Mimicking Gen3 Prius Exhaust Heat Recovery

[Daox]

lol, it did indeed! However, this tank is going to be bad from lots of trouble shooting and what not. I figured I'd get it all done in one horrible tank's worth.

[Stan]

Originally Posted by Daox

I took exhaust pipe temperatures from just behind the cat. The exhaust manifold on the car comes out the front of the engine, then wraps around and goes toward the back. The cat is probably a good 5 feet from the head.

I have to wonder if the distance is the issue. Can you get thermal readings right off the head? If so, I bet you'd see the temp coming up within seconds. The reason being that there is a great deal of thermal mass in all that piping and cat to bring up to temp before the tail pipe gets hot where you measured it.

Thanks for sharing, though...good stuff!

[Blue Bomber Man]

An easy way to use a heat exchanger would be to have the loop extend to your exhaust and have crossover line with directional valves on each side of the loop, when you no longer need heat, throw valve so it cuts the exhaust loop out.

[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

[basslover911]

So how many "wraps" around the exhaust did you do?

Does your car have two downpipes or just one?

Also, you might want to insulate the tubing (wrap the tubing and exhaust) with exhaust wrap to contain the heat better.

[chuckm]

Here's an idea for recovering some exhaust heat: Wrap your exhaust headers in 1/2 or 5/8" copper tube. Run one end back up to a point behind the radiator, bell it out and put a fan sucking air through the radiator and pushing it through the tube. The other end of the pipe could be run to a low point in the coolant reservoir and through a bubbler. Put insulation around the pipe running to the reservoir. Once your coolant temp is up, turn off the fan.
Probably not very efficient, but relatively simple.
I suppose you could even pump the coolant from the reservoir and back again. The problem of coolant remaining in the tube could be resolved by opening an air valve on either the suction or discharge and letting the fluid siphon back down into the reservoir.

[chuckm]

Promoting circulation of the coolant from the reservoir is an exercise left up to the student.

[Piwoslaw]

Bumping this thread.
Maybe someone has found more info on the Prius' exhaust-coolant heat exchanger?

Originally Posted by Bearleener

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

[Rokeby]

Piwoslaw,

A very interesting thread that is bump-worthy.

A Prius driver seriously interested in keeping MPGs up in the colder months
soon comes to understand that in large part any efforts will hinge on
retaining heat in the ICE and catalytic converter. Getting to the most
efficient operating regime, S4, depends on getting the two up to temp, and
in stop and go driving at least, the HSD computers will restart the ICE as it
needs to keep temps up whether you like it or not. Thinking about any heat
(= fuel = $$) that leaks off into the cold, cruel world is frustrating.

There are a number of threads here on passive ways to retain what little
heat the ICE puts off: grill blocks, pre-start-up block heaters, engine
blankets, etc. All good stuff. Still, sitting at a traffic stop in the windy cold,
ICE-off, with the block temps dropping rapidly (ScanGauge readout), I see
the ICE kicking on apparently only to warm up the ICE or CC arbitrarily at
~145 degF, ~135 degF, and always if temps get as low as 110 degF. You
wonder, Where the heck did all the BTUs go? Out the proverbial window of
course.

Back on target:

Here is a link to some pictures of the GEN III exhaust heat recovery
hardware. (Thank you, Bob Wilson.) Not great, but a start:

Detroit Auto Show and Prius 2010

A little farther afield, here is a plan to recover Prius exhaust heat with a
small steam turbine generator:

Steam-Hybrid Prius

I see that this system is "total loss" as regards the water/steam. It ends up
going out the exhaust pipe. I wonder what the MPGs are for the water.

And finally, a proposal published in Dec. 2007 to use "theromelectrics" to
turn the excess exhaust heat directly into electricity.

http://www.nrel.gov/docs/fy08osti/42256.pdf

"Identifying an optimal system size is challenging. Sufficient waste heat is
available at highway speeds, but little is available in city driving where the effect of engine start-stop cycles is expected to reduce waste heat by 30%. As a compromise, this study sized TE generators based on the amount of waste heat available at 40 mph (64 km/hr) constant speed driving. In transient simulations, these systems were slightly undersized to capture peak waste heat during accelerations. Mass and cost constraints for TE systems simulated were on the order of 50 kg/kW and $150/kW to capture ~1.5% fuel savings in the near-term and ~2.9% fuel savings in the future.
Cold starts, repeated engine starts, and cold temperature operation, none of which were explored in the present study, can be expected to negate some of this benefit."

All very interesting. But as far as I know, neither of the latter two is
available.

[MetroMPG]

A Canadian auto journalist (a master automotive mechanic and automotive technology teacher) has chosen Toyota's heat transfer technology as his "stand out" technology from the past year:

Exhaust heat recirculation

Not sure there's much technically new in his piece that hasn't already come up in this thread, other than the author saying:

There is no documentation on how much the system improves engine efficiency on the Prius, but from experience, I can tell you that the engine and passenger compartment do warm up much faster. I look forward to seeing it used on more vehicles.