Fuel economy - Page 4 - PeachParts Mercedes-Benz Forum

babymog · #46 07-29-2008, 08:27 PM

What about a cabin-adjustable wastegate? You could dump exhaust through the wastegate for low-power cruising if you wanted to decrease the boost.

Quote:

Originally Posted by dozer

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 lieu of the $$$ VGT's that I can't afford, I've often wondered about experimenting with a tandem turbo setup. Something tiny from a sub-2L, in tandem with either a stock 617 turbo, or even a slightly larger one (to keep backpressure down at the upper-end of the power/rpm range).

My first guess is to use backpressure to control the gates that'd switch between the two turbos...but I've really not thought it all out in detail yet.

So many experiments to try, so little time....

tomnik · #47 07-30-2008, 12:42 AM

just vent the boost line.
Btw. ALDA enrichment should be the the negative effect on economy while cruising in my opinion.
A switch to operate an electro valve venting the boost line could help.

Tom

#48 07-30-2008, 12:49 AM

The ALDA enrichment won't make a difference, your right foot ultimately controls the fuel rate while cruising. If you vent it you'll just have to give it more throttle to maintain speed and it will still consume the same fuel rate.

dozer · #49 07-30-2008, 12:56 AM

Quote:

Originally Posted by cjlipps

Very interesting thread.

FI, you have loads more experience with turbochargers than I do and I'm chiming in with the utmost respect for said experience but I have to throw in with dozer here.

The expansion valve analogy isn't appropriate because the refrigeration in that case comes primarily from the phase change.
....

....If the exhaust gas is cooler at the turbo outlet than the inlet, where did the energy go?

This is a reason that exhaust driven superchargers are more efficient than engine driven superchargers. You are using leftover heat energy from the combustion process to compress the inlet charge, instead of new mechanical energy (requiring more fuel consumption) from the crank.

If it was purely a gas pressure-driven scenario, the pumping losses through the exhaust side of the turbo would not offset the gain from the increased inlet airflow.

Keep the dialogue going. I'm learning from everyone here.

thanks CJ....those are all good, correct, and nicely put, points.

(except that one -can- refrigerate/chill without a phase-change or a liquid-to-gas expansion valve...by simply expanding gas itself. In this case, the 'valve' is a 'throttle', and the process is called 'throttling', and it's not so efficient.)

I wish I had thought of that one simple question that you asked, since it hits the nail on the head so well....

"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. Or, to perhaps put it more precisely, it's the expansion-half of a Brayton heat-engine cycle.

In the case of our TD's, the piston-engine itself is acting as the compressor and 'burner' portions of this 'outer loop' Brayton-cycle heat engine.

The turbo's turbine is the expander half of this
'outer loop' heat-engine cycle; and the turbine's shaft, where it drives the turbo-compressor, is the mechanical-power extraction point.

But even if the output air from the turbo's compressor wasn't even being fed to the piston-engine, but was used externally to, say, blow up tires

; it would STILL be a Brayton cycle heat engine.

(The mechanical-work extraction point would now be the compressed-air output of the turbo-compressor)

In fact, the compressor half of the turbo isn't even -necessary- for this heat-engine/cycle to be complete. ANY load on the turbine-shaft will do.

You could instead hook an alternator to the turbine-shaft and charge batteries with it.....and this combination of piston-engine compressor/burner and turbo-turbine expander would STILL be a complete Brayton cycle heat-engine.

It is further illuminating to recognize that if you -don't- put a mechanical-load on that turbine-shaft, it will NOT cool the gases!

Which makes totally intuitive sense if you're properly viewing a turbo-turbine as a heat-engine expander instead of a simple fan.

It's clear that, without a load, you're not extracting any energy, right?

Right....and exactly as you'd expect from heat-engine and turboexpander theory, the usual 200F temp-drop simply -disappears- when you disconnect the shaft-load !

(of course, a very short time later, your turbine hits 400krpm and goes into orbit...

)

hey Babymog, glad to hear from a fellow Cat-man!

I have to correct that steam-turbine thing tho....

The expansion-ratio between water and steam isn't relevant to the operation of the steam-turbine itself. The large volume-difference between water and steam that you mentioned is true, but it does not take place within the turbine. The turbine never sees liquid.

