Fuel economy - Page 6 - PeachParts Mercedes-Benz Forum

jt20 · #76 08-02-2008, 10:55 AM

Quote:

Originally Posted by GREASY_BEAST

Forced, rather than guessing, can you prove this statement (hint: math)?

EDIT: Another hint: which has more energy? a hot gas at x velocity or a cold gas at x velocity? How are you measuring velocity? Where are you measuring velocity? Does velocity in this whole thing really mean what you think it means?

That is all the information you should require to figure this "puzzle" out scientifically, rather than just guessing. Or you could save yourself some time and mental strain and just read a bit about turbo machinery theory and the physics involved therein.

Perhaps a way you might expose your mind to this concept would be to think of heat giving a gas velocity? why might that happen?

you are missing a crucial element again. Referring to Cervan's laws: neither has more 'energy' per volume of gas. The heated one has more energy per molecular mass which takes up more volume which destroys your ability to test the two comparitively by volume.

A heated gas has less 'material' or mass and is therefore less dense. The resistance it provides is less than that of a cold gas. This means that cold gas per volume can most likely push a propeller with more strength than a heated gas... the other side is that per mass of the same gas, the heated one will have greater volume. Perhaps the balance b/w these two characteristics determines the design for a specified application of a turbo.

Turbos need heat to make gains or they are no more than a supercharger.

jt20 · #77 08-02-2008, 12:16 PM

Quote:

Originally Posted by dozer

That's OK Cervan......I can't even remember now what the original point was myself....

First, FI has been misquoting me all along....using words like "just", and "only", and "all".....when what I actually said was that the >majority< of the shaft-power produced by the turbine was from the extraction/conversion of the heat, not simply from pressure like a 'fan'. I never said "all" or "only".

I also didn't say that the heat was 'doing' the work. I said that the shaft-power was mainly derived from conversion of the heat-energy into mechanical work.

Second, to answer your question....yes, exactly, the turbo IS mostly powered by the heat-energy contained in the compressed gas.

The turbine-portion of a turbo is technically known as a "turboexpander"; and contrary to FI's assertion, the 150-200 degree drop in gas-temps across the turbo is mostly NOT caused just by heat radiating off the turbo. It IS caused mainly by the heat-energy stored in 1000F gas being usefully -extracted-, i.e. converted into mechanical shaft-power via the process of "expansion" in heat-engine terms.

As I posted earlier, nobody has to take my word for it....you can confirm all this yourself -easily- with just a simple 5-second web-search; or a 5-minute consultation of any turbomachinery handbook ever written.

And no, a brake is not a heat-engine. That's just plain silly. A brake is nothing but a dissipative load. -Exactly- like a resistor in electronics. Nobody would claim that a resistor is a generator.

"Heat Engine" is a specific term defining a machine which converts heat-energy to mechanical work; or converts mechanical-work to cooling.

The extraction of mechanical energy from a gas -requires- expansion. Expansion is how you get work out of any engine.

Thus, any heat-engine -requires- a compression phase -prior- to adding the heat-energy that we want to turn into mechanical work later in the expander.

Here we go.

As the HOT exhaust leaves the high pressure area and enters low pressure - it is given the space it needs to release potential energy. By expanding it cools down. It is the place whrere this expansion ocurrs that allows a turbo to harness the otherwise wasted engergy (heat energy) in exhaust.

by expanding the gasses across impellers of different ratios and vector fields (quote FI here), the basic fans (impellers) capitalize on TWO DIFFERENT VOLUMES of A gas body at different temperatures as it (gas body) changes across them (simple fans)

turbos need heat (or an available means of pressure differential)
turbos need simple fans

jt20 · #78 08-02-2008, 01:04 PM

Quote:

Originally Posted by dozer

I'd think that a change of turbo settings or geometry would improve your MPG then.

If your boost is zero at cruise, then you're not using the 'free' exhaust energy to overcome the less-than-100% VE, correct?

In another post, someone mentioned running around 5psi at cruise. That sounds like about the optimum point for best economy.

So I've always been interested in setting things up to get boost at very low power-settings. Yet I need a turbo system that works well at near full power as well; because every 10 curves there's another mountain to climb...

in an effort to finish this..

It is safe to assume that a turbo-ed engine is more efficient than a non-turbo-ed.

But it is only 'kinda free' exhaust energy

It sounds like Forced pays really close attention what he is doing, although some serious mathematical analysis may help further maximize his turbo settings.. his mileage is pretty impressive already.

dozer · #79 08-02-2008, 01:25 PM

Cervan, if the turbo (either primary or secondary) is of the proper size and geometry, it extracts far more HP from the exhaust-heat than the pumping-work it adds to the piston-engine.

FI, thank you!! Yes, that was the point that got it all started.

You claimed that having the turbo doing zero pumping-work at cruise-power (20hp as I recall) was more efficient.

I pointed out that even at partial-output like 20hp, the piston-engine would run more efficiently if the turbo (using wasted heat-energy) was set up to do the work of overcoming intake-resistance; rather than using the pistons (whose work comes from burning additonal fuel).

JT, I think you summed it up well....

turbos need heat.....and obviously, turbos need a 'fan'...

jt20 · #80 08-02-2008, 02:12 PM

I said "turbos need heat" OR another available source of PRESSURE DIFFERENTIAL.

