SOHC vs DOHC engines: A detailed guide

Since the inception of the internal combustion engine, various innovations have led to the formation of multiple engine configurations. Modern engines are more refined, efficient, and powerful than before. Two such engine configurations in modern times are the Single Overhead Camshaft (SOHC) and Dual Overhead Camshaft (DOHC) engines. Let’s delve deeper into the differences between SOHC vs DOHC engines, their applications, advantages and disadvantages, and their unique characteristics.

What is a camshaft?

The camshaft is a long shaft with egg-shaped lobes in the head section of an engine, responsible for regulating the timing and operation of the intake and exhaust valves. It works by opening and closing the valves at specific intervals during the engine's cycle. The valves control the intake of air and fuel into the combustion chamber and the expulsion of exhaust gases from it.

Functions of the camshaft in an engine

Valve Timing: The timing of the opening and closing of the intake and exhaust valves, relative to the position of the piston, is controlled by the camshaft. It ensures proper engine timing for maximum power and efficiency. 

Engine Performance: Cam lobes are the components responsible for the opening of the valves. The shape of the valve lobe greatly determines the power output, influencing torque and efficiency. In performance oriented engines, cam lobes are designed to keep the valves open for longer, allowing for more air to flow into the engine, ultimately increasing performance.

Synchronisation: In multi-cylinder engines, the camshaft helps synchronise the opening and closing of valves across different cylinders. This is crucial to ensure each cylinder ignition is timed perfectly, and that the pistons do not crash into open valves.

Single Overhead Camshaft (SOHC) Engines

SOHC engines come with a single camshaft installed in the engine head, responsible for the opening and closing of both the intake and exhaust valves. This simplifies the engine layout, and reduces complexity, as compared to DOHC engines. SOHC engines are also lighter than DOHC engines due to their reduced mechanical components. SOHC engines are commonly found in economy cars, compact vehicles, and some light trucks where cost-effectiveness and reliability are prioritised over high performance.

Advantages of SOHC engines:

1. SOCH engines feature a simplified design, making for easier maintenance and repairs. Simpler designs also mean they are cheaper to manufacture.
2. The simple design helps SOCH engines to be compact in size, making them better suited to be used in smaller vehicles.
3. SOHC engines provide decent performance in the lower to middle RPM ranges, making them great for city driving and everyday applications.
4. SOHC engines are more fuel-efficient in lower RPM ranges, making them beneficial for city driving.

5. The simple construction of SOHC engines provides for simpler tuning options, making mechanical tuning easier than DOHC engines. However, the extent of the tuning may be limited.

    

Disadvantages of SOHC engines

1. The fewer valves and the single camshaft limit the engine's ability to draw in and exhale air, leading to reduced power output and efficiency as compared to DOHC engines.
2. The simplicity of the design makes it harder to implement advanced tuning techniques.

Double Overhead Camshaft (DOHC) Engine

A DOHC engine comes with two camshafts in the engine head, one for the intake valves and another for the exhaust valves. DOHC engines enable more control of the timing of the intake and exhaust valves, granting the ability to tune the engine for specific needs and extract more power. DOHC engines are usually found in sports cars and performance vehicles where power and performance are the primary requirement. The first ever DOHC engine was a Peugeot engine, an inline four intended for racing. It was the engine that powered the car which won the 1912 French Grand Prix. 

Advantages of DOHC engines:

1. Individual camshafts for the inlet and exhaust valves allow for better airflow and power output at higher RPMs.
2. Many DOHC engines incorporate technologies like VVT (Variable Valve Timing), which changes the valve timing at different RPMS, enabling more air to enter the engine.
3. The improved combustion efficiency offered by DOHC engines can result in better fuel economy and lower emissions.
4. DOHC engines perform better at high RPMs, providing more power and throttle response, ideal for performance-oriented vehicles.

5. The more complex design allows for more advanced tuning options, making it possible to tailor engine output for different driving conditions.

    

Disadvantages of DOHC Engines

1. More parts mean a more complicated design, which can lead to higher manufacturing and maintenance costs.
2. The additional mechanical components in DOHC engines make them heavier, potentially affecting vehicle dynamics.

SOHC vs DOHC 

What is VVT (Variable Valve Timing)?

Variable Valve Timing (VVT) is a technology that enables altering the timing of the valves in real-time, without having to alter the engine configuration manually. This is achieved through a VVT gear that changes its position on the cam in accordance to the crankshaft, advancing or retarding the valve timing. At higher RPMs, engines require massive air to produce optimal power. However, if the intake valves close before the combustion chamber has enough air, it can lead to less than optimal power output. This is exactly where VVT comes in. With VVT, and its precise control, intake valves can be allowed to remain open for longer at higher RPMs, leading to more air gushing into the combustion chamber. This, in turn, leads to higher power output.

Benefits of VVT in DOHC Engines

1. By optimising the timing of valve openings, VVT helps achieve better torque characteristics at low RPMs while maximising power output at higher RPMs.
2. VVT improves combustion efficiency, which translates to better fuel economy and reduced emissions. 
3. VVT allows for dynamic adjustments to the engine's performance, enabling it to adapt to different driving conditions, whether it's stop-and-go city driving or high-speed highway cruising.

In contrast, SOHC engines are typically unable to fully implement VVT due to their simpler design. The single camshaft limits the ability to independently control the timing of intake and exhaust valves, which restricts the potential benefits of variable timing technologies.

SOHC vs DOHC Reliability

SOHC and DOHC engines both have their advantages and limitations when it comes to reliability. 

Reliability of SOHC Engines

The simpler design of SOHC engines tends to have fewer mechanical failures. The fewer moving parts mean there’s less that can go wrong, which often results in greater reliability over time. Repairs tend to be less expensive due to the engine's simpler construction, making it easier for mechanics to diagnose and fix issues.

Reliability of DOHC Engines

While DOHC engines are designed for longevity and performance, their complexity can lead to increased potential points of failure. Regular maintenance is essential to ensure reliability. When maintained properly, DOHC engines can provide superior performance over long periods, making them reliable choices for performance-oriented applications.

FAQs

Q. What’s the main difference between SOHC and DOHC engines?

The main difference is in the number of camshafts. SOHC engines have one camshaft that controls both intake and exhaust valves. DOHC engines have two camshafts, one for intake valves and another for exhaust valves. This allows DOHC engines more precise control over valve timing and can enhance performance.

Q. Why is DOHC faster than SOHC?

Generally, DOHC engines are favoured for performance because they allow for better airflow and more precise timing. This means they can produce more power, especially at higher RPMs. SOHC engines can still be efficient and powerful, but they may not match the peak performance of a DOHC configuration.

Q. Are SOHC engines more reliable than DOHC engines?

Reliability often depends more on the overall engine design and maintenance than on whether it’s SOHC or DOHC. SOHC engines can be simpler with fewer moving parts, which might contribute to ease of maintenance. However, both types can be very reliable if properly cared for.

Q. Is SOHC more fuel-efficient than DOHC?

Fuel efficiency can vary based on design, tuning, and driving conditions rather than just the camshaft layout. SOHC engines may offer better efficiency in some lower RPM applications, while DOHC engines can be designed to optimise fuel economy at higher speeds. It largely depends on how each engine is configured.

Q. Which engine is easier to maintain?

SOHC engines can be easier to maintain due to their simpler design with fewer moving parts. This can make tasks like timing belt changes or valve adjustments a bit more straightforward. That said, many mechanics are well-versed in DOHC systems, and both can be maintained effectively with the right care.