Fuel tanks could also ignite from multiple hits.
With bombers exploding from hits, this could happen from the bombload being ignited. Explosion would be spectacular... and dangerous.
Often happened vs nightfighters because of the angle of the Schräge Musik cannons.
In principle greybeard is about right, I think. There was an American training video made by Disney, to US Naval aviators in WW2. It's on youtube. And most importanltly it was narrated by the infamous John Thach. He is the tactical innovator behind the Thach Weave manouver.
Thach recommends in the video, to always aim for the frontal engine (vs an enemy fighter aircraft). Unless that is, you can confidently pop some bullets directly into the cockpit of the enemy.
The reasoning being, that once you have solid aiming point in the engine, first bullets, even if only a few of them, will hit the vulnerable engine. Any engine will lose performance, once damaged, I think.
Other bullets in the burst will sweep across the other vulnerable parts such as the cockpit. The only reliably effective cockpit armor vs heavy machine guns or cannons, is armor steel or titanium. Neither of which, exist to protect the pilot in the plexiglass canopy, out of which the pilot is observing the world around him.
Yes, the radial engine would act as sort of armor, blocking shots from the front, to the pilot. But engine might get knocked and lose power so it's not exactly a win-win scenario here...
Oftentimes there would be fuel stores just in the centerline fuselage, behind the cockpit. (such arrangement exist in me109 and fw190, also spitfires for the Allies)
Basically when you hit the engines with a good solid burst, you will have a chance of damaging any of the following three: engine, pilot, internal fuel tanks.
You can also damage the control surfaces more to rear, the tailplane, if it was a longer burst.
Although despite Thach's advice about aiming, it was also specifically very effective to aim for the wingroots of A6M zeroes, as they had vulnerable fuel stores there (of course every respectable IL-2 player knows this already!)
