ShinerBock wrote:
4x4ord wrote:
ShinerBock wrote:
4x4ord wrote:
This is the torque curve for the PACCAR MX 13 that’s Peterbilt uses in their medium duty truck
Looks like PACCAR is unaware of the 1800 rpm fuel saving rule.
I think you are a little confused as to what that 1,800 rpm rule is. It is the rpm at which diesel burns most efficiently and completely and there for will generally have the highest thermal efficiency for the amount of power it is creating around that rpm. Basically, it is the point at which you get the most out of a drop of diesel fuel. This is not to be mistaken for the spot at which an engine is using the least amount fuel or the different rpm ranges to make power for different applications, industry needs, or rpm limitations due to engine size.
I might be a little confused. This quote from your original post in this thread:
Knowing all this, manufacturers of diesel engines, turbochargers, fuel injection systems and engine control systems have all directed their efforts at producing engines that make the most of this 1,800-rpm phenomenon. So, drive your diesel at or near to 1,800 rpm if you’re looking for the very best in both engine torque and fuel economy."
has me understanding that engine manufacturers would be designing their diesel engines to be used at 1800 rpm to make best use of the “rule”. The PACCAR 13 litre totally misses out as it won’t even rev to 1800. Would you expect the PACCAR 13 litre to be more fuel efficient at 1700 than 1100 rpm?
Re-read the sentence above especially the part in red. In order to create more horsepower within reliability and emissions specs, you have to go big. The bigger you go, the more you are limited on rpm.
I believe you are your own worst enemy here.
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"
