Mimicking Gen3 Prius Exhaust Heat Recovery
 

I've been thinking about the new Prius' elimination of the coolant thermos and how they went to using exhaust heat to speed up the warming up of engine coolant. Obviously they think its a superior way of doing things. So, I started googling and found a drivetrain display picture taken by some autoshow goer.

Detroit Auto Show and Prius 2010

Of course my question is how can we make a cheaper rip off of it? :D If you ran your engine coolant to this heat exchanger it would work, but at some point you'd want to stop drawing heat from the exhaust. So, you make a seperate loop that you can close off with some valve (automatic or not). But, now we still have a problem because that coolant is now going to get really hot and probably build up some pressure once the valve(s) is closed. So, do we run an addition closed loop with different high temperature coolant through the exhaust heat exchanger and run that to another heat exchanger to warm the coolant. This is seemingly what Toyota did, but now that loop is going to need a pump... blah blah its getting more complicated ($) by the minute.

Anyway, these are my initial ideas on it. I'd love to hear some of yours to see if we can use their technology on our rides.

Ooooh, I was just thinking. Maybe one might use some sort of drain back system to remove the coolant from the exhaust heat exchanger. Must think more.

If you used a heat pipe with a valve in it then you could just close the heat pipe off when your coolent was hot enough, you would have less fluid moving around so it would all heat up faster as you would have less mass to heat.

A drain back system might introduce air into the loop, and we don't want that. How about instead of an extra coolant loop you divert the exaust? Once the correct temperature is reached the exaust goes back to its usual route, thereby not heating the coolant any more. The coolant is still flowing through the exchanger, which by now may be used as an extra radiator.

Excellent point about the air Piwoslaw. That would create problems. I'm not a fan of diverting exhaust though. Its usually too restrictive with space issues. Plus, over time we're looking at carbon deposits gunking things up.

I really like the heat pipe idea Ryland. Any ideas on how to physically implement that idea? You'd need a high temperature valve, and the system would need to hold up to fair amounts of pressure.

To take Rylands idea and twist it a bit. What about using a physical copper block as a heat pipe, and physically disconnect it to remove heat to the coolant heat exchanger. Here is a pic of the idea.

http://ecomodder.com/forum/attachmen...1&d=1234536870

Is it even a problem? After all, you have a huge radiator up in front of the car that is designed to get rid of waste heat. Just let it do its job.

Well, if you want good aero, you block off as much of your grill as much as you can. We don't want to be increasing grill sizes.

I was thinking about using some thermo-electric coolers to generate electricity from the temp difference between the exhaust and the ambient air. I figured about 3 or 4 TEs mounted to the exhaust with some heatsinks on the other side. I would use them to charge the car battery and take some load off the alternator. This would work best on long drives while a coolant heater would improve warm-ups.

Didn't even know what a heat pipe was before seeing it here.

If you simply coiled some metal tubing around the exhaust pipe, insulated it, and put the valves in the pipe (vs. any flexible heater hose you might use for routing), I'd think you'd be OK with pressure issues.

It really depends on the fluid you use in the heat pipe, but any valve that is rated for steam should work and there are alot of steam valves out there for heating systems that have servos on them so it could be set up as an automatic system.

Know of any solenoid valves that can flow enough coolant and aren't $100+? I searched mcmaster.com for solenoid steam valves and they are EXPENSIVE.

"...and how they went to using exhaust heat to speed up the warming up of engine..."

My '66 Corvair has that feature! LOL

And how much faster the coolant heats up? 5 seconds? not worth it... 1 minute faster? WORTH IT

Perhaps I'll do a test tonight. I'll fire up the car in the garage and let it idle. I have an IR thermometer and I'll measure how fast the exhaust pipe heats up. I'm betting its pretty darn quick.

Make some kind of moving heat exchanger. To heat up it moves to touch the exhaust and when hot enough, a motor removes the contact, separating it from the exhaust.:D

That would be awesome because I am contemplating doing this MAJORLY. And not only for the coolant, but the transmission and oil liquids as well.

Let us know pleeease

Thinking about it, that's what the cooled EGR on my TDI do.

Thats pretty much what I was thinking. I have some initial rough sketches. Might as well share them. The top assembly would be soldered together, and with more tubing. It would ride up and down guided by pins or bolts or something, one on each end. The aluminum tube cut in half would help capture more heat and keep the unit smaller and cheaper. It would also allow for easy mounting with a clamp on each end.

http://ecomodder.com/forum/attachmen...1&d=1234809364

Prius does it cuz engine is on/off so much.

