One more time I turn to you, this competent group, don't have anything
like it in europe..;-)
I've got an EFI related question. I have a 1.6litres 16V engine
(Zetec-E) in my car and I like to know what's typically the maximum
duty cycle on the injectors could be, and in what situation the engine
demands the max duty cycle, like low rpm, WOT or high rpm, medium load.
I'm looking for general information, it doesn't have to be specific for
my engine, which I assume you never heard of anyways....
Fuel injectors are not duty cycled.... They are pulse width modulated and
there is a world of difference between the two.
Trying to read the dwell time of an injector is not possible since the
frequency changes (one of the basics of duty cycle is that the time to
complete one cycle never varies).
Having said that, I am not familiar with the particular vehicle BUT.....
typical fuel injector pulse width on most any engine will be on the order of
2ish or 3ish milliseconds at idle and may increase to about 12 or 14
milliseconds at times of maximum fuel demand. I doubt you'll see pulse
widths greater than that since events inside the engine happen so
quickly..... using some high school math and whatnot, we can compute the
length of time (in milliseconds) that an intake valve is open at 5000 rpm
(or any rpm, for that matter)..... or the length of time it takes a piston
to travel from dwell at TDC to dwell at BDC (the numbers are
? It is rare to vary the frequency in a PWM application. PWM
essentially acts as a digital-to-analog converter - the effective work
seen at the output is varied by modulating the percentage of the cycle
Yep... OK... you bet... I see you have a good grasp on the situation....
A digital signal is either on or off... an analog signal is variable.....
digital to analog is a physical change in a signal....... Oh, fuck it... you
wont understand anyway....
I fix cars for a living.... I'm good enough to make a handsome profit AND
have happy customers.... What do you do for a living????
Since I design this sort of thing for a living - yes, I do. A PWM
output is a single digital output run through a low-pass filter to a
load. Often used for varying motor speeds or generating analog
waveforms, e.g. for audio.
A typical PWM implementation has a clock signal fed to a timer, a
compare register and an output pin. Assume the timer has an 8-bit
resolution. Start with the timer at 0 and the output ON, and the
compare register = 128. After 128 clocks, the compare register will
match the timer contents. The PWM hardware will turn the output OFF at
this point. After 256 clocks, the timer will overflow. The PWM hardware
will turn the output ON at this point. Hence you get a square wave with
a 50% duty cycle.
Varying the contents of the compare register varies the duty cycle
stepwise according to the precision of the timer. Hence it also varies
- depending on the external circuitry - either the current or voltage
seen by the external load.
The timer frequency is selected in conjunction with the filter
components and with consideration to the load being driven and PWM
update frequency. It is, as the OP mentioned, normally the _duty cycle_
(compare register) that is modulated. Of course it is not impossible to
change the PWM frequency, but it is unusual - which is why I asked what
special circumstances exist for this particular application.
If you look at <http://www.larwe.com/technical/current.html you'll see
a vending machine application I developed using PWM (64kHz PWM
frequency, 8-bit resolution, 8kHz sample rate) for audio output. This
is operating, effectively, as an 8-bit ADC putting out
If you read my second book, published a couple of years ago and readily
<(Amazon.com product link shortened)>, you can see
an application, with schematics and sourcecode, where I used PWM and an
H-bridge to provide variable speed DC motor control.
I do hope you're feeling as much the fool as you look right now; you
need to learn when not to jump in with two left feet.
Thanks for your reply, but I don't agree with you on some points. Of
course, I know the injectors are pulse width modulated, but this can
still give you a duty cycle measure. The look-up tables in ecu's are
often written as duty-cycle, more than milliseconds. The ecu knows the
time between two injector pulses, (operating frequency), if the duty
cycle is set to 50% the on-time will match the off-time independent of
engine rpm, this will of course result in variable on-times as rpm goes
OK, guys.... what do YOU expect to see as a "duty cycle" measure from
something that has a changing frequency AND a changing on time - in one
instance we can see long on times and low frequency (rpm), in another we see
long on times and high rpm and in another we see short on times and high
rpm..... just WTF are you going to measure? The only measure that will mean
anything will be pulse width.....
Let's put it this way.... I am here answering questions... you guys ask
them.... 'Nuff said???
FWIW, feel free to disagree with me.... but if you are going to disagree, do
it because I am wrong - not because my answer isn't what you wanted to
Enough... Both wrong both right
Duty cycle is a misnomer, as it isnt a constant velocity issue.
Neither is it simply a matter of 'engine rotation degrees open'.
The pulse would be longer at WOT at any given RPM, independent of load,
because of intentional richening.
Given WOT - the higher the RPM, the longer the 'open time'
And THAT is the best answer we can come up with here.
I'm sure a better math guy than me can come up with a decent formula
based on throttle angle, air flow, and RPM biased by engine temp.
That said... this is not the forum to ask, nor is asking an open ended
question - not specifying the REASON or issue to be solved on topic for
Yeh, I'm a Krusty old Geezer, putting up with my 'smartass' is the price
you pay..DEAL with it!
I've crossposted this to comp.arch.embedded. Automotive electronics
engineers post and lurk in here, you may get some general information
as I know this topic has been discussed recently in a different
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