mmarshall

Here in the northern hemisphere, we're pretty deep into fall now, with winter only about a month or so away, and, of course, temperatures are dropping. Some parts of the country, in the northern Great Lakes/Plains/Rockies, have already seen snow....even blizzards. This time of year, car owners typically start to complain about dropping MPG figures, even with no change in the car's physical condition, driving habits, or weight-loading. In many cases (probably most), for a conventional gas or diesel-powered car, that is normal, though hybrids, with their complex gas/electric-motor interactions, may be a different issue.

It is considered normal for several reasons. First, gasoline does not vaporize as easily at lower temperatures as it does when warm, so to compensate, with a cold engine, the engine-computer has to signal the fuel-injectors for a richer (more fuel, less air) mixture going into the cylinders, also bumping up the idle-speed to help prevent stalling. As the engine warms up, the mixture leans out (more air, less fuel) and the idle speed drops, both conserving fuel (engines are considered at minimally normal temperature with the coolant at 140 degrees). Second, at lower temperatures, the engine oil doesn't flow as freely, adding some drag to the engine and requiring more fuel to be burned to overcome it....though, with the very light-vicsosity oils used today in newer cars, such as 0W-20 and 5W-20, the amount of drag is much less than with the old 10W-40 and 20W-50 of years ago. Third, at lower temperatures, the air itself becomes more dense, and requires more fuel (by volume) to compensate just to maintain the same mixture-ratio. Fourth, at lower temperatures, the tire-pressure drops (roughly 1 PSI for each 10 degrees), so the tires bulge out more on the bottom and have more rolling-resistance, which adds to drag. Fifth, the transmission, power-steering and differential/final-drive hydraulic fluids, like the engine oil, are heavier and also add more drag at lower temperatures (this is one reason why auto manufacturers are converting to non-hydraulic electric power-steering systems). Sixth, low-pressure weather systems, bringing widespread rain and snow events, become more common this time of year, especially in central and northern states......and a significant amount of rain or slush on the road can lower mileage by adding more drag to the tires...at least up to the point where they start hydroplaning and then start to lose traction.

So, what can you do about it as a car owner? Well, obviously, one cannot do much about the weather, but, of course, most of the same rules for saving gas still apply whether it is hot or cold....consolidating trips as much as possible, allowing the engine to get fully-warm as much as possible when you DO drive, driving at constant moderate speeds, avoiding traffic jams and stop/go driving as much as possible (which is almost impossible in many large cities), clearing all unnecesary junk and weight out of the trunk (weight consumes fuel), and avoiding jackrabbit starts/stops. Keep the tire PSIs at recommended cold-inflation pressure (cold-inflation means sitting stopped and out of the sun for more than three hours, as direct sunlight heats up the black rubber in the tire and warms the air inside, increasing its pressure). Newer cars, of course, with the now-standard tire-inflation sensors, make this job a little easier, as with some (not all) of them, they display the direct PSI in each tire for you. Cheaper (but still legal) sensor-systems simply compare the rotation-rate of each tire to the other three, and when one is significantly off (indicating a loss of air), flash a warning light instead....and then you have to find out which one is the culprit. Also, on a cold morning, avoid the temptation (many car-owners don't) to remote-start the engine and stay inside, or go back inside while it sits, idling, warming up in the driveway. An idling car gets zero MPG, and the transmission/differential fluid doesn't warm up while the car sits still either. Instead, start it up, allow the oil-pressure to come up a little (maybe a minute or so if the temperature is below freezing, two or three if below zero), and then SLOWLY drive off....once oil pressure is up, the engine will warm up faster and more efficiently moving under a light load than standing still. Some newer automatic transmissions, to warm up the transmission faster, will electronically delay the gear-upshifting in higher gears until the engine coolant is at a certain temperature, so that's a further reason to keep the speed and RPMs down when cold). In today's upscale cars, of course, heated seats and steering wheels help to keep your buns and pinkies warm, so you aren't necessarily sitting there shivering while the engine warms, either.

