Once upon a time, automakers would give names to their cars that revealed a great deal about the car itself. For instance, a BMW 325xi used to mean a BMW 3-series car with a 2.5-liter engine and BMW’s X-drive system. The current 328xi comes with a 2.0-liter engine, so chalk it up to marketing; another example, a Honda Civic with a “DOHC VTEC” badge actually had an engine that featured dual overhead cams and Honda’s proprietary VTEC system.
Now, if you don’t know what those are, then the fact that they are called out on the car isn’t too helpful. I’m here to explain what a Dual OverHead Camshaft (DOHC) setup is, and how it differs from other camshaft arrangements. As for Variable Valve Timing and Lift Electronic Control (which Honda abbreviates as VTEC), we will get to that in another post.
Engines are extremely mechanically complicated, but they are also simple in concept: Fuel and air need to be introduced into a cylinder, compressed, ignited, and then exhausted. Hence the rather colorful phrase describing the four-stroke engine cycle: suck, squeeze, bang, blow.
The compression and ignition parts are relatively straightforward: The piston moves toward the top of the cylinder, and the physical space is compressed. This leads to an increase in temperature and pressure for the air-fuel mixture. (Side note: The volume of the cylinder when the piston is at the top of its stroke compared to the volume of the cylinder when the piston is at the bottom is how you determine the compression ratio of the engine.) A spark plug, located near the top of the cylinder, ignites the fuel mixture. How do we introduce and exhaust the fuel? We use intake and exhaust valves. And how are those valves controlled? With camshafts.
“Camshaft” is a fairly descriptive name. Merriam Webster’s definition of a cam is “a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice versa.” A camshaft, therefore, is simply a shaft with many cams on it that transform rotary motion, supplied by the crankshaft, into the linear motion that is needed to operate the valves.
The valves in automotive engines, which are known as poppet valves, pop open, allowing for gas flow, due to the shape of the cam and then are returned to their seat, preventing flow into and out of the cylinder, by the valve spring. Depending on the size and shape of the cam, also called a lobe or eccentric lobe, the valves will open at different times and can open further to allow more time for gas flow.
We explained that the crankshaft supplies the rotary motion of the camshaft. This means that it was essential in early engine design to have the camshaft near the crankshaft. The camshaft was connected to the valve through the valve stem and pushrods, which effectively served as additional length for the valve stems. If you have ever heard the term “pushrod V8,” it means that the camshaft is located between the two banks of cylinders and uses pushrods to transmit the motion of the cam to the valves, via rocker arms, which are located above the cylinders, as shown in the image above. The modern Corvette uses a pushrod V8, which garnered a lot of press attention due to the fact that pushrod engines are no longer commonly used.
In the ’60s, it was determined that the camshaft could be moved to the cylinder head, which reduced the number of components involved in producing the needed valve action. This setup, called a Single OverHead Cam (SOHC), provided automakers with greater flexibility of engine design and reduced the overall mass of the engine. The camshaft was connected to the crankshaft by either a timing belt or a timing chain, as it was too far to be efficiently connected via a gear set.
SOHC arrangements worked very well for what are known as I-head engines, which is where both valves are aligned perpendicular to the stroke of the cylinder and parallel to each other. However, it was determined that engines functioned more efficiently if the valves were in a V-shape away from each other, as was made popular by the Hemi engine. As the angle of the valve V increased, it became more difficult to engineer systems that could effectively drive both valves. This problem was compounded when designers started adding more than two valves per cylinder.
The solution to this was to add a second camshaft to the arrangement. In this way, one camshaft could drive the intake valves while the second would drive the exhaust valves. With that, the Dual Overhead Camshaft setup was born; it continues to be the most popular arrangement, and it is found on the majority of cars today. You can see the valves and the DOHC setup in the image of the V8 from the Porsche 918 above. This arrangement allows for an extremely high level of control for tuning the engine, which translates into performance, efficiency, and control of emissions.
There are a few automakers out there, such as Koenigsegg and BMW, that are exploring various ways to drive the valves without the need for a camshaft, instead using electromagnetic actuators and solenoids. This allows for the engine to be smaller, as it doesn’t need a camshaft, and more flexible, as the timing for any cylinder can be adjusted at any time. This is probably more important to have on a multi-million dollar hypercar than it is on the family minivan, but we have no doubt that this technology will filter down in the decades to come.
Like classics? It’s always Throwback Thursday somewhere.