Does the Heater Waste Gas?
 

This is a discussion that I had with a coworker a while back. He has a mechanical engineering degree so he should be quite knowledgeable and have a fair bit of common sense and intuition.
The question is weather or not having the heater on burns more fuel. His answer was flat out "no", that it is waste heat and would be lost anyway.
My assessment was that during engine warm up, having the heater turned on would prolong the amount of time that the engine is below operating temperature and therefore increase fuel consumption. Once operating temperature is reached I agree with him that there is no loss in efficiency.
Does anyone here have any insight into this? Thanks.

You are exactly right.

And its not only the cooler temperature that reduces efficiency, it takes more of a hit. When below a certain temperature, the car's computer often runs in warmup mode. Which defaults to a richer fuel mixture, and the torque converter does not lock up on an automatic transmission.

I am a mechanical engineer too, and some of my coworkers who have more seniority still dont know jack about cars. Its a different field in a lot of ways.

The heat has to come from somewhere. I see about 8 degrees different in the water temperature when running the Heater. One of the reasons to run a grill block, beside aero, is to help warm up quicker and run hotter. The hotter the engine, to a point, the better the FE.

The Old beaters I drive when it starts to overheat I can turn the Heater on Full blast and the temp will drop.

if I don't turn my heater fan down when at a stop, I can move my engine temp needle down a fair amount, I tend to keep the fan on the lowest setting or my car will never register on the temp gauge before I get to work, and if it's not warming up it's using more gas.

I sure wouldn't call is waste, I like to stay warm, but during a long car ride my girlfriend asked me the same question, does it use more gas? I had some time to think about it... there are two other very small power consumers to consider:

A stretch, but the fan you use to get that heat into the cabin takes electrical power, requiring the alternator to work a little harder.

A real stretch, the waterpump works harder to push the coolant through another heat exchanger, increasing the surface area in which the coolant must flow over.

Again, these are both big stretches, and small consumers of ultimately slightly more fuel when you run your car's heater.

I think your engineer friend was refering to when the car was already up to operating temperature. in that case, its very unlikely that the heat itself will cost you power, the blower fan will.
if you wait untill your car is up to temperature then turn on the heat you can have your heat and drive it too, so to speak. once the car is up to operating temps (NOT warmup, NOT extending warmup, already warmed up) then the extra heat IS just waste heat that would only heat the radiator.

btw the radiator is not ALWAYS flowing engine coolant, there is only flow when the coolant is above the preset temperature control (usually a wax that melts at a specific temperature) valve and allows the flow to go through the radiator, its a paralell line on the coolant circuit.

though I belive many cars use the engine oil to heat the car, or used to at least

Thanks for the replies, you all bring up some interesting points. I know from my ScanGauge II that my engine consumes over 2.0gph at idle when cold and around 0.7gph when hot, so there is a significant difference.

I'm sure it wastes fuel because now everyone idles their car for a half hour before driving anywhere. :rolleyes:

But that behavior aside, and warmup time aside, it is just waste heat once the engine is up to temperature.

yeah i leave my heater off till it gets to 170ish, stops at 182ish

but with the heater blowing at full, it does take alot longer to warm up. thus useing more fuel.

but once warm nah.

2 gph at start up wow.....

None that I've ever heard of.

Only time I ever heard of using oil to heat the car was a mining equipment guy in Grand Junction CO, who drove an old VW Thing. Great snow car, but a lousy heater. He rigged up an oil cooler and a fan in a sheet metal box and controlled it with a switch and a manual valve. Worked like a champ.

Any oil-heated cars would be the work of determined DIY freaks.

On my cars the coolant always flows through the heater core. When on COLD it just pushes a flap so the fan doesnt blow through the core.

Wonder if the Thing used exhaust to heat the car like the old Beetle did. Id prefer oil heat too, at least if it fails its probably not going to kill me.

Really? Cause mine definitely does. Otherwise it'd run at 1750 RPM at 55 instead of 1550.

Im sure that it is not applicable to all vehicles. The ones I am familiar with, it will not lock in warmup mode.

All of the old VWs used the exhaust pipes to heat the cabin. An oil cooler would be, at best, a "supplemental" cooler--or one that took the place of rusted-out heater boxes on the exhaust. Takes quite a while to get warm enough to do any good, though.

BTW, I believe that there is still a hit to your FE from using the heater even when the coolant is up to temp. It will vary upon the amount of heat that your car needs to shed to stay at operating temp and the amount of heat you are putting into the coolant, and it is likely too small to measure reliably anyway, but I do think it is there.

Could be wrong, though...

-soD

Lazarus -

That's egg-zactly what I used to do with my old Datsun B210 Honeybee. Until I implemented my manual fan radiator control switch, I used to do that in the summer with my SW2. I still advise it to friends as a stop-gap when they're having problems. It makes me wish there was a setting on the vent-control-switch that would "vent to the outside". That way, the heat wouldn't enter the cabin when I'm trying to protect the engine.

I never turn on the cabin heat unless the engine coolant is at least "almost warmed up".

CarloSW2

I forgot about that... most modern cars have contious coolant flow through the heater core, and control where the warm air goes via the under dash air plenum. I had worked on some older cars where the heater core was valved off when there was no call for heat, the dash control for warm air opened up the valve and allows coolant to flow through the heater core.

VW's; I've owned 20-some-odd VW's, including a Thing. The VW Thing used a beetle confiuration engine, cooling, and heating. The factory heaters on those use a heat exchanger, aluminum fins cast around two of the exhaust pipes, then encapsulated in sheetmetal to form an isolated plenum (called a heater box) for fresh air to be warmed and piped into the cabin. Carbon monoxide poisoning in those cars is not very common. I remember seeing an aftermarket system where engine oil flows through an oil cooler, an electric fan blows warmed air into the cabin, but it was a DIY kit sold in hotVW's magazine 15 years ago. I wish I had one to use this winter, I've been invited along on this year's Mt.Shasta VW Snow trip. 10-15 teams driving pre-'68 VW buses, offroad, mud, ice and snow, hillclimbs and overland trials. I understand numb fingers, rolled busses and hangovers are common.

The exhaust heat exchanger idea, I've personally used in a golf kart. Air-cooled Kohler engine, and the pipe was just ingeniously run right behind the seat, dead center. It was quite easy to take some of the accessory power from the electrical system, cut holes in between the seats, and apply some sinks to the pipe, with a nice vent box around it that allowed external air to be heated at low velocity as it was sucked into the passenger compartment (enclosed, obviously). I wish I'd kept pics of it, but they're gone with most of my old half-finished projects.