Locomotives with superheaters are usually fitted with piston valves or poppet valves. Each cylinder bank had a single overhead cam actuating the poppet valves. The valve core is a poppet valve assisted by a spring. Presta and Schrader valves used on pneumatic tyres are examples of poppet valves. Sickels patented a valve gear for double-beat poppet valves in 1842. In internal combustion engines with pistons, the camshaft is used to operate poppet valves. The plug looks similar to a poppet valve. The problem was traced to a malfunction of the vernier LOX poppet valve. The prototype uses two overhead camshafts with standard poppet valves. The valvetrain consisted of 12 poppet valves timed by two chain-driven overhead camshafts. These inefficiencies drove the widespread experimentation in poppet valve gears for locomotives. Some SI engines are crankcase scavenged and do not use poppet valves. The T1 was also the first production series locomotive designed to use the poppet valve. In most cases it is beneficial to have a "balanced poppet" in a direct-acting valve. The change to poppet valves began with the Fifteen of 1933.