# Is a car more efficient in the Winter?

## Main Question or Discussion Point

I was thinking about the Carnot Theorem, and thinking how that applies to a car's engine. T_hot would be the temperature of combustion for the fuel/air mixture, which, once the car's engine has been operating for a few minutes under load, should reach a stable maximum. I think that T_cold would end up being a function of the temperature of the outside air in the environment around the vehicle (the temperature of the engine coolant would be a function of how cold it can get as it flows through the radiator, and cannot get colder than the air flowing over the radiator; it seems reasonable [though I may be wrong] to conclude that the colder the outside air, the colder the engine coolant will get).

So, T_hot seems like it would be, probably, about the same in the summer in Mexico, as in the Winter in Alaska, while T_cold could go down by a pretty decent chunk in a place in the extreme North, where temperatures might get down to -45 Celsius.

I realize that for Carnot's Theorem, you use temperatures on an absolute scale (that is, Kelvin), which is still 227.6 Kelvin, so it's not a huge jump, but seems like it might be a measurable difference?

Are there any practical complicating factors which might negate the expected increase in efficiency?

## Answers and Replies

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xts
Theoretically - you are right.
Practically - cars must have an universal construction, which is optimised to work with inflow air at moderate temperature (old-fashioned free suction Volvo cars, designed to work even in -30C conditions, had a thermostatic valve, wintertime mixing in some air picked from above of engine exhaust, to make it warmer)

But - even practically - the effect you think about is used at airplanes. Jets usually fly at about 10,000m not only because higher => less drag, but also because it is a tropopause, where air temperature is lowest.

I've just checked it by looking under the hood: my pretty modern Peugeot sucks the air from behind of the radiator - so, as soon as engine reaches its normal operation temperature, it is usually warmed to 80-90C.
Probably the engineers designing the car gave priority to stable conditions in which the engine works than to small improvement of efficiency.

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Bill_K
In summer as the temperature rises the vapor pressure of the gasoline rises, causing more evaporation and therefore more air pollution. To counteract this, the gasoline sold in the summertime is a different blend, making it more difficult to compare the engine efficiency.

Borg
Gold Member
Are there any practical complicating factors which might negate the expected increase in efficiency?
Poor software design. Today's cars have lots of software that control the timing of engine firing, fuel intake, exhaust, etc. depending on conditions such as temperature, humidity and other factors.

For example, a Prius is designed to shut off the engine during light load conditions such as when stopped at a light. However, they get about 5-10 miles per gallon (almost 20%) less in the winter because their software constantly turns the engine on in order to warm up the catalytic converter which cools off faster in the winter. I understand this has nothing to do with Carnot cycle efficiency but it's a good example of how bad software can really screw up the goal of getting the most out of your fuel. After all, why does the engine need to be turned on if it wasn't running in the first place?

xts
I agree with Borg that nowadays it is mostly (but not only) poor software issue.

In old times (as of my already mentioned Volvo of 1970), using carburatter engines, you had to use a special feature ('choke' - is it right English word?) to enrich gasoline mixture to keep the engine operating before it reached standard operational temperature. Until the temperature reached standard level, the fuel consumption was actually much (3 times...) higher, than normally.

But even nowadays not all issues may be controlled by software, so it is safer to design the engine to operate at stable temperature.

I didn't know about this 'eco-feature' of Prius keeping its catylser hot... I really like it

@Bill_K - where do you live to have different gasoline for Winter and Summer?
I - from my Lithuanian perspective - used to have different Diesel fuels (Summer one freezes at -15C, while Winter one should be still liquid at -45C), but I was sure the gasoline is the same all around the year...

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AlephZero
Homework Helper
There are also issues like the change of rolling resistance of tires with temperature (not to mention using tire chains in severe climates), the extra power consumed for heating and lights, etc.

My personal experience in the UK (where the difference in summer/winter temperatures is small, say 20K) with a modern gasoline engined car is that fuel consumption is about 5% worse in winter, not better.

