It is well documented that engines make more power when the inlet air is cooler. To understand what goes on during both the induction cycle and the power cycle when inlet air temperature is reduced, we need to consider both normally-aspirated and Supercharged petrol engines, We will also limit this discussion to four-cycle engines.
Before going any further, there are a couple of terms to define. For this article, it can be suggested that Supercharging is anything that increases the amount of oxygen available in the cylinder to support combustion of fuel above what could be expected from cylinder filling due to atmospheric pressure only. Take the assumption that atmospheric pressure at sea level to be 14.7 PSI and that “normal” air contains approximately 21 percent oxygen. Excluding oxygen-bearing fuels, such as Nitromethane, as a form of Supercharging, but inclusion of any form of mechanical compressor that pumps more air into an engine, such a belt- or gear-driven Supercharger, or an exhaust- or turbine-driven Turbocharger is included, as well as the injection of Nitrous oxide.
Airflow helps engines make power in a number of ways, but the truth is that the more air you can flow through an engine, the more oxygen mass that will be available for burning fuel at the correct fuel air ratio and that means more power. This is most evident when dealing with an ordinary normally-aspirated Petrol engine. Many performance methods relate to getting more air (read oxygen) into the cylinder. Whether it’s by installing a less restrictive throttle body or carburettor, a better flowing intake manifold, porting the cylinder head, larger valves, increasing camshaft lift or duration, the purpose is still the same – get more oxygen into the cylinders. The tuner is looking at getting maximum oxygen into the cylinder at wide open throttle for peak power. This is partly why Nitrous oxide (an oxygen-rich gas) injection is so effective. Nitrous oxide effectively increases the percentage of oxygen in the working fluid (which becomes a mixture of air, Nitrous oxide, and fuel) above the 21 percent oxygen in air alone. That means more fuel can be mixed into the working fluid also for greater combustion heat to expand the working fluid and increase pressure in the cylinder. Additionally, when the compressed Nitrous oxide, which is stored in its pressurized container as a liquid, is injected, it depressurizes and changes state from a liquid to gas, cooling the working fluid for an accompanying density increase. Of course, it would take an incredible amount of nitrous oxide to be able to achieve this at all times, so as you would expect, nitrous oxide injection is only used on demand at wide open throttle. But what if we could get more oxygen into the engine at all throttle positions all the time?
The air throttle body on a Petrol engine controls the density of the intake charge that enters the cylinder. It is also explained how Superchargers and Turbochargers increase the density under boost conditions. In some regards, it can be viewed as density as the amount of oxygen crammed into a given volume of air (the working fluid). Increased density means the molecules in the air are closer together in the same space – more air mass (and oxygen) in the same space. However, there has to be consideration for increased air density in both unconfined and confined spaces. Looking at an unconfined space, such as the atmosphere, because that is what is available to the normally-aspirated engine. Two things affect air density in the atmosphere – pressure and temperature. As atmospheric pressure goes up, indicated by higher barometric pressure on a barometer, the density increases if the temperature stays the same. In other words, at any given temperature, if the barometric pressure rises, so does the air density. By the same token, as temperature goes down, the density increases if the atmospheric pressure stays the same. Atmospheric air density is very important to normally aspirated engines. Obviously, there is not much that can be done to increase the atmospheric air pressure in regard to a normally-aspirated engine, it could potentially be enhanced slightly with some form of Ram air taken either from the front of the vehicle or from a dynamically high pressure area, assuming that the vehicle is travelling at sufficient speed to enable a dynamic pressure rise. More importantly, in most cases you can do something about the temperature of the inlet air. The object is to get the coolest air possible to the engine’s intake system. Some engines can induct air that has passed through the radiator or over other warm areas of the engine, significantly heating the air and reducing its density.
Ensuring the air intakes/ ducts/ filters source air that hasn’t been warmed by the engine bay, density is significantly increased by comparison to the engine inducting hotter air. For example, it is not uncommon for air to increase up to 40 degrees above atmospheric ambient inside the engine bay having passed through the radiator, air conditioning condenser and other heat exchangers on vehicles, then factor in exhaust manifolds and this problem is further exacerbated. This less dense air can have effects on the weight of air mass through the compressor and potentially change compressor performance in relation to the compressor map.
If cooler, or higher density air is available at all throttle positions, it means that the engine is capable of producing given amounts of power at lesser throttle openings. This generally equates to better fuel economy. It also means the engine has greater power potential for accelerating or climbing gradients. Cooler intake air also aids in the suppression of detonation since the Cylinder charge air doesn’t reach as high peak temperature on the compression stroke – again, a plus for accelerating or climbing with the engine under high load. Superchargers and Turbochargers significantly heat the intake air as they compress it to create boost, depending on the compressor efficiency and the pressure ratios that the compressor is being expected to deliver. The higher boost pressure increases the air density, but large increases of temperature of the air from low efficiency compressors can largely offset and even negate this density gain. In this case, it is relating to the effects of pressure and temperature in a confined space, the intake system. Consequently, it is desirable to cool the compressed air before it enters the engine. In most cases, especially where boost levels exceed 7 PSI, cooling the compressed air with a Charge air cooler, often called an intercooler, increases the air density more than any density losses that occur due to the accompanying pressure drop due to cooling or flow restrictions through an intercooler. In other words, intercooling/chargecooling results in a net density increase for the air entering the cylinder.