Inlet Air Temperature in IC Engines (Petrol Engines)

If it's that noticeable get the car tuned!

Warmer air is less dense, so the engine draws in less weight of air. I too am surprised that this is very noticeable.

This effect will be magnified at higher dewpoints, you will not see as great an effect when the dew point is near 0 C. This effect led to the use of water injection to improve the performance of aircraft engines during takeoff with heavy loads or short runways during WW II. The cooler air increases the charge volume, and the water vapor gives you a steam kick.

A general rule we used to use was for every 10 degree drop in inlet temp you gain about 1% HP...colder denser air gives more power.

On most modern cars these days the engine is controlled by a computer.

This computer takes information from a variety of sensors to adjust the fuel / air ratio, a mass airflow sensor and a temperature probe is used to allow the computer to adjust the fuel/air ratio to conteract any temperature and pressure changes.

You shouldn't be having these problems with a modern car, even an old style carburettor fed engine shouldn't experience much of a problem with just a 4% change in air density.

Do you run the air conditioner in both the morning and the afternoon? An air conditioning compressor can use a significant fraction of the power available to the car's wheels. It could very well be a difference you notice when driving the vehicle in the afternoon.

I would be surprised if you noticed a significant power loss through this small temperature change. As someone else noted, hotter air is less dense and this will reduce power (make sure you don't have the air inlet diverted to the exhaust manifold as some older carburetted designs used to be able to do, to preheat the air).

There was the thought that cooler air contained more moisture and this gave more power. One correspondent mentioned this as connected to water injection on WW2 aircraft. This is wrong. Tests on engines with high humidity air show reduced power as the humidity displaces oxygen, reducing the effective air flow. How the water injection works is that it is sprayed in the manifold, where evaporation takes place and this cools the air, making it more dense and allowing more to be drawn in. As well, the water cools the whole combustion process, effectively reducing the tendency to knock. In those years fuel quality was a real issue so any improvement was appreciated. I understand that the water injection was only used on take-off and climb. Water usage was about 1/4 of fuel usage. I converted a car I had to test this about 20 years ago, only spraying during full throttle. The heads come out as clean as a whistle, any carbon deposits are removed by the water/gas reaction.

Water injection, with fuel is making a big comeback in the area of marine diesel engines.Because of the very strict controls of MARPOL 73/78 Annex VI which covers air pollution, two of the main products of combustion from diesel engine exhaust SOx and NOx must be reduced..........many ways are being tried to achieve the goals set for these reductions and one method being used by some engine manufacturers to control NOx emissions is water injection with the fuel..........it is also improving engine efficiencies.

For most properly running gasoline engines, cool denser air allows more power to be available, just as many comments here have explained.

Somewhat counter-intuitively, for most properly running gasoline engines, warm, less dense air can allow for somewhat higher efficiency.

The reason is that most gasoline engines regulate engine speed by throttling the air available to the engine (diesel engines don't throttle air, and so do not realize the effect).

There are significant pumping losses when the throttle is only partially open, which is a major reason why gasoline engines produce power much more efficiently near maximum power, than at some small fraction thereof.

When air is hotter and less dense, to achieve the same power, the throttle must be open wider than it would on a cool day. So although maximum available output is decreased, because the throttle must be open wider to make similar power, pumping losses are lower making higher efficiency possible.

Some high efficiency gasoline vehicles use a warm air intake preheated by the headers.