Engineering question: Cold air. Power vs. Aerodynamics.

milltown

Was just wondering on my way home yesterday evening as my car beeped to tell me it was 3 degrees C outside; cold air is denser and therefore gives us more power per combustion stroke from our engines. How much more is one part of my question. Being denser, cold air must also be harder for the car to move through.

On an average car with a Cd of say .30, which is the bigger factor?

I'm well aware that in actual driving, having colder tyres will be a much more limiting factor but, knowing there's a good few eggheads on the motors forum with a knowledge of the scientific side of auto engineering, I'd be interested in the hypothetical answer.

Discuss, and show your work.

Anan1

I'd imagine it varies with the car. Turbocharged engines, for example, should benefit more from cold air than NA ones. Speed will also play a role - acceleration may be stronger at lower speeds but may then tail off more as speed rises.

endplate

I think it's a trade off. Denser cold air better engine preformance but poorer aerodynamics and warmer less dense air poorer engine preformance but better aero

jayok

I'm not sure how much of a difference the colder air will make. I would imagine that the drag of the car from points such as the wind-screen, grille, road friction, etc will be so significant that a difference in temperature of say 20 degrees will be negligable. Perhaps the tiny increase in power will compensate for the increased air density?

Consider that an effect from and increase in density will probably been wiped out by a 5 km/hr wind breeze.

chris85

milltown wrote: »

Was just wondering on my way home yesterday evening as my car beeped to tell me it was 3 degrees C outside; cold air is denser and therefore gives us more power per combustion stroke from our engines. How much more is one part of my question. Being denser, cold air must also be harder for the car to move through.

On an average car with a Cd of say .30, which is the bigger factor?

I'm well aware that in actual driving, having colder tyres will be a much more limiting factor but, knowing there's a good few eggheads on the motors forum with a knowledge of the scientific side of auto engineering, I'd be interested in the hypothetical answer.

Discuss, and show your work.

Ok well technically there is a difference between the density of the air and the effect on the drag on the car.

The formula for the drag is shown in the link below (dont usually trust wikipedia but its correct)

http://en.wikipedia.org/wiki/Drag_equation

If the air is denser this results in the drag becoming greater as it increases the density value in the equation.


Lets assume your car has a Cd of 0.3 and a frontal surface area of 2m^2 and a velocity of 20m/s.

If you are traveling at 3 degrees C outside, then your drag force is roughly 154 Newtons.

If you are travelling at 10 degrees C outside, then your drage force is roughly 144 Newtons.


Ok basically as the density of the air increases the drag will decrease. But the biggest factor in the drag force is the speed the car is going.

Andrewf20

On cold days my car defo feels slightly faster.

Pherekydes

chris85 wrote: »

If you are traveling at 3 degrees C outside, then your drag force is roughly 154 Newtons.

If you are travelling at 10 degrees C outside, then your drag force is roughly 144 Newtons.


Ok basically as the density of the air increases the drag will decrease. But the biggest factor in the drag force is the speed the car is going.

Increased density will increase drag. As the T rises the density drops and the drag drops as noted in the two figures above, yes?

Oilrig

Ballpark crossover where aerodynamic drag outdoes rolling friction happens at about 90 kph, most motors have max torque in top around this speed as its the most efficient cruising speed.

Oilrig... old, old, text books...:p

chris85

Slow coach wrote: »

Increased density will increase drag. As the T rises the density drops and the drag drops as noted in the two figures above, yes?

Yeah thats it.

we cant control the temp outside so the best way to go reduce drag is to have smooth lines (reduces Cd value) and smaller frontal area.


Both these have dropped in the last 50 years, except for america where they saw cars with bigger frontal areas come in in the 70s/80s. This is when they went crazy on the muscle cars

kona

milltown wrote: »

Was just wondering on my way home yesterday evening as my car beeped to tell me it was 3 degrees C outside; cold air is denser and therefore gives us more power per combustion stroke from our engines. How much more is one part of my question. Being denser, cold air must also be harder for the car to move through.

On an average car with a Cd of say .30, which is the bigger factor?

I'm well aware that in actual driving, having colder tyres will be a much more limiting factor but, knowing there's a good few eggheads on the motors forum with a knowledge of the scientific side of auto engineering, I'd be interested in the hypothetical answer.

Discuss, and show your work.

well, by the time you let the engine warm up to its optimal temperature, the heat of the engine bay would have the air heated to a certain degree.
A cold engine can be your worst nightmare, the materials of a engine are designed to work, at a set temperature range, ragging the engine when cold will wreck it.
Turbo cars have a intercooler to cool the air to make it denser, so that is a modification, but, yes if your engine is warmed, your filters arent restricing the airlow then you should get more power, although i dont imagine it would be significantly more. likewise brakes need to be warmed to work to their max performance.
also if you crash in the cold, metal is more brittle, so in crashes in the cold you are more likely to be a mess.
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