# Car Exhasut heat to electricity

ramonegumpert
Hi Experts,

May I know what is stopping cars from converting exhaust heat to electricity?

Eg. using :

1. Thermo-electric generator TEG.
2. steam turbine

Just curious to know why this energy is not tapped.

Thanks.

regards
Ramone

Gold Member
Whenever energy is converted from one form to a different form some energy is lost. The efficiency of a power conversion system is found by dividing the power out by the power in. That fraction then gets multiplied by .01 to express the efficiency in per cent.

When heated exhaust is passed through a thermo-electric generator the output electrical power will be less than input heat power. That is governed by the efficiency of the TEG. You then ask if it would be practical and cost-effective. The same exercise with a steam turbine/generator will probably result is a very low yield of electrical power. My guess is that neither system is very efficient and only small amounts of power could be recovered from exhaust heat. But if you can find some way to improve the efficiency of the mechanisms, then prove my guess wrong!

There are systems that use the car exhaust to preheat the engine’s air/fuel mixture that result in increased engine output power.

2ndofafew
lets go back to basics here what is the purpose of the car exhaust its to remove carbon monoxide and excess heat from the engine if you mounted any sort of turbine or heat absorbent plating your increasing the amount of resistance the exhaust has which could do 1 of 2 things stall out your engine or blow it up

Homework Helper
Perhaps a thermal-couple could be embedded into the exhaust system, but I don't know how much energy it would produce.

2ndofafew
not sure how much heat is actually absorbed by the exhaust pipe due carbon(soot or blackish dust) coating the inside walls of the pipe.

mushinskull
lets go back to basics here what is the purpose of the car exhaust its to remove carbon monoxide and excess heat from the engine if you mounted any sort of turbine or heat absorbent plating your increasing the amount of resistance the exhaust has which could do 1 of 2 things stall out your engine or blow it up

or use the the energy derived from the turbine and use it to drive a compressor to increase the amount of air entering the engine, like every turbocharger out there is designed to do.

2ndofafew
you still have the issue of carbon buildup in the engine choking it

You also build up extra heat might be fun to watch or maby run for about 2 minutes but after that kapoot

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Mentor
lets go back to basics here what is the purpose of the car exhaust its to remove carbon monoxide and excess heat from the engine if you mounted any sort of turbine or heat absorbent plating your increasing the amount of resistance the exhaust has which could do 1 of 2 things stall out your engine or blow it up
You missed the point: the OP isn't talking about a turbine in the exhaust, but rather a heat exchanger wrapped around the exhaust, which is then used to boil water to run a turbine.

Homework Helper
While money is an issue, the amount of electricity that may be obtained may not be a lot. However I am sure some cars use the waste heat from the engine to run the air conditioning system of the car.

mrspeedybob

While money is an issue, the amount of electricity that may be obtained may not be a lot. However I am sure some cars use the waste heat from the engine to run the air conditioning system of the car.

Not so much air conditioning but the old VW bugs used waste engine heat to warm the passenger compartment. It was a good idea, poorly implemented.

Mentor
?? Don't all cars use waste engine heat to heat the passenger compartment?

mrspeedybob
?? Don't all cars use waste engine heat to heat the passenger compartment?

Pretty much. I should have been more clear. Most cars use heat from the engine coolant which absorbs it from the block and heads. VW used an air cooled engine so there was no coolant. They blew hot air directly from the engine compartment into the passenger compartment.

As a side note, a few cars use electric heat so that you can start warming the cabin before the engine warms up. Most of the time this is a secondary system that is superseded by an engine heat system once the engine is warm. On a Volt it is the primary system because the engine only ever runs on long trips.

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Lsos
lets go back to basics here what is the purpose of the car exhaust its to remove carbon monoxide and excess heat from the engine if you mounted any sort of turbine or heat absorbent plating your increasing the amount of resistance the exhaust has which could do 1 of 2 things stall out your engine or blow it up

Almost every exhaust system and muffler increases the resistance to the exhaust, and yet somehow doesn't cause catastrophic engine failure. The third thing that can happen (and which actually DOES happen in countless examples) is that you will lose a little bit of power to covercome this resistance. Depending on how much and what you gain out of it, it might or might not be worth it.

2ndofafew
Almost every exhaust system and muffler increases the resistance to the exhaust, and yet somehow doesn't cause catastrophic engine failure. The third thing that can happen (and which actually DOES happen in countless examples) is that you will lose a little bit of power to covercome this resistance. Depending on how much and what you gain out of it, it might or might not be worth it.

Yes those systems add a small amount of resistance but a turbine will add a whole lot of resistance your even increasing the amount of pressure on the inside of the system There is no way to over come this without using more energy then your creating and when you get pressure you get heat so catastrophic failure is possible if not probable

Also as i said earlyer excess carbon coating the exhaust would reduce an already small amount if power generation

But i do agree that the reason this is not done is its low returns for investment

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Mentor
Again, you're missing the point and it makes you wrong: adding a heat exchanger around an exhaust pipe would decrease resistance by cooling the exhaust.

xxChrisxx
Electric turbo componding does exactly this, it uses a velocity turbine to extract electrical energy. As it's a blowdown turbine, it doesn't restrict exhaust flow as much as a pressure turbine.

The reason why it isn't used is tha it's easier and cheaper to use a pressure (conventional) turbo, and downsize the engine.

ramonegumpert
Again, you're missing the point and it makes you wrong: adding a heat exchanger around an exhaust pipe would decrease resistance by cooling the exhaust.

Dear Russ,

May I know which resistance are you referring to ?
Is it the resistance between the 2 peltier plates?

If the cool plate is maintained at say 100 degrees celsius and the hot side is attached to the exhaust that is at least a few hundred degrees celsius, would electricity flow since the temperature difference is a few hundred degrees?

Thanks!

Gold Member
Hi Experts,

May I know what is stopping cars from converting exhaust heat to electricity?

Eg. using :

1. Thermo-electric generator TEG.
2. steam turbine

Just curious to know why this energy is not tapped.

Thanks.

regards
Ramone

TEG: http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2006/session6/2006_deer_fairbanks.pdf

Steam Turbine: http://en.wikipedia.org/wiki/Turbosteamer

I hate BMW, in an envious kind of way. I thought of both of these ideas, months before BMW announced they were working on them, and half a decade after they claimed they had started research.....

When I start my car company, it's initials will be CNX. (In homage to HAL, the mythical computer.)

Dr Lots-o'watts
They can tap a roughly 600 degree temperature gradient in an exhaust system, that can result in a roughly 3-4% fuel economy (with a proper TEG).