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Old 02-13-2009, 09:35 AM   #1 (permalink)
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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? 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.

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Old 02-13-2009, 10:00 AM   #2 (permalink)
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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.
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Old 02-13-2009, 10:19 AM   #3 (permalink)
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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.
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Old 02-13-2009, 10:21 AM   #4 (permalink)
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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.
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Old 02-13-2009, 10:55 AM   #5 (permalink)
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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.

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Old 02-13-2009, 11:52 AM   #6 (permalink)
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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.
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Old 02-13-2009, 11:57 AM   #7 (permalink)
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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.
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Old 02-13-2009, 01:10 PM   #8 (permalink)
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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.
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Old 02-13-2009, 03:32 PM   #9 (permalink)
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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.
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Old 02-13-2009, 03:38 PM   #10 (permalink)
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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.

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Originally Posted by MetroMPG View Post
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.

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