Gotta wonder if it warms up much faster than regular stuff from initial start up?

I mean, with thermostats and especially us with grille blocks, it's only a matter of minutes anyway. I bet coolant doesn't even get overly cool with big P&G.

I think it's a hybrid thing.

Here's a PowerPoint about a simulation of various auxiliary heating systems.
http://www.vda-wintermeeting.de/file...DA-Handout.pps
The focus was primarily on warming up the passenger compartment faster when you have an efficient diesel engine (not enough waste heat), but there are interesting comparisons on Slides 14 and 15.
Below is Slide 14. The yellow curve is exhaust heat recovery. Doesn't look too great, but on Slide 15 it has negative fuel consumption, i.e. it saves fuel.

I'm thinking an oil heater: a separate water circuit from the exhaust heat exchanger to tubing wrapped around the oil filter. A pump would circulate the water only when desired, greatly reducing additional, unwanted heat at the oil filter when the engine is warm. An overflow reservoir could accomodate the thermal expansion of the coolant-water mix.

http://i300.photobucket.com/albums/n...ntaryheate.gif

Well, it does make sense for me. My wife drives around a lot during the day servicing different companies. The dang aluminum block cools off pretty quick in that car and this may allow very fast warm ups which would really help out in my case. I've also noted temperature drops of up to 35 degrees when going from highway to slow city driving.

Interesting presentation Bearleener, thanks for sharing. Of course, exhaust heat will be much more plentiful on a gas engine vs diesel due to it being less efficient. Also, they didn't show their setup. I wonder how they did it...

I did the test the other night. It didn't turn out so great, but I think it still gives you an idea of how much energy there is to capture. The outlook isn't so bright.

For the testing, I used the Paseo. 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. The main problem I encountered was the new exhaust. It is so shiny, that my IR thermometer had problems getting consistent readings. I got as accurate of readings as I could. The coolant temperature was measured with the scangauge.

It took 3.5 minutes for the exhaust pipe temperature to exceed the coolant temperature. At this point the temperature does rise up above coolant temps pretty quickly. However, this is also just sitting and idling. If we were to be actually driving, both coolant and exhaust pipe would heat up much faster. Unfortunately, I have no way of testing this.

On the other end of the stick. I did take one temperature reading of the exhaust manifold right after the test with the engine still running. It was right around 580F, so roughly twice as hot as the pipe behind the cat. This I do think would be a viable area to extract heat from. However, the result would be the cat warming up slower and increased emissions.

Here is the graph I made. Temperature is in degrees fahrenheit. Time is the bottom scale and is in seconds.

http://ecomodder.com/forum/attachmen...1&d=1235090015

Directing intake air from the exhaust manifold may work as well with less cost or work.

A hood blanket/belly pan will also retain engine warmth and reduce warm up times.

That is probably quite true.

About the exhaust manifold vs the cat.

It would take longer for the cat to warm up, but the engine would also be warming up MUCH faster which I think would offset that downtime of the cat... ? Get what I am saying?

I can tell you from my supra experiences that EGT's are with in 200 deg of where the will be at full temp in 30 seconds (Say 700F in 30 seconds of light driving).

Also had an idea in regards to drain back.
How about a drain back + heat exchanger?

Basin/ tank low mounted, submersible pump in tank, pump water up to exaust manifold heat collector, through a exhaust heat water -> coolant water heat exchanger, then back to the tank.

If we could weld some tubing on to the exhaust manifold we'd get way better heat transfer.

For the Drainback/ exhaust heat -> coolant heat exchanger perhaps a heater core with engine coolant flow ruining inside it with a box around it containing drain back water. Just an idea.

Issue will be how much heat can we steal off the exhaust stream before the catalytic converter has issues.


I *think* that would all work.

Other question is how much gain will you get out of it? very cold climate + pulse an glide probably a good bit. Warm climate + turbo diesel probably not a lot.

What do you think would be a good enough gain if this works?

The car warming up 30 seconds faster? would that justify itself? Or would 2 minutes faster warm up be the minimum...

I think that has to depend on the car, your driving style, and your goals.

Oh, how it must have pained you to idle your car like that.

But we appreciate your reduced fuel economy in the pursuit of science! :D

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.

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! :thumbup:

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.

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

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.

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.

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

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

Piwoslaw,

A very interesting thread that is bump-worthy. :thumbup:

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. :p

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. :(

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: