DOHC stands for dual overhead camshaft while SOHC stands for single overhead camshaft. Camshafts are responsible for the opening and closing of valves on a cylinder. The camshafts are long cylindrical shaft which will have some lobes protruding from it. These lobes, on rotation o camshafts, push the valves, making it open and close. The valve springs provide the return for the valves to pull back to their original position. In SOHC, there will be a single camshaft above the single line of cylinders (cylinder bank) or a single camshaft that controls both the exhaust and inlet valve and in DOHC, there will be two camshafts.
In earlier models, the camshafts were placed below the engine block and were known as OHVS or Overhead Valve Engines. In OHV the valves on the engine were operated with the help of pushrods and rockers and were also known as pushrod engines. Hence in OHCS, there are fewer moving parts compared to OHVS.
The first OHC engine on a production car was built around 1910, Later, during World War 1 overhead camshaft engines were widely used by both the Allied as well as German military forces in their military aircraft. In the aircraft engines, the camshafts were driven by a shaft with a sliding spline connected to the crankshaft. The sliding spline allowed it to withstand thermal expansion which would not be possible in the case of pushrods. In automobiles, the camshafts are driven with the help of chains or belts that connect them to the crankshafts. Since the 1940s, the use of DOHC increased gradually and is widely used among cars these days.
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Advantages of DOHC over SOHC
- The DOHC will have double the number of valves than the SOHC and allows them to be placed in a better position for maximum performance. In DOHC, there will be at least four valves or more depending on the make and model. This helps in better airflow and equal distribution of operations resulting in a smooth and noise-free experience.
- Valve timing can be easily optimized in DOHC. The timing of valves is crucial for being performance and efficiency of the engine. Most modern car engines are equipped with variable valve timing technology to optimize the operation of valves. As SOHC has a single camshaft for both the inlet and exhaust valves, they have much lesser control in valve timing while in DOHC, the timings can be better optimized as there are separate shafts for inlet and exhaust valves.
- DOHC also allow better placement of valves as well as a spark plug. The valves can be placed at the most appropriate angle for maximum airflow and the spark plugs can be placed in the middle, resulting in better combustion. This is not possible in the case of SOHC, as the camshaft operating both the valves need to be placed in the middle.
- As DOHC has more components and hence higher mass, even though the initial torque is lower, the top end performance will be much better.
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Disadvantages of DOHC over SOHC
- DOHC will have double the number of components and hence the design is complex than that of SOHC and hence more expensive.
- Due to the larger number of parts, DOHC is difficult to repair and can result in higher maintenance costs.