i am a little confused here. today my physics professor told me that the hotter the engine the more efficient it runs, and the hotter then temp is outside the better gas mileage you get. this makes no since since you make more power and run faster 1/4 mile times when its colder out right? and also if it was more efficient to run hotter air then why would we bother having intercoolers, and why would people use Nitrous Oxide or Meth/Water injection? isnt the purpose of water injection to furthur cool down the charged air temperature? well anyways if someone could give me a physics explination on why it is more efficient to run in "hotter" weather please let me know.

 

is it possible that because the hotter air runs more efficiently gas mileage wise...less power, better fuel economy?

 

I'm guessing your professor is referring to the temperature of the internal combustion? I would think the air/fuel mixture explodes more efficiently when it's hotter, because less energy will be lost on heating it up. Kinda similar to it being easier to start a fire in summer than winter.

But in turbo engines, colder air = denser/ more air, and more air allow you to use more fuel. So colder air outside = bigger power. But that doesn't mean the engine is more "efficient", which is what your professor is suggesting.

Just my $.02, I'm no expert tho

 

hes reffering to efficiency in a gas mileage sense. less dense air = less fuel used = better gas mileage. thats incredibly overly simplistic, as there is a certain range that engines can operate under that gives the best compromise of reliability and efficiency. and im not going to get into the thermodynamics aspect either. ill just say that he is grossly oversimplifying.

 

Another way to view it is that you have to use a bigger throttle opening at a steady state cruising speed to get the same amount of air. Bigger throttle opening means lower pumping losses.

 

Also on top of the pumping losses in winter cold weather greas is thicker so all the drive train parts experiance more viscosity related friction which increases fuel consumption.

The air density is a factor as well. When they do top speed runs out on the salt flats, the hotter the weather the better. The cars make less power, but the lower density of the air makes up for the power loss at high speeds. Air resistance is NOT a strait line function, it goes up with the square of the velocity.

Keith

 

the actual combustion process is more efficient at higher temperatures. more energy per mol of fuel is liberated at high temps than at low temperatures given the same fuel/air ratio and density.

this is also the case with turbine engines as well. they are designed to run with the first stage turbines and stators as close as possible to plastic defformation temperature in order to get the most out of the engine efficiency.

 

As we use to say in the old muscle car days,heat is power! but not overheating.

 

hahahahah..... I'll have to remember that one!

Another thing, the hotter the cylinder walls, the less heat loss through the walls which means more heat goes into pushing the piston down.

 

I've seen some dyno tests on this, on a NA engine, and the hotter is better theory is correct; but, the % gains in power were very small. The theory is that the hotter the surroundings the less heat lost from combustion into the coolant, the more efficient the process is. IC engines have very low efficiency because of high heat loss. For the test they used a special coolant and ran it ~100F hotter than the conventional coolant setup IIRC. Keep in mind that since it was on a dyno you would not see the same effect of higher underhood temps on the intake tract that you would in the real world. In the real world, engine under the hood, the increase in operating temp would spill over into increased intake manifold temps and the reduction in intake air density might cancel out the combustion efficiency gains.