All You Ever Wanted To Know About OHV, SOHC, and DOHC
Engine technology is continually evolving and improving, which means that several options exist for engine designers and automakers to use when developing a new engine. Three common types of camshaft engines are in popular use today and many are confused about what the differences are between them:
All camshaft-type engines have three basic components in their heads which actuate the intake and exhaust for each cylinder: camshafts, rocker arms, and valves. The camshaft rotates in time with the engine's central crankshaft and has egg-shaped lobes on it which, when the taller portion comes around, push against rocker arms (or pushrods in the OHV case) which rock against their spring, pressing the valve down to open it at its head, allowing air, fuel, or exhaust to enter or exit the combustion chamber or cylinder. Most engines utilize at least two valves per cylinder (one intake, one exhaust) while some have many more, depending on design.
OHV or Pushrod
Overhead valve (OHV) or pushrod cams are common on simplified V-style engines. The camshaft resides at the center of the V, usually at its base, and as it rotates, it pushes rods (hence the "pushrod" name) that in turn push against rocker arms that open and close valves in the engine head to either side of the V.
This design is less complex, more compact, and less finicky about oil. These engines are common in some forms of racing and in very small applications.
Single overhead camshaft (SOHC) engines are very common. They work with both inline and V configurations and operate exactly as their name implies. A single camshaft, located over the heads, rotates in time with the crankshaft to actuate rocker arms on one or both sides (depending on valve configuration).
SOHC engines are relatively simple to produce, have fewer moving parts than DOHC, and are relatively robust. They are commonly configured with three or more valves per cylinder for greater efficiency and control.
Dual overhead camshaft (DOHC) engines are also common. These work with either inline or V-shaped engines, but are more common on Vs. They also operate as their name implies. Two (dual) camshafts reside over the heads, rotating in time with the crankshaft. These will have four valves per cylinder, two controlled by each overhead camshaft. The camshafts are situated as mated pairs next to one another. DOHC allows more efficiency and better control over the intake/exhaust process since each camshaft can be tuned separately from its mate.
DOHC engines are more complex than SOHC, but are very efficient and are thus gaining popularity in automotive design geared towards better economy and lower exhaust emissions.