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EFI Tuning Fundamentals: Calculating Fuel Mass Flow

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Calculating Fuel Mass Flow


00:00 - Remeber that what we're trying to do is work out the mass of fuel required to achieve a certain air/fuel ratio.
00:06 If we have the mass airflow per cylinder, and we know how much fuel the injector can supply, it becomes pretty easy to calculate how long to open the injector, in order to achieve the air/fuel ratio we want.
00:18 The equation we need to use is shown here.
00:20 The required fuel flow in pounds per minute is what we're calculating.
00:24 The mass airflow per cylinder is what we just worked out in the last section.
00:28 And finally, we have the desired air/fuel ratio, which you'll remember was 12.5:1.
00:33 To avoid confusion, remember that we're assuming the eight-cylinder engine is equipped with one injector per cylinder, so all the calculations we're doing, are based on a single cylinder, and hence we're going to calculate the pulse width the ECU needs to apply to a single injector.
00:50 Now we can put the numbers into the equation, and if we solve it, we get a result of 0.462 pounds per minute.
00:58 This means that to achieve a target air-fuel ratio of 12.5:1, we would need the injector to supply 0.462 pounds of fuel per minute.
01:07 Let's now consider the injectors fitted to the engine.
01:10 To keep the maths nice and easy, let's assume the engine is fitted with 60 pound per hour injectors.
01:16 Since the injectors are rated in pounds per hour, we need to start by changing the units to pounds per minute, so it matches the units we're using for airflow and fuel flow.
01:26 To do this, we simply divide the injector size by 60, since there are 60 minutes in an hour.
01:32 Now you can see why we chose 60 pound per hour injectors 60 divided by 60 is one, this means that our injectors can flow one pound of fuel per minute.
01:42 Now just knowing the mass of fuel that the injector needs to supply isn't particularly useful.
01:48 What we really need to know, is how long the injector needs to be open for, to supply this amount of fuel.
01:54 There's one more step we need to take before we can calculate the required pulse width though, and that's calculating how long the engine cycle takes, at 6,000 RPM.