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EFI Tuning Fundamentals: Mass Flow vs Air Flow

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Mass Flow vs Air Flow


00:00 - When it comes to the operation of the engine management system and its ability to achieve and maintain an accurate air-fuel ratio, it's essential for the ECU to accurately know the mass of air entering the engine at any time.
00:12 There are two ways the ECU can go about obtaining this information, and I want to talk briefly about both methods here.
00:19 In the vast majority of modern factory engines, the mass air flow is directly measured by way of a mass air flow meter.
00:26 I'm going to discuss these in more depth later in the course, so for now, it's sufficient just to know that they exist, and what they do.
00:33 If you're reflashing a factory ECU, then often you'll be using an ECU that bases its load input off the reading from the mass air flow meter.
00:41 The other option, which is much more common in the standalone ECU market, is to calculate the mass air flow, using what's known as the speed density principle.
00:50 In this case we aren't actually measuring the mass air flow directly, but rather we're calculating it based on the volume flow and air density.
00:58 Let's move on with this module, and look at how we can make this calculation.
01:03 Once we know the air density, it becomes a simple process to calculate the actual mass of air flowing through the engine.
01:09 Mass flow, is just equal to the volume flow, multiplied by the air density.
01:15 The equation is V multiplied by D, where V is the volume flow in cubic feet per minute, and D is the air density in pounds per cubic foot.
01:24 Notice here that we're measuring mass flow in pounds per minute, the metric equivalent would be grams per second.
01:31 V is equal to the volume flow, which we calculated earlier, and D is equal to the air density, which we've just learnt about.
01:39 Let's continue our earlier example, where we measured the actual volume flow of our engine at 572 cubic feet per minute.
01:47 If we assume that we are operating at a standard air density of 0.076 pounds per cubic foot, we can now put these numbers into the equation, and calculate the mass flow through the engine.
01:59 Running through this equation and solving it, we find that the mass flow is 572 multiplied by 0.076, which equals 43.47 pounds per minute.
02:12 So we now know that we have 43.47 pounds per minute of air, passing through our engine.
02:19 The key point to remember from this module, is that it's the mass air flow that's important to the ECU in order to be able to accurately achieve a consistent air-fuel ratio, and calculating mass air flow, can be done by multiplying the volume air flow by the air density.
02:35 Remember that many factory engine management systems use a mass air flow meter, to directly measure the mass air flow, rather than calculating it like we've just done.

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