diy mpg gauge

[skewbe]

44100/8 bit/mono recording using a diode clamp interface and a 10k resistor on a 133mhz laptop with integral sound card via line in left channel . Car in neutral in driveway. Probe connected to yellow wire between fuel injector resistor and fuel injector.

3 samples, 1 low rpm, 1 accalerating to higher rpm, 1 high rpm. Images stretched horizontally for easier duty cycle visualization/comparison.


steadyish @ ~1200
period 1714 bytes
pulse width 119 bytes
duty cycle 0.0694



just starting to ramp up on the way to 2000rpm in neutral
period 1273 bytes
pulse width 137 bytes
duty cycle 0.1076



steadyish @ ~2000rpm
period 833 bytes
pulse width 73 bytes
duty cycle 0.0876
(duty cycle increased for higher rpm and throttle setting.

sanity checks:
(3 pulses/2 revolutions)*2000 rpm=3000 injector pulses/minute = 50 pulses/second
44100/833 = 52.94 pulses/second. Enh, close enough for a single wave analysis.

accelerating had the largest duty cycle (most time spent squirting fuel)

hi rpm has a proportionally larger duty cycle than low rpm.

833 byte sized samples @ 2000 rpm equates to a potential 99.75% accuracy at 4000rpm

Note: this signal doesn't look very clamped to me, maybe 10k is too much or my diodes fried. Agc is having fun with the signal too.