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Camshafts Part II: Single Overhead, Double Overhead and Pushrod Cylinder Head Engine

July 31, 2004 | By: Abdul Rehman (UK)

In the first article you read about the basics of camshafts and how they function. Furthermore, you also got brief details on certain specifications of the camshaft which would help the cams perform better such as the valve lift and duration hence get more power out of the engine. In this article you will learn about the different arrangements of camshafts which are available and brief details of how each of them work. There are several different arrangements of camshafts on engines but we'll just talk about the most common and widespread. You've probably already heard the terminology or read about it some where on the internet or in a magazine, if not, here's your chance to learn some thing new. The most commons camshafts are:

• Single overhead cam(SOHC)
• Double overhead cam(DOHC)
• Pushrod

Single Overhead Cam Engines

This arrangement denotes an engine with one cam per head. So if the engine is an inline 4-cylinder or inline 6-cylinder, it will have one cam. If it's a V-6 or V-8, it will have two cams (one for each head).

The cam actuates rocker arms that press down on the valves causing them to open. Springs then return the valves to their closed position again. These springs have to be very strong because at high engine speeds, the valves are pushed down very quickly, and it is the springs that keep the valves in contact with the rocker arms. If the springs were not strong enough, the valves might come away from the rocker arms and snap back. This is an undesirable situation that would result in extra wear on the cams and rocker arms.

On single and double overhead cam engines, the cams are driven by the crankshaft, via either a belt or chain called the timing belt or timing chain. These belts and chains need to be replaced or adjusted at regular intervals. If a timing belt breaks, the cam will stop spinning and the piston could hit the open valves.

Double Overhead Cam Engines

A double overhead cam engine has two cams per head. So inline engines have two cams, and V engines have four. Usually, double overhead cams are used on engines with four or more valves per cylinder. A single camshaft simply cannot fit enough cam lobes to actuate all of those valves.

The main reason to use double overhead cams is to allow for more intake and exhaust valves. More valves mean that intake and exhaust gases can flow more freely because there are more openings for them to flow through. This increases the power of the engine. You may have also heard of Twincam engines. This is the same as a double overhead cam engine because the engine has dual overhead cams just like in a double overhead cam engine.

Pushrod Engines

Like SOHC and DOHC engines, the valves in a pushrod engine are located in the head, above the cylinder. The key difference is that the camshaft on a pushrod engine is inside the engine block, rather than in the head.

The cam actuates long rods that go up through the block and into the head to move the rockers. These long rods add mass to the system, which increases the load on the valve springs. This can limit the speed of pushrod engines. The overhead camshaft, which eliminates the pushrod from the system, is one of the engine technologies that made higher engine speeds possible.

The camshaft in a pushrod engine is often driven by gears or a short chain. Gear-drives are generally less prone to breakage than belt drives, which are often found in overhead cam engines.

Now that you've learnt about the basics of how camshafts function and the different arrangements they come in, you will need to know about valve timing and why manufacturer's have developed schemes to vary the cam profile as the engine speed changes. In the final article we will look at valve timing and the systems currently used in some of the engines today.