DOHC engine: what is it

DOHC (Double OverHead Camshaft) is an internal combustion engine that is equipped with a double camshaft mechanism. Engines of this type are now the most common, since they have a relatively simple structure and have high power at a moderate weight. In this article, we will look at the main subspecies of DOHC engines, their differences from other types of engines and design features.

What is DOHC

There are two types of DOHC motors. The first group consists of engines with two valves per cylinder. The second group is motors with four valves per cylinder. Each group has a number of characteristics.

DOHC with two valves per cylinder

This type of cylinder is a sophisticated version of the conventional OHC (Overhead Camshaft). The principle of operation of this cylinder is the operation of two camshafts located in the cylinder head - one camshaft drives the exhaust valves, and the other drives the intake valves.

Engines with two valves per cylinder were actively used in the 60-70s of the last century. The main car brands where such engines were used were Alfa Romeo, Jaguar, Fiat 125, Ford and the domestic Moskvich. However, starting in 1994, the use of two-valve DOHC motors began to decline.

DOHC with four valves per cylinder

On DOHC engines with a four-valve cylinder, each camshaft drives a different valve bank. The first camshaft drives the intake valves (2 pcs.), And the second camshaft drives the exhaust valves (2 pcs.).

DOHC engines with a four-valve cylinder are installed on passenger cars:

  • UAZ;
  • Gazelle, Volga (on vehicles manufactured before 2008);
  • VAZ-2112.

How DOHC differs from SOHC

DOHC and SOHC engines have been in use since the 1960s. They have a similar purpose and can be diesel or gasoline. However, there is one significant difference between them.

SOHC engine (left) and DOHC engines

SOHC engines are designed with one camshaft, and DOHC engines are equipped with two camshafts shafts. As a result, in the SOHC engine, all valves are driven by one camshaft, and in DOHC engines, by two camshafts.

Pros and cons of DOHC

The advantages of DOHC engines include:

  • increase in engine power by an average of 10-25 horsepower - the increase was made possible due to the distribution of engine efforts, equally on both shafts;
  • improving the dynamism of the motor systems - this leads to a decrease in oil consumption and to an improvement in the smoothness of the machine;
  • improved auto acceleration characteristics;
  • the presence of a hydraulic compensator - it helps to reduce the outgoing noise during engine operation;
  • reduction of fuel consumption by up to 30%, with the same engine power.

The disadvantages of the DOHC system include:

  • the complexity of the design - it consists in the complicated process of regulating the nodes of the gas distribution system and the level maintainability of the engine;
  • the need to use only high quality synthetic motor oils in engine operation;
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  • the need to frequently change engine oil;
  • the need to periodically adjust valve clearances - in order to adjust them, you need to perform a number of actions: remove the camshaft, adjust the thickness of the adjusting washer, disrupt the valve timing, assemble the motor in reverse order;
  • the need to use a more complex timing system.

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Features of the drive in the camshaft of DOHC engines

The drive is the element that drives the camshaft and the engine as a whole. DOHC motors use three types of drive: belt, chain and gear.

The gear drive is the most reliable mechanism. However, it has a huge drawback - this is the maximum level of outgoing noise from a running drive.

Chain drive is a reliable mechanism. Compared to gears, there is significantly less noise from the chain. However, the chain drive also has a lot of disadvantages:

  • periodic pulling that occurs during operation - in order to eliminate this in automatic mode, you will need to install an automatic chain tensioner in the engine;
  • the need to regularly lubricate the chain with special lubricants - to ensure a regular supply of lubricant to the chain, you will need to install a sealed container (crankcase) for oil in the camshaft.

Toothed belt drive is a reliable mechanism. Its cost is significantly less than that of a chain or gear drive. And also the level of the emitted noise from the belt drive is minimal and there is no tendency to stretch at all. However, despite the positive aspects, the belt drive has a small drawback - if the belt fails, then it can cause the piston and an uncontrolled valve to come into contact, which will lead to the destruction of both of these elements. Therefore, in order to avoid the failure of the toothed belt, it will be necessary to replace the belt and adjust the tension rollers every 50-150 thousand kilometers traveled by the car.

See also : What is CRDI engine, its structure and features.