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I have around 4 miles of city driving to get to work every morning and in the winter my car never reaches operating temperature during my short trip. In fact, I am lucky if I get the engine temp above 50C, and having warm air out the AC vent is one of my wildest dreams.

I've seen footage from a thermal camera (flir) of the underside of a car as it was warming up from a cold start. The exhaust temp went from 5C to 90C in 1 minute and in 3 minutes it was up to 200C.

Is it feasible to have a heat exchanger between the exhaust and the engine coolant to transfer that exhaust heat to the coolant, heating up the engine?

There would have to be some sort of mechanism (diverter valve?) to ensure just enough of the exhaust gas gets to the exchanger so as to prevent overheating but engineers have faced far greater challenges.

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    Have you considered remote start? Aftermarket kits are available for many vehicle models. – Tyzoid Aug 31 '17 at 16:27
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    Simply put, you have the wrong kind of car for that service. Next car, get a strong hybrid. It isn't worth starting an engine for a 4 mile commute, and battery can provide heat instantly (the better ones reverse the direction the A/C pumps heat, chilling the great outdoors to heat the interior). – Harper Aug 31 '17 at 22:09
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    My fully electric car can preheat the cabin for a scheduled set off time, nothing like getting into a toasty warm car, fully defrosted, on a cold winter's morning - stick on the heating immediately and the heated seats for maximum warmth... Mmmmm as an added bonus, you can set it from your smartphone and in summer can pre cool the cabin, awesomes! – RemarkLima Aug 31 '17 at 22:28
  • I did consider remote start but the only good it would do is to probably heat the windshield a bit and make it easier to scape the ice off. The reason being that even with the engine fully warm, in around 20 minutes the temp drops back to around 40C-50C. I don't know about fully electric because I would have to run a power cord down from my apartment to my car as there really isn't any infrastructure to support electric cars where I live. – user1969903 Sep 1 '17 at 6:18
  • Hybrids are a nice option. I remember driving a small turbo petrol hybrid. Can't remember the make but even with that small an engine the car accelerated decently fast with the help of the electric motors and helped mitigate the turbo lag. I would definitely consider hybrids, especially for such short commutes. – user1969903 Sep 1 '17 at 6:19
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The problem you'll have in extracting heat from your exhaust gases to heat your engine is twofold:

  1. Gases don't generally have a high specific heat capacity so though it's hot, it's not a lot of heat energy, and it'll take some time to heat up a large mass. The walls of your exhaust pipe are incredibly thin, and quite effectively insulated by the air around them so they will indeed heat quickly. If you took a fan and blew air over it to warm the air, you might be surprised how quickly your glowing cherry red exhaust cools down again. If you wrapped the exhaust pipe in a 2 inch jacket of water, it will take much longer to heat this contraption than it would the bare exhaust pipe

  2. The condensation caused by actively cooling the exhaust gases would be considerable and it's quite corrosive to the metal of your exhaust. You might find that it accelerates the rate at which you have to replace the heat exchanger or exhaust, and the associated cost of doing so makes it an economically unviable option

Investigate whether your heater system is functioning properly; if your thermostat is stuck (partially) open (as they're designed to when they fail) then it will take a lot longer for your engine to reach operating temperature.

It's in the best interests of fuel economy (and hence manufacturer reputation, so there's a market drive for this) for an engine to warm up quickly, and they're thus designed to have a really small cooling circuit (the engine block, and the heater matrix/exchanger that heats the cabin via the vents) thanks to the main thermostat being totally closed when cold, causing the engine to heat quickly, before the main radiator starts being used to dump the excess heat.

Consider too, running the blower at a lower speed so you don't suffer human factors such as windchill meaning you perceive the air from the vents to be cooler than it is, because it is moving at high speed. Run the recirculate mode, so your heater is not drawing fresh cold air from outside, but keep the AC on to dehumidify the air, especially if you're running it in recirculate rather than fresh. Excess moisture on surfaces inside the car will sap the heat from the air, as the evaporation of the water needs to get its energy from somewhere - running recirc without AC will mean that over time the moisture in the fabric of the car interior and on its surfaces will increase. You'll be wiping the windshield every morning with a rag so you can see, etc..

