Why does the heater core transfer heat from the coolant, not the exhaust?
 

It would be hot almost instantly and would allow for an air cooled engine whose temperature could be controlled precisely by the amount of air that was let into the engine compartment.

The only reason I can think of is that it would be TOO hot, but that can be solved by just placing less heat exchange material on the pipe. It's probably be just fine to run the regular pipe through the airstream.

Too complicated is my best guess...

If it leaks, it's poisonous gas leaking in the cabin versus engine coolant ;)

That, and it's a whole lot easier to route a relatively clean liquid than a much hotter gas ;)

trebuchet hit the nail on the head....toxic fume leaks.

Why don't they run an absorption cycle A/C from it?

I've been curious of the same thing for a few years now.... But I haven't educated myself enough on the concept to build one myself :p

I have a number of vehicles where the cabin heat is gotten from exhaust heat exchangers and/or from cylinder waste heat (VW, Corvair, general aviation aircraft (but I don't own one now)). That system can be great WHEN EVERYTHING IS IN PERFECT WORKING ORDER. But, reality interrupts our little daydream (doesn't it always?). Over time exhaust systems rot out and yes, noxious fumes come into the cabin. Also, when (not if) there is an oil or gasoline leak in or near any component of this sort of heating system, all that stench comes in the cabin too.

both good answers, I will also add.

-heat is exchanged easier, and more efficiently in a fluid
-antifreeze has an anti corrosive that should ideally keep the system working properly longer
-exhaust is wizzing out of your car as fast as possible, back pressure makes your car perform less goodish, where as the radiator fluid is restricted/slowed down by the therm to make the engine coolant exchange work better, and it doesn't _really_ effect engine performance.

Fire: I disagree with your points 1 and 3. Experience shows those not to be factors.

That's assuming that you and I agree that VW and Porsche heaters are sufficient. We don't.

All of these are good reasons, but they're classic examples of in the box thinking. I'm sure there's a way to safely and reliably transfer heat from an exhaust, we just came upon the coolant idea first and it's easier.

Example: If you don't like toxic fumes and smells being transferred to the cabin along with the heat, use another medium as a buffer. Water/antifreeze responds to convection currents just like air does. Put heat exchanger from water mix to exhaust plumb to the coolant to air exchanger in the cabin. Bingo, best of both worlds! Sure it's not terribly efficient and would now be subject to a warm up time again, but you can't tell me that there's not more than enough heat in the exhaust and since the fluid for this purpose would be only for this purpose, it's quantity could be much less, therefore, warm up much faster.

I guess I'll spell it out in more detail then:

1. There is nothing, repeat, NOTHING restrictive to the exhaust flow in a VW/Corvair/aircraft exchanger system.

2. The air coming out of a VW/Corvair/aircraft heater in proper working order is so hot you can't even hold your hand next to the outlet. Efficient or not, it is plenty hot enough to do the job.

P.S. I'm not saying the air/air systems are superior. There is nothing more pathetic than the defroster on a Microbus.

But to be sure, the exhaust gas is a fluid also. A fluid can be a gas or liquid. ;)

You would need alot of fluid, otherwise you will scorch the working fluid and it will brake down rather quickly.

the Honda N600 also has an air cooled engine with exhaust heat, a pin hole in the heat exchanger and you start to get light headed while driving if you turn the heat on, on VW's the heat exchanger was a costly part that wore out fast, something like a heat pipe could work well.

[QUOTE=extragoode;66678]

Example: If you don't like toxic fumes and smells being transferred to the cabin along with the heat, use another medium as a buffer. Water/antifreeze responds to convection currents just like air does. Put heat exchanger from water mix to exhaust plumb to the coolant to air exchanger in the cabin. Bingo, best of both worlds! <snip>

Having a double - wall heat exchanger with fluid between them would bring a lot of the kind of safety we get with things like double-bottom tankers. However, the cost would go up a lot, and eventually, somebody would still let it fail.

There is plenty of heat conveniently available from the cooling system on most cars. When I see a motor so small that the rad is the same size as the heater core, I'll think about a valve to direct the coolant where it is needed. My metro warms up quite rapidly, because it works hard for its size.

The rule of thumb is that 25% of the heat in gasoline goes out the shaft, 25% out the rad, and 50% out the exhaust, so it is a rich source. I concur that it is most interesting as a way to feed an absorption cooler, or perhaps run the electrical generation.

;) Check out <http://www.google.com/patents?id=yOBqAAAAEBAJ&pg=PA1&dq=manifold+heater+ automobile&source=gbs_selected_pages&cad=0_1>

A manifold heater was the common way to heat a Cessna in history past and air cooled VW's. Don't know how the IBP (itty bitty planes) are heated today.