bitkahuna

iTrader: (20)

agree with some of your points, but not others.

letting the engine 'fully-warm as much as possible' - a lot of cars don't even have engine temp guages these days first of all, plus, obviously waiting longer at idle after starting is using more gas, so not sure how that can help.

about engine/transmission and lubricants in general - they've improved so much that unless you're in EXTREME FROZEN cold i doubt it makes any difference.

i've heard the opposite about fuel/air - that colder air is DENSER than warm air, so there's more combustion thus requiring less fuel. i believe race cars are known to be more efficient in colder weather, not hot.

about tires - people should keep the right pressures obviously when the car has not been driven for a few hours.

i'd say people in colder regions notice a slight mpg drop simply because they let the car interior warm up, not for the engine's sake, but for THEIR sake (don't want to be in a cold car).

and finally, it obviously completely depends on where you live. in minnesota it might be 0F in the middle of the day during winter and 80F in summer, and here in central FL it might be 50F in winter and 90F in summer (a MUCH smaller temp swing). i wouldn't be surprised if i get better mpg in the 'winter' here than the summer!

mmarshall

Yes, it uses a little more gas, but my point was that some people sit there in the driveway until the car's engine and interior are toasty-warm, THEN take off. That, in general, uses more gas, (and doesn't help a cold engine or transmission warm up any faster) than simply driving it slowly and gently while cold.

Synthetic lubricants, of course, flow freely at much lower (and much higher) temperatures than conventional dino/petroleum-based ones, so I agree that is not as much of an issue.

But, anyhow, thanks for bringing that point up ...I should have noted in the article, which I didn't, that it referred mostly to conventional lubricants, though many new cars have synthetics or (as in my Verano) semi-synthetic oils.


Cold air is denser (you're right)...and I stated that in the article. But the way I've heard it from auto engineers (and the way we were taught in flight-training, where pilots had to manually adjust mixture on their engines as the plane climbed or descended) is that an equal VOLUME of air required a coresponding equal VOLUME of fuel to maintain the same ratio. In an aircraft (or in a car climbling into high mountains) that is important because the average density of air decreases around 2% for every 1000 feet of altitude, affecting a number of things....air-fuel ratio, engine-power, and pitot-static instrument airspeed readings.

Yes.....but tires can sit still in direct sunlight, increasing the PSI whle the car is not moving. That's why tires should be checked overnight or after sitting in a shady place for at least three hours.

The nice thing, of course, about individual-PSI sensors on new cars is that they do that for you at the touch of a button...no fumbling around with hand-held gauges. My Verano has the individual PSI-sensors...and I really like them.

True...but today's bun-warmers in the seat, and pinkie-warmer steering wheels help make a cold warm-up more tolerable. Engine thermostats also run hotter than in older cars, allowing more potential heat to come out of the interior vents as the engine warms.

In the article, of course, I'm generally referring to places that have a REAL winter. Most of the time, in most of the state, that's not really an issue with Florida.....though it can sometimes get pretty cold in the Panhandle around Tallahassee.......away from the coast.

I8ABMR

All I have been feeling is MORE POWER. AZ is finally in the 80s and 70s so the cars ECU is no longer retarding the timing and the LS feels like a monster relative to driving it around at 115 degrees during the summer . The only thing that has killed my mpg recently is the Michelins. They are sticky and have a greater rolling resistance. Im down 1-2 mpg easily since they went on

praetor

Meh. I think it has to do more with driving conditions and the change to winter blend fuel. I read the winter blend fuel produces slightly less power due to the different additives used. And in the colder months, there is often a lot more idling and traffic due to warming up the car, snow, ice and other road conditions. My last car, I would remote start for at least 5 minutes before even stepping out of the house.

