Mimicking Gen3 Prius Exhaust Heat Recovery

[engineered]

Originally Posted by Vman455

Aqueous ethylene glycol decomposes into carbon monoxide and diatomic hydrogen (hydrogen gas) if it gets hot enough.

At what temp? Water would also split into oxygen and hydrogen if you heat it to 2000'C.

[Daox]

Originally Posted by engineered

Since this has been revived, why not use a 12V block heater and/or 12V oil pan heater to heat up the fluids? They could be used manually or controlled automatically to come on with the car and turn off at a certain coolant temp (or from the tstat).

The purpose of this thread is to use 'free' exhaust heat to speed up the warming of the engine to decrease fuel consumption.

A block heater does work, if you have 110V power. If you don't, this device can and will help.

Using 12V power from the alternator will actually increase fuel consumption, not decrease it. So that kind of defeats the purpose we are going for at least, though you would get heat quicker. VW actually does do this with their TDI engines to get them up to temperature faster.

How would coolant water (and glycol) degrade? It doesn't break down like motor oil. I'm mean if you overheat it, it would eventually vaporize, but 98'C water is always going to be the same water.

Vman is correct. Ethlyene glycol is only supposed to be good to 250F / 121C. It will start cracking at around 285F / 140C according to this paper (page 16). That is certainly achievable by warming the coolant with the exhaust. I'm not sure if the water will start boiling out before that point or not, but if it does, thats not good either.

[dremd]

I’d also note that the quality of 12volt energy is limited. Even with VW’s large battery and alternator the difference in warm up time isn’t noticeable to the average driver with the system disabled. The gen 3 Prius warm up time is genuinely impressive until you move to EV’s and expect heat instantly.

[engineered]

Originally Posted by Daox

The purpose of this thread is to use 'free' exhaust heat to speed up the warming of the engine to decrease fuel consumption.

A block heater does work, if you have 110V power. If you don't, this device can and will help.

Using 12V power from the alternator will actually increase fuel consumption, not decrease it. So that kind of defeats the purpose we are going for at least, though you would get heat quicker. VW actually does do this with their TDI engines to get them up to temperature faster.


Vman is correct. Ethlyene glycol is only supposed to be (page 16). That is certainly achievable by warming the coolant with the exhaust. I'm not sure if the water will start boiling out before that point or not, but if it does, thats not good either.

I agree that the exhaust heat is 100% free, while a 12V heater is not. I guess I was thinking people were looking to heat the engine up quicker to reach operating temps faster.

Originally Posted by dremd

I’d also note that the quality of 12volt energy is limited. Even with VW’s large battery and alternator the difference in warm up time isn’t noticeable to the average driver with the system disabled. The gen 3 Prius warm up time is genuinely impressive until you move to EV’s and expect heat instantly.

With a 100A alternator, a 12V heater could put out more than 1kW of heat into the coolant, which is very little compared to an idling engine, say 5hp (3.7kW), so the engine is already putting 4x more heat into the fluids. Still, 1kw heater would add 27% more heat, but barely noticeable to a driver. Gently driving around using 100hp(75kW) would put out 75x more heat and make the 12V heater very insignificant.

[ratgreen]

Why complicate things. Just get an EGR cooler that has the valve built in. Then you just need a 12v vacuum pump to activate (based on the temperature sensing relay) to keep it open to let hot exhaust through during warmup. Then close it once up to temperature to stop it boiling the coolant in the egr cooler.

[Daox]

I believe that is basically what the Borg Warner unit is. Only, it is electronically actuated, not vacuum actuated.

[Ecky]

G1 Insight:




2006 Civic:




2008+ Accord, 2009+ TSX, 2012+ Civic Si:




2009+ Honda Fit:

[arcosine]

v8 engines of the sixties and earlier had a bimetallic spring and a flapper valve on one exhaust manifold that would divert exhaust through the intake manifold during warm up, nothing new here.

Just use an on-off valve to let coolant into your exhaust heat exhanger when cold, turn the valve off when warm. Have a check valve at the bottom to let coolant out of the exchanger when it boils. When it all boils out, no problem. Make it from welded stainless steel.