So here is a picture illustrating what the fuel injection does.
This is taken from a flat-out acceleration run, starting at 3000rpm in top gear, with the throttle held wide open for thirty seconds. The road was flat, there was no noticeable wind, the engine was fully warmed up and the ambient temperature was around 20C.
The picture shows a number of snapshots going down the page; each is numbered with the time at which it was taken (beginning at 0s). For each snapshot you will see a diagram of the pulse timing, the throttle opening and the engine rpm.
The pulse diagram has time along the x axis, and the blue blobs show the start and end of the pulses. The x axes are to scale, which is why they get shorter as the engine rpm gets faster. Above the blobs is a note of the total injection pulse duration.
The x axis lasts for ONE FOUR-STROKE CYCLE (two revolutions of the crank). Unfortunately I couldn't conveniently trigger from exactly TDC; rather I've triggered from the spark event which is perhaps 20 degrees before TDC(??). Therefore the coloured line above the diagram showing the strokes is only approximate. To summarise: each snapshot goes from one spark event to the next spark event.
Unfortunately I didn't realise the engine often injects right across TDC. This makes the diagrams harder to interpret, but I'll explain next.
Please begin by looking at the snapshot labelled "3s". Because of the difficulty I mentioned earlier, the first, longer blob on the left is actually the remainder of the pulse which started in the previous cycle, and the small blob on the right is the start of the next injection pulse, which continues into the next cycle. So in reality we are seeing ONE PULSE PER CYCLE, it's just that it is spread across TDC on the compression stroke. (Triggering off the spark event was a bad idea!) Because the pulse length doesn't change much between consecutive cycles we can still add the blobs together to get a good idea of the total injection duration per cycle, and you can read this from the middle of each pulse diagram.
As you scan down the page you will see the injection pulse duration remains pretty constant, between 12.1ms and 12.5ms, even though the engine speed is increasing. This duration presumably provides the perfect mixture for steady-state wide-open-throttle acceleration.
After 30s I roll off the throttle. At 31.3s the throttle is at 54%, although the engine had barely slowed down. The fuel injection pulse is now down to 10.6ms. As I continue to close the throttle the injection pulse gets shorter, until it is only 3.4ms at 32s. Note how the pulse is shortened by delaying its start - the end point remains the same.
We skipped the first three seconds earlier. At 0s - when I had just snapped open the throttle - the injection time was longer, at 14.8ms. This shows the
temporary acceleration enrichment, which allows the engine to respond instantly to rapid throttle demands. Over the next three seconds it gradually falls to 12.6ms, where it goes into the steady-state explained earlier.
Finally, notice how the extra fuel is supplied by an additional pulse, this time around TDC on the exhaust stroke. This second pulse gradually gets shorter, and then disappears, over the three second period.
I've got some more diagrams which I'll upload separately. These show what happens at idle, and a few other places. This use of a second pulse is quite common. The second pulse is always around TDC on the exhaust stroke.
