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  #13  
Old 11-07-2012, 05:37 PM
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Re: Gas mileage decrease when....

Most vehicles get less mpg's in the colder months, even my motorcycles.
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Old 11-07-2012, 08:25 PM
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Re: Gas mileage decrease when....

Quote: "Most vehicles get less mpg's in the colder months, even my motorcycles."

I do not know where you found this myth, but some factors may contribute to how you came up with this hypothesis. I'll come back to this later.

Your vehicle actually burns fuel more efficiently in winter months because cold air mixes easier with gasoline, promoting a more efficient burn. This is also why so many aftermarket companies attempt to sell you on Cold Air Intakes to improve horsepower and gas mileage. In addition, there is generally speaking less Ethanol in the gasoline during winter months, which actually helps your vehicle in increasing gas mileage because Ethanol is not a very efficient fuel compared to gasoline. If you do not believe me, you are welcome to read numerous studies on this matter published all over, including by the EPA. An article in Motor Trend about the attempt to bring the 2013 Shelby GT500 Mustang to 200mph also verifies this point of view in terms of the effect of outside temperature on horsepower and performance.

Another factor that could cause you to assume that you are getting worse gas mileage in the winter compared to summer months is in regards to distance travelled. If you drive your car only a few miles forth and back in the winter, your engine will not reach normal operating temperature nearly as quick as during the summer months, which also means that it burns more fuel. Again, parking your vehicle during the winter, even for relatively short periods of time, can cool the engine enough so that the gas is not burned very efficiently upon restart. The reality is, that most people also do not like to get in a cold car, which means cars are often left idling for longer periods of time during winter months in order to preheat the cabin and prevent your butt from accumulating ice patches. Idling is extremely bad for fuel economy, especially when an engine is cold. The seal between pistons and cylinders is only effective at normal operating temperatures, thus increasing fuel consumption when the engine is cold. Also, most fuels are designed to burn at very specific temperatures, which requires the engine to be at normal operating temperature before the gasoline can burn at its maximum potential. If you do not check you tires during the winter, your tires may also deflate significantly as a result of decreasing temperatures. This can also decrease your mpg as more friction takes place due to a larger contact patch with the road.

In defense of your argument that vehicles burn more fuel in the winter compared to summer months, there are some factors that can contribute to increased fuel consumption during winter months. The strain of outside temperatures on both batteries and alternators will cause both of these items having to work harder in order supply the vehicle with electricity. Cold starts are also notoriously bad for batteries, effectively making the alternator having to work harder to recharge the battery, which increases gas consumption. Again, the entire powertrain, including transmission, transfer case and differentials all contain lubricants that operate better under warmer temperatures compared to the colder temperatures during the winter. As a result, you may very well see a decrease in mpg if you are not operating your vehicle over longer distances or for long periods of time.

Now to your hypothesis in regards to less mpg in the winter on your motorcycles. I reckon that your experience with less mpg on your motorcyles may correlate with the fact that you are unable to endure long enough trips during the winter to outweigh the negative impact of cold starts and the time and fuel it takes to heat up your engine. Do yourself a favor and buy a heated set of clothing from Gerbings and do a 200 mile loop on your bike in the winter and then the summer. You will be amazed at the results, both from the Gerbing clothes and on the average miles per gallon consumed between winter and summer months. I average an increase of approx. 8-12% in terms of miles per gallon on my bike running longer stints during the winter compared to the summer months, but this data is very sensitive to the motions of my right hand

In conclusion, your vehicle does not actually consume more fuel in the winter, unless you travel distances that do not allow the vehicle to make up for the additional strains caused by colder temperatures.

I hope this clears up any misconceptions that you may have in regards to fuel consumption and outside temperatures.
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Old 11-07-2012, 08:48 PM
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Re: Gas mileage decrease when....

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Originally Posted by WK2007 View Post
In conclusion, your vehicle does not actually consume more fuel in the winter, unless you travel distances that do not allow the vehicle to make up for the additional strains caused by colder temperatures.
Except for the fact that winter blend fuels typically contain more butane to make them more volatile in the colder temps and higher atmospheric pressures. This results in a fuel that has less BTU per gallon and results in lower fuel economy. So you really do get lower fuel economy with everything else being equal.

Supposedly the energy content in winter blend fuels is only 2% lower, but I have noticed that my fuel economy drops nearly 10% once they switch over. It's possible there is something else that is going on. I'm guessing the other additives to "oxygenate" the fuel perhaps cause the 02 sensors to report slightly different values and cause slightly richer air/fuel mixtures in closed loop operation.

I know it's anecdotal, but it my personal experience winter blend fuels definitely have a noticeable impact on fuel economy.
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Old 11-07-2012, 09:13 PM
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Re: Gas mileage decrease when....

I think the compressor cycles when the heat is on. I'm going to see for sure, and hopefully find a setting where it doesn't.
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Old 11-07-2012, 09:19 PM
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Re: Gas mileage decrease when....

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Originally Posted by stashgto View Post
I think the compressor cycles when the heat is on. I'm going to see for sure, and hopefully find a setting where it doesn't.
Check your owner's manual. Some systems cycle the compressor to control humidity.
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Old 11-07-2012, 09:21 PM
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Re: Gas mileage decrease when....

Doc, I agree and can support the subjective impression that fuel consumption increases during winter months. However, this is due to confounding variables, not actual fuel consumption by the engine. Because I drive relatively short distances, I too see my mileage get worse in the winter. But this is in reality not due to the colder temperatures causing the engine to burn more gas. It is simply because cold temperatures take a toll on other elements within the vehicle, as I mentioned. If I were to do a long run in the winter and then in the summer, the winter run would prove to increase gas mileage.

