Megasquirt-3 MS3 Injector Dead Time Measurement

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Introduction - Measuring - Video - Non linear region

Introduction

The dead time (or offset) of an injector is effectively the portion of the pulsewidth when no fuel is injected. Setting the correct dead time and and voltage correction curve is important for smooth operation at low pulsewidths and to enable the "corrections" to operate as desired.

For information on setting dead times and some background theory see here

Measurement

CAUTION! Working with pressurised fuel is dangerous. Test at your own risk. Take fire precautions.

One method to determine the dead-time is by physically flow testing injectors on a flow bench. A most rudimentary flow bench consists of a pump, fuel rail, regulator, injector under test and graduated cylinder, with a Megasquirt and tuning computer connected. Be sure to use the same injector drivers as you will use during your install. e.g. if you are planning on running low-z injectors with resistors from the MS3X outputs, then do that NOW. The drive circuit has an impact on the measured dead-time, so do your flow testing with the same circuit you will use in your install.

The flow testing uses the Output test mode. Read up on that.

Having plumbed in your fuel (or injector test fluid) system, connected up the Megasquirt and reviewed your safety arrangements, you can begin testing. In normal testing, set the interval to 20ms and measure flow at 2,4,6,8,10ms pulsewidths. Adjust the number of cycles at each pulsewidth so your measuring cylinder is at least say 75% full for improved accuracy.

Calculate the volume (cc) per pulse.
e.g. at 2ms PW * 4000 pulses = 80cc dispensed.
cc/pulse = 80/4000 = 0.020 cc/pulse
Calculate this for all measurements and plot a graph. X = PW, Y = cc/pulse
Draw a best fit line.
The dead time at this voltage is where the line crosses the PW X-axis.
Before entry into the dead-times screen you need to normalise it to a 13.2V correction.

Actual data taken from Low-impedance Holley injector with 3.3R series resistor @ 12.0V battery voltage.

Actual data taken from High impedance Siemens Deka injector @ 13.2V battery voltage. (For this test, the author had a battery charger connected to a car battery and was able to maintain 13.2 +/- 0.1V.) This allows the measured dead time to be entered directly.

In the near future we will be providing updated sample battery voltage correction curves - the defaults are linear for backward compatability with older Megasquirt versions, however actual behaviour is non-linear with low voltages giving rise to far larger dead-times.

Video

A video is available www.youtube.com/watch?v=lJgi8l-2XG4 showing the process.

Non linear region

At higher pulsewidths, injectors have a fairly linear behaviour where an increased pulsewidth gives a proportionally larger flow. However, at low pulsewidths (typically less than 2ms) the behaviour ceases to be linear. For this reason, it is best to avoid operating the injectors in this pulsewidth range as tunability problems usually arise. The non-linear small-pulsewidths feature in MS3 can possibly be used to aid this non-linearity if the actual injector behaviour is known.
Shown here are two actual plots showing just how non-linear (or even unpredictable) injectors are at low pulsewidths.



The raw data spreadsheets are available here:
Low-z Holley 90lb
High-z Siemens Deka 630cc


If you have a question, comment, or suggestion for this FAQ please post it on the forum.

No part of this manual may be reproduced or changed without written permission from James Murray and Ken Culver.