Minimum Injector impedance and max Pk/Hold currents?

KevlarUK · Post by **KevlarUK** » Tue Jan 24, 2006 11:23 am

Hi all,

Have the oppurtunity to use a McLaren electronics (

) high pressure (12Bar) fuel injector but it has a DC coil resistance of 0.65 ohms, would this draw too much current for my MegaSquirt ECU? its a MS1 processor and V3 board.

It'll be running on a Husqvarna 577cc single cylinder, so just the one injector to worry about.

Also, it requires 4A/1A peak/hold currents, is MS capable of this? I have had a hunt around but couldnt find anything.. Can the peak/hold currents be adjusted in megatune?

Thanks in advance

Kev

PS - I know duplicating posts is often frowned upon in forums, i did just post about this in the general section but my last post in there didnt get a response and i thought this was more relevant in here... sorry

Bernard Fife · Post by **Bernard Fife** » Tue Jan 24, 2006 11:38 am

Kev,

The impedance itself isn't the primary issue, the total curent per driver and the flyback enegy are. However, with just one injector, you aren't likely to have a problem, especially if you have installed the PWM flyback damping circuit in step #69.

You can adjust the 'peak' current with MegaSquirt, but using the 'PWM time threshold'. This is the time during which the full supply voltage (nominally 12 Volts, usually about 14 Volts in the vehicle) is applied to the injector. Basically you need to calculate the injectors time to reach 4 Amps, based on the inductance and resistance.

The 1 Amp hold current is a function of the PWM%. You could calculate this as well, based on the inductance, resistance and effective supply voltage (for example, 25% PWM is effectively 25%*14 = 3.5V)

These PWM parameters are usually better tuned in the car than calculated though, the procedures are in the manual.

Lance.

KevlarUK · Post by **KevlarUK** » Wed Jan 25, 2006 5:41 pm

Hi Lance, thanks for the response.

Yep, I have installed the PWM flyback damping circuit, so I'm going to go for it and use this Low impedance injector when it arrives.

Is there a way to work out how quickly the current rises through the injector? I've forgotten most of the electronics from my 1st year at uni

I'll be able to hook the injector up to a scope to measure the time accurately once it gets here if not.

And just to confirm i understand you.. if I've got a 0.65Ω injector and want 1 amp, that would mean supplying 0.65 volts, which means at 14V I'd want to set the PWM to 4.5% ?

Thanks for your help

Kev

Bernard Fife · Post by **Bernard Fife** » Wed Jan 25, 2006 5:52 pm

if I've got a 0.65Ω injector and want 1 amp, that would mean supplying 0.65 volts, which means at 14V I'd want to set the PWM to 4.5% ?

Kevlar,

The rise time is similar to a coil calculation, which is:

T = (-L/R) * ln( 1 - (R * I / E))

Where:

T = time (sec)
L = inductance (H)
R = resistance (Ohms)
I = current
E = voltage (Volts)

(ln is the 'natural logarithm', often available as 'LN' on calculators).

For 1 Amp on a 0.65 Ohms resistance, the Voltage required is:

V=I*R = 1*0.65 = 0.65, as you say, which is theoretically a PWM% of 0.65/12 = 5.4% (though the supply is likely to be more than 12V).

However, this is applying a static analysis to a complex electro-mechanical situation that involves more than just the resistance, and in any case you need to tune the PWM% on the vehicle so that it is adequate under all conditions. It's not hard to do, the procedure is in the manual.

Lance.

KevlarUK · Post by **KevlarUK** » Thu Jan 26, 2006 2:49 am

Cheers Lance! - saves me rooting through all my lecture notes!

One other quick thing.. I've just noticed on the spec that this injector (I got an email yesterday explaining that it is second hand and from an old F1 engine!!) has a MLPT of just 0.9ms, so I'm guessing the rise time will be pretty quick. Please forgive my ignorance as I have not yet started tuning the engine, but will MS be able to go as low as 0.9ms? What is the shortest duration MS is capable if I'm using peak/hold?

thanks again - Kev