How do OHV Small Engines Work

Overhead valve engine features

Overhead valve engines (OHV) are designed with the camshaft housed in the cylinder block of the engine. The compact design makes the OHV engine a durable machine. The OHV is so compact that even the intake and exhaust valves are housed in the cylinder head, just over the combustion chambers. The cylinder block is located differently for straight engines and V engines. To locate the cylinder block in a straight engine, look next to the crankshaft. In a V engine, the cylinder block is located directly over the crankshaft. Valves moved by the pushrods are designed to be parallel to the pistons in the OHV engine.

The pushrod

An OHV engine is often referred to as a pushrod engine, because the valves are moved by pushrods. The terms OHV and pushrod engine are interchangeable. The valves in the OHV/ pushrod engine are operated by lifters and rockers from inside the engine block. The OHV/ pushrod engine houses a camshaft that is driven by chains. These chains are renowned for their reliability over the more common timing belt in other types of engines. Pushrods set the rocker arms into motion. The rocker arms in turn open and close the valves in the OHV/ pushrod engine. Tappets, also called lifters, are housed within the camshaft, next to the camshaft.

The OHV is a simple drive system

Other types of engines require tensioners, and sometimes more than one chain. OHV engines (pushrod engines) are built with the camshaft next to the crankshaft. The OHV is run with a small chain, and sometimes by a straight gear connection.

RPM and inertia

Valve timing is a problem during high revolutions per minute (RPM). Greater inertia occurs due to the number of valve train components, including the rocker, pushrod, and lifter. Variable valve timing is not possible with the limitation of valves for the cylinders in the OHV engine. The difference between OHV engine designs for production cars and racing cars is significant. Revolving the engine for a sport cars used in races is restricted to about 10,500 RPM. The production car (for consumers) is further restricted. Production car engines may revolve at a mere 6,000 to 8,000 RPM. Because of the limitations on the number of cam lobes, a double camshaft is desirable for racing. Due to the limitations of RPM in OHV/pushrod engines, modern racing cars are built with an overhead cam engine rather than an overhead valve engine. Even today's modern consumer automobiles are manufactured with a double overhead cam, giving the engine the ability to produce more power while reducing the engine volume. The more openings (many valves) in an engine allow exhaust to be released faster, which produces all that race car power in a smaller volume engine. OHV/ pushrod engines, however, have survived in automobile manufacturing for decades due to the reliability of the chains versus the modern timing belt in double cam engines. The OHV/pushrod engine is also much simpler in design.