Let's look at the turbocharger elements from a different perspective as far as producing power and protecting the powertrain components. I'm an engineer too (electronics), but I want to try to add some understandability to this whole discussion without getting too much into the nuts and bolts of thermodynamics.

The fundamental reason to go to forced induction is to increase the air density so that there will be more Oxygen molecules that can be burned (oxidized) and produce more heat to expand the gas field inside the cylinders and convert the thermal energy into kinetic energy (rotation of the crankshaft with improved torque). The more RPM's and torque producing events with each revolution, the more power is being produced. OK, this much is probably evident to everyone, but I just needed to state it to build the foundation of the rest of what I have to say.

I mentioned the expansion of the gas field within the cylinders. Well, the expansion of the gas field (and energy that can still be harnessed) isn't over just because the exhaust valve is open. The gas field continues to expand as the exhaust travels through the exhaust manifold/header/exhaust pipe until sufficient cooling has occurred that the expansion ceases and contraction begins. The turbo charger needs to be placed as close as practically possible to the exhaust ports of the cylinder head to take maximum advantage of the expansion of the exhaust gas field. This means that with a turbocharger, long primary tubes go out the window in order to collect the exhaust heat energy together while it is still actively expanding. The greater the temperature and pressure differential between the inlet and outlet of the exhaust turbine of the turbocharger, the more kinetic energy that can be applied to the shaft to rotate the compressor side of the turbo.

If the mixture is really rich the engine doesn't produce much power in the first place and the exhaust gas temperature(EGT) is relatively low and won't expand very much either. As the mixture is leaned (up to a point), the engine produces more power directly and the EGT is hotter, providing more thermal energy in the form of external exhaust gas expansion.

I mentioned leaning up to a point, let's talk about that point. If you run an air to fuel ratio (AFR) that gives you peak EGT, you will be too lean to produce max power directly and you will also be in the temperature region where exhaust valves are burned, detonation is a constant threat and the brother of EGT, turbine inlet temperature (TIT) can harm the turbo in a couple of ways. One way is that the exhaust turbine is simply running too hot for the metal of the housing, turbine impeller, shaft, etc to be durable. The clearances between the housing and the impeller that's whizzing around inside shrink to too close for comfort. Also, the lubricating oil in the shaft housing can be changed at a chemical/molecular level to a compound called coke (similar to the hydrocarbon coal). This coking can foul up the lubrication to the turbine shaft and ruin the turbocharger.

So, TIT is just one consideration . . . one of several. I hope that this little explanation helps with the understanding without adding to confusion.

Steve

 

ok, i don't know really what this stuff is but i do understand it. in simple terms, i thought that the hotter the exhaust flow is the faster it moves, it has more energy. i thought that was the whole purpose of exhaust coatings and wraps. is this the basic idea. more flow, more energy, more turbine speed, more power?

 

Pressure difference on the turbine itself is what spins the turbo over and the higher the pressure across those blades the more power is transferred to them until the turbine is spinning too fast at which point it's not too efficient aerodynamically anymore and it just becomes a restriction with no more boost produced.

Basically there's a whole lot of thermodynamics and aerodynamics and mechanics to what makes a turbo operate efficiently and it changes with the turbo and it's design as well. The pressure drop across it along with the mass flow across it will tell you about how much power can be transfered into boost production if you know the efficiency of the turbo itself at that point. It might already be turning crazy rpm and be way out of it's sweet spot so it may make less of everything except heat.

Then theres the info of running a turbo ENGINE efficiently and thats a whole nother ballpark.

 

This *may* be a little off topic (let me know), but since there's a lot of talk about pressure and temperature differentials spinning the turbine, has anyone thought of using an air pump to evacuate exhaust immediately after the turbine? I have seen them used on the crankcase, and was considering using one on my car. What if you were to use the pump to not only decrease crankcase pressure, but also to decrease pressure in the exhaust after the turbine? Is this feasable, or am I just off on one of my crazy ideas again?

 

To pump the volume of air needed to make a difference the additional load will most likely negate any gains.

You can add another turbine and extract more power from the exhaust and put it to some use. This is known as turbo compounding.