Research report | Assessment of Governor Control Parameter Settings of a Submarine Diesel Engine
Modern conventional submarines use diesel generators to provide power for propulsion and the hotel load. The governor, often a proportional-integral controller, attempts to maintain a constant speed by regulating the fuel flow to compensate for the back pressure disturbances due to the underwater exhaust. Poor control can cause fluctuating exhaust gas temperatures, leading to increased wear and reduced reliability. This paper develops a low order engine model which is then used to investigate the performance benefits that can be obtained through proper tuning of the governor control parameters. It is found that the engine exhibits stable behaviour over a very wide range of controller gains, and that tuning the governor solely to minimise the engine speed fluctuations may not minimise the exhaust gas temperature fluctuations.
Modern conventional submarines use diesel generators to provide power for propulsion and the hotel load. The governor, often a proportional-integral (PI) controller, attempts to maintain a constant speed by regulating the fuel flow to compensate for the back pressure disturbances due to the underwater exhaust. Poor control can cause fluctuating exhaust gas temperatures, leading to increased wear and reduced reliability.
This paper develops a low order model of a generic engine and governor, and then uses the model to examine the effect of varying the proportional and integral gains on the ability of the controller to reject the disturbance and maintain a constant engine speed. The effect of the governor gains on selected other engine parameters is also investigated.
The key conclusions from this work are as follows:
- for a speed governed engine, with the control system implemented using a PI controller, the engine exhibits stable behaviour over very wide ranges of both the proportional and integral gains;
- it is possible to tune the controller in such a way as to effectively eliminate engine speed fluctuations due to back pressure disturbances;
- tuning the control system to focus on minimising speed fluctuations may, or may not, also minimise the cylinder exit temperature fluctuations (for the engine described in this paper, there is a large region for which the speed variations are minimised, but the cylinder exit temperature fluctuations are only minimised over a small part of this region); and
- even at the optimal control point, a PI controller based on speed error alone still results in significant temperature variations due to exhaust back-pressure disturbances. This effect cannot be simply tuned out. A different control structure is required for reduced engine temperature variations.
It should be noted that the results presented in this paper only apply to the engine modelled here, and other engines may respond differently to changes in the controller gains. This paper describes a technique for optimising engine performance in response to back pressure fluctuations by tuning the controller parameters. In order to apply this technique to another engine, a validated model of that engine is required.