@Bill_K - where do you live to have different gasoline for Winter and Summer?
I - from my Lithuanian perspective - used to have different Diesel fuels (Summer one freezes at -15C, while Winter one should be still liquid at -45C), but I was sure the gasoline is the same all around the year...
Well, I had forgotten about that (it's something that's not often discussed), but I believe pretty much the entire U.S. has this winter/summer gas difference. I think it's required by the EPA. Might be similar in UK/Western Europe, not sure. My impression is that Eastern Europe, Russia, and Asia don't take as aggressive regulatory action towards air quality?

xts
Might be similar in UK/Western Europe, not sure.
I am sure the whole European Union follows the same regulations. If are there any differences - they must be dependent on climate - so maybe at Cyprus they have different gasoline for winter/summer (to avoid excessive evapouration in +40C summertime), while in Northern Europe we don't care about evapouration, but wintertime we need different (not freezing) diesel fuel.

I can't believe US have homogenous regulationsfor such issues: summer at Alaska (and I believe - even at Montana) is not warmer than winter at Florida.

Other issue about car efficiency are tyres - winter tyres cause much stronger drag (over 10% increase of fuel consumption in my car case).

Maybe the gasoline regulations are different because of different octane index standards? In EU "standard" gasoline is RON-95, while (correct me if I am wrong!) in US it is 90. So "standard" EU gas is much heavier (thus having smaller vapour pressure) than US one - that's why in Europe we do not need to change it even for summer hots?

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Summer gasoline beldn in the summer in the US IS less volatile....
colder intake air aids combustion efficiency....hence INTERCOOLED turbo chargers on engines and sometimes even dry ice in the intakes of race cars.

My personal experience in the UK (where the difference in summer/winter temperatures is small, say 20K) with a modern gasoline engined car is that fuel consumption is about 5% worse in winter, not better.
No-one seems to have mentioned the major difference between winter air and summer air. which is not temperature but density and humidity, both of which reduce the winter efficiency over the summer.

There was a time when carburettors had winter and summer mixture settings to help offset this. I don't know if that is still the case. Drivers are probably too lazy to make the changeover these days.

The worst is driving in (freezing) fog conditions where fuel consumption rises dramatically in any vehicle I have ever owned.

go well

Borg
Gold Member
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Borg
Gold Member
Summer gasoline beldn in the summer in the US IS less volatile....
colder intake air aids combustion efficiency....hence INTERCOOLED turbo chargers on engines and sometimes even dry ice in the intakes of race cars.
No-one seems to have mentioned the major difference between winter air and summer air. which is not temperature but density and humidity, both of which reduce the winter efficiency over the summer.

There was a time when carburettors had winter and summer mixture settings to help offset this. I don't know if that is still the case. Drivers are probably too lazy to make the changeover these days.

The worst is driving in (freezing) fog conditions where fuel consumption rises dramatically in any vehicle I have ever owned.

go well
http://epa.gov/otaq/regs/fuels/ostp-3.pdf" - page 3-7:
Ambient Temperature Effects
Wintertime driving results in large decreases in fuel economy when compared to other
times of the year. These large decreases are due to increased stop and go driving, more
friction between vehicle mechanical parts, idling to heat up the vehicle prior to a trip,
increased rolling resistance due to poor road conditions, a greater power load on the engine,
and longer periods spent in cold engine operating modes at richer fuel/air mixtures. A
combination of these variables can produce a profound effect on mileage. For example,
the difference between city versus highway driving, excluding the wintertime temperature
effects of increased wind resistance and tire rolling resistance, can cause a variation in fuel
economy in the range of 5 to 10 miles per gallon. These cumulative effects can account
for as much as a 35 to 40% difference in expected fuel economy and far outweigh the effect
of the small change in gasoline energy density that is described below.