I have never heard of Butane being added to any type of gasoline made for normal vehicles as it is a gas. Instead, I believe you are thinking of Butanol, or n-butyl, which is actually an alcohol in the family of Oxygenates. N-butyl is not the only additive that is applied to gasoline in order to improve burn. Flame retardents, anti-knock additives and much more is mixed with gasoline in order to make the burn as efficient and controlled as possible. I am not trying to embellish what I know and I am certainly not a chemical engineer, but I have friends in very low places, aka the oil industry , who specialize in this and they all tell the same story. Gasoline burns better and more efficient when mixed with cold air compared to gasoline mixed with hotter air. Cold air is more dense and contains more oxygen compared to hot air, which lowers the amount of gas needed to provide an efficient combustion. It's an undisputable fact...
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Old 11-08-2012, 07:26 AM
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Re: Gas mileage decrease when....

I believe you are thinking about this incorrectly. In colder higher density air an engine can make more power but it won't be more fuel efficient.

Let's say we have two identical small single cylinder fuel injected engines. One is in very dense cold air. The other is in very thin hot air. Both engines have their throttle plates open 10%. I'm going to use some silly numbers to make the math easy.

The engine in the cold dense air pulls in 28,000 molecules of air per revolution. It injects 2,000 molecules of fuel to maintain a stoichiometric mixture of 14:1. The engine produces 2 HP.

The engine in the hot thin air pulls only pulls in 14,000 molecules of air per revolution. The fuel injection system only injects 1,000 molecules of fuel to maintain a stoichiometric mixture of 14:1. The engine produces 1 HP.

To get the engine in the hot thin air to put out the same power as the engine cold dense air we have open it's throttle plate to 20%. Now the engine in hot dense air will pull in 28,000 molecules of air per revolution and inject 2,000 molecules of fuel to make 2 HP.

Both engine are making the same power with the same amount of fuel. The only difference is the amount the throttle plates are open. Technically speaking the engine with the larger throttle plate opening will experience lower pumping losses and be more efficient. I'll ignore the fact that the engine in the hot air is technically more efficient due to the higher pumping losses the engine in the cold dense air is experiencing and just call them exactly even for this mental exercise.

So here is the rub. Now we introduce winter fuel that contains 10% less energy per unit to the engine running in the cold dense air. That engine is still pulling in 28,000 molecules of air and injecting 2,000 molecules of fuel but it is now only making 1.8 horsepower. So now we have to increase the throttle plate an additional 10% to compensate for the 10% lower energy density of the winter blend fuel. At a 12% throttle plate opening this engine is now pulling in 30,800 molecules of air and injecting 2,200 molecules of fuel to make the same 2 HP. So now this engine is burning 10% more fuel to make the same power.

So you can see while higher air density can allow an engine to make more power at any given throttle opening, it doesn't make it any more fuel efficient. Technically because of the lower volumetric efficiency of the engine with the smaller throttle opening (cold air engine) it is less efficient due to higher pumping losses but that is a dissertation for another post. When you reduce the energy density of the fuel, the engine's fuel efficiency is reduced. I know I exaggerated the 10% reduced energy density for the sake of making the math easy. Supposedly it's only 2%.

Phew! Sorry for the long explanation. I hope it helps.
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Old 11-08-2012, 08:34 AM
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Re: Gas mileage decrease when....

Winter mpg varies with temperature. This discussion about winter fuel is great and does its part to reduce mileage, but the major contributor to the mileage decrease is the longer cold run cycle. Do you think winter fuel energy changes with the kind of temperatures we see? As Doc explains, short of full throttle, the change in temperature isn't going to effect the power you get for the mpg.

Back in the day when instant mpg indicators were included, it was just surprising to me how long it took for the engine to kick out of the cold enrichment and higher idle cycle. Often 30 minutes on the highway. Near 50 minutes in sub freezing temperatures. The colder it was, the more impact on mpg.
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  #21  
Old 11-08-2012, 09:01 AM
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Re: Gas mileage decrease when....

Thanks Doc for the reply.

I am inclined to think that you are right as I read up on the topic. I had also not accounted for the change in energy per unit between winter and summer blends as I was commenting on a standardized comparison with no change in any other variables, except air temperature. Now I am getting really curious about the exact effect of air temperature on the fuel consumption of the engine itself and I have sent this discussion to a few, and hopefully accurate, resources for further evaluation. Like I said, I may actually have gotten this wrong in terms of gas consumption by the engine itself. Must be early senility on my part, but let's see what the resources fire back with.

Again, thanks for adding to the conversation, Doc!
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Old 11-08-2012, 10:26 AM
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Re: Gas mileage decrease when....

Great info WK2007 and Doc. I think everyone in this thread is correct in their own right, and the answer to the OPs quandry is that it's a blend of ALL these things, not just one or the other.
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Old 11-08-2012, 11:24 AM
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Re: Gas mileage decrease when....

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Great info WK2007 and Doc. I think everyone in this thread is correct in their own right, and the answer to the OPs quandry is that it's a blend of ALL these things, not just one or the other.
I agree. The lower energy content is the winter blend fuel can only account for a 2% reduction in fuel economy. As you, WK2007 and others have stayed other factors account for the rest.
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Old 11-08-2012, 06:04 PM
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Re: Gas mileage decrease when....

Well, I noticed an approximate 3 mpg decrease today after installing the snow tires, filling with winter blended fuel and driving into the wind from a NorEaster. Have to see if there's an improvement on Sunday PM. Good thing fuel was only $3.359.
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