RoyJ wrote:
I think what he's saying, and I'll let him speak if I'm wrong, is on larger engines, 1800 may not be the most efficient rpm.
I used a ship engine as an extreme example, and yes I realize big ships run heavy bunker fuel, but large yatchs, tugs, often run diesel. Regardless of fuels the ideal rpm drops as engine stroke increases.
But I agree lugging an ISB at 1100 rpm is just as bad as revving an ISX to 2100 rpm. In both cases, BSFC is really poor, because the piston speeds are too far away from ideal diesel burn rate. I think we're saying the same thing, just worded differently.
I am not saying that larger engines are most efficient(using the least amount of fuel) at 1,800 rpm either. I am saying that they generally have the highest thermal efficiency at that rpm. The most thermal efficient rpm is the point at which an engine utilizes the most energy out of a drop of fuel while the rpm at which may be different that the rpm where the engine is using the least amount of fuel.
Take the study Caterpillar did for the DoE a few years ago with their C15 engine that reaches peak torque at 1,200 rpm. As you can see below, even though it makes peak torque at 1,200 rpm and uses less fuel at 1,200 rpm with any load, 1,800 rpm is where it converts the least amount of fuel into to the most amount of power. There have been multiple studies from various other manufacturers with many engines size stating the same thing that the most thermal efficient rpm was 1,800 rpm because the engine was able to burn the amount of fuel being injected more completely utilizing the most energy from it.
Again, I think he is confusing thermal efficiency with the point at which a diesel will use the least amount of fuel for a given load. This point varies with every engine and is based on engine design characteristics such as bore stroke ratio, cam profiles, turbo efficiencies, size and so on. These characteristics change depending on how the engine will be used. The lowest rpm may not always be the most fuel efficient point with a given load.
He is also arguing that I will always achieve better fuel economy while towing my 13k trailer in 5th rather than 6th at 65 mph which is what started this bout in this and other threads. Below is the BSFC of my truck. Yellow is 6th gear at 65 mph and red is 5th. What do you think? On flat land with normal wind drag, I agree that 6th is better, but not on grades or higher than normal wind drag. These scenarios push me into the dark blue of the map.
