Multi-Engine Speed Matching
Keeping the speeds of engines working together close. As GS740-3P channels do not communicate, matching is done by hand, channel by channel, on a tachometer.
Multi-engine speed matching is keeping the speeds of engines working together close to one another. Because there is no communication between the channels on the GS740-3P this matching is not automatic; the operator makes it by setting the potentiometer of each channel separately. Since the matching is made through the throttle lever position, the speeds can separate again when the load changes.
Why the same potentiometer position does not give the same speed
The age of the engines, the fuel setting, the characteristic of the mechanical governor, the cable friction and the load on them all differ from one another. That is why bringing the potentiometers to the same mark does not mean equal speed. The marks on the panel are not a means of control, only a reference that lets you return to the setting you found.
Typical reasons the matching goes off
- Cable friction: if the cable of one channel goes through a more roundabout route, the lever moves less at the same command.
- Fuel path: a blocked filter or an air leak holds that engine behind the others at the same throttle position.
- Different load: if the load on the pumps is not equal the governors settle at different points and the speeds separate.
- Mechanical play: play in the throttle lever joint swallows part of the command; even if you return to the same potentiometer position you will not find the same speed.
The right way to match them
- Bring the engines up to working temperature; a setting made on a cold engine shifts once it warms up.
- Read the speed of each engine with an independent tachometer. Matching by ear misleads, especially from inside the cab.
- Choose one engine as the reference and bring the others to its reading.
- Repeat the setting at the real working load; matching done off load goes off under load.
- Note down the resulting potentiometer positions. It is normal for the positions to differ from one another and it is not a sign of a fault.
The upper speed limit of the channels is taught separately from this; the calibration made with the blue wire is carried out independently for each channel. A common mistake is to confuse matching with calibration: calibration teaches the channel over what range it will drive the throttle lever and is done once, while matching is a setting made during work within that range. Repeating the calibration because the speeds have separated does not solve the problem, and on top of that it invalidates the potentiometer positions you found earlier.
Why the matching is not permanent
When the load on an engine increases its mechanical governor increases the fuel, but the speed still falls somewhat; this is droop behaviour. A position-based system cannot see this and does not correct it, because the only thing it measures is the command it gives itself. Holding the speed independently of the load is possible only with feedback that measures the engine speed; for the GS740 this comes under the optional speed holding module with active feedback.
The practical meaning of this is as follows: in an installation where the loads stay close to one another and steady, matching done by hand serves through the shift. In an installation where the load changes often and channel by channel, the matching is short-lived; the operator has to correct it several times during the day. Take this difference into account from the start when choosing the system, do not try to close it afterwards with adjustment.
How it is applied in the field
On jobs where the pumps work together, do the matching once at the start of the shift and look at it again when the character of the load changes. How the speed of three pumps is synchronised is explained step by step. If one channel stays constantly behind the others, examine that engine first rather than the control side: the fuel filter, an air leak and cable friction are the first places to look. Set the expectation right as well: on a position-based system the aim is not to lock the speeds together second by second but to bring the working bands closer to one another.
The context in which this term is used: Why is constant speed important on a generator?