physics of turbo question

[ATLD]

as more fuel is burned, both the temperature, volume, and mass of the gases in the exhaust increase

It should be noted, that in any reaction, mass is ALWAYS a constant. The mass of the reactants is always equal to the mass of the products; ALWAYS. Remember, energy cannot be created or destroyed, the same goes with mass. The only variable factors are: gas volume, temperature, pressure, humidity, ambient temperature, atmospheric pressure, and rate of combustion (which includes the amount of fuel burnt). Actually, the operation of a turbocharger in a gas engine is practically the same as for a diesel engine, just a few less safety features (knock sensors/ computer etc.) but perhaps not with MB's new CDI's.

The thermodynamics governing the turbocharger are very complex, involving many variables and sometimes can be skewed, if considered from the multiple frames of reference all at once. That's why it is important to use the same frame of reference in each explanation. I explained the function of the turbocharger with a fixed reference to STP (standard atmospheric pressure and temperature).

I'm sorry if my very technical explanation confused anyone. I don't want to make this topic any more nebulous.

ATLD
Dr. Adam Ted Luke Delecki

[JimSmith]

This is an interesting thread and, while I have resisted jumping in up to this point, I think I will put my $.02 in now.

psfred's response comes closest to the mark. He never suggested the turbo worked on a principle that violated the conservation of mass or energy rules. What he said was, as the turbo spins up using the energy of the exhaust stroke, on the inlet side, the inlet pressure is raised so on the next exhaust stroke there is more air and fuel that burned to pass through the turbine wheel than on the previous stroke. That is, assuming the driver still has his or her foot in it. And he is right.

The other thing is, a turbine designer has a few variables to play with when selecting the details of the inlet, blade and exhaust shapes. On a turbocharger the emphasis is to use energy that is otherwise being wasted to compress air in a compressor connected to the inlet of the engine. If the designer elected to accomplish this task using differential pressure alone, he would add exhaust system flow losses that would detract from the available power to drive the wheels. Therefore, the intent of the turbocharger turbine designer is to use the heat and pressure available. The shape of the radial (possibly a slightly mixed) flow turbine wheel increases the volume rapidly as the gas travels through the wheel, which decreases pressure and cools the exhaust.

The intent is to do this with the least possible increase in back-pressure, as that just adds pumping losses to the engine (higher pressure on top of the piston as it forces the exhaust out of the cylinder takes more energy), detracting from thermal efficiency. So, the hotter the exhaust gas at a given rpm, the faster the turbine spins as there is more energy available to take out. And, as psfred noted, the higher the inlet pressure, so the more air and diesel there is to burn on the next power stroke, which increases the exhaust volume and its temperature on the exhaust stroke, which make the turbine spin even faster, and so on. Until the driver comes to his/her senses or the load, which is running up a cubic curve for horsepower to speed (twice the speed takes 8 times the horsepower)tries to exceed the available power from the turbine.

The analogy to water turbines is erroneous because water is not a compressible fluid and exhaust gas is, which makes all the difference in how you approach the detailed design. A better analogy would be a gas turbine. Gas turbines also depend on extracting the thermal energy of the combustion process in the power turbine, by expanding the gas generated by combustion across turbine stages that are mostly axial flow designs, but that have an increasing volume from inlet to outlet. These machines are very complex, much more so than a turbocharger so even this analogy is of limited value. But in general, the higher the combustion temperatures, the higher the thermal efficiency.

Also, the response of a gas engine to throttle position is different than a Diesel, which is what psfred was suggesting. A Diesel does not throttle the inlet air based on the demand for power. A gas engine does, so it varies the charge of air as well as the fuel added, depending on throttle position. I am not ready to speculate about how this affects the turbocharger function, but I will bet the designers at Porsche and other manufacturers took this into account so the net effect is an improvement in performance and thermal efficiency.

Jim

[240 Ed]

Jim

Thanks for your 'splanation of turbos, and thanks to Adam for his very well written response. I have always wondered about why the turbo doesn't spool when not under load.

Nother question:

It sounds like a diesel is the most efficient at WOT than when it is not, because the amount of air would be perfectly matched to the amount of fuel the designers have figured would be at the final full throttle mix...? (does that make sense?)

Whereas the gas engine is more matched throughout its operating range because of the ability to vary air as well as fuel?

ARE diesels most efficient at WOT?

[psfred]

Ed:

There is no throttle in a diesel -- output is regulated by varying the amount of fuel injected rather than the amount of air/fuel mixture admitted to the the engine.

A naturally aspirated diesel (non-turbocharged) has maximum air flow at fast idle, with flow dropping off as rpms increase due to pumping losses (restriction of flow due to having to go through valves, etc).

A turbo will have the greatest air flow at highest load and fuel delivery.

Maximum efficiency is usually a bit below full fuel delivery, at the point just before the black smoke appears. More power is available at the cost of less specific horsepower as more fuel is added past that point -- black smoke is unburned fuel, and that fuel, being unburned, didn't produce as much power as would have been available if burned.

Another note of interest -- all diesel engines have a governor -- this adds some extra wrinkles to fuel delivery and boost control.

Peter

[RunningTooHot]

Actually it’s the other way around. A gasoline engine approaches peak efficiency (from a brake specific output point of view) at WOT by defeating the pumping losses caused by the throttle – a condition that never exists in a diesel. Indeed, a diesel’s BSFC increases at very heavy loads, but part-load efficiency is outstanding as compared to a gasoline engine at part-load.

RTH

[whunter]

Thanks for the help finding this thread.