Cams: so important for the four-stroke motor…
As its name suggests, an over-head cam sits ‘over-the-head’ of the engine itself and the job of the cam-shaft is the opening and closing of the valves themselves.
This design itself originates from the many advances that happened in the aero engine during wartime. During the First World War, aero engines not only became more efficient and reliable they also increased in power on both sides. This was vital if the air-war was to be won.
Power increases were pretty darn big… at the start of the war in August 1914 many aero engines would be hard pushed to make 50bhp, but by the end of the war mammoth motors pumping out almost 350-400bhp had been built. Much of this was down to engine design itself changing but some was thanks to the development of the OHC or ‘Over Head Cam’ system.
Many WW1 fighters used air-cooled, push-rod rotary engines, where the propeller and cylinders spun around a fixed crank and while both sides had engines like these, other more complex motors were also being developed.
Over-head cams first originated in the liquid-cooled, inline six-cylinder Mercedes engines used in the Albatros series of German fighters. The DIII engine produced around 160bhp and a similar design of OHC was copied from the German engine and used in the American ‘Liberty’ V8 aero engine which produced around 400bhp!
So what are the advantages of an over-head cam system? Well, the over-head cam set-up will use fewer components, even if the system itself with the cams may be complex things themselves. This complexity was accepted as one of the downsides of better performance and allowing the motor to higher revs and therefore torque and power….
In over-head cam systems the camshaft operates the valves via its lobes and a rocker arm, whereas the over-head valve is moved via tappets, pushrods and rocker arms. Over-head cams allowed engine designers some leeway when it came to the configurations of the ports themselves, as those previously pesky pushrods weren’t in the way.
Designers soon began to use DOHC, or double/dual over-head cams, per cylinder bank as a way to make yet another performance leap forward. One cam moved the inlet valves and one the exhaust valves. Again, the use of DOHC originally can be traced back to aero engines of the 1920s and car and bike motors followed soon after. DOHC works well in multi-valve layouts where the engine has more than one inlet or exhaust valve per cylinder, but they have been used in two-valve per cylinder motors too.
The cams themselves can be driven by belts (think Ducati) chain (think lots of Jap bikes) or gears (think some Honda V4s…)