Rather, as CJ correctly noted, a steam-turbine works by converting the HEAT energy content of the steam to mechanical shaft-power; which of course has the effect of -removing- that heat and thereby chilling the steam.

If this wasn't true, then as CJ also noted, why would we need to burn so much coal to heat the damn stuff up in the first place?

The steam to the inlet of a turbine is -superheated- to add energy to it; well above the temp where water first evaporates. Inlet steam temps are rarely less than 500F...and up to 1000F are typical.

Water is used not because of the volume-ratio between its liquid and vapor; but simply because it's the cheapest and safest of the various HEAT-energy-carrier gases available to us (e.g., butane, ammonia, freon, sulfur dioxide, etc. etc).

In any case, FI still doesn't believe me, and nobody can say I haven't given it my best shot, right?

....so I'll admit defeat and give up now, while we're all still friends....

FI: a pleasure debating with you....and I still urge you to get yourself a good turbomachinery handbook and look up turboexpanders and brayton cycles, just for your own satisfaction.

#50 07-30-2008, 01:15 AM

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. Or, to perhaps put it more precisely, it's the expansion-half of a Brayton heat-engine cycle.

The above is false information, a turbo is NOT a heat engine. Where did the heat go? Two places, some is dissipated as the gas expands and some is absorbed into the turbine and housing as it contacts their surfaces.

Quote:

It is further illuminating to recognize that if you -don't- put a mechanical-load on that turbine-shaft, it will NOT cool the gases!

This is also incorrect. If it were true then the exhaust gasses would stay hot well after it exits the exhaust pipe.

Quote:

Which makes totally intuitive sense if you're properly viewing a turbo-turbine as a heat-engine expander instead of a simple fan.

However, it IS a simple fan.

Quote:

It's clear that, without a load, you're not extracting any energy, right?

No, aerodynamic energy is being used to spin the turbine even if its not producing work (boost). Take the inlet off your turbo and start the engine, notice its spinning even with no significant heat in the exhaust stream.

Quote:

Right....and exactly as you'd expect from heat-engine and turboexpander theory, the usual 200F temp-drop simply -disappears- when you disconnect the shaft-load !

Incorrect. There is still a temperature drop, although it is reduced because the velocity of the gasses is not changing as dramatically.

Quote:

The expansion-ratio between water and steam isn't relevant to the operation of the steam-turbine itself. The large volume-difference between water and steam that you mentioned is true, but it does not take place within the turbine. The turbine never sees liquid.

Thats because steam is a vapor. If it sees liquid then the system is not operating efficiently.

Quote:

Rather, as CJ correctly noted, a steam-turbine works by converting the HEAT energy content of the steam to mechanical shaft-power; which of course has the effect of -removing- that heat and thereby chilling the steam.

Incorrect again. The steam is just an efficient large volume working medium moving from a high pressure environment to a low pressure environment.

Quote:

If this wasn't true, then as CJ also noted, why would we need to burn so much coal to heat the damn stuff up in the first place?

Because it takes lots of energy to raise the water temperature.

Quote:

The steam to the inlet of a turbine is always -superheated-, well above the temp where water first evaporates. Inlet steam temps are rarely less than 500F...and up to 1000F are typical.

That is to keep the steam from condensing and maintain it at its maximum expansion ratio. Freshly boiled steam is still wet, it has liquid vapor suspended in it (This is the white steam people see). A superheater boils any remaining liquid.

Quote:

Water is used not because of the volume-ratio between its liquid and vapor; but simply because it's the cheapest and safest of the various HEAT-energy-carrier gases available to us (e.g., butane, ammonia, freon, sulfur dioxide, etc. etc).

Incorrect again. See above.

I'm sorry to repeat it again but, I'm sorry, you study up a little bit more on the fundamentals of how these things work.

dozer · #51 07-30-2008, 01:27 AM

ps; I'm so used to my old textbooks that sometimes I forget there's google! ...lol...

Which I just tried...and found that anyone who wants to be sure of their facts can do so in less than 5 seconds.

A google of "centrifugal turbine expander heat engine", and variations on those terms, instantly provided the following facts; among thousands of others.....