...and I just realized everything I have said was already stated by FI. - sorry

dozer · #81 08-02-2008, 02:23 PM

JT, I didn't see an "or".

ps; I'd be interested in hearing your thoughts on the original question/point:

Is a TD running at partial-load (say, 20-30hp) more efficient if the turbo isn't providing any pressure; or if the turbo is set up to provide enough boost (say, 3psi) to overcome intake-resistance losses?

thanks

jt20 · #82 08-02-2008, 02:27 PM

Quote:

Originally Posted by dozer

"since the output gas from the turbine is much cooler, where did all that heat-energy go ??"

Or to put it another way, if it isn't mainly heat-energy that's being extracted as shaft-power, then why is X grams of gas per second suddenly 200 degrees cooler just 2" further along at the outlet of the turbine? Where else did that heat GO?....if not to shaft-power?

Obviously, the answer is that the shaft-power DID come from that heat-energy.

A turbo-expander IS a heat-engine.

NO. The gas cooled by expanding. As a by-product, it pushed on the vanes as it left. The work the heat did was carry potential energy, the pressure did the work. Heat=Potential Pressure

Ok, I promise not to return to this thread.

Mike, I hope you get better gas mileage

jt20 · #83 08-02-2008, 02:29 PM

Quote:

Originally Posted by jt20

Here we go.

As the HOT exhaust leaves the high pressure area and enters low pressure - it is given the space it needs to release potential energy. By expanding it cools down. It is the place whrere this expansion ocurrs that allows a turbo to harness the otherwise wasted engergy (heat energy) in exhaust.

by expanding the gasses across impellers of different ratios and vector fields (quote FI here), the basic fans (impellers) capitalize on TWO DIFFERENT VOLUMES of A gas body at different temperatures as it (gas body) changes across them (simple fans)

turbos need heat (or an available means of pressure differential)
turbos need simple fans

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

jt20 · #84 08-02-2008, 02:31 PM

Quote:

Originally Posted by dozer

JT, I didn't see an "or".

ps; I'd be interested in hearing your thoughts on the original question/point:

Is a TD running at partial-load (say, 20-30hp) more efficient if the turbo isn't providing any pressure; or if the turbo is set up to provide enough boost (say, 3psi) to overcome intake-resistance losses?

thanks

to be perfectly honest, I do not have enough information on that. I was trying to learn more about turbos. Thank you for the excellent dialogue.

My belief is that a turbo-ed engine is more efficient than a n/a. pretty lame.

jt20 · #85 08-02-2008, 02:37 PM

Quote:

Originally Posted by dozer

JT, I didn't see an "or".

ps; I'd be interested in hearing your thoughts on the original question/point:

Is a TD running at partial-load (say, 20-30hp) more efficient if the turbo isn't providing any pressure; or if the turbo is set up to provide enough boost (say, 3psi) to overcome intake-resistance losses?

thanks

I dont think the backpressure that a turbo imposes is ever a loss at the crank.
partial load boost can't be bad

#86 08-02-2008, 02:40 PM

Quote:

Originally Posted by jt20

I don't think the backpressure that a turbo imposes is ever a loss at the crank.
partial load boost can't be bad

It is. Thats how exhaust brakes work, backpressure.

dozer · #87 08-02-2008, 02:50 PM

Quote:

Originally Posted by jt20

to be perfectly honest, I do not have enough information on that. I was trying to learn more about turbos. Thank you for the excellent dialogue.

My belief is that a turbo-ed engine is more efficient than a n/a. pretty lame.

My thanks back your way JT...appreciate your posts and input.

jt20 · #88 08-02-2008, 03:05 PM

Quote:

Originally Posted by ForcedInduction

It is. Thats how exhaust brakes work, backpressure.

I don't know much about exhaust brakes, clarify. (use of expanding gases, or backpressure to crank?)

it doesn't make sense that a turbo could be beneficial to an engine if it didn't make use of more than one form of energy at all times.

you said it yourself. The expansion of the gases is the important part of a turbo's ability to compress air. As long as you put some exhaust into that turbo, you gain from the natural tendency of that gas to expand in your turbo and, in turn, compress air on the incoming side.

How could the push of the piston forcing hot air into another chamber that now captures an additional form of energy return a loss at the crank that would have completed its revolution w/o that captured energy?

jt20 · #89 08-02-2008, 04:10 PM

Quote:

Originally Posted by ForcedInduction

It is. Thats how exhaust brakes work, backpressure.

Back pressure on crank = belt driven supercharger on crank.

Compressing the intake air from energy stolen at the crank is even worth the gains from a simple supercharger.

A turbo does this job AND harnesses expanding exhaust gasses across the turbine shaft through aerodynamic propagation.

#90 08-02-2008, 04:23 PM

An exhaust brake works by causing backpressure in the exhaust manifold. That makes the pistons work harder to push the exhaust out of the cylinders.

A turbo or supercharger does not add any power to the engine. The additional power comes from more fuel being burned per cycle because the turbo/SC provides extra air.

Thats why a turbo added to a 240D or non-turbo 300D with an otherwise unmodified engine does not add any power. Its only after the injection pump settings are changed that it can get a change in output. The same applies with making the turbo produce more boost than the engine receives fuel for, it adds restriction and actually reduces power.