Alternatively, it's possible to obtain (in my country at least) aftermarket electrical accessories such as heated seat covers and fan heaters that plug into your cigarette lighter socket. It's also possible to equip your car so you can plug it into your house energy supply overnight to preheat the cabin and engine with electrical resistance heating (cold countries find such systems a necessity even for getting the engine to turn over in a morning). Keeping it inside a garage may help too as it won't go so initially cold.

If you're after a cheap and easy experiment of a similar ilk to what youre proposing here, consider ducting the air from around the engine, particularly on the exhaust manifold side, into your cabin air intake instead. Older caburetted cars often had a cowl around the exhaust that was part of the air intake to the carburettor, controlled by a bimetallic strip, with the aim of reducing temperature variations of intake air (reducing condensation of fuel on the surfaces of the intake) and make it require less choke in early running; this system would be similar, but doesn't have so many condensation complications as stripping exhaust gas of its heat

Lastly, if you do go for the processing-exhasut-gases route and take it to commercial production, it could actually have considerable benefits for future exhaust design; if you can get the exhaust gas temperature down sufficiently that exhausts can be made of plastic, the condensation worry goes away.. but then cars would only need one exhaust to last their lifetime and the exhaust companies would buy and bury your patent </conspiracyTheory> :)

Edit: from the comments that indicate the goal is to warm the engine faster

You'll have a hard time getting exhaust gases to do this, for reasons of the aforementioned lack of serious amount of heat energy embodied in the air. Sure, if you could keep the gases in contact with the block longer then it'd warm slightly faster, but I don't think you'd see an appreciable difference on a short commute

There are crazy approaches you could take, like insulating your engine with spray foam, adding a very large thermos vacuum flask type thing to the coolant system (some luxury cars have this built in to provide cabin heat sooner on cold days), adding some kind of fuel burning device to heat the coolant circuit as you've noted. You could even buy a smaller engined car, or if your commute is short, an electric one (like a Renault Twizy) then you don't have to worry..

..but overall, I think you should stop worrying about it. An engine will have reached a reasonable operating temp long before the needle on the coolant temp begins to rise. Those coolant gauges typically have a very narrow range, like 90 to 110, with operating being 100. The parts doing the serious work will already be hotter than this long before, and it's the increment delay in heating your coolant that taking the time. It's not the cause that an engine had to reach a point where every part of the block is at 100 degrees before it's running efficiently and will be worthy of turning into an f1 car one day (BMW used old saloon engines to base f1 engines on, as the theory was they'd been through many heat cool cycles and the metal would be more free of internal stresses) so the short answer would essentially be "don't worry about it" - yes, some things might get knackered sooner but generally cars are so cheap and throwaway by the time they reach the "double the ten year lifespan they're designed for" point, that they just get scrapped. Running your engine up to full temp for every commute isn't going to stave off that death by much, "its old-age that kills cars, not mile-age"

Won't hurt to take a longer high speed blast a few times a month, especially if your car has a cat, diesel particulate filter or similar emissions control system that requires a good blast through, but don't worry about it having to be every journey; my commute shortened drastically from 50 miles to 1.5 five years ago, and the 04 diesel Volvo I've had for 12 years still behaves the same.. I don't make any special efforts to blast it regularly, but when it gives up irreparably I'll scrap it and hope to get an E60 M5. I firmly believe by having this dream my current car will run forever, just to keep it from coming to fruition :)

  • Would like to add, cooling the exhaust gas increases the Catalytic Converter heat up time. This could over time cause a CEL fault. – mikes Aug 31 '17 at 10:36
  • That's a good point; such heat exchange might have to be done after the cat, as runing a catalytic converter on a regular basis would definitely make it an expense way of having warm fingers on the drive to work – Caius Jard Aug 31 '17 at 10:37
  • Another factor in rapid warm-up is emission controls. It's hard for an engine to run clean when cold. Any exhaust heat reclaiming must happen after the post-cat oxygen sensor. You are not allowed to modify the emissions envelope (MAF to tail O2 sensor) in the way you want. – Harper Aug 31 '17 at 22:05
  • The only sort of auxiliary heater would be one of those eberspacher or webasto fuel driven heater but those are expensive as hell. Any other sort and I would have to run a power cord from my neighbor living beneath me on the ground floor :). The thermostat was checked and rechecked so it's not that. It's not necessarily about comfort as I'm used to it. It's more about engine wear as I do mostly short commutes and the engine never really reaches operating temps. – user1969903 Sep 1 '17 at 6:26
  • If I keep the AC on medium to high, it starts sapping the heat from the engine so before I leave I keep it on low, recirculating, and only on the windshield so I have an easier time scraping the ice of. Once I get going, I need to turn it up to atleast medium speed or else the windshield gets foggy. I only use the AC to blow warm air toward me once the engine is warm. – user1969903 Sep 1 '17 at 6:35
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It is not feasible. Possible, yes. Feasible, no. You would spend too much money for too little gain in heat.

