Let's clarify things a little bit.
There are two basic ways to measure intake manifold air pressure:
- 1: intake manifold air pressure relative to the air pressure at mean sea level. This is usually referred to as "relative" pressure. Since the pressure indicated is relative to sea level pressure, vacuum is indicated with negative values.
- 2: intake manifold air pressure relative to an absolute vacuum. This is usually referred to as "absolute" pressure. Since the pressure is indicated relative to an absolute vacuum, there can only be positive values.
The 2nd way of measuring air pressure is easier on computers because the value starts at 0 and increases as the pressure increases: there's no having to deal with negative numbers and the effects that can have on a computer.
Now... MAP means "Manifold Absolute Pressure". See where we're going here? A MAP sensor outputs a voltage that relates to the ABSOLUTE air pressure in the intake manifold. Theoretically, a "perfect" MAP sensor would output 0v if exposed to an absolute vacuum- something totally unobtainable even in the vacuum of outer space. And it will output increasing voltage as the pressure increases. When the pressure is approximately 101 kilopascals, that's about the same as the ambient air pressure at mean sea level: 0 BAR air pressure, 0 millibar, and so on. Atmospheric disturbances are often measured in millibar because they aren't strong enough to be measured in other units.
Now let's say you're using a "relative" pressure sensor connected to the engine and the manifold is pressurized from a turbo then you shut the throttle. You have a positive number from the previous "boosted" situation, and when the throttle is closed you have to work with a negative number. What happens when you subtract a negative? ... you add the values. See the problem? The program has to take extra steps to make sure the calculation is handled correctly- not efficient and slows the processor down.
So, MAP sensor it is!
Now, off somewhere else for a moment.
Proper air density calculations... it is really air density we need to know to accurately calculate how much fuel to inject, along with the fuel density and such. Calculating air density in the intake manifold requires: manifold air pressure, manifold air temperature, ambient air pressure (barometric), ambient air temperature, ambient air humidity... lots of variables, right? In general, though, knowing manifold air pressure and air temperature can get things close enough to work well since knowing the air temperature and the pressure after it entered the manifold, and that barometric and humidity don't generally change enough to cause any big problem, somewhat basic math works.
In some cases, compensating for barometric pressure, humidity and ambient temp can get more accuracy, but with an suitable tune and O2 / Lambda feedback, those can be somewhat accounted for after the fact, and in kind engine temperature is a factor that requires compensation because that adds heat to the intake air once its entered the manifold and it's a well-known fact that cold engines require more fuel.
To get even more specific, we should also know the fuel pressure and the fuel's specific heat of evaporation too.
I'm done.
Very cool - I will checkout the repos this week. Really appreciate your work ... now I need one of the boards. In the shop it is not available anymore.
- By hoopstar