Mimicking Gen3 Prius Exhaust Heat Recovery

[Joenavy85]

Originally Posted by Daox

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.

how about once the engine is up to temp you divert the coolant from the heat exchanger to a heater core in the filter box to warm up the intake air, i'm sure you could do it in such a way that you wouldn't have to worry about a leak in the core getting into the intake. you could use a small high temp water pump to circulate it, or even have it be like a second heater core which wouldn't even need a second pump, that way any excessive heat build up would still go to the radiator.

[fjasper]

Originally Posted by 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 ?

I guess the issue becomes the balance between getting enough heat early in the cycle without overheating the system in hot conditions. I like the breakable heat pipe idea. Simple, effective, and doesn't involve fluids that can leak, freeze, or react badly to heat. Would probably be sensitive to alignment and dirt issues, though.

[Air-Hybrid]

How about just redirecting the return hose to pass near to the exhaust manifold. Machine a flat on the exhaust manifold and have a ally heat exchanger that can be move to contact with the flat surface on the manifold. The flex of the rubber hoses should be enough to allow a two inch gap to be made between the exchanger & the manifold when the temperature is reached. For extra control insulation could be slid in as the heat-xchr moved out.

(just realised - would need to move the thermostat valve outside the block - with a separate circulating hose bypassing the rad)

[fjasper]

I thought of yet another possibility: Drill a hole in the manifold and weld on an O2 sensor bung (or use an existing socket). Make a heat probe, either machined from solid copper or by using an old O2 sensor body, that puts the end of the copper heat pipe directly in the exhaust flow.

Make the receiving-end of the heat junction out of a heat-conductive metal with a higher rate of thermal expansion than the heat-sending end, so when it gets hot (coolant at operating temp), it expands and breaks (or at least reduces) the heat transfer path. Copper plug (thermal conductivity 400) on the sending end (coefficient of thermal expansion 9), aluminum socket (thermal conductivity 250) on the receiving end (coefficient of thermal expansion 12). Machined aluminum block that has a socket for the heat pipe and a coolant path. Maybe some heat-transfer fins in the coolant path to improve transfer, maybe not. Spring clip to hold it all in place.

When it's all cold, they may or may not be clamped together-I'd have to run the numbers. As the copper warms and expands, it squeezes against the aluminum, which transfers heat to the coolant. As the coolant reaches operating temperature, the aluminum warms and expands enough to relax the contact with the copper, slowing heat transfer when it's not needed.

So there you go, that's the most elegant solution I can come up with. No break in the coolant path, no delicate parts, no moving parts. Failure mode would either be regular old hose clamp leak, or the copper plug breaking off. First is easy to fix, second doesn't have any ill effect beyond stopping the heat transfer. Installs with a 7/8" O2 sensor wrench and a screwdriver. Pointing out galvanic corrosion issues at this point would just make you look like a spoilsport.

[fjasper]

Originally Posted by Air-Hybrid

(just realised - would need to move the thermostat valve outside the block - with a separate circulating hose bypassing the rad)

I think the hose going to/from the heater core would work-it's in the same coolant loop as the engine (as opposed to the radiator). I bet you wouldn't have to move the block more than a few mm to stop the heat transfer.

I like this idea. maybe use a couple small springs to hold the block away from the manifold, and a pull-cable wrapped around the whole assembly to cinch it together when you want heat transfer. Maybe something like one of those spring-steel universal oil filter wrenches.

[drmouse]

Hi all

I have been condering some way of using the exhaust heat to decrease warm-up time for a while. There are some interesting ideas here

However, I have always subscribed to K.I.S.S. (Keep it simple, stupid), and have a couple of ideas I would like to float.

The first is something I have seen on many (mostly older) cars, and wondered if it would be worth it on a more modern vehicle. There was a cowelling around the exhaust, and the intake air was drawn in through that, henced pre-warmed by the exhaust heat. I know this was mainly to improve cold-running ability, but it seems a very simple method of warming up quicker.

The other idea is an extension of this, but would only work well in a car with a full grill block. Do the same with a cowel around the exaust manifold, but use a fan to circulate air accross it inside the (pretty much sealed) engine compartment. This would quickly raise the engine bay temparatures, which would at least help warm up (albeit probably not by that much). The fan can be turned off at any point, which would effectively break the thermal link between the exhaust and engine bay.

This second idea could be further enhanced by using an oil cooler in the path of the heated air, hence tranferring more of the exhaust heat directly to the engine, and by adding fins to the exhaust manifold (which would probably require the cowel to be insulated so as not to degrade warm-running performance).

As I said, I am not sure how effective these methods would be, but they are certainly simple. If they would work they would be cost- and effort-effective methods.

[Daox]

I did some warm up testing with my Paseo. I wasn't overly impressed by using exhaust heat to warm up intake air.

Testing: WAI's effect on engine warm up

I also did this testing earlier in the thread if you didn't read it. It took 4 minutes of idling for the coolant temp to exceed the intake temp. On the road this would surely be less, but its not real promising for KISS type setup.