RoyJ wrote:
Risking another long debate with you, I have to agree with 4x4ord.
Your original quote was based on diesel flame propagation speed (at some reasonable AFR, CR, etc.). It implies at 1800 rpm, the piston best captures the force developed by the speed of combustion (aka propagation speed).
But how can that be when the piston speed @ 1800 rpm can be vastly different depending on the stroke?
On one extreme, a heavy ship engine, if magically turned at 1800 rpm, would probably have piston speeds going supersonic. A remote control sized engine, at 1800 rpm, would be so slow it may not run.
Hence, when you look at BSFC curves for diesels, the longer the stroke, the lower rpm it develops peak fuel efficiency.
Not saying YOU are wrong, but the person you quoted was being too generic. Perhaps they should've said "typical sized engines found on light cars and trucks, with stroke of xx inches, develops peak fuel efficiency around 1800 due to their piston speeds matching well with the diesel flame propagation speed"
Large ship diesel engines do not use the same kind of diesel we use in our trucks. They use marine diesel oil which is a blend of gasoil and heavy fuel oil, straight heavy fuel oil, or various blends of the two. All have completely different properties and burn rates than on highway diesel. I don't think you would even be able to use regular diesel oil after a certain size, but I could be wrong.
Marine Diesel Oil (MDO) & Intermediate Fuel Oil (IFO)However, even 4x4ford is saying that you are wrong when you stated that the 6.7L Cummins could achieve better fuel economy at higher rpms(1,600-1,900 rpm) based on the BSFC map I posted. He is adamant that the 6.7L and other diesels only achieve the best fuel economy at the lowest rpm regardless of load.