Seasonal Fuel Composition
Wintertime gasoline has a lower density and therefore less energy per gallon than
summertime gasoline, regardless of whether it is oxygenated. Lower energy results in
lower fuel economy.
One of the most important characteristics of gasoline is its rate of evaporation. The
property of a liquid that defines its evaporation characteristics is called "volatility". A
gasoline with low volatility will evaporate comparatively slowly and will cause engine start
up and warm up problems. These problems are caused by either insufficient fuel reaching
the combustion chamber or insufficient mixing of the air-fuel charge. Evaporation that is
too rapid (high volatility) will cause warm engine operating problems, primarily because
the liquid gasoline turns to a gas prior to reaching the carburetor or fuel injectors, thereby
blocking fuel flow. Such a condition is called "vapor lock". Fuel manufacturers vary the
proportions of gasoline components to produce volatilities within appropriate ranges. The
wintertime mixture is made up of less dense, lower molecular weight hydrocarbons in order
to provide higher volatility for cold weather engine operation.

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xts
@Borg:
In most of US summer gasoline must have vapour pressure not exceeding 7 PSI (hottest states, Alabama) to 9 PSI (colder states)
I've just checked - it is not required by law, but standard all-year gasoline 95 sold in Poland and Lithuania has its vapour pressure not exceeding 60kPa = 8.7 PSI - the same as summer one in not so hot States.

Borg
Gold Member
@Borg:
In most of US summer gasoline must have vapour pressure not exceeding 7 PSI (hottest states, Alabama) to 9 PSI (colder states)
I've just checked - it is not required by law, but standard all-year gasoline 95 has its vapour pressure not exceeding 60kPa = 8.7 PSI - the same as summer one in not so hot States.
I didn't dig too deep but that doesn't sound too surprising. I also didn't see a winter blend for the Alaska area but I would think that their harsh winters demand some pretty special fuels that might fall under a different section of the EPA guidelines.

xts
Oh, Come on... Lithuanian climate is not like Alaskan, it is more like Pennsylvania...

I really doubt (by I am not a chemist...) if we really need special mix of gasoline for winter. I think it is that, because of evapouration => pollution, you pay a bit more for not so volatile gasoline for Summer. As in EU we use higher octane index, gasoline has to be heavier anyway, so me may keep this standard all the year anyway.

But, maybe, in Southern Europe, the "standard" gasoline is a bit heavier than in LT.
Anyway I haven't noticed any change in fuel consumption of my car between travels with tank filled at Shell stations in Sweden or Italy.

But, on the other hand, we must pay for special nonfreezing (more expensive) winter diesel oil.

Staff Emeritus
2019 Award
I just looked at my records. The difference between winter and summer mileage for my cars is 8% for one and 10% for the other - lower in winter. I think this is typical, and I think it's because the Carnot efficiency is tiny compared to other things, as mentioned above.

One item in particular I suspect gets too little attention is tire pressure. In a place with cold winters, your tires are likely to be significantly underinflated for the first few miles.

Summer gasoline beldn in the summer in the US IS less volatile....
colder intake air aids combustion efficiency....hence INTERCOOLED turbo chargers on engines and sometimes even dry ice in the intakes of race cars.
Intercooling is done because the turbocharger delivers hot air (since the compression is approx. adiabatic).

Cold air intake is to increase air density during intake, doesn't have anything to do with combustion efficiency.

256bits
Gold Member
I think that T_cold would end up being a function of the temperature of the outside air in the environment around the vehicle (the temperature of the engine coolant would be a function of how cold it can get as it flows through the radiator, and cannot get colder than the air flowing over the radiator; it seems reasonable [though I may be wrong] to conclude that the colder the outside air, the colder the engine coolant will get).
The temperature of the coolant is regulated by a temperature sensitive valve. Engines run so that the block is at a certain temperature and the heat rejection is to the coolant is designed to be stable. If you notice your temperature guage in your car it stays at roughly the same temperature ( or should be ), summer or winter. In summer, as the outside air is hotter more fluid will have to pass through the engine block to keep the temperature stable. In winter, as the outside air temperature is cooler, the valve may not open as much and less fluid will pass through the engine block.

