Why better MPG in warmer weather?

<div class='quotetop'>QUOTE(goodsam @ May 4 2006, 08:12 PM) [snapback]250050[/snapback]</div>

Two different issues here. Better MPG does not equate to "more power."
Yes, cold air will serve up better performance because of dense air entering the intake. But more power does not mean better mileage! Off the top of my head, the benefits to MPG with warm air include:

1. Warm air is less dense so the car can push it aside more easily (and this air density business is exactly why colder air gives you more power - denser air can be mixed with more gasline to give more power. More power, yes, but NOT better mileage!)
2. All bearing grease is thinner and allows for less drive train and rolling resistance.
3. Tires are harder with higher pressure due to heat
4. Pleasantly warm temps allows cruising with no heat/AC
5. And in most places the summer gasoline mix offers up better mileage than the winter mix.

 

you forgot the most common one... the engine has much less warming up to do when it's warm out than when it's cold!

 

Expanding on darelldd's #1:
I have heard that no matter the temperature, the Gasoline/Air mixture must remain the same to maintain combustion. The denser the air (colder), the more gas you must use to keep the ratio correct.

(hmmmm...so...would that also imply that you get the best mileage in the middle of the dessert at high noon?)(or at the top of everest....if you could drive up there?)

 

<div class='quotetop'>QUOTE(Tripod137 @ May 5 2006, 06:09 AM) [snapback]250133[/snapback]</div>

The ratio stays the same, except for being richer when starting, running with a cold engine, or accelerating. Looking just at the steady state case, denser air means more fuel for the same volume of air, but the mass ratios remain the same, which is why cars use mass airflow sensors, but I digress. More fuel and air means more power, so you would find yourself backing off on the accelerator to keep the speed the same. This would reduce the fuel burned back down to its original amount, so denser air does not cause you to consume more fuel relating to mixture ratio.

What does happen, again just looking at the engine and not the tires, etc., is that the denser air effectively gives you a more powerful engine. A more powerful engine running at lower power levels is less efficient than a smaller engine running at higher power levels. This is do to increased pumping losses through the now more closed throttle.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ May 5 2006, 04:47 AM) [snapback]250138[/snapback]</div>

If increased pumping losses at low temperature is a significant factor in mpg reduction at low temperatures, then, Atkinson cycle engines should be less affected than Otto cycle engines; Atkinson cycle engines inherently have lower pumping losses than Otto cycle engines. I believe that only the Prius, HCH, Insight, and hybrid Escape use the Atkinson cycle and all other gasoline cars use the Otto cycle.

 

<div class='quotetop'>QUOTE(Tripod137 @ May 5 2006, 03:09 AM) [snapback]250133[/snapback]</div>

Technically, yes.

Air is less dense at altitude, as you say. Consider a light aircraft with a piston engine, without turbocharging. At altitude, the lower air density requires that the pilot "lean out" the air/fuel mixture, reducing the fuel flow to maintain a proper combustion mixture. The result is a lower fuel flow, but also lower horsepower.

Fuel flow generally equals horsepower. An aircraft at 10000 feet is burning less fuel and producing less power than an aircraft at sea level.

Turbocharging increases air pressure (and density) at the engine intake to compensate for altitude, so the engine can produce higher power (and use more fuel) at altitude. The benefit is increased speed.

Some aircraft engines are turbocharged, so they always have intake pressure greater than ambient air pressure. Others are turbo-normalized; the turbocharger is set to provide sea-level intake pressure at all altitudes. A turbo-normalized engine always "breathes" air at sea-level pressure.

Anyway, that's more technical info than most folks need...