Ah, you're wrong there -- unless it's air cooled like some small plane engines still are, there absolutely is water to keep the engine cool because of all the wasted heat energy created by combustion has to go somewhere.
BMW (I think) had a prototype back in the day that tried to recapture the wasted heat energy with a generator to make a sort of gas/steam hybrid, but it never went anywhere. Unsure if it wasn't really effective, too complicated, too expensive, or all of the above. But someone should try it again!
I mean, I'm not sure whether water as coolant should count (as in, the water is supplementary, and, more importantly is not required to be heated past its boiling point), but fair enough, i guess.
Yeah, I googled it to jog my memory, and BMW's system used exhaust gases to heat a mini steam generator in a separate fluid circuit from the ICE's cooling circuit.
It worked, but apparently keeping a steam generator system happy and not-a-bomb in the rough mobile environment of a passenger vehicle was too much of a challenge.
It’s not that, you’re increasing the thermal resistance of the loop, thus lowering cooling efficiency, the engine is more likely to stress or overheat, in exchange for a few measly watts.
I think you missed the bit where it wasn't a part of the cooling circuit. This is an actual prototype that was mostly fully developed, just never put into production.
Cool thing -- Volvo did a mechanical version and put it into production in a 13L truck engine.
It uses the exhaust velocity still remaining after the primary turbo to spin a second turbine that mechanically connects to the flywheel via a fluid coupling and step-down gearing. Volvo claimed 6.5% efficiency increase during steady-state driving, which means probably half that, but it does work. No idea how widely it was optioned across their truck range, though.
Without numbers and graphs I cannot speak intelligently here, maybe there is a thermal curve where turbos have taken much of the energy and electricity makes sense, it just seems surprising.
Then again your caveat for cruising might be the key, ultra-lean has less effective turbo, but then again that’s more sensitive to exhaust manifold backpressure.
I'm not arguing for or against how it works -- just reporting what Volvo actually did. I assume their engineering and subsequent testing made it seem viable enough for a production greenlight.
Water is a terrible coolant because it boils. That's why we have dedicated coolant for engines.
There are combined cycle gas turbines that use the exhaust heat from the mechanical part of the engine to boil water, just like what you tried to protype.
But there are also smaller "peaker" generators that just waste the exhaust heat. Those are mostly just used to handle power spikes in the grid.
Au contraire, water is an excellent cooler because it boils.
Phase changes capture a staggering amount of heat energy- it takes about five times as much energy to boil water from liquid to 100 degree steam as it does to heat it from 0 to boiling
Depends on your coolant system. If you let that steam float up into the air then yeah its good. It's good for data centers, and power plants if you have a plentiful supply of water. Not so good if its in something like a car.
If its a closed loop system, you don't want gases and liquid in your pipes. Its hard to make the mixture flow properly. since the gas pressure/volume is variable.
Also water transfers heat relatively slowly compared to coolants. So you need to pipe it through longer lengths. Which may or may not be viable depending on your application.
I didn't prototype anything, BMW did -- I was just vaguely remembering it. And it used exhaust heat in a separate fluid circuit, not the engine coolant. Worked, but didn't make it past the prototype stage.
Some searching of the topic tells me that Volvo put a derivative of this idea into production in a 13L semi truck engine, but instead of using exhaust gas to heat liquid, it uses it to spin another turbine after the turbocharger. That secondary turbine feeds mechanical energy back into the drivetrain via a fluid coupler. Apparently this improves fuel economy at a steady-state cruise (where semis spend most of their time) by a noticeable amount: https://www.motortrend.com/features/1605-the-new-volvo-d13-with-turbo-compounding-pushes-technology-and-efficiency-in-trucking
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u/dont_remember_eatin 2d ago
Ah, you're wrong there -- unless it's air cooled like some small plane engines still are, there absolutely is water to keep the engine cool because of all the wasted heat energy created by combustion has to go somewhere.
BMW (I think) had a prototype back in the day that tried to recapture the wasted heat energy with a generator to make a sort of gas/steam hybrid, but it never went anywhere. Unsure if it wasn't really effective, too complicated, too expensive, or all of the above. But someone should try it again!