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

[some_other_dave]

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

[Peter7307]

Originally Posted by Big Dave

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.

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.

[extragoode]

Originally Posted by jesse.rizzo

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.

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.

[bikin' Ed]

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.