![]() In this way, air density can be recorded on a moving vehicle, in real-world conditions.Īnyone interested in getting more out of a given engine-any combination of more power and better mileage-should be very interested in air density. Here they are shielded to protect from the sun, but open to air flow for accurate readings atop a moving vehicle. ![]() These instruments are used to capture temperature, ambient pressure, and humidity on a moving vehicle. We also use lab-grade digital versions of the classic mercury barometer. When it comes to performance, increasing humidity reduces effective density. So there is less air, and less oxygen, to use for burning fuel. In fact, the addition of water vapor crowds out some of the air molecules. Interestingly enough, humidity does not increase density. Anything that cools air, such as an intercooler, is essentially a density machine. With colder air, it becomes possible to get more into a given volume. Logically, as temperature goes down, density increases. It’s the relationship between the two variables-temperature and pressure-that determines air density. You can pack more air into a given space if you push it in with a turbo, supercharger, or ram-air system. It can be calculated using a formula that relates pressure and temperature, two key variables affecting air density.Īs air flow (pressure) increases, density goes up. What Is Air Density?Īir density is the mass per unit volume. A classic mercury barometer, this type of instrument is used by weather stations, airports, research labs, universities and educational institutions worldwide. This 3-foot-tall Princo barometer sits outside our engine dyno cell. ![]() The more accurate your instruments, the better. To calculate air density, you need accurate measurements of temperature, relative humidity, and ambient barometric pressure. Why does air density matter to the rest of us? Because denser air brings in more oxygen with every stroke, so fuel will burn better, or allow for burning more fuel in a given cycle, maintaining a proper and safe air fuel ratio. That’s why the weather conditions-atmospheric pressure, temperature and humidity-are important factors to account for when doing accurate dyno testing. Just because of minor seasonal weather variations, there are certain tracks, at certain times of the year, that tend to become venues for record-setting. Racers, of course, study density intently, because it makes such a significant difference. The computer adjusts the air/fuel ratio accordingly, so your engine runs about the same regardless of changes in air density. More modern, fuel-injected engines have air mass sensors that look at temperature and air pressure, and account for altitude. With thin air at the top, the air/fuel ratio turns rich, fuel fouls the plugs and the engine won’t idle. An obvious case is when a carbureted Jeep drives over a 13,000-foot pass in Colorado. Whether we’re talking about gas or diesel performance, understanding air density is a key concept.Ĭhanges in air density affect pilots, racers, and to some extent, practically everyone who drives. That’s because those conditions tend to increase air density. It’s the kind of situation that lets them tune for a little extra power. You may have heard that professional drag race teams prefer cool, clear days-and sea-level altitudes. It all has to do with air pressure, air temperature, and the relationship between them. Understanding air density explains why some air-flow products work better than others.
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