According to a search of NTSB data, in 2010, there were only four general aviation fixed-wing accidents investigated that involved in-flight engine compartment fires. That’s a representative annual number. Initiating events commonly include things such as fatigue failure of cylinder fuel lines, leaking fittings and deteriorating hoses that should have been removed from service.
There are many opportunities to feed fires in engine compartments where you’ll find hoses and lines full of fuel, oil and hydraulic fluid. The FAA has two terms to describe the flammability performance level of materials used in engine compartments: fireproof and fire-resistant.