What is the Meaning of DOHC?by Joseph Nicholson
Double overhead cam, or DOHC, engines are associated with higher horsepower and increased performance. The unique design makes these engines quieter, smooth-running and more efficient.
A double overhead cam engine is exactly what the name suggests: a gasoline combustion engine with two cam shafts located above each row of cylinder heads. The DOHC design is often compared to overhead valve (OHV) designs that put the cylinders above the cam shafts, and single overhead cam engines, or SOHC, in which only one camshaft drives the valves.
It's not necessary to have two camshafts to have multiple inlet or exhaust valves on each cylinder, nor do all DOHC engines have multi-valve cylinder heads. In fact, the first double cam engines had only two valves per cylinder. Most today, however have between three and five valves per cylinder. Engines with eight cylinders or more, like the Bugatti 16-cylinder engines, have more than one bank of cylinder heads, but because each has two camshafts per cylinder head they are considered DOHC engines even though they have more than two camshafts in total.
The DOHC engine design is used to increase the efficiency of the engine. An engine can only produce as much drive power as it can generate energy in its combustion chambers, and this is largely determined by the airflow in the chambers. The dual camshafts allows for twice as many valves per cylinder, maximizing the engine's output without increasing its overall volume.
At highway cruising speeds an engine can easily be spinning at 3000 revolutions per minute, meaning that a valve in the engine would be opening 1500 times per minute, or 25 times per second. Using more camshafts to actuate the valves makes the valve train lighter and reduces the drag on the system. It also allows more valves to be present on each cylinder. Cars with more valves tend to be quieter and stronger torque at high speeds.
DOHC engines were first designed on two cylinder engines, but evolved with the straight-8 and the modern V6 and V8 engines. Many designs come in different versions, with the single cam varieties having exactly half as many total valves and appearing in lower performance models. Engines that have five valves per cylinder will have two on one camshaft and three on the other. An example of the added power of DOHC engines is the difference between the sedan and family models of Dodge and Chrysler, which use a single overhead cam version of the Mitsubishi Cyclone V6, and the 3000GT and Stealth980s which use a DOHC version of the same engine to provide extra horsepower.
- Matthew Newton (CC-BY-SA-2.5)