[Diy_efi] ignition timing variation

[Eric Fahlgren]

Stewart Prince wrote:
> 
> Does anyone have any information on "acceptable" variation of ignition
> timing?  We're getting about +/- 2 degrees of jitter on our home-built
> EFI system (Formula SAE).  By my calcs, 100 microseconds of error
> corresponds to 1.8 degrees of rotation at 3000 rpm, and 7.2 at 12,000
> rpm (assume constant angular velocity). I believe the students are using
> the time between the last two teeth on an 8-tooth wheel (crank mounted)
> to calculate engine speed.  Is it possible that some sort of averaging
> scheme should be used to account for the speed variation due to combustion?

Stewart,

2 d is too much error, and 7.2 d is totally unusable when you are in
the powerband.

So the last 1/8th of a turn is being used to compute "RPM" and the
jitter is 2 d per revolution (or maybe cycle?).  That seems
reasonable, as you suggest, due to accelerations during combustion.
Have the kids use a running average, say accumulate time into
8 bins (one per tooth) and still compute cycle time every 1/8th
turn, but based on a full rev (or again, maybe a full cycle).
That should smooth the output nicely, but still allow real changes
in engine speed to be seen rapidly.

Eric 
-- 
Eric Fahlgren                                        MSC.Software
Simulating Reality                       Ann Arbor, Michigan, USA

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