Simple and cheap DIY exhaust heat to coolant heater idea - using air to transfer heat

[Daox]

I ran some numbers to see what kind of heat transfer we could possibly get from this thing. The calculations are pretty simple. This is based off the heat differential between the air and the coolant. Air (assumed dry, moist air will hold more heat) holds X amount of energy per cubic foot. If you move X number of cubic feet that is Y degrees above coolant temperature you have Z amount of energy. This is the chart I came up with. The chart units are degrees F (horizontal), CFM (vertical), and watts (chart body) to compare to a block heater for example.

When I googled 'car blower fan cfm' it seemed like 200-300 seemed normal. I have an extra blower fan kicking around that I plan on using should testing continue to prove this worthwhile.

Manifold temperature will be a future test. I know my Paseo's manifold got to 580F after 5 minutes of idling. Thus I know a higher temp is possible, so the chart goes to 700F.

One last but very important note, these numbers do not take into consideration the efficiency of the air to coolant heat exchanger (I'm thinking a heater core or two will do the job). The heat exchanger will definitely create a efficiency hit. Just off the top of my head, I'd estimate 50%.



The numbers aren't super amazing, but they are substantial I think. I think its reasonable to assume we can get 400°F temp air with 200 CFM giving us a possible 5600W, but at 50% heat exchanger efficiency we're looking at 2800W. If we compare this to MetroMPG's block heater vs time chart (test on his Metro), that would give us a 70F coolant temp rise in 12 minutes. Obviously, this also isn't realistic because everything has a warm up time. The manifold isn't going to instantly be making 400F air, and certainly not at 200 CFM.

Moving forward, testing will refine these numbers and show what is really possible out of this setup.

[California98Civic]

Originally Posted by Daox

Yes, it would offset it. The offset is just very marginal. In order to heat up the coolant faster, we need to move large amounts of heat. Heat is energy. The fan also uses energy, but its (hopefully) moving a LOT more energy than its using.

Its akin to a furnace blower. My home furnace blower fan uses ~300 watts of power, but it moves 80k BTU/hr which is the equivalent of about 23,500 watts.

Also, the fan is only going to be used during warm up. Once the engine coolant is warm, it'll turn off. So, its not a continuous draw.

You could reverse your plan. Put a solenoid-controlled or manually-controlled shutter on the front grill block and allow forced air to go through to the airbox, then the exchanger, then a fan used to generate power. Restriction from the piping, exchanger, and fan could be tuned to slow the air down giving more time for the "heat exchange" The fan/generator offsets solenoid's power use. If your shutter were manually operated you'd have a side benefit of (minor) energy production.

EDIT: Nevermind, given this chart... forced air would never move as much air as a fan.

[Daox]

Yeah, you really need more airflow than you can get by ducting air from the front of the car.

[Isaac Zackary]

Not just air flow, you need convection. The VW Beetles can put out only a little over 200F air in the cabin vents. And that's with heavily finned aluminum OEM heat exchangers (fins on both the inside and outside of the exhaust manifolds!) with not a lot of air flow. Maybe if you had finned exhaust from the tail pipe clear onto the exhaust manifold, but that still probably wouldn't get you enough heat.

[Daox]

Really? I find that quite odd. I'd imagine if you have a 600-700F manifold, it shouldn't be too much of a problem to get 300+F air. Well, after I test the exhaust manifold temp warm up times, building the box/heat exchanger would be the next step to see how hot we can get the air to.

Thanks for the info. That means the heat exchanger will definitely need to be made with much thought put into it.

[Isaac Zackary]

Of course there's a ways from the heat exchangers in the engine in the rear of the car all the way to the vents in the front which allows for some heat loss. But still, this is with exhaust manifolds that have fins both inside and out. It's not bad for heating the inside up to nearly 90F on a 30F day. But when guys get aftermarket heat exchangers is when they notice a huge difference, because then they don't get enough heat to make any difference in cabin temps. That's because aftermarket heat exchangers don't have fins. In other words I don't think this thread's idea would work, at least not without a finned exhaust manifold.

[Ecky]

My thoughts:

Divert one of the engine's coolant hoses (inside the loop behind the thermostat) through a copper tube wrapped around the exhaust manifold. Add a heatercore with a fan that's wired in parallel with the radiator's fan to help with cooling once everything is warmed up.

Honda built the G1 Insight with a waterjacket on the exhaust manifold and beefed up the main radiator a bit to compensate.

[Daox]

Originally Posted by Isaac Zackary

Of course there's a ways from the heat exchangers in the engine in the rear of the car all the way to the vents in the front which allows for some heat loss. But still, this is with exhaust manifolds that have fins both inside and out. It's not bad for heating the inside up to nearly 90F on a 30F day. But when guys get aftermarket heat exchangers is when they notice a huge difference, because then they don't get enough heat to make any difference in cabin temps. That's because aftermarket heat exchangers don't have fins. In other words I don't think this thread's idea would work, at least not without a finned exhaust manifold.

Do you have any pictures of this exhaust manifold for reference? I did a quick google image search, but I really don't know exactly what I'm looking for.

[Daox]

Originally Posted by Ecky

My thoughts:

Divert one of the engine's coolant hoses (inside the loop behind the thermostat) through a copper tube wrapped around the exhaust manifold. Add a heatercore with a fan that's wired in parallel with the radiator's fan to help with cooling once everything is warmed up.

Honda built the G1 Insight with a waterjacket on the exhaust manifold and beefed up the main radiator a bit to compensate.

That is definitely a doable idea. I guess I just don't want the aero or electrical fan load penalty of having to get rid of that heat once up to temperature.

[2009Toyotoad]

Diesel motorhomes and certain luxury water craft have tackle this idea from a different direction and for a different purpose. They use the exhaust heat for domestic hot water, But you can use the same idea. What they've done is essentially created a tube within a tube with coolant circulation through thea coil in the domestic hotater heater. I'm not sure when it was first used but the GMC gas motor homes of the 1970s had it as a popular option. So oking at that literature may give you some ideas for the heat exchanger design.

Rather than route the coolant to a separate radiator you could route it back through the transmission cooler loop in the radiator, assuming you aren't using it to cool the automatic transmission already.

The trick would be to find the right fittings, fab up the heat exchanger, and probably a valve to close off the heating for longer trips where you would not want extra heat circulating back to the radiator.
If you are doing it up right you'll also need some tee fitting to create a "to" "from" loop to the expansion tank.