The conceptual sketch below shows a model for a closed loop control of a lawnmower engine. Unlike the lawn mower motor of exercise 7, there is a feedback device. It takes advantage of a cooling fan that is connected directly to the output shaft of the lawn mower. This fan blows on a flap indicated in the diagram, which is preset to a desired position and kept in that position against a spring tension if the fan is rotating at a desired speed. The desired speed is set by initializing the spring with a tension SSP corresponding to a desired operating condition such as 3,000 RPM.

If the fan rotates too fast (i.e., the engine speed is too high), the flap is blown clockwise, causing a mechanical linkage to close the throttle and reduce engine speed. If the speed is too low, the flap is pulled counterclockwise by the spring. Draw a block diagram for this control system, assuming:

(1) That the control unit sends an error signal in terms of the spring tension (SSP (set point) – S) to throttle a proportional amount of air/fuel GTh(SSP – S) kg/s;

(2) Resulting in engine power P = G_E × AF; 

(3) The speed response function for the motor speed is N = G_SR × P. Finally the controller converts engine speed to spring tension by the relationship S = G_STN.

Exercise 7

Fuel Throttle Control Cooling Fan AF Engine Linkage Spring Return Flap