OBTW, a gasoline burning heater was common in the 30's. Fire hazard? You bet plus you could smell the fumes inside the car.

Manifold heaters would be a giant step backwards in safety & comfort.;)

I was fascinated to discover that turboshaft helicopters are heated by air that is bled off from the compressor ahead of the combustion zone. The heat of compression is wasted, but the effect of the combined inefficiencies even that far along are sufficient to produce nice warm air. This is a lighhtweight and technically simple solution to cabin heat, not necessarily, or even likely, an efficient one.

If you've flown in a commercial jet or turbuprop aircraft you've experienced pressurization with bleed air. It's passed through an air-to-air heat exchanger to alter the high temperature. Bleed air is used to deice the wing leading edges, pressurize fuel tanks, and other jobs that can be done with pneumatics.

The Boeing 787 is the first commercial aircraft to do away with bleed air to improve compressor efficiency. It removes a heavy, complex and leak-prone duct system to be replaced by new compact, lightweight, high-efficiency HVAC/pressurization systems originally developed for the Space Shuttle.

People who operated Beetles in cold weather country often use an auxiliary oil cooler as a heater. That worked a lot better than the OEM heater.

Most if not all "itty bitty planes" still use exhaust heat to heat the cabin. Since they are air cooled this is probably the best way to do it.

During your training, instructors hammer it into your head to be on the lookout for carbon monoxide poisoning, and how to combat it. Since no training is required for driving a car, exhaust leaks would be much more of a problem than they are in aircraft.

The later Vee-Dubs and Porsches went to a fail-safe system for the most common failure mode of the exhaust heat exchangers. They made the exhaust pipes out of (a low grade of) stainless steel, and the heat exchanger "wrapping" out of mild steel. The wrapping would rust out first--which would be visible from the outside, and would have noticeable effects on the heat delivered by the system. The exhaust could then be replaced, in theory before any leaks could develop from the exhaust pipe into the wrapping.

I will second that there is no extra backpressure created by an exhaust heater system. The heat exchanger part simply is wrapped around standard exhaust pipes, with no changes to the inside of the pipes at all.

The heat exchanger system in my 914 works so well that I have to open the windows any time I have it on! Driving on a nice cold evening (~35-40F) with the top off, I was quite comfy in my jeans, t-shirt, and leather jacket. Though my toes got very uncomfortably hot...

I think that one of the reasons that the liquid coolant is used to heat the cabin air is that it is easier in most cars. The radiator core is pretty compact, and only needs two (relatively small) fluid lines going to it. Exhaust heat exchangers need to move the heat from the exhaust system into the cabin, which can require significantly larger air hoses.

For vehicles with large interior spaces (e.g., Microbuses, minivans, SUVs, large sedans) you can also put heater cores here and there and anywhere and use a fan to circulate cabin air through them. For an exhaust-heat system, you would have to run more large air hoses to where you wanted the warm air to be, and you would have to push that much more air across the heat exchanger. That gets impractical pretty quickly, I would think. I also think that was one of the primary problems with the old Buses, they were trying to heat up a larger volume of air than they could reasonably heat up. Vehicles with smaller interior spaces get quite well warmed, in contrast.

Finally, there is the perception that people have of exhaust heat exchangers as "unreliable, smelly, and prone to kill you". That mostly hasn't been true for a while (and the true parts can be addressed with some redesigning) but the perception exists, and will make it difficult for anyone to sell such a system. (Check the US consumer's reaction to diesel cars over the last 20 years, most opinions were poisoned by the loud stinky underperforming Chrysler diesels and big Benz diesels from the 80s. Even to this day, when new diesels are clean and quiet and torquey as heck, there's still a resistance to them.)

-soD

The very early Subaru cars used something similar.
The oil cooler air could be directed to the cabin or the outside via a simple vent.
The engine was air cooled at the rear (Yes it was still a Subie) like a VW but with only two cylinders and about 600 cc.

Pete.

Training IS required for driving a car, it's just that most of us would probably agree that it's horribly insufficient for a lot of reasons, not just this one.

Point Missed
 

I think a major point of this thing is that the engine needs to be cooled. Liquid cooled engines can be built to tighter tolerances than air cooled and can thus be made more efficient. It makes more sense to scavage "wasted heat" from a system already in place.

Also in my experience a cold winter day in a liquid heated car has been much more temperate than the same day in one of my old corvairs or my dad's piper with "air" heat/cooling. I believe it has something to do with the liquid being able to carry more heat, thus being the more efficient medium for heat exchange.