The factors which limit the power that a given cylinder can develop are the piston speed and the mep. The piston speed, as stated before, is limited by the inertia forces set up by the moving parts and by frictional heat. In the case of the mep, the limiting factors are as follows:

1. Heat losses and efficiency of combustion.

2. Volumetric efficiency (the amount of air charged into the cylinder and the degree of scavenging).

3. Mixing of the fuel and air.

The limiting meps, both bmep and imep, are prescribed by the manufacturer or NAVSEA. They should never be exceeded. In a direct-drive ship, the meps developed are determined by the rpm of the power shaft. In electric-drive ships, the horsepower and bmep are determined by a computation based on readings from electrical instruments and from generator efficiency.

CYLINDER LOAD BALANCE

In order to ensure a balanced, smooth-operating engine, the general mechanical condition of the engine must be properly maintained so that the power output of the individual cylinders is within the prescribed limits at all loads and speeds. In order to have a balanced load on the engine, each cylinder must produce its share of the total power developed. If the engine is developing its rated full power, or nearly so, and one cylinder or more is producing less than its share, the remainder of the cylinders will become overloaded.

Using the rated speed and bhp, it is possible to determine for each INDIVIDUAL CYLINDER a rated bmep which may not be exceeded without overloading the cylinder. If the ENGINE rpm drops below the rated speed, then the cylinder bmep generally drops to a lower value. The bmep should never exceed the normal mep at lower engine speed. Usually, it should be somewhat lower if the engine speed is decreased. Some engine manufacturers design the fuel systems so that it is impossible to exceed the rated bmep. This is done by installing a positive stop to limit the maximum throttle or fuel control. This positive stop regulates the maximum amount of fuel that can enter the cylinder and limits the maximum load of the cylinder.

In order to meet emergency situations, engines used by the Navy are generally rated lower than those designed for industrial use. The economical speed for most of the Navys diesel engines is approximately 90% of the rated speed. For such speed, the best load conditions have been found to be from 70% to 80% of the rated load or out-put. When an engine is operated at an 80-90 combination (80% of rated load at 90% rated speed) the parts last longer and the engine remains cleaner and in better operating condition. Diesel engines do not operate well at exceedingly low bmep such as that occurring at idling speeds. You are well aware that idling an engine tends to gum up parts associated with the combustion spaces. Operating an engine at idling speeds for long periods will result in the necessity for cleaning and overhauling the engine much sooner than when operating at 50 to 100% of load.