Just a suggestion, you might consider a block heater. This would allow you to keep the block warm on those very cold nights. This would allow for quicker warm up in the morning.

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    Why would keep the block warm during the night? That seems a big waste of energy. Warming an hour before starting in the morning, that might perhaps make sense. – leftaroundabout Aug 31 '17 at 15:22
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    @leftaroundabout I don't see where I suggested wasting energy. How much energy will it take? Have you researched it? I guess that depends on how cold it is and what the OP wants to do. A timer might be a great suggestion to automatically turn it on at a specific time. Suggestions are better than criticisms, IMO. – CharlieRB Aug 31 '17 at 17:27
  • @leftaroundabout back when I used them block heaters needed several hours to heat up the whole block so leaving them in over night was really the best option. The heater on our car was 400 watts, basically two big light bulbs worth of heat, that will not heat up a cold iron block in an hour, but overnight it will help. Now, I just have a garage. – Ukko Aug 31 '17 at 19:48
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Yes, it is feasible. In fact, many cars do already have such a system. My car (Toyota RAV4 hybrid with 2.5 litre 2AR-FXE engine) has such a system. It is called "cooled EGR" or "coolant cooled EGR", and moves heat from exhaust gases to the engine coolant, exactly like how you described. It is also possible to move heat from the exhaust gases to the engine oil ("oil cooled EGR").

Here's a figure from a research paper that shows how well coolant cooled EGR and oil cooled EGR result in faster warmup: https://www.researchgate.net/figure/257207795_fig12_Figure-12-The-EGR-gas-temperature-after-the-EGR-cooler-for-oil-cooled-EGR-and

Now, if you're planning to install such a system on a car that doesn't have it already, it probably isn't that easy. It would require modifications to the coolant flow, and the mounting of such a device might be tricky if the car hasn't been designed for the installation of such a device.

I think the most sensible option for your case is a block heater, as CharlieRB noted.

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Due to emissions controls/law/performance on modern engines, any exhaust modifications must occur after the post-cat oxygen sensor. Otherwise you will blow codes, fail smog, and have poor performance.

There is such a thing as heating a car interior with exhaust. One example is the Old Beetle or any air cooled engine. There may be aftermarket kits for this but a quick googling didn't turn up any.

They also make exhaust heat recovery systems which interchange exhaust heat into glycol coolant. The problem with these systems is you are forced to circulate the glycol continuously, or drain it entirely -- if the glycol is static in the heat exchanger, the high exhaust heat will quickly boil it, causing excessive pressure in the system and leaving deposits. You can't include it in the engine's coolant loop because the radiator isn't big enough to cool both the engine and the exhaust cooler.

Another option is electric heat, using a rather large alternator. 100 amps at 14V is 1400 watts or about the output of a cheapie Walmart heater-fan. They make alternators as large as 400A. However wires are enormous.

It helps even more if you heat the driver instead of the whole cockpit. Do this with heated seats, that is an easy bolt-up solution.

For that matter you could toss a mains heater inside your cabin and have it switched on by a WeMo or other home automation system. I would be very selective on heater type, e.g those "oil radiator" types that don't get very hot. You'd want to remove the heater during travel, as the daily vibrations of being in a car would wreck a household grade heater.

The core problem is gas-engine cars are simply not designed for a 4-mile commute. The right thing for that is an electric or strong enough hybrid that it doesn't need to run the engine at all, e.g. Chevy Volt. These vehicles have systems for providing cab comfort without the engine, and the relevant thing here is those systems are instant-on. Either straight electric heat, or if possible, switching the A/C system to pump heat into the car instead of out of it. This is very efficient, able to bring in 3-5 BTU at energy cost of 1.

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