Right - but their efficiency is related to the temperature difference (specifically, to the ratio of the absolute temperatures of the hot & cold ends). So if you want useful power, the Sterling engine will need to be of reasonable size and weight (and thus cost).
If you want additional output (say, for faster battery charging), in most cases you’ll do better simplay to specify a slightly larger IC engine in your hybrid.
Like all heat engines, the stirling engine transfers heat from its “hot reservoir” (burning fuel) to its “cold reservoir” (the outside world.) If it’s equalising in temperature, that’s because it hasn’t got a big enough radiator, not an intrinsic problem of the engine itself. Trouble is, on a vehicle you may have to turn the entire skin of the thing into a cooling radiator to use a stirling engine of halfway respectable power. An article here (scroll down) mentions poor specific power (power-to-weight) and “high heat rejection requirements” as problems.
Regarding the OP - the maximum theoretical efficiency for a typical IC engine (a carnot cycle with the same operating temperatures) is around 60% IIRC. Real engines are down around 25% due to a less efficient cycle, heat losses to the metal of the engine, “pumping losses” etc. So there’s a fair bit of thermal energy available for use. The question is, how to extract it? most of the energy isn’t lost to the metal of the engine, but is sent out through the exhaust manifold. When the exhaust gases leave the engine they are still hot and have some pressure left in them, that’s why you can run a turbo off them. Exhaust manifold temperature is 430-540 deg C (800-1000 deg. F), a good hot temperature for a a stirling engine.
Ways to extract that energy include a turbocharger, there’s no reason why a turbo shouldn’t contribute to the engine output instead of running an intake compressor. There’s also the Atkinson cycle, where the power stroke of the engine is longer than the intake stroke, allowing the gases to expand and cool more before they exhaust. It’s used on the Prius. And in the old VW Beetle, exhaust gas heat exchangers were used to warm the car. Not too effectively, in my experience…
So yes, you could put a stirling engine on the exhaust manifold of an IC engine. But you’d be looking to at least double the size of the engine for a fairly small output increase, and you’d have to find a lot of radiator space!
An alternative way to think about it is to try to combine the benefits of both engines with a hybrid, and my link above states General Motors attempted to do just that. NASA developed a 60 kW (80 hp) stirling engine for automotive use, although they don’t say in the summary how big it was and how much it weighed! If you used a smaller stirling engine, say 20hp, running of the exhaust heat of a small gas engine (motorcycle engine?), say 40 hp, maybe you’d improve the power-to-weight and large radiator requirement… or maybe you’d get the worst of both worlds.
Thanks everyone. So it may not be an entirely stupid idea.
I was thinking of something of about 5 hp. I would think that would be enough to charge the batteries. But, then it may not be worth it. The more battery life you have, the less you would have to use the IC engine. But then, the less heat you would produce to run the stirling.
I didn’t think about the possible cooling problems of the stirling. But since it would be cooling the engine, I would think that the engines cooling components could be down sized a little bit.
All that wasted heat……. Seems there should be a way to do something with it.
That would be a perfect application of such an engine - the “business end” would get nice and warm while the other part would be at room temperature. Eco-dildos! We’re going to be rich, RICH I tell you!
Argent Towers was probably referring to the ‘steely dan’ dildo in William S. Burrough’s novel Naked Lunch which was the source of inspiration for the name of Fagan and Becker’s music group.
Back to the OP topic, I built a Stirling engine from a model kit during my freshman year studying engineering (2001). The warmth of a hand would power the engine a little bit but the axle turned faster when I used a cigarette lighter and an ice cube. Stirling engines will likely never be useful for automotive purposes, but they possibly could provide clean, efficient geothermal generation of electricity in the future. The heat source would be underground magma and the heat sink would be cool ocean water. I could see Stirling engine power plants being a boon to future energy needs after years of deep drilling advances and infrastructure changes. The geothermal electricity could then power electric vehicles (EVs).
Would Stirling engines be superior to steam systems? Various Googling produces links (including this Wiki article) that suggest steam is used for most medium- and large-scale geothermal power generation today.
There really is no reason to use a Stirling enginge for geothermal as geothermal is already a clean resource when it is run on steam. However, Stirling solar generators have been developed and ther are plans to build large central station arrays of them, although those projects are finding it difficult ot get the financing together.
Stirling engines have greater efficiency potential (i.e. greater output of energy given the same amount of heat input) than steam engines. However, it could be a decade or three, if ever, before Stirling power stations become a reality due to current deep drilling technology limits and the associated need for research and development.
Back in the early 1960’s two big firms (NV PHILIPS, the Netherlands), and the FORD MOTOR Co. (USA) jointly worked ona Sirling engine. FORD wanted it to power cars, PHILIPS wanted it for rural power systems. After 9 years and many milions, they came up with something that was ALMOST as good as a 35 HP IC engine.
Too expensive, too complex, and slow revving, the engine was not a success.
Stirling engines wouldn’t need different drilling tech as far as I know, and I didn’t intent to imply that. Greater drilling technology would make geothermal energy (steam or otherwise) viable for more regions on Earth, not just those with hot rocks within a couple miles below the surface, if we could cost effectively drill deeper.
Also, ground instability is sometimes a risk when pumping water at high pressure into deep bore holes to create the necessary steam for a steam power station. This may not be a problem (although I’m being hypothetical here) with a Stirling engine as it may not require the employment of high pressure water and steam in a geothermal application.
Honda Is looking at a small steam engine to recapture the heat energy from the engine to help charge the batteries on a hybrid.
I thought that a stirling engine would do the trick. I suppose I am wrong. Thermodynamics being what they are, I guess that the cool side of a stirling would not stay cool long enough (thought perhaps with highway speeds it could be air cooled).
Missed this the first time around. Yes, I know that the reference is to Burrough’s book, which is why I said that there was at least three of them. The one most discussed in the book is referred to as “Steely Dan number three” with one of the previous incarnations having been broken by a dyke who was over excited.
I can’t believe that I forgot to link to BMWs steam hybrid which predates Honda’s announcement by a few years.
That link broke for me Tuckerfan, though this one seems to get to it.
The Beamer seams to be a IC engine and steam. Directly applying tractable power to the wheels instead of storing it.
IC engines and electric, the ‘common’ hybrid as we know it seems to work pretty well. The suggestion that I make, and Honda seems to be looking at, is a system that would use engine heat to charge batteries on a long trip. Or at all times. Put that waste heat to work.