Exhaust Heat Recovery: Simulation of a Rankine Cycle System

[Smokeduv]

Originally Posted by markweatherill

I get it now! So why not a turbocharger connected to a generator?

That's almost the thing they are running now on Formula 1. They are running turbos, but only for obtaining a similar power output as the older engines, but the interesting thing is in the exhaust turbine, where they are not boost limited, but instead of a wastegate they run it into some kind of electric motor/generator (called ERS-H) to recharge the batteries so they can spool quickly, eliminate turbo lag and adding 160 hp when combined with the normal engine and the ERS-K (which takes energy from braking).

[oil pan 4]

Originally Posted by markweatherill

I get it now! So why not a turbocharger connected to a generator?

Because the alternator belt system in use now is about 98% efficient.
A turbine is no where near that efficient.

[serialk11r]

A turbocharger does NOT act like a heat engine on a car. It will capture some of the excess energy that is the exhaust gases being at higher than atmospheric pressure when the exhaust valve open. This is useful for race cars that are run at WOT most of the time (I read that it boosts efficiency by 7% at WOT) but not for street cars which have very little excess pressure running at 30% load.

Look up BMW Turbosteamer. They were able to generate a consistent few kW from the exhaust with a Rankine cycle generator, 10kW peak. Not a DIY project though.

I imagine in the future as high temperature thermoelectric tech advances we'll be able to get most of the benefit in a simple bolt on package. Thermoelectric generators that are half as efficient as the BMW steam turbine cycle could completely alleviate all accessory loads on the engine and keep a hybrid system's battery topped up.

[oil pan 4]

Diesel engines run WOT all the time.

[serialk11r]

Originally Posted by oil pan 4

Diesel engines run WOT all the time.

Touche

But this is more about excess pressure at blowdown, which for a diesel still corresponds to load. At lower loads a turbo-generator will not be efficient at all on a diesel engine and the restriction it introduces could negate a lot of the benefit. That said the setup that they use in Formula 1 where the turbine is very free-flowing and uses the electric motor to augment boost would probably work very well on a diesel street car, albeit very expensive.

[maxc]

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