The slide valve engines were later fitted with piston valves. The slide valve was removed and steam sent down. It was a slide valve coal gas engine. It had two overlapping cylinders that formed a combined double slide valve ("Doppelschieber"). The balanced slide valve was invented by the Scottish engineer Alexander Allan. Eccentrics on this shaft actuated a transverse slide valve for each cylinder. The slide valve is controlled by a fixed crank on the crankshaft.