Electronic regulators as used in modern railway sets where the voltage is raised or lowered to control the speed of the engine Mechanical systems such as valves as used in fluid control systems. Purely mechanical pre-automotive systems included such designs as the Watt centrifugal governor whereas modern systems may have electronic fluid speed sensing components directing solenoids to set the valve to the desired rate. Complex electro-mechanical speed control systems used to maintain speeds in modern cars (cruise control) - often including hydraulic components, An aircraft engine's constant speed unit changes the propeller pitch to maintain engine speed.