OHV, SOHC And DOHC Valve Trains: How They Work, And Their Respective Pros And Cons

For an internal combustion to function optimally, one of the key areas is to ensure that the air-fuel mixture burns properly, and is let out equally efficiently. To facilitate this air flow, the internal combustion engine has a complex set-up where a whole bunch of components control inlet and exhaust valves at different stages of the complete four-stroke cycle. All these parts, which form a system that helps open and close the inlet and exhaust valves in an internal combustion engine are comprised of what’s called a valve train. Now, the specific way in which a valve train works may be slightly different for bigger automobiles as compared to two-wheelers, which have relatively smaller engine capacities, and often rev higher, but the basic principles remain the same. In this article, we will discuss the various types of valve trains which are used on everyday motorcycles and two-wheelers, along with their respective advantages and disadvantages.

Now, before we talk about the various types of valve trains, it’s important that we understand the various different components that come together to make a valve train function efficiently. While the use of the components described below may differ based on the specific type of mechanism they are a part of, these, generally are the components which are the essential parts of a typical valve train.

Camshaft

Camshaft, as the name suggests, is a shaft with precisely machined lobes on it. The camshaft rotates in sync with the crankshaft, and plays a crucial role in ensuring the timely opening and closing of the valves through its lobes, termed as cam lobes. Now while some engines use only one camshaft to control both inlet and exhaust valves, others use two different camshafts for the two functions. Thus, the number of lobes on a camshaft is often decided by the number of valves it has to control. A two-valves per cylinder setup typically has a single camshaft per cylinder, while a four-valves per cylinder engine has a pair of camshafts to control its valves.

Push Rod

It’s used in relatively older valve train designs, but is still very much in production. It’s a rod that’s used to connect the camshaft located in the engine block with the rocker arms to open the valves

Timing Chain / timing belt

Timing chains or belts connect the camshaft with a drive sprocket, which in turn is directly connected to the engine’s crankshaft. The timing belt decides the precise time to turn the camshaft based on the crankshaft’s cycle so that the valves open and close at optimum times.

OHV or Push-rod valve train

An OHV or Over Head Valve engine has its valves, well, over the head of the engine. Now, this is not a distinction as compared with the OHC valve trains which we’ll come to in a bit. The key difference here, however, is the fact that the camshaft that controls the opening and closing of the valves is located inside the engine block. The OHV’s distinction with the other variety of valve train, or OHC, as you would imagine now is that the latter has over head camshafts too, along with over head valves.

In case of OHV or push-rod systems, there are long rods which have to be pushed by the camshaft lobes to move the valve rockers, which in turn open the valves – thus the name ‘push-rod’. The long rods and the mechanical nature of the push rod system make it heavy and it’s not compatible with engines which run at higher revolutions per minute. Now while OHV is an older design, it has its advantages in terms of simplicity of design, compact packaging and a simpler lubrication system requirement as compared to an OHC system.

The disadvantages, of a push rod system, however, are many. To start with, the engines with an OHV system cannot run very high RPMs and such valve trains are suitable mostly for low engine speed applications such as heavy cruisers.  Owing to the heavy components, the noise and friction on such systems is much more than an OHC system. Also, any issues with the camshaft require the entire engine to be opened up, as the camshaft sits inside the engine block, which increases the maintenance effort and cost in case of a breakdown.  Finally, OHV engines lend their design well primarily to two-valves per cylinder layout. It’s not that there aren’t any three or four valves per cylinder engines with OHV, but that setup becomes way more complex, and OHC systems offer much more flexibility with multiple valves per cylinders.

OHC Valve trains

To overcome the shortcomings of the pushrod valve trains, OHC valve train was developed. As the name suggests, it’s a valve train configuration where the camshaft for the engine is placed over the head of the engine, above the pistons and valves. This design allows for a very direct contact between the camshaft lobes and the valves or a lifter, thus reducing mass, reducing components and allowing better engine performance as well as more flexibility with the overall engine design.

Single Overhead Cam

For this variety of valve trains, there is a single camshaft for each row of engine heads. So a single cylinder OHC engine will have one camshaft. However, if it’s an engine with multiple rows, say a V6, then it will have two camshafts – one for each row of heads, or each bank. For SOHC engines, the camshaft is connected directly to the crankshaft via a timing belt or chain to ensure that the opening and closing of the valves is perfectly in sync with the various strokes of the engine for each cylinder.

Now, with SOHC, there is an option to either open or close the valves directly with a shim between the cam lobe and the valve stem, or via a rocker arm. Valves have springs which return them back to their closed position once the pressure from the camshaft lobe is off. SOHC engines are also suited better for 2 or 3 valves per cylinder configuration. Not that an SOHC valve train cannot run on a 4 valve per cylinder layout, but the whole set-up then becomes too complex for the design of rocker arms and lobes and it’s generally considered better to employ a DOHC valve train is such scenarios.

DOHC

DOHC or dual over head camshaft design includes two camshafts for every row of cylinder heads. Talking about the example we took for SOHC, a DOHC setup for a single cylinder engine will have two camshafts. However, if it’s a V6, it will have 4 camshafts, two for each row of engine heads, or banks. The primary advantage of such a setup is that it allows manufacturers to have a well engineered answer to handling a 4-valves per cylinder. Generally, one of the camshafts handles the intake valves, while the second one handles the exhaust valves. The 4-valve per cylinder setup allows for better breathing for the engine, and better performance in most cases, making DOHC a choice for engines that need to rev higher. A DOHC setup also allows for putting the spark plug bang in the middle of the cylinder head, which facilitates better combustion, and enhances performance, and fuel efficiency of the engine. With SOHC, such a setup is not possible for 4-valves per head, as it has to sit in the middle of the cylinder head so as to operate both intake and exhaust valves. As mentioned before, though, SOHC engines too can handle four valves per cylinder, and while the construction of such valve trains is complex, it’s desirable in some cases. DOHC brings along the extra weight of the additional cam, though by allowing the positioning of spark plug in the middle of the cylinder head it also enhances optimum combustion of fuel.  In a nutshell, DOHC is more suited for high performance engines which need to rev higher and perform in higher rev range. SOHC systems have somewhat better lower end torque though.

Finally, a DOHC system, with its more fine grained control over valves is more suitable to implement variable valve timing for engines. Such systems utilize variable camshaft profiles for different engine speeds to enhance performance across the entire rev band. The control over the speed and position of valves opening and closing is better in case of DOHC, and in today’s electronics driven world, great benefits can be extracted using that fact. DOHC valve train is more expensive than SOHV though, and coupled with its suitability for 4 valves per cylinder, it makes it feasible to employ that setup only on automobiles above a certain price point. For applications where everyday usability, low and mid-range torque, simplicity of design, easy construction and cost are important factors, SOHC system works well.

Finally, a DOHC system, with its more fine grained control over valves is more suitable to implement variable valve timing for engines. Such systems utilize variable camshaft profiles for different engine speeds to enhance performance across the entire rev band. The control over the speed and position of valves opening and closing is better in case of DOHC, and in today’s electronics driven world, great benefits can be extracted using that fact. DOHC valve train is more expensive than SOHV though, and coupled with its suitability for 4 valves per cylinder, it makes it feasible to employ that setup only on automobiles above a certain price point. For applications where everyday usability, low and mid-range torque, simplicity of design, easy construction and cost are important factors, SOHC system works well.