In open loop, if IPW is determined by the fuel map and the fuel map is determined by the load, then why does my load appear to follow the same pattern as my IPW, and to a lesser extent - airflow?

And where does the MAF fall in the equation in open loop?

Is IPW the commanded IDC out to the injectors or is IPW the result of the duty cycle on injector when fuel pressure and boost pressure are accounted for.

Basically, can someone explain open loop operation? I'm hitting a dead end

Thanks

 

the MAF sensor measure the air coming in. in open loop, it is simply using a lookup table to see how long the injectors need to be open (plus possibly a few modifiers such as air temp compensation and acceleration gains).


so, using the MAF it knows how much air is coming in (load), it checks the rpm (crank position), and leaves the injectors open for the length of time in the cell in the table (interpolated between a few cells actually though).

the actual ECU shouldn't use IPW to calculate load like in evoscan. the actual ecu will use airtemp, MAF frequency, throttle position and maybe some other things... but it uses what it calcuates as load to determine the IPW via the tables in open loop.


edit* also, open loop generally refers to the fact that it does not use any feedback to modify the signals going to the injectors. I.E. it does not use the o2 sensors to keep it at stoic. ratio, it just takes rpm + load and spits out a IPW.

 

IDC = injector duty cycle

IPW = injector pulse width.

they are essentially the same. the duty cycle is simply a calculated value from injector pulse width. the injector pulse width is the width of the "trigger signal" going to the injector.... for example, if you wanted the injector to open for 10 milliseconds, you would send it a IPW of 10ms. now in the case were you have a limited amount of time before the next cycle, you can determine what percent of the time availible is the injector firing... this is called IDC. heres how to calcluate it:

lets say at 7000 rpm on the engine. that is equal to 117 revolutions per second.

that means each revolution takes .0085 seconds (take the inverse of 117 to get the period). we know that this is a 4 stroke engine so that means it gets 2 revolutions to fire, so multiply .0085 by 2 = .017 seconds is how long an injector gets at 7000 rpm to inject the fuel at a maximum.

now lets say at 7000 rpm, your injectors have a pulse width of 14ms. that means it is using only 14ms of the maximum 17ms, so therefor your IDC is calcuated like this 100* 14/17 = 82% duty cycle. if your obd logger shows a IPW of more then 17, that means you are over 100 % duty cycle, and no matter how much richer you try to make the tune there, the injectors are already maxed out and have no more time they can fire.

 

Thanks for the info,

 

So each A:F map cell value will always result in the same IPW no matter the fuel pressure and boost pressure?

It seems that increases in ECU precieved load results in an increase in IPW no matter what the value in A:F cell. My MAF and IPW values are proprtional (not perfectly, just in synch) to each other which of course result in an inversly proportional A:F lambda

 

fuel pressure does not effect what the pulse width directly... but having a higher fuel pressure will increase the amount of fuel getting injected, and on a closed loop operation, the o2 sensor will see that it is rich, and it will decrease the injector pulse width. that would be how fuel pressure would indirectly effect IPW in closed loop operation.

in open loop, having a higher fuel pressure should not effect the injector pulse width. it will however increase your fuel delivery though.

boost pressure will increase the injector pulse width even in open loop. having a higher boost pressure is increasing the amount of air injested per revolution, and therefor the load should increase. the ecu does measure the air flow, so it knows if there is more boost or not. think of it this way, with volumetric efficiency aside, the engine can only suck so much air in per revolution. about 1liter per revolution. now again assuming 100%VE, if you are running 2liters of air per revolution, the ecu measured that air, and knows that obviously your are running higher then atmospheric pressure, and it will use the fuel table value according to the load it calculates (again, i don't know exactly how the ecu calculates the load).

you will see some aftermarket ecus use boost x rpm tables instead of load like ours. they do this, because both mass air flow, and manifold pressure are closely related (manifold pressure ideally would want to know IAT as well to calculate density, or have a temp compensation modifier). with boost tables it is much easier to know where on the table you are, also, nothing else is used in calcuated where on the table you are so it is very straight forward how to tune a car using those type of tables. also, they typically enter the actual IPW value in ms for the cell values instead of a A/F number.


and i see where your getting at as far as why you can put 11.5 in the cell with 100 load and 5000 rpm, but the IPW is going to be shorter then the IPW at 260 load and 5000rpm even if the cell value is 11.5 there as well.

my guess is that it uses the load value as a multiplier to the final IPW as well as the value in the table. for example, at the 100 load its at 5ms, at 200 load it adds 10ms, at 260 load it adds 13ms.... obviously thats not exactly how it does it, but i would guess it is something along the lines of that.