In the summertime, I routinely get 550 km and sometimes more on a tank of gas (~50L) in my '03 Chevy Malibu. The two times I’ve done long distance highway driving, I broke 600km.
I noticed last winter (my first with a car) that my mileage was poor, but improved in the spring. This winter, my mileage is quite shockingly low (I barely got over 400km on this last tank). My driving speed / behaviour hasn’t changed very much, and although I did spring for the synthetic oil on my last oil change (twice as expensive, but I figured it would be better for the cold), I figured I’d ask here.
My three WAGs are: air temperature affecting something, air density affecting something, and gasoline manufacturers changing formulations for winter. (Though I’m willing to entertain the possibility that my car needs a tuneup of some description)
Gasoline is different between summer and winter. Also in some places they oxygenate the fuel in winter which does reduce fuel mileage.
But probably the biggest single thing is the temps in the morning when you go out to your car. The colder it is the more fuel must be injected to start and warm up the car. Also the colder it is, the longer it takes for the engine to reach full operating temp, so the longer the cold start enrichment stays on.
If you do a lot of short trips in the winter, your fuel mileage will suck great big donkey balls.
I won’t pretend to have anything I can back up with a physics formula for this. I will, however, cite the fact I live in North Dakota. We’ve been known to have a chilly spell here and there.
A warm engine should usually burn fuel at a better rate of efficiency. The engine is made of metal. The gas (or diesel) is burned in that metal casing at the same temperature. A cold engine should therefore waste more of its energy in heat dissipation warming the metal than in an engine that can expend more energy in power production.
I have no doubt that someone will come along and prove this to be false, and if so I look forward to it. It’s something I’ve thought of for some time.
In personal experience, I’ve noticed that (at least with large V-8’s) a warm engine will outperform a cold V-8. It can be seen in motorsports that employ such engines. That’s why NASCAR, and other races start with warm up laps after firing the engines. It isn’t just getting the tires scuffed.
Not sure how Formula 1 and CART and IRL cars figure into that since they use different engines, but it seems that loss of energy heating the engine causes a loss of energy per combustion cycle.
OTOH, we usually leave our cars running while going into a store and have an insanely low rate of vehicle theft.
By and chance, do you let your car warm up before you drive it?
I tried to explain to my wife that letting her car warm up for 5 minutes before the 5 minute drive to work essential cut her milage in half but it didn’t go over so well.
I suppose that all depends on the model of your car. I have driven many vehicles with the “Mileage Remaining” indicators, and most of them go down slightly when you turn on the heater, especially if you already have a load on the alternator, such as with headlights, etc.
I haven’t used thermo in awhile, but doesn’t a greater temperature difference between the engine (at steady state) and the ambient temperature have a greater Carnot effeciency?
I’ve wondered about that. How much gas does it take to warm up the car vs. drive it (warm or cold)? Of course it’s more comfortable and it defrosts the windows, but what’s the breakpoint? I can either warm it for a few minutes while making breakfast which doesn’t do too much, or 20 minutes while I shower and dress, which seems like too much?
Am I really wasting much if I warm it for 20 minutes and then drive a nice warm car to work for 20 minutes? Assuming nights in the teens or so.
AFAIK, heat in an internal combustion engine car is as close to free as can be. I’d be more worried about the extra power used by my headlights in the darker days.
If I let my car warm up, I’d be even more late for work! I have an unheated ancient shack that I park in to avoid frost; if it’s below -20C or so, then coming home I let it warm up for however long it takes me to scrape the windows off. (Hasn’t been an issue for December though).
>I haven’t used thermo in awhile, but doesn’t a greater temperature difference between the engine (at steady state) and the ambient temperature have a greater Carnot effeciency?
Right you are. The possible power you can get from moving heat from a higher temperature to a lower temperature is limited to the total amount of heat you start with, multiplied by 1 minus the output temperature over the input temperature.
In fact, using the heater should lower the engine temperature (at least, if the thermostat in the cooling system has finite gain and no integral action), and this would also improve the Carnot efficiency.
Rick nails it in 1. To what he said you could also add: 1) your tires have higher rolling resistance in cold weather, and if you don’t readjust your inflation pressure they could be underinflated in cold winter months too, and 2) in addition to the engine heating up, you also have to warm the other fluids in the drivetrain - trans fluid and differential fluid (for RWD), and these are warmed up by creating a greater driving load for the engine.
Correct - heat is free. I believe that your headlights also would be in the noise as far as mileage calculations go - they would be a second-order effect that would be hard to distinguish after recognizing the more significant things that, for example, Rick mentioned.
In control terms, the thermostat is a proportional controller, but it saturates at the high-temp end and doesn’t respond (start opening) until fluid temperatures reach (typically) 180-195 degF. So, once the car reaches operating temperature the heater should not affect coolant temperatures - rather than dumping waste heat through the radiator you are now sending it into the car interior.
And, Carnot efficiency deals with the temperature of the working fluid, which in the case of an internal combustion engine is air, and reducing the engine block temperature is not going to increase efficiency. You get a boost in efficiency with a warm engine, since the combustion mixture isn’t cooled as much by the block/head. Power-wise, you can run more advance with a cold engine since pre-ignition is less likely (this is one reason why drag racers likes to start with cooler engines).
The colder the air, the denser it is so theoretically since more air is entering the engine in cold weather, that should increase the power output.
In practice the difference in air density is minuscule and cold mixtures do not ignite very well which ultimately hurts performance. Only forced induction cars (turbocharged or supercharged) benefit significantly from the colder air and most of that is due to the more efficient cooling of the intercooler.
I have a magnetic oil pan heating element attached to my cool weather vehicle and I put a $20, thermostat controlled little room heater on the passenger floorboards on a a board and at night set to about 70º. I plug that all in, ( Arranged so I can do it all with one extension cord through the door. ) and I have clear windshields, warm seats, warm oil at the git-go and I only need to worry about tire inflation, and gearbox and transmission fluids being cold and sluggish. I was lucky in that most times at work when it was really nasty I could also use an extension cord I carried to plug in as they liked my 4X4 to help others leave when it gets deep or slick.
Believe it or not, in reality, less advance is used during cold start and warm up. The object here is to get the coolant and oxygen sensors, and converters to operating temperature as soon as possible. By retarding the ignition timing (and if equipped changing the valve timing) more heat is dumped into the exhaust and coolant. Less power is made, which also contributes to poor mileage, but the object is to get the emissions systems working ASAP.
As far as the temp of the intake air goes, the system will adjust the mixture as air temp changes. So if your intake charge is say 0F you will get more fuel injected than if your intake charge was at 100F. (assuming a fixed throttle angle) Of course, since you will get more bang for your buck, you will use less throttle with the cold air to accelerate at a given rate. So the intake air temp is pretty much a wash. It is the temp of the block that causes issues. A cast iron block full of coolant at 0F is a huge heat sink.
