Ignition Handbook page 882

"In actual automobile fire tests, it is found that gasoline or brake fluid leaking onto a hot manifold will not ignite without a flame or spark (eg., errant spark plug wire), but that automatic transmission fluid and power steering fluid will. Engine oil can also be ignited in this manner."

I think the sentence before may have confused the issue. In a vehicle fire investigation course I took, they heated up a stand-alone exhaust manifold with a large torch until it was a similar temperature as found on a vehicle. A different engine fluids were sprayed on the exhaust. The gasoline did not ignite. The alternator is a competent ignition source of a gasoline fuel leak in an engine compartment.

-Pat