Can we use the heat to boil water to turn a turbine? Or would the water never cool enough to be used again?
What about using the heat to create pressure and then releasing the pressure to help move the fly wheel?
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Because of size issues.
You need the boiler, the turbines, the dynamo. And you need a constant influx of water. And a stock of water to evaporate.
For all it takes, you’d consume more energy to make it work than you’ll get.
I mean, we *do* use heat from combustion to make cars work in the first place, but around 2/3rds of it is lost either as heat in the exhaust or heat lost through the engine block.
For a modern, liquid-cooled engine, you *could* hypothetically recover heat from the coolant, but that would either require you to replace the radiator with another heat exchanger to put the heat into something with a lower boiling point, *or* allow the engine to boil the coolant (which will almost certainly cause thermal stresses on the engine block). Then that boiled vapor can be expanded through a small turbine for power.
As for the exhaust heat; there is the potential to extract that exhaust heat, but it’s somewhat tricky to do because the exhaust gas is already expanding back to ambient pressure, causing the temperature to fall (which causes you to lose efficiency on any kind of work extraction as a result of Carnot’s Law). More to the point; we already use turbines to attempt it; they’re called turbochargers. Alternatively, you could use variations of the same low-boiling point energy recovery (e.g. Organic Rankine Cycle tech) on the exhaust.
The other problem is actually emissions; a lot of the technology that cleans your car’s exhaust is dependent on heat as an energy input to drive the process. If you extract more energy as work, you necessarily lose heat in the exhaust, which makes exhaust catalysis much more difficult, thus the car’s emissions will likely get worse.
Point being; all of this requires lots of extra equipment, weight, and complexity added to the vehicle. So yeah, you can do it…but it makes absolutely no economic or thermodynamic sense.
To some degree turbocharging does this by recovering the waste heat from the exhaust gas to drive a compressor. The result is a higher intake pressure which increases engine efficiency at the cost of increased weight and complexity. Before the adoption of jet engines the last generation of large radial aircraft engines used a technique called turbocompounding, were in the turbo not only drove the compressor, but the crankshaft as well. These engines powerful and efficient, but very complex and a maintenance nightmare.
Mercedes M278 4.0 Liter Twin Turbos consume way less fuel and generates way more power than the older M273 engine 5.5 liter normally aspirated V8. As someone else pointed out turbocharging does use hot exhaust energy to improve efficiency though without generating electricity. Direct Inject plays a large part in the improved efficiency along with turbocharging and automatic transmission improvements.
At the extreme end of performance vehicles, they do capture exhaust heat and store it as electrical energy to power hybrid electric motors. See https://en.m.wikipedia.org/wiki/Porsche_919_Hybrid
I imagine this is too expensive to be used in standard production vehicle.
It’s possible. Formula 1 cars use a component called the MGU-H to turn exhaust heat into electrical energy that can be deployed through the hybrid motor. The reason it hasn’t been adopted by scale is that it’s extremely expensive to do.
>What about using the heat to create pressure and then releasing the pressure to help move the fly wheel?
You are describing how a steam turbine works.
>Can we use the heat to boil water to turn a turbine?
BMW actually did this, called the Turbosteamer. https://en.m.wikipedia.org/wiki/Turbosteamer
The technology started development around 2000 but it never made it into production. It’s a lot of cost, weight, and complexity for what it does. Their version attached a steam turbine to the transmission/driveshaft, the steam generated from waste heat from the engine would spin the turbine so the engine doesnt have to work as hard to move the car.
Another reason this isnt done is that steam must be under pressure in order to spin a turbine. Getting in an accident could potentially damage parts of the system that are under pressure, causing them to leak scalding hot steam or explode. If every car has a high pressure steam tank, people are going to accidently hurt themselves with them.
Using a few batteries and an electric motor accomplishes the exact same thing but is way cheaper, more compact, more efficient, and way safer than using a steam turbine.
F1 cars do harness the heat energy and turn it into electricity. I read that an F1 power unit is about 50% efficient compared to a normal road car which is about 30%.
What you’ve described is a type of system used in power generation called combined cycle. They use a gas turbine to spin a generator, then they take the exhaust from the turbine and use it to generate steam. The steam drives a turbine, which is also connected to the generator. The steam then goes to condensers, and returns to the steam generator still very hot, but as liquid. These systems run under high pressures to increase efficiency.
We don’t do this in cars because the systems required to support this cycle are very large. As you scale them down, the their efficiency drops. There isn’t any way around that efficiency drop, because it’s down to some fundamental geometry and physics.
For example, the volume of a cube is x^(3) where x is the length of one side, but the surface area is 6x^(2). This means that smaller systems will have an unfavorable ratio of surface area — through which heat is lost — to volume — through which heat is retained and transported.
This may not be exactly what you meant, but cars _do_ use the heat from the engine to be more efficient. Namely, when you turn on the heater in your car, it’s using heat extracted from the engine (via the coolant) to heat your car “for free” rather than requiring any alternative heat source.
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