[Bearleener]

I tried to build an exhaust heat recovery unit but it didn't work. It's probably too small. It's basically thin copper tubing (5 mm) bent into a double back-and-forth loop so it has an overall length of about 14 cm and a total contact length to the exhaust pipe of about 50 cm. I attached this to the exhaust pipe just downstream from the header (i.e. upstream of the catalytic converter) using 2 hose clamps and let water flow through it and thin rubber & tygon tubing. The idea was to pump the heated water through tubing wrapped around a steel pipe on the engine through which the coolant returns from the heater core to the water pump.

About 30 seconds after starting the cold engine the exhaust pipe was definitely too hot to touch. But after several minutes of water through my heat exchanger (at a rate of about 500 ml/min.) the water was lukewarm at best. After about 3-4 minutes the normal coolant tubing was already quite hot.

That is, my heat recovery doesn't work. Either the thermal contact is bad, the contact surface is too small, or it's difficult to extract lots of exhaust gas heat through the exhaust pipe wall. Also, I'd probably need to use much thicker tubing, larger contact area, and circulate the coolant directly rather than using an intermediate water circuit. It may also help to use the exhaust header directly (greater heat capacity --> warms up more slowly but doesn't get cooled down as much by the heat exchanger.

My questions for you guys are:

1. Are there any details on how the Prius does it?

2. One way to ensure good thermal contact would be some sort of metallized bag that you strap around the exhaust pipe and when it's filled with coolant it sort of molds around the pipe, giving you good contact. When the bag is empty it has bad contact, as desired. Any ideas what material might work?

3. What can you use as heat transfer tubing, i.e. good through-the-wall thermal conductivity like copper but flexible like plastic tubing?

Thanks! -- Mike