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How Engines Respond to Air Temperature
[ April 24th, 2008 ] By: Mark Ozimek Posted in » Technical Articles

I’m sure that everyone has heard or experienced first hand how cars have more power when it is colder outside. This may strike some of you as being counter-intuitive, but from a technical standpoint, it makes a lot of sense. Let’s break it down to see what is happening when it gets cold out.

The engines in cars burn gasoline (or diesel) and oxygen to create energy. The heat created by this explosion causes the air inside the cylinder to expand, pressing down on the piston, which creates a torque in the crankshaft. There is a very specific ratio between the amount of gasoline and air that provides peak power. This ratio is called the Air/Fuel Ratio or AFR, for obvious reasons. Get too far away from this ratio, and the gasoline won’t even ignite! As a general rule of thumb, engines run with around 12 to 16 pounds of air per pound of gasoline. Below 14.7:1 is called “rich”, while above that is “lean”, while 14.7:1 is “stoichiometric” for normal gasoline. Typically cars run lean for better economy when subjected to low load conditions, and rich when the pedal is to the floor, to help prevent overheating of the engine. Getting into optimal AFRs will be an article for another day though.

Why does all that matter? Well, notice that it is pounds of air and fuel. Engines have a constant volumetric displacement, but it’s possible to vary the mass of air flowing into the engine by changing the density of the air flowing into the engine. In fact, this is exactly how a throttle valve works. When the throttle is partially open, the air flow is restricted such that the density of air in the cylinders is very low, so there is very little mass. The ECU is aware of how much air is in there and injects an amount of fuel to match to get close to the AFR that it wants. A turbocharger or supercharger takes this in the opposite direction and increases the density of air by compressing it.

All of this has a point, don’t worry! Many of you have surely taken some sort of chemistry class, either in high school or college. To make things simple, air can be modeled as an “ideal gas”, which means we can easily say how four very important things relate as you change one of them: Density, Volume, Temperature and Pressure. The density and temperature terms are the critical ones for explaining why engines have more power when it is cold out. As temperature decreases, the density increases. That means when it is cold out, the engine will be able to contain a higher mass of air than at a higher ambient temperature, assuming the same throttle position and engine speed. When there is more air, there is more force from it’s expansion when the gasoline burns, which translates to more torque and power.

We can take this a bit further though. Notice the pressure term. At sea level, the air pressure is much higher than it will be somewhere like Independence Pass, Colorado, at 12,095ft. At this point, the pressure is down to about 60% of the pressure at sea level. Pressure and density have a direct relationship, so at 60% of sea level pressure, the engine is flowing 40% less mass, which will then correspond to a loss of about 40% of the engine’s power that it had at sea level (This is a rough approximation and it will be more or less depending on load and engine design. Having a turbocharger is a good way to get around this loss in pressure, but a significant power reduction will still be there). Pretty substantial, isn’t it? Now you know why the WRC racers were complaining about how slow the cars felt in Mexico, as they were racing at around 6,000ft for much of the course.

Another thing to note: With lower air temperatures, while the engine power increases due to the increased density, the drag from air resistance increases as well. The body of the car has to push more dense air out of the way as it is moving forward. This will negate a lot of the benefits of the extra power at higher speeds, but drag is a very minor part of the forces a car must overcome when accelerating at low speed. A similar effect is going on at high altitudes.