(underlines are mine)

Brayton cycle: Definition from Answers.com
1791 - John Barber received the first patent for a heat engine in which a bellows (compressor) and a turbine (expander) were connected...
www.answers.com/topic/brayton-cycle

Heat engine - Patent 6336316
6134876, Gas turbine engine with exhaust expander and compressor, Hines et al. ..... 4 comprises, as an example, a centrifugal turbine T 32 ,

(turbo-turbines are heat-engine turboexpanders)

"Elliott Company • Products • Steam Turbines/Expanders
The TH power recovery expander turbine is available to utilize the energy in high temperature, low pressure gas streams to drive..."

http://en.wikipedia.org/wiki/Steam_turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work.

(it's the heat, not the pressure, producing the majority of the shaft-power)

etc. etc..

#52 07-30-2008, 01:47 AM

Quote:

Originally Posted by dozer

it's the heat, not the pressure, producing the power.

Incorrect, again. A turbine is a simple fan.

cjlipps · #53 07-30-2008, 10:37 AM

Dozer, it's clear that we've each brought a knife to a gunfight here.

I'm just chagrined that much of what I learned in my mechanical engineering tech bachelors degree and nearly 20 years at an anhydrous ammonia manufacturing facility (where we generated and used about 800,000 #/hr of steam per unit) is totally false and incorrect.

Guess I'll go look for a new book of steam tables because I now know that the old one (that's been in use for many years) is wrong.

dozer · #54 07-31-2008, 01:55 PM

Quote:

Originally Posted by cjlipps

Dozer, it's clear that we've each brought a knife to a gunfight here.

I'm just chagrined that much of what I learned in my mechanical engineering tech bachelors degree and nearly 20 years at an anhydrous ammonia manufacturing facility (where we generated and used about 800,000 #/hr of steam per unit) is totally false and incorrect.

Guess I'll go look for a new book of steam tables because I now know that the old one (that's been in use for many years) is wrong.

Apparently so CJ....and dinky little knives at that...

An ammonia-guy huh? A rare-breed these days! I'll call you brother; as I cut my heat-engine teeth on ammonia-systems in ice-cream plants in the 60's; working with my dad, who was a refrigeration engineer.

I look forward to joining you in future threads on fuel-economy and other ways to usefully extract the surplus heat-energy via the turboexpander.

#55 07-31-2008, 05:19 PM

Its just a fan. Get over it.

jt20 · #56 07-31-2008, 10:10 PM

wow. an incredible volley.

Isn't it fair to say that you could never make any major positive gains without the heat that dozer alludes to?

although, 'heat' is not what spins the turbo. Turbos are efficient b/c they make gains off otherwise wasted energy: The heat from combustion is going to happen regardless, it is going to get dumped, regardless. After combustion there is still more potential energy in that exhaust gas than before it went through the turbo. That potential is now harnessed by the turbo shaft to pressurize the next batch of fresh air. What was waste is now more potential energy in the cylinder in the form of compressed, combustible gas. The turbo makes a profit off of its very own work - compounding interest.

true, the fans suck more air in via the exhaust stroke on the crank, but that is used, additional energy. Almost the the same as a supercharger. If the a turbo is no more than a supercharger, why do they consistently produce higher gains?

And it isn't about the engine being hot. There's an explosion happening in each cylinder, and that air is hotter than before, independant of the chamber it is in.

not that I have extensive experience with what I'm talking about though. Just looking for clarity.

#57 07-31-2008, 11:18 PM

Dozer's argument is that a turbo is just a heat engine and that the heat is all that drives it. What really happens is the physical push of the high velocity gasses is what drives it, heat only increases the efficiency by making the gasses expand to a larger volume.

jt20 · #58 07-31-2008, 11:32 PM

kinda seemed like he just wsn't saying it right. But what was almost said is a crucial part of the turbo-compression process.
??

Cervan · #59 08-01-2008, 12:26 AM

Quote:

Originally Posted by ForcedInduction

Dozer's argument is that a turbo is just a heat engine and that the heat is all that drives it. What really happens is the physical push of the high velocity gasses is what drives it, heat only increases the efficiency by making the gasses expand to a larger volume.

The ideal gas laws

This law has the following important consequences:

If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.

#60 08-01-2008, 01:07 AM

There you go.