I8ABMR

So glad we dont deal with snow in AZ but it does suck that our cars are in limp mode for 4-5 months out of the year.

mmarshall

Cooler, denser air, if it maintains the same air-fuel ratio (which the ECU also manages) will burn more fuel per unit of cylinder-volume, therefore producing more power. That is one reason why many turbo engines have intercoolers....to reduce the temperature of the incoming charge-air to the combustion-system.

mmarshall

That's not as much of a factor as it once was, since the EPA requires an E10 gas/ethanol blend for many areas, regardless of the season. Ethanol, of course, is notorious for robbing mileage...it has only some 60% of the available BTU energy that typical gasoline has.

Yes, it makes the car's interior warmer and more toasty for you when you got in, but it also takes longer for the engine itself to warm up itself (for reasons which I explained above), and doesn't warm up the other powertrain fluids (transmission, final-drive, etc...) at all. In short, it wastes gas.

I8ABMR

feels great but the drop I had started when I mounted the Michelins during the summer. Now its cooler but the AC is not on as much so I wonder if its just balancing out and thats why its staying about the same

mmarshall

Yes, A/C compressor-factors could be making a difference. So could simply new (or newer) tires like your Michelins if they have less rolling-resistance. High-performance tires, though, in general, tend to have more rolling resistance...because the rubber needs to have more friction and grip the pavement more for better cornering/braking. That's why some of them also wear out relatively quickly.

Infra

iTrader: (1)

mmarshall is right about the stoichiometric ratio not changing regardless of air density. "Volume" is the wrong term, however, if you'll indulge my pedantic nature.

Mass would be a better term as the volume of air inside a cylinder is a fixed parameter. Mass/Volume gives density.

Or simply "# of molecules". From the ideal gas law, PV = nrT, if you decrease temperature, you can correspondingly increase n. More air in the same volume allows more fuel, thereby creating more power.

One thing you forgot, though, is that many moderns cars now must heat up the catalytic converter for emissions. This is done by using a very rich mixture that does not fully combust, and some fuel remains to burn in the exhaust, thereby quickly heating up the cats. This is why some cars will be very LOUD for the first minute or so, in addition to running at a greater idle RPM.

mmarshall

Thanks, Infra, for explaining the formula and technicalities. I know the rough basics of fuel-metering, but I am not a chemist or a physicist.

A lot of catalyists today use a three-way function, too, for quicker warm-up. But the engineers have to be careful how they program those rich mixtures, even during warm-up. A typical catalyst, when fully hot, operates with an inside temperature of some 1700-1800 degrees Farenheit....that's how it burns up most of the pollutants and/or changes them into water vapor (steam). Obviously, with heat that fierce, if you have any gas in the mixture that doesn't fully combust before hand, it can cause an exhaust-fire or blow the catalyst right off the underside of the car. That is why catalyst-equipped cars, particularly in the days before on-board computers and OBD II systems, often had a warning in the Owners' Manual warning the driver not to continue driving with a fully-warm engine/catalyst if there were any signs of a misfire, where the spark plugs weren't burning the fuel before it went into the exhaust. With the newer engines, of course, a warning light on the dash comes on if the engine computer detects a misfire, and the system shuts off (or greatly leans out) the fuel to that cylinder to prevent a catalyst explosion.

GoGiantsNJ

My '12 RX drops about 2 mpg in the Winter. I have it garaged at home so that helps. I see people all the time letting their cars idle for too long for their creature comforts. Those same people complain about terrible gas mileage. Go figure.

Toys4RJill

The biggest factor on reduced MPG is the change in fuel blends of colder climates. The driving conditions do have a factor as well. I wouldn't worry about cold air and such.

Warming up the car is a no brainer, however it is a personal choice, so like to remote start their cars while others drive off. I have a garage so it does not worry me much.

J.P.

Read stats before all over the place but if the average is .5 GPH at idle, and you're warming your car up for 15min, that's like 43 cents to warm your car up in the morning, big deal? no