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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. |
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