Overhead camshaft

The OHC valve control means a design of reciprocating four-stroke engine, in which the valve controlled by an overhead camshaft (English: Overhead Camshaft ) takes place; this is ( V, boxer and, in principle at even with radial engines ) counted per cylinder bank.

The characteristic feature of the OHC motor is the position of the cams, the cylinder above the separator line - must be cylinder head; in boxer engines, other engines with several rows of cylinders and in-line engines with hanging built meant "above " the combustion chamber on the side facing the dividing line; overhead camshafts are therefore at Hanging installed in the airplane engines actually below the combustion chambers. The valves can thereby be controlled by the cam directly on bucket tappets indirectly via toggle or rocker arms. Alternatives to the OHC design are generally SV - valve timing and OHV valve control.

Variants and abbreviations

DOHC refers to a design in which the valves of two overhead camshafts (English: Double overhead camshaft, twin cam also ) are controlled by cylinder number. (Ie DOHC V and Boxer engines have four camshafts. ) The valves of DOHC engines are almost always controlled directly by bucket tappets or cam followers.

SOHC (English: Single Overhead Camshaft ) refers to two different types in the four- stroke engine. Firstly, the conventional two-valve control of the OHC engine is sometimes referred to, in which a single cam shaft located directly above the valves are arranged in a line. However, the term is commonly used SOHC for engines with three or four valves per cylinder, in which the valves are controlled differently in the DOHC engine, only a central overhead camshaft per cylinder bank. The cam shaft is fitted to save space between the V- shaped arranged inlet and outlet valves, the operation of valves is generally about (roller ) rocker arm. The cylinder head of a SOHC engine is thus much more compact than that of a DOHC engine. Examples of four-valve SOHC engines are the Triumph Dolomite Sprint, the Husaberg FE 570 and the Chrysler Neon, three-valve SOHC engine found in the Honda NTV and various Mitsubishi Colt.

OHC engines, the camshaft is located above the side of the cylinder head is called specifically CIH engines ( Camshaft in Head ). This design with conventional ( hydro) tappets can be found for example in four-and six-cylinder engines of the Opel brand, which were produced from 1965 until the early 1990 .. These engines however are not as tight as usual speed OHC engines. You have rocker arms and tappets, but no bumpers. In CIH engines, all valves are in a row.

A very similar solution, but hanging with V-shaped intake and exhaust valves - this arrangement allows a semi-spherical ( hemispherical ) combustion chamber - called Ford CVH engine ( Compound Valve Hemi, translated in the network ( arranged with the camshaft ) valves, hemispherical combustion chamber ), which was offered in Europe from 1.1 to 1.8 liters and with about 50 to 132 hp rated power from 1980 to about 2000. In this OHC engine driven via a toothed belt camshaft sits centrally above the crankshaft in an aluminum alloy cylinder head between the valves. These are driven by hydraulic tappets ( the Escort RS1600i mechanically, the Sierra 1.8 roller plunger ) and rocker arms. The compact design - as with CIH - compared to the classical OHC engines was an advantage to the maximum design maximum speed of 8000 min -1 is a disadvantage. Similar structures existed in the 1960s, BMW, NSU, and later at Peugeot.

All of these four-stroke reciprocating piston engines are also OHV engines since their valves - regardless of the position of the camshaft (s) - in the cylinder head are arranged " suspended". OHV engines, however, will be to ensure a clear distinction of the valve controls, only so called, if they have no overhead camshafts.

SV engines have "standing" valves and no overhead camshaft.

In a radial engine, the camshaft as " drum cam " or ring coaxial with the crankshaft is arranged. Therefore this special motor design shall be considered as "bottom -driven " motor. In principle, camshafts in the cylinder heads can also be arranged in radial engines, what is not realized because of the high construction costs.

In principle, valve controls without camshaft (eg via a pneumatic ), and even valveless control of gas exchange processes in the four-stroke engine with slider controls possible.

Drive

Overhead camshafts are usually driven by a toothed belt or a timing chain. Stirnradkaskaden and push rod pairs ( Albert harvester ULTRAMAX control) can only be found in the historic engines, vertical shafts are expensive and rare ( Ducati until around 1986, [ [ Kawasaki W800 ] and W650 ] since 2011 ). A combination of spur gear and chain drive is the so-called Weller instinct of the Englishman John Weller (1877-1966; simultaneously co-founder of the British car manufacturer AC Cars Ltd. ). A spur gear pair thereby reducing the crankshaft speed to half, then drives a chain in the ratio 1: 1, the overhead (s ), camshaft ( s). This drive was in 1927 by the British motorcycle factory AJS introduced. John Weller has improved the tension and guide devices of chain drives in many details.

Compared to chain belts are cheaper, require no lubrication (and thus no oil-tight enclosure) and run quieter. As the first user of the timing belt in the series is considered the Dingolfinger agricultural machinery and automobile manufacturers Hans glass. Disadvantage is the need to exact tension of the belt and the opposite chains greater risk of tearing incorrect belt tension or fatigue; often are potential engine damage is the result of a timing belt crack. Regular inspection and timely replacement are therefore necessary, typical replacement intervals, depending on the type of engine in the order Between 50,000 and 150,000 km route path or five to ten years.

An innovation is the belt drive of the new Ford EcoBoost 3 -cylinder engine is: The drive belt of the engine running in an oil bath. This is to reduce running noise and consumption, while increasing the mileage of the belt.

History

One or two overhead camshafts are known for high performance engines since the beginning of the 20th century. An engine with overhead camshaft was installed in 1909 in the produced by the company Deutz Prince Henry Type 9C. Another early example of such an engine was developed in 1913 with Alfa Romeo and arrived at the Grand Prix model of the Italian car manufacturer for the first time in 1914 for use. Also in 1914 falls the development of the first aircraft engine with overhead camshaft Hispano-Suiza 8 When racing bikes, two overhead camshafts in the Peugeot 500 Sport from 1913 are known.

In built before the Second World War cars, this construction hardly played a role. Because they were easier to maintain and were considered reliable, SV engines were used instead. In these one -ending valve or failure of the camshaft drive caused no serious consequential damage. In addition, the waves were then common king with two bevel gear pairs far more complex to manufacture than spur gears or short timing chains. The vibration damper for long timing chains or timing belts of today's engines were not yet invented or ready for production.

A kind of pioneering in terms of the development of engines with overhead camshaft is the Alfa Romeo, almost all of their engines endowed from the featured model in the 1922 RL as normal. Overall, there were standard until the 1940s only a few engines with overhead camshafts, including in addition to the Alfa Romeos that time in 1925 the engine of the Fiat 509 and the Morris Minor 1928, the Jaguar XK