This is a new twist to an old idea. The basic concept goes like this: To extract work from heat, the heat must flow from a hotter area to a cooler area. All engines produce some waste heat that flows out at the end. With a normal ICE this waste heat goes out the tail pipe and the radiator. If you take that waste heat and use it to drive another engine, you can extract more work, essentially for free. The problem is that the heat becomes lower and lower grade as it moves through each engine. The second engine must be able to work with a lower temperature span, or in other words, with lower grade heat.

Most of the WWII era steam ships used triple expansion steam engines. The steam went through three pistons in sequence, each one larger than the first to use the lower grade steam. With three pistons, the steam expanded three times, which is why they are called triple expansion engines. This is similar to what Honda is doing, but with one more expansion.

Tom

I thought having two engines was a little complicated. The Rankine-cycle engine would be in addition to the ICE and electric motor bringing the grand total up to three engines under the hood. I wonder what it will cost to train an engine technician to repair internal combustion, electric, and steam engines.

Yup, most people think of steam engines as being "primitive", but the triple expansion types could hit a massive 50% thermal efficiency!

Compare that with the peak thermal efficiency of the average gasoline engine of around 28%, or Atkinson cycle Prius engine at 35%, or VW 3L TDi engine at 48% efficiency and it's quite amazing!

What I find very promising in the Rankine Cycle system is the fact it can charge the traction battery while the vehicle is being driven normally, unless I am reading the articles wrong.

For regenitive braking to charge the battery well you have to be slowing down correct? While on the freeway the ICE is recharging the battery but that requires HP so we have some loss here. If the Rankine Cycle system is utilized to add HP (or recharge the battery, however you want to view this) then we can recoupe this loss while under "steady state" driving and especially under higher speed gliding situations where the regen braking is not active and we have the ICE off but due to high speed we are still pulling power from the battery.

So in effect I think this would be a great way to boost efficiency for the average driver who canot be bothered with any change of driving style (gliding, P&G, etc.). As to how much extra cost this would add to a vehicle's build price I could not even guess but as with everything else, with enough production the costs will come down.

Here is the article I read from GreenCarCongress.