exceeding 100 % duty cycle

[geewiz]

During review of several logs, I have found that the duty cycle is exceeding 100 % under full throttle and above 5,200 RPM. A 14.3 AFR is maintained from 5,200 - 7,000 RPM in an attempt to reach a 12.1 AFR. The PW during this period is displaying from 22.7 msec to 23.5 msec. During the initial set up for the 12 ohm injectors being used, a suggested 25.5 msec was entered as the PWM time threshold. Is there any reason that this threshold can not be increased to 35 msec for the high resistance injectors being used. [This change should allow enough open time for the system to reach the target AFR.] I am using two injectors running in an alternate fire sequence on a six cylinder engine so the overall heating of the injectors should not be an issue. It seems to me that increasing this threshold would correct my lack of fuel problem with out creating other tuning change requirements.

[Matt Cramer]

If your injectors are going static, adjusting the settings won't help - you need more fuel pressure or larger injectors. A setting of 2 squirts alternating is going to have only one opening event per cycle, so there's not much else you can do.

[geewiz]

Matt, I apparently did not make myself clear. I believe that the adjustment I wish to make will prevent the injector from going "static". Perhaps I am wrong ,but, It is my understanding that the duty cycle being reported by Megatune of say 110%, is that the pulse time for the individual injection 'required' has exceeded the allowed "maximum open time" [by 10 %]. The pulse is emitted correctly for the "required' time but is cut off prematurely by the "maximum open time' . This "allowed maximum open time" is a constant that is set into "injector characteristics" portion of the program during the initial set up, [for injectors that do not require PW modulation]. Therefore increasing this "allowed maximum" should allow the longer "required' injection cycle to complete.

[Matt Cramer]

Perhaps I am wrong ,but, It is my understanding that the duty cycle being reported by Megatune of say 110%, is that the pulse time for the individual injection 'required' has exceeded the allowed "maximum open time" [by 10 %]. The pulse is emitted correctly for the "required' time but is cut off prematurely by the "maximum open time' .

That is correct so far, but the maximum open time is determined by the amount of time physically available, not any settings.

This "allowed maximum open time" is a constant that is set into "injector characteristics" portion of the program during the initial set up, [for injectors that do not require PW modulation]. Therefore increasing this "allowed maximum" should allow the longer "required' injection cycle to complete.

Not really - the PWM Time Threshold is only the amount of time before PWM current limiting kicks in (if PWM current limit is set under 100%). It is not the maximum opening time.

[geewiz]

Matt, I computed the time required as follows: With two squirts alternating on a six cylinder engine running 6000 RPM, there is one squirt per revolution. So, there are 6000 squirts per minute from each injector or 100 per second. 1/100 of 1 second = 10 Msec. Therefore when the primary injector reaches the total open time of 10 Msec it has stayed open for 360 degrees. At that point the other injector triggers. But even if the primary injector stays open for the next 360 degrees, that still totals only 20 Msec and allows no close time whatever. Using 85% X 20 Msec =17 Msec - 1 Msec for opening =16 Msec This should be my maximum open time. [I am still amazed that the time is totally consumed .] I would prefer that the injection cycles ended closer to the 360 degree mark so I am going to choose simultaneous instead of alternating & perhaps bump the pressure up a bit. Matt,I thank you again for your reply.

[geewiz]

I am posting the results of the change for those who might use it for reference. Because I was showing 130% duty cycle at high RPM'S, I multiplied the injection pressure by 130 %, and raised it from 37 PSI max. to 49 PSI max. I then went from alternating to simultaneous to reduce the time between injections when no fuel is squirted. This "off" period will occur at the same time for each revolution and could feasibly starve a cylinder of fuel - even at fairly high RPM's. Results thus far show 85% DC at 6400 RPM yielding 13.6/1. Therefore when fully tuned to 12.5/1, I should be running at nearly 100% DC with very little or no "off" time and none of the cylinders will be shorted of the needed fuel. Before installing Megasquirt, I tuned this engine as well as possible with carburetors. It was always rough running,in high gear, at speeds under 60 MPH- due to it's 300+ degree camshaft. The injection system vastly improved the engine, but with the present set-up- it is the best that it has ever been. I can mash the throttle to the floor in 2nd. or 3rd gear at 10 or 15 MPH and it will lug smoothly away , with no jumps or jerks [as if it had a tractor camshaft]!!!! The overall power has increased at all RPM'S. So I am again -in amazement. The car, that would not run smoothly at 45 MPH, now runs well at !0 MPH !!! The Megasquirt has proved to be a wonderful addition to the car.

[jcarruthers]

I believe you're better off aiming for 80% duty cycle at max power?

(have to say I love how smooth my engine is with a 300 deg cam too!)