# Fuel consumption increases during winter?

1. Dec 3, 2012

### cosmic dust

I guess that if the answer is "yes" then the difference will be neglectable, but anyway...
I was wondering if during winter, fuel is less efficient than summer. During summer, there is some extra energy content in the fuel, due to increased temperature. This extra energy is added to the energy of the gas after the combustion and contributes in the production of work, so fuel during summer will offer more work. Is there something that I am missing here?

2. Dec 3, 2012

### cjl

Well, part of it depends on how you measure fuel efficiency. Colder fuel is a bit more dense, so the volumetric energy density is actually better with colder fuel. Per weight, you should be right (at least I can't think of any reason why you would be wrong), but I'd expect the difference to be pretty negligible.

3. Dec 3, 2012

### Staff: Mentor

Are you talking about cars? One big reason is that cars run rich and idle high until they warm up and since that can take a lot longer in the winter, it will cause the fuel economy on short trips to be much worse.

4. Dec 3, 2012

Staff Emeritus
Russ is right. Also, if you live in the snow belt, you have more days in stop and go traffic which also affects the economy. Finally, the cold temperatures tend to underinflate your tires.

5. Dec 3, 2012

### Khashishi

The maximum efficiency of any thermal engine is equal to that of a Carnot engine.
$\eta < 1 - T_C/T_H$
During the winter, the maximum thermal efficiency actually improves because the ambient temperature T_C decreases, while presumably T_H can be heated to the same temperature. But this is the efficiency of an ideal engine after the operating temperature is already reached...

6. Dec 4, 2012

7. Dec 4, 2012

### cosmic dust

@cjl
I had not think about this. Since engines have cylinders with standar volume then, per cycle, they consume more fuel but offer more work. Maybe, w.r.t. my question, the answer is positive, but here occurs something else: since fuel is charged per litre, then buying fuel during winter is more efficient (assuming that you buy it at ambient temperature).

Actually, I was referring to every kind of vehicle, although I was motivated by my motorbike. Of course, what you mentioned is very important about fuel consumption and surely the difference will not be neglectable. But my question was about the engine it's self, neglecting other factors such as traffic or warming up.

Last edited: Dec 4, 2012
8. Dec 4, 2012

### cosmic dust

I think that this formula applies to engines that produce work due to heat exchange. Here, the compression is guided mechanically and the expansion occurs because of the release of chemical energy.

9. Dec 4, 2012

### D H

Staff Emeritus
Rolling resistance is considerably worse in winter than in summer even if you keep your tires properly inflated. Cold rubber is much less flexible than is warm rubber.

Other problems:
Aerodynamic drag.
Air pressure doesn't change all that much over the course of a year, which means that density of air tends to be higher in winter. Greater density => greater drag.

The winter fuel mix.
Gasoline doesn't atomize as well at cold temperatures. Using the summer mix in winter would result in considerable air pollution from unburnt hydrocarbons in vehicle exhaust. To overcome this problem, the winter fuel mix has more ethanol than does the summer mix. Fuel efficiency as measured in miles per gallon drops because ethanol has less energy content per unit volume than does gasoline.

Oil viscosity.
It takes more energy to pump that thick, viscous wintertime oil throughout the engine block than is needed in summertime.

Darkness.
Anything that saps energy from the engine reduces fuel efficiency. Using the heater, window defrosters, or headlights saps energy from the engine and thus reduces efficiency. Live far enough north and its dark outside to and from work during winter. Another problem with darkness is that driver intelligence is impaired by adverse conditions, and darkness is an adverse driving condition. People drive stupid in winter. This leads to more traffic jams, and even when the traffic is moving, it moves slower in winter than in the summer. Miles per gallon is zero when you are going zero miles per hour down what is supposed to be a 60 MPH freeway.

Last edited: Dec 4, 2012
10. Dec 4, 2012

### Khashishi

Isn't rolling resistance lower when the tires are stiffer?

11. Dec 4, 2012

### jbriggs444

In the limit of complete rigidity (if one makes the counter-factual assumption that the tires would remain circular) you are correct that rolling resistance would be lower. Think, for instance, if you jacked your car up, inflated the tires with water, allowed it to freeze and then unjacked the car. You'd roll down the road as if on steel wheels with very low rolling resistance.

[But if you forgot to jack the car up first, you'd have a flat spot and rolling resistance would be complicated by a nasty thump, thump, thump as you rolled down the road]

In the limit of complete elasticity, rolling resistance is also made lower. Assuming that the tread does not squirm on the road, what you want is for the restoring force as the tire body unflexes to be the same as the the force that caused it to flex in the first place.

The point being made is that a "stiffer" tire in cold weather is probably one that still flexes almost the same amount but is not supple enough to unflex well.

12. Dec 5, 2012

### Naty1

It's common practice for auto racers to use dry ice to cool combustion air: it provides more efficient combustion. Analogously, turbo charged marine diesels often use seawater to cool incoming air prior to combustion....

13. Dec 5, 2012

### Staff: Mentor

Yes.... and turbocharged auto and truck motors are usually intercooled (air out of the compressor, heated by compression, passes through a radiator to shed some heat before entering intake manifold). The point of these exercises is not, however, to improve combustion efficiency. It is to increase the amount of air that goes into the cylinder as that's what limits the developed power, and in the spark ignition case, to prevent detonation.

14. Dec 5, 2012

### nitsuj

Rolling resistance, to the point of another poster would be less. (obviously with all else equal such as no snow on the road)

This must be false, the air is much more dry in the winter. I'm sure summer air is heavier. Im gunna go check it out now for certainty.

I would suspect there is a diminishing return with cold air intake, it's not about the intake air being cold...

I'd agree with Russ, warming the engine taking a toll on fuel efficiency.

15. Dec 5, 2012

### D H

Staff Emeritus
I'll dig up a reference on this later.

This one is easy: PV=nRT. If pressure remains unchanged (and it does; pressure at sea level is about one atmosphere year round), density has to increase as temperature decreases.

You apparently missed what I said about the winter fuel mix. Per EPA dicta, gasoline distributors increase the amount of ethanol in gasoline during cool weather months. This immediately decreases fuel efficiency expressed in terms of distance per unit volume because ethanol has less energy per unit volume than does gasoline.

The rationale behind those dicta is that gasoline doesn't vaporize as well at low temperatures. Vehicle exhaust would contain a greater percentage of unburnt hydrocarbons during wintertime if that ethanol was not added to the mix. Adding that ethanol mitigates the vaporization problem, but does so at the expense of fuel efficiency.

Moist air is less dense than is dry air at the same temperature.

Last edited: Dec 5, 2012
16. Dec 5, 2012

### Pythagorean

We've even monitoring our fuel. Went from 15 mi/gallon to 10 mi/gal in the transition to winter.

Also, it's -40 here so lots of idle time to warm up car for Human comfort.

17. Dec 6, 2012

### nitsuj

To DH holy snap I was wrong on all points ('cept for cold air intake, that point was for those saying cold air improves efficiency).

Tire rolling resistance info

And Avogadro's law for humid air being less dense than drier air.

I did miss your comment about the winter fuel mix. That seems like it'd really significant.

18. Dec 6, 2012

### sophiecentaur

Problem with simulations is that they just demonstrate someones idea and don't actually 'prove' anything. They are only as good as the model they use - and that's hidden.
It was pointed out to me in a recent thread, that even the mpg indication on my dashboard is not actually measured (the fuel isn't actually metered, for instance) - it's just inferred from some of the parameters of the engine and its conditions.

19. Dec 18, 2012