Mimicking Gen3 Prius Exhaust Heat Recovery

[Daox]

From talking with Prius III owners, they aren't convinced that the heat exchanger is any better than the coolant thermos in the Prius II.

[Frank Lee]

I think it's yet another YMMV thing.

I really really could have benefitted from the thermos system back when I had a daily commute.

Now I go for days, sometimes weeks without firing a cylinder. The thermos wouldn't help me now but the heat exchanger would.

[Piwoslaw]

I think (hope) that it's only a matter of time before one of us finds a Prius III in a scrap yard and reverse engineers the heat recovery thing. Let's keep our eyes on the accident column in local newspapers. I'm really into this, but Prii are very rare here, and none are gen III (Not that I wish the Prius III owners any wrecks, but maybe a branch could fall in a certain way, so noone gets injured...)

[fatman57]

the thermos is only good for a short while but might be difficult to make - flooding a cool engine with hot water could cause thermal shock.............in this sense the heat recovery form the exhaust is better I think as it works on demand.

from what has been said so far the best system might be a form of water cooled EGR, so you let the warm gasses in, and since this is the stuff doing the warming it should warm the coolant up just as quickly.

Or............instead of wriapping copper microbore around the exhaust put the exhaust into a pipe and seal the ends, then pass water through the pipe.

A big problem is when the exhaust gets too warm, any water will be very close to if not already boiling no matter how much antifreeze is put in.....this is another reason why I favour an EGR approach (or even running a dead end off the exhaust which is smaller so will not get as hot which is cooled via the coolant system???)

If doing anything like this I was thinking of putting the outlet of the warm water on the inlet side of the engine but it would need time to mix well too or there would be seams of VERY hot water running through which isn't altogether good for the cylinder head.

sorry for the long post.

[Piwoslaw]

Originally Posted by fatman57

from what has been said so far the best system might be a form of water cooled EGR, so you let the warm gasses in, and since this is the stuff doing the warming it should warm the coolant up just as quickly.

If we want to catch more heat from the exhaust and put it back into the engine, then why cool the EGR? Just leaving it warm would be more efficient than transfering the heat to the coolant, which may lose some heat before it returns to the engine block. Also, I doubt there is enough flow in EGR to make much of a difference, since EGR is only a small part of all exhaust flow. Daox has a project going with cooled EGR.

Originally Posted by fatman57

sorry for the long post.

Don't worry, we see much longer posts here. As long as it has relevant info and/or good ideas it's OK

[tasdrouille]

The cooled EGR in VW TDIs does make a significant difference in warm up time in cold weather.

I'm pretty sure getting the heat transferred to the coolant by a purposely designed heat exchanger and then having the coolant circulated works better than just sending back hot exhaust gases in the combustion chamber. Plus you wouldn't get the same NOX reduction.

[dremd]

Originally Posted by tasdrouille

The cooled EGR in VW TDIs does make a significant difference in warm up time in cold weather.

I'm pretty sure getting the heat transferred to the coolant by a purposely designed heat exchanger and then having the coolant circulated works better than just sending back hot exhaust gases in the combustion chamber. Plus you wouldn't get the same NOX reduction.

Although the EGR cooler isn't designed for the purpose it does take heat from exhaust and puts it in to the coolant.

[fatman57]

Originally Posted by dremd

Although the EGR cooler isn't designed for the purpose it does take heat from exhaust and puts it in to the coolant.

thats where I was coming from - its safer to use this way as it is a controlled amount of exhaust heat.

as an extra you can regulate the amount of exhaust gases going into the combustion chamber to make the right temperature for efficent combustion in both winter and summer.

even if you used a dead end running off the exhaust it would still heat up quite a lot allowing you to transfer heat to the coolant.

[fjasper]

I guess you could put in a coolant temp switch that turns on a block heater when the coolant is below a certain temperature, say 150F, maybe. Run a 120v one off an inverter, or look for a 12v one. It wouldn't harness exhaust heat, but it would speed warmup (to an unknown degree).

Like This

Whether or not it would help overall mileage may depend on the efficiency of the alternator, the relative efficiency gains of warmer coolant, driving cycle, etc.

[euromodder]

As the exhaust gasses cool down along the exhaust pipe, why not harvest the heat a bit further down the pipe so it won't overheat the coolant ?
It'd require a bit more coolant and a bit more coolant piping, adding weight, but also adding some more buffer capacity to the system.