MAP sensor

[SAMII]

at what instant, MAP sensor's reading is used by an ECU for the Injector's Pulse Width Calculation beacuse it changes its value several times in a single Engine cycle and gives distorted sinusoid on Oscilloscpe

[Owen Thomas]

Presumably, the MAP sensor signal is used just before the injector is fired.

You seem to be confused about how this kind of control system works, and from what I've gathered, the following is what I believe: The sensors will be supplying a constant stream of data to the ECU, and the ECU simply takes the instantaneous value from whatever sensor it needs at whatever time it needs.

Simply put, the value you need to verify as correct is the one which occurs around the injector timing, which is measured in crank angle space. If you can do it, plotting CA degrees and MAP sensor voltage in your ECU and comparing the charts should be pretty helpful. I suspect the amount of noise generated in both signals will not make this easy for you, and I personally don't think it is worth your time, but hey it's your engine.

Oh, and just FYI, people don't typically like it when you ditch a thread and then ask a similar question.

[jlangholzj]

I guess nobodies asked WHY you're concerned about this.
-are you getting inconsistent readings in the ECU?
-is the ECU not seeing the correct pressures?
-is the engine even running?
-are you trying to get a running engine and can't?

whats the problem here...the big picture...

As tobias mentioned in the "other" post, more often than not there's some sort of filtering going on but also keep in mind that some sensors do require a load to function properly. I've tried to read a hall effect sensor not hooked into the ECU before and it give very sporadic readings.

[Jay Lawrence]

What is your tuning regime? Are you trying to use alpha n (a table that tells the injectors to open for a time determined by throttle position and RPM (typically)), manifold density (knowing the properties of the air in the manifold, and having the injectors operate accordingly) or mass flow (using a mass flow meter to know exactly the amount of air and therefore fuel that is needed).

Of these 3, alpha n is the simplest and is most recommended for this application. If you are using alpha n, your MAP sensor is somewhat irrelevant (except for various trims you may want to apply).

As for the 'problem' that you are seeing, if your MAP sensor is where it should be (in the manifold...) and you have your throttle closed, ask yourself what is happening to the air (and therefore sensor) when you cycle the engine? Repeat this experiment with the throttle open.

[Tilman]

SAMII,

if you have a sensor measuring your current throttle angle (alpha) you are usually running an alpha-n-mode (n = engine rpm). In this mode, the ECU determines the fuel needed by the current engine rotational speed and the throttle angle. In this setup, you can or should have an absolute pressure sensor for atmospheric pressure, not manifold absolute pressure as the manifold pressure is mainly a function of engine rpm, throttle angle and air temperature in the manifold. You need/should have the absolute atmospheric pressure in an alpha-n-setup because air density varies with pressure & temperature (and therefore with altitude) and the ECU can correct the amount of fuel injected. With more altitude, air is less dense, the engine can suck less air into the cylinder, you need less fuel but the power output is lower, too.

There is another mode other than alpha-n to determine the current engine load and request for power by the driver, in our ECU it is called "MAP as load". In this setup, the ECU calculates the fuel injected by the manifold absolute pressure and the engine rotational speed. For this mode, you defenitely need a MAP sensor in your manifold.

By the way: Can you imagine why the reading from the MAP sensor is not constant at constant throttle and constant engine speed?

[B Lewis @ PE Engine Management]

Hello All,

A third type of load indication method that works well (if your ECU supports it) is Alpha-N with MAP compensation. This gives you a third degree of fine tuning while still maintaining a dependence on manifold pressure. This method tends to 'flatten' and linearize the TPS vs RPM fuel table while making tuning a little easier. You still do most of your tuning in the TPS vs RPM table, but the MAP compensation automatically accounts for pressure variation via the ideal gas law, half the pressure = half the fuel. I tend to prefer this method to both Speed-Density and Alpha-N.

As for the question of how the ECU uses the MAP measurement, there are a couple of possibilities. As someone mentioned earlier you can sample the measurement at specific crank angles during the cycle to give repeatable measurements or you can simply filter or average the MAP reading over the entire cycle. Both methods can be made to work well.