The irregular injection may be caused by jiggling of the governor, and/or play in the connections in the fuel pump rack control system. The effect in either case is that the fuel pumps, when operating so close to the minimum injection amount, may sometimes just have enough index to inject fuel, at other times just not enough index to do so.
By the introduction of a system where approximately half of the cylinders are cut out at low speed, the injection into the remaining working cylinders is improved considerably, giving more stable combustion and, consequently, stable running and keeping particle emission in the low speed range at a minimum.
To avoid that excessive amounts of cylinder lubricating oil are collected in cylinders that are temporarily deactivated, the cutting out is made by turns between two groups of cylinders in order to burn surplus lubricating oil and keep the same thermal load on all cylinders. Turns between the groups are made on a time basis.
The group separation is determined in order to halve the number of active cylinders and to get the smoothest possible firing order. In order to obtain a safe start, the cutout system is disabled during the starting period and until the engine has been stabilised.
The system has been in service for a year on a series of 11K90MC-C engines, and stable operation down to 13 r/min (MCR is at 104 r/min) is achieved by means of this system.