Steve wrote:

In almost ALL real-world engines, the actual limit is set by the point
at which some mechanical component would fail. The engine's torque *may*
drop off well before the mechanical failure point if it can't ingest
enough fuel or air at high speed. In the case of a diesel, you can
pretty much increase the burn rate to as high as the mechanical parts
can tolerate by increasing turbocharger boost (and injection rate to
match). Since detonation isn't possible (no fuel exists in the cylinder
until combustion is supposed to begin anyway) the only limits to boost
pressure are mechanical in nature. In practical terms, no one really
wants or needs a 9000 RPM diesel, though.

Actually, detonation can and does occur in Diesels. With today's Diesel
fuels and injection methods the fuel does not burn INSTANTLY upon
injection. The droplets are too large. The droplets tend to evaporate
from their surface area. It IS possible for unburned gases evaporated
from the fuel to exist in the chamber and detonate in local regions. The
effect is nowhere as severe as the detonation that occurs in spark
engines at times. Interesting traces made with high frequency pressure
sensors indicate these local "peaks" in combustion pressure when this
occurs. It is the cause of the "tinkling" sound that Diesels
occasionally make.

Detonation is not a full or nothing situation. Local regions can
undergo detonation even while the majority of the charge undergoes
normal wavefront burning, in either Diesel or SI.