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By Jim Kerr; photo courtesy Lawnmowerblog.Blogspot.com
There have been many engine designs over the years, from sleeve valve to the Wankel rotary, but only two designs have endured; pushrod engines and overhead camshaft engines. There are advantages and disadvantages of both designs so, which is better? Let’s take a look and you can decide.
Pushrod engines are the simplest design. A camshaft in the engine block moves the lifters, which transfer the movement to the pushrods. Pushrods are slim metal tubes with rounded ends that pass through holes in the cylinder block and head and move the rocker arms. The rocker arms are like a teeter-totter, changing the direction of movement and pushing down on the valves and valve springs. When the camshaft rotates, the lifter and all the connecting parts move up and down, opening and closing the valves. It sounds like there is a lot going on, and this is supposed to be simple! It is, when compared to overhead camshaft designs.
Overhead camshaft engines use one or two camshafts per cylinder head. If one camshaft is used, it is often referred to as a SOHC or single overhead camshaft engine. DOHC, or double overhead camshaft, engines allow the designer to vary camshaft timing for the intake and exhaust valves separately because one camshaft operates the intake valves while the other operates the exhaust valves.
Some overhead camshafts push directly on a “bucket” (an inverted cup) that in turn pushes the valve open. Other designs have the camshaft move a rocker arm that then pushes on the valve. This type of rocker arm doesn’t change the direction of movement but because of its leverage, it does increase the movement at the valve end.
The complexity of overhead camshaft engines comes in the drive mechanism used to turn the camshafts. While pushrod engines can use gears or short chains because the camshaft is close to the crankshaft, overhead camshaft engines typically use long roller chains for each bank or a single toothed timing belt. These long drive systems may require hydraulic tensioners, guide plates, idler pulleys and complex covers to complete the drive system. Because all this drive system must be disconnected before a cylinder head can be removed, they are also much more complex when performing major engine work.
Timing belts used to be the most common drive mechanism but roller timing chains have become predominant again. Roller chains last longer, are contained inside sealed covers so they are quieter, and they are narrower than a timing belt. A narrower camshaft drive enables engineers to make the engine shorter so it will fit in tighter engine compartments.