The problem is injection duration, not just fuel quantity. The injector sprays from the center of the cylinder at a nearly flat angle, so as the piston moves down after top dead center, the relationship between the spray and the piston changes fast. Early on, the fuel stays in the bowl where it belongs. Keep the injector open too long, and the burning spray crosses the bowl edge, superheats the piston lip, and can scorch the cylinder wall. By roughly 40 degrees after top dead center, the piston is far enough down the bore that the spray is no longer safely contained. That is how you burn pistons: too much injection duration too late in the cycle.
The injector sits at the center of the cylinder and sprays fuel outward at an almost flat angle.
That spray pattern makes piston position critical. As the piston moves down the bore, the fuel plume must remain directed into the piston bowl rather than crossing the bowl edge or reaching the cylinder wall.
Injection begins as the piston approaches top dead center and continues as the piston starts moving down the bore.
The key question is how long injection can continue after top dead center before the piston moves too far away from the intended spray pattern. In this example, the practical limit is approximately 40 degrees after top dead center.
At top dead center, the piston is positioned so the injector spray is directed into the combustion bowl.
At 20 degrees after top dead center, the piston has started descending, but the fuel spray is still largely contained within the bowl. As the crank continues rotating, however, piston position changes rapidly.
At 20 degrees after top dead center, the spray can begin approaching the edge of the piston bowl.
As injection continues and the piston moves farther down, the fuel plume can cross the bowl edge. This concentrates heat around the radius at the lip of the bowl and can superheat that portion of the piston.
Piston movement increases rapidly after top dead center.
By 30 degrees after TDC, the piston is considerably farther down the bore than it was at 20 degrees. This demonstrates why relatively small changes in injection duration can significantly change where the fuel spray lands.
By approximately 40 degrees after top dead center, the piston has moved far enough that the injector spray is no longer contained within the bowl.
Instead, fuel can be directed toward the cylinder walls. At this point, the spray geometry and piston position are no longer properly aligned.
Extending injection too far after top dead center can expose the piston bowl edge to excessive localized heat.
As the spray crosses the bowl radius, that area can become severely overheated. Extending injection even farther introduces cylinder-wall wetting as well.
The central lesson is that injection duration must keep combustion properly contained within the piston bowl. If injection continues too late, the spray crosses the bowl edge, overheats the piston, and can ultimately burn or damage it.