What will be more efficient, in a manner of speaking, is the radiator that cools the engine coolant, as the air that passes through will be at a lower temperature.

Thanks for all the input everyone. I found this to be quite an interesting discussion. I'm glad I asked! I kind of suspected that other factors might "overcome" any possible Carnot efficiency gains in the winter.

I know some folks talked about how the engines are essentially designed to run at a 'fixed' temperature regardless of outside temp (that is, warmer than the outside air at almost all times). Can someone explain to me why this would be? Why don't engine designers *try* to leverage Carnot efficiency gains in colder weather?

It was definitely interesting reading from you folks how the fuel differs enough in the Winter to increase fuel consumption simply by virtue of having less energy density, and that other aspects of the air (density and temperature) contribute to reduced efficiency in the Winter.

For about the past 15 years I've noticed a difference in gas mileage for winter and summer. For me, it's always better in the summer. When driving home there is a long (about one mile) mostly downhill stretch of road just before I get to my house. When driving my 5-speed I routinely shift into neutral for a down hill coast. The temperature has a very noticeable impact on how fast I will coast. On hot summer days I have to use the brakes. On cold winder days I can barely make it without coming to a stop. Other posters have already mentioned tire rolling resistance. But I'm wondering if the changing viscosity of the grease in the wheel bearings and drivetrain will also have an impact?

Thanks for all the input everyone. I found this to be quite an interesting discussion. I'm glad I asked! I kind of suspected that other factors might "overcome" any possible Carnot efficiency gains in the winter.

I know some folks talked about how the engines are essentially designed to run at a 'fixed' temperature regardless of outside temp (that is, warmer than the outside air at almost all times). Can someone explain to me why this would be? Why don't engine designers *try* to leverage Carnot efficiency gains in colder weather?

It was definitely interesting reading from you folks how the fuel differs enough in the Winter to increase fuel consumption simply by virtue of having less energy density, and that other aspects of the air (density and temperature) contribute to reduced efficiency in the Winter.
Cars use engines based on the Otto cycle, not the Carnot cycle. In fact, no engine (that I know of) is based on the Carnot cycle.
The outside temperature is irrelevant in the ideal Otto cycle efficiency. Engines run at a fixed temperature because it is designed to run that way, for reasons of thermal expansion, oil lubrication, among others.

Cold air intake is to increase air density during intake, doesn't have anything to do with combustion efficiency.
Depends how you define combustion efficiency, but certainly denser intake mixtures produce more power per stroke. This is well known among racers, you'll make the most power on cold days, and if it's allowed you might spend a lot of extra money to achieve a dramatic version of this effect by adding nitrous oxide to the intake.

Cars use engines based on the Otto cycle, not the Carnot cycle. In fact, no engine (that I know of) is based on the Carnot cycle.
The outside temperature is irrelevant in the ideal Otto cycle efficiency. Engines run at a fixed temperature because it is designed to run that way, for reasons of thermal expansion, oil lubrication, among others.
I don't think any engine in the world is based directly upon Carnot - isn't Carnot just an idealization of what the best possible thermal cycle is? But, it's my understanding that, at least for some engine types, the Carnot Theorem applies (for example, I've heard people talking about electric plants - gas, nuclear, etc - and discussing the concept of high temperature gas reactors, where they get better efficiency by operating at a higher reactor temperature).

So, I'd thought that maybe there was a possibility that car engines might benefit from colder outside air, but alas.

The otto cycle engine gets heat addition by burning fuel, not from a temperature reservoir. When you burn a fuel, you get X amount of joules added to the system. If the temperature is higher to begin with, it will also be higher after the burn. So it turns out it doesn't matter how hot the gas is when you started.