Originally posted by: PowerEngineer
Originally posted by: Jahee
Also in cold conditions your engine is less efficient bear in mind.
This is the real culprit...
Uhm... no?
An engine has a higher volumetric efficiency and higher performance in colder air due to the higher density of the air moving through the constant volume of the engines displacement, allowing it to burn more fuel per volume of air. Cooler air also results in a cooler air/fuel mixture that is less prone to auto ignition and detonation, allowing for more advanced timing, to extract more power out of the same quantity of fuel.
Come one now, you don't think your car pulls better on a cold winter morning when its 30F out than it does in the middle of a 110F summer afternoon?
Why do you think people use intercoolers, ice packs on their intake, or race in cooler weather, etc, to lower their 1/4 mile times?
But more power decreases mileage right? Not really. This doesn?t translate to lower mileage and fuel economy in normal practice. With denser air, your engine can pump the same mass of air at a lower RPM, so the over all air flow and fuel consumption is the same. And because a lower RPM is required to pump the same mass of air at a higher density, there is less frictional load on the engine. Hence an engine will always run more efficiently in colder air.
The net effect to the driver is that on a colder day you don't have to rev has high to get the same push in the seat that you do in the summer. But if you like the way your car pulls a little better in cold weather and you get addicted to accelerating harder than you are used to, then yeah, its going to consume more fuel than you are used to. You are running it at the same revs as in the summer, with denser air, so more fuel will be required to balance the mixture.
The likely causes:
1) Fuel blend. Additives that occupy some percentage of volume of the gasoline displaces power producting gasoline molecules, reducing available power. Engine computers only know the injector pulse width and timing values for the octane of fuel they were programmed with. Other than O2 feedback they have no real means to compensate. The computer trusts that you are only putting fuel into your tank and that 1 liter of fuel contains 1 litre of 93 octane gasoline. If you stick 10% water in your tank, no problem, the computer will see that only 90% of the oxygen burned and compensate accordingly by adding more "fuel" (your 10/90 mixture rather), resulting in a higher depletion rate of fuel volume per horsepower and mile driven. Combustion will be cooler due to 10% of the fuel mass being inert and absorping energy rather than producing it. The result is that every cycle of the engine only produces 90% of its potential and CO2 emissions due to the other 10% being harmless H2O and unused O2 passing on through. Emissions will be lower per engine cycle, but overall you will have a less efficient engine that has to use more fuel volume flow to attain the desired performance, thus MPG suffers. If you drive the same speed you did before with only 90% of the liquid in your tank consisting of useable fuel, you will empty your tank faster and cover a shorter distance. To put it simply, a junkie snorting 'cut' coke that has been diluted is going to have to consume more of the diluted coke to get the same high.
2) Elevation. Without knowing what cities we are talking about... What is the elevation in CA vs. VA and the associated atmospheric pressure? Again, due to air density described above, engines running at a higher elevation are less efficient. Just looking at each states mean elevation shows that VA is at 290m and CA is at 884m, but of course this is skewed by extremes like tall mountains or beaches and valleys. It depends where you live specifically. Moving to a higher elevation is compounded by moving closer to the equator where temperatures are also higher, making the already lower density air even less dense. In addition, if the heat causes premature ignition and detonation, timing is retarded, reducing power per volume of fuel even further.
3) Terrain. Driving through 3 miles of hills consumes more energy than driving those same 3 miles on flat roads. You now add gravity to the forces of friction and air resistance that the car has to overcome. Also using engine power to accelerate back down a hill at the same speed you went up does not regain any energy lost going up the hill. Due to speed limits and traffic, your brakes and impatience will dissipate any energy that would have been gained back if you accelerate to that speed using engine power. If you turn the engine off and coast downhill until you reach the speed limit, it works out that way, but who does that? In other words, going up a 3 mile hill on the way to work, and down a 3 mile hill on the way home, will overall, consume more energy driving 3 miles both ways on a completely level road.
4) Mix and match any of the above for some seriously noticible losses.
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