Toro MX 4250 starter current captured using a Picoscope

A recent exercise was to become familiar with a recent acquisition, a Picoscope USB oscilloscope.

It has quite different software to those of other USB oscilloscopes that I have used, so a bit of learning and adaptation.

The test scenario here is capture of battery current and battery voltage whilst cranking my lawnmower which as a 24.5HP V twin petrol engine.

Above is the captured data expanded to explore the initial part of cranking (with disconnected spark plugs).

One can observe the very high initial current of the stationary armature, reducing as the armature spins up. When the bendix spiral has sent the bendix gear into mesh with the ring gear (about 42ms), there is a sudden increase in current as the armature is near stalled again, and current decreases as cranking speed increases. Peaks in current occur where the cylinders pass through the compression stroke.

Note that like many / most small engines, this engine has compression relief to reduce the cranking load.

Above is a different view of the same capture, this time showing that the peak current for each cylinder is approximately uniform. A previous capture showed they were a little different, and the cause was out-of-spec exhaust valve clearance on #1 which has since been corrected.