- 47% mechanical
- 21% electrical
- 7% intentional
- 6% exposure
- 4% crash, overturn, run-over
- 2% smoking
So crashes are a relatively rare cause. They also break it down by the area the fire started:
- 63% engine area, running gear, or wheel area
- 11% passenger/operator area
- 5% cargo/trunk
- 3% exterior
- 2% fuel tank or line
Interpolating between those numbers, you might guess that between 20% and 40% of fires could be described as "engine spontaneously combusting." That would be 45,000 - 90,000 per year in the US.
So despite you never having heard of it, it does happen.
[0] https://www.nfpa.org/vehiclefires, numbers above from report table 8 & 9
Most interesting is that the first part of an ICE to catch on fire is most frequently the electrical wiring. And even if it is a flammable liquid (the second most likely cause), it's only a 42% chance that liquid was gasoline!
Thanks again, it's interesting reading.