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V3 MicroSquirt® - QuickStart Guide

Number of Squirts and Injector Staging

There are a number of considerations in choosing the number of squirts:

More squirts are more likely to help if the engine:

However, if the engine is tuned so that the idle MAP is minimized (likely 13.5:1 AFR or so), and has a higher idle speed (say 800 rpm compared to 600 rpm), then more squirts is less likely to help.

There are definitely some down sides to more squirts:

So more squirts may help. They may not, however. And this can be a bandage for poor idle or accel tuning. So users should start with 2 squirt/alternating for port injection, and tune it as well as they can.

Then, if a user can't solve some tuning issues, they should try more squirts. But jumping straight to more squirts is probably not a good idea - most often it just masks the need for a richer idle mixture or more accel enrichment.

Example: Suppose you have a 4 cylinder 4-stroke engine. Then in 720° (two revolutions) you have 4 spark events (at 0°, 180°, 360° and 540°, then starting over at 720°=0°). MegaSquirt can only inject on an ignition event, so 1, 2, or 4 squirts/cycle for this engine.

If you have 2 squirts/simultaneous, you have:

crankshaft
degrees
 0° 90°180°270°360°450°540°630°720°
=0°
...→
cylinder at TDC
(not injector)
1
3
4
2
1
...→
bank 1injectinjectinject...→
bank 2injectinjectinject...→

but with 2 squirts /alternating, you have:

crankshaft
degrees
 0° 90°180°270°360°450°540°630°720°
=0°
...→
cylinder at TDC
(not injector)
1
3
4
2
1
...→
bank 1injectinject...→
bank 2inject...→

Both are two squirts per cycle, but alternating only has 1/2 as many total squirts per injector (i.e., since only one channel is active per squirt), so the pulse width must be doubled.

4 squirts/simultaneous would look like this:

crankshaft
degrees
 0° 90°180°270°360°450°540°630°720°
=0°
...→
cylinder at TDC
(not injector)
1
3
4
2
1
...→
bank 1injectinjectinjectinjectinject...→
bank 2injectinjectinjectinjectinject...→

There are 4x as many injection events with 4 squirts/simultaneous compared to 2 squirts/alternating, so the pulse width is just 1/4 as long (neglecting the opening times).

The req_fuel calculator will show this - watch the top and bottom numbers as you make changes to the alt/sim and number of squirts. The top number stays the same (it is the 'unadjusted number' for 1 squirt simultaneous), while the bottom 'adjusted' number used for the pulse width calculations changes with the number of squirts and simultaneous/alternating.

Note that the above tables show which cylinder is at TDC at specific crank angles. Which injector fires when depends on which injector bank they are connect to.


If you have any questions or problems that can't be answered from the links above, or a search the MicroSquirt® manual:
,

you can ask questions at the MicroSquirt® support forum which is at: www.microsquirt.com Click the links for more information.



MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
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©2011 Bruce Bowling and Al Grippo. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of V3 MicroSquirt® boards from Bowling and Grippo.