The invention relates to ship propulsion systems of the type incorporating air actuated clutches, and more specifically to an improved control system which provides an automatic engine speed advance during the engagement cycle for the pneumatically operated clutches.
A common form of marine propulsion system employs ahead and astern air actuated clutches for connecting the prime mover to a reversing reduction gear unit for each propeller. The air actuated clutches are engaged by the inflation of an inflatable rubber and fabric air gland bonded to an outer steel rim. Friction lining on the inner surface of the gland engages a cylindrical clutch drum when the gland is inflated. When the gland is fully deflated there is no clutch engagement, and when the gland is fully inflated there is complete clutch engagement. Between these two extremes the degree of clutch engagement corresponds to the amount of inflation of the gland.
In certain propulsion systems a controlled slip of the clutch is provided by controlling the degree of clutch engagement. Such controlled slip permits a lower propeller shaft speed than would be otherwise available at engine idle with full clutch engagement. This is particularly advantageous for maneuvering the ship when docking or traveling in a congested area. An example of such a controlled slip system is found in my earlier U.S. Pat. No. 3,727,737 issued Apr. 17, 1973, for "Pressure Modulating System for Reversing Clutches and Throttle Control".
In the system of my earlier patent, I provided a pneumatic clutch control assembly for a ship's propulsion system that was sequentially operated to regulate the inflation of ahead and astern air inflatable clutches and to also control the prime mover speed. The control assembly was actuated by a single throttle lever located on a pilot house control stand. Movement of the lever in one direction provided forward rotation of a propeller at a speed which increased with handle travel away from neutral. Movement of the handle in the opposite direction provided astern rotation of the propeller with speed increasing as the handle was moved farther from neutral. The center position provided a neutral setting in which the engine was disconnected from the propeller and no power was transmitted, although the engine continued to idle.
The single lever control of both direction and speed was accomplished in the following manner: As the lever was pivoted in either direction from the neutral, air was supplied to a selector valve which selected one or the other of the ahead and astern clutches. Thereafter, and up to a first control pressure, air pressure proportional to the position of the lever away from neutral fed through a first valve to the clutch and thereby began inflating the selected clutch. During this time the engine would remain at idle speed. After a first control pressure was reached, the first valve was piloted and it connected a second path for air to the clutch. This second path had provision for an initial programmed rate of feed of air to the clutch through a choke valve so as to softly inflate the clutch. Upon reaching a second higher control pressure, full supply air pressure was connected to the clutch. After the first control pressure was reached, the continued inflation of the clutches was not dependent upon the position of the throttle lever.
When the air pressure within the clutch rose to a predetermined level, the control of my earlier patent piloted a governor valve which, in effect, connected the throttle lever control to the speed governor of the engine so that the pressure supplied to the governor directly corresponded to the position of the throttle lever and the speed could be controlled by movement of the throttle lever. The throttle lever setting determined only the final operating speed and direction and all intermediate steps of clutch engagement and inflation and engine governor speed were handled automatically by the control system.
Certain engines used for marine propulsion will stall at idle speed when the clutch is engaged. This problem is solved by increasing the speed of the engine during the period of time in which the clutch is engaged. The increased engine speed increases the engine torque capacity and the inertia of the engine while the clutch is being engaged and thereby avoids stalling of the engine.
Present control systems which provide for the automatic increase in speed of the engine during the time of clutch engagement have accomplished this by automatically accelerating the engine to a specific rpm for a specific period of time. While such control systems allow for adjustment of both the specific rpm to which the engine is increased and the period of time of the engine speed increase, the available ranges of speed and time are arbitrary and are not inherently related to any specific operating conditions for the propulsion system. Thus, if the throttle lever is set for a speed which is less than the increased speed which the control will call for during clutch engagement, the engine will be accelerated to a speed beyond that at which it will run after the clutch is engaged. The time period can also continue well beyond the clutch engaging cycle thereby resulting in a surge condition where the vessel accelerates too quickly and not under control of the vessel operator.
I have provided by the present invention an improved control system which provides for an increase in the speed of the engine during the clutch engagement, but in which the engine speed boost timing is governed specifically by the clutch engaging cycle and the engine boost speed cannot exceed the speed that the engine is set to run at after the clutch engaging cycle is completed.