other.

Start a turbo vehicle from dead cold and immediately give it full throttle in first gear. Notice both the power it gives, and the rpms it takes until the "turbo kicks-in".
Now try it once the engine has had 30mins to warm up. As long as the intake/intercooler isn't suffering from heat soak, the power it will give will be greater and the rpm's at which the "turbo kicks-in" will be lower, because the exhaust manifold etc. will be much hotter and keep the thermal energy of the air.

 

Also the air is cooling every inch of metal pipe it is traveling through.

 

But all the molecules are moving in directions that cancel each other out.

If they weren't, a hot frying pan would move off the stove.

 

There is no debate. . .its an application of the physics laws. The turbo needs energy to run just like any pump. The only energy source is heat from combustion. If you think drive pressure is what fuels the turbo you just aren't lookin at the big picture. Everything on the entire truck is fueled by thermal energy. That includes the turbo. . . . .there is no magical source of energy that makes the turbo work. . . .its thermal energy. I.e. heat. Thermodynamics 101.

 

Correct! Anyone who has built a rear mounted turbo setup knows this. Turbos used in rear mounted kits (for the street) are smaller, and have tighter a/r ' s. Also. . . .you'll catch many a racer wrapping his entire pre turbo exhaust with expensive header wrap. . .need i say more? Drive pressure is a bi-product. . .not the source.