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A T-head engine is an early type of internal combustion engine that became obsolete after World War I. It is a sidevalve engine distinguished from the more common L-head engine by its valve placement. In T-head engines, the intake valves are located on one side of the engine block and the exhaust valves on the other. When viewed from the end of the crankshaft, especially in a cutaway view, the cylinder and combustion chamber resemble a 'T', leading to the name "T-head". In contrast, an L-head engine has all valves on the same side.
One reason for the T-head design's popularity from the late 19th century to the 1920s was the type of gasoline available then, which ignited at lower temperatures than contemporary fuels. If gasoline vapor became too hot or was overly compressed, it risked pre-ignition or engine knocking before the spark plug could ignite it. Detonation, a leading cause of catastrophic engine failure, necessitated managing both the temperature and compression of the highly volatile gasoline vapor for maximum reliability.
The T-head addressed these issues by placing the valves in open alcoves on opposite sides of the cylinder head. Additionally, cool water from the radiator was directed over the intake valves as a further safety measure. This design minimized heat transfer from the exhaust ports and reduced the engine's compression ratio, thereby preventing detonation. The relative inefficiency of the T-head engine was offset by significant reliability gains and suited the lower travel and racing speeds of the era. With the entire cylinder bore and cooling passages often contained within the head, many T-head engines required no head gasket and fewer gaskets overall, reducing potential leakage. This reliability made T-head engines popular in early auto racing, where engine reliability was more crucial than peak performance. However, as other engine designs enhanced their reliability during World War I, the T-head's performance limitations led to its decline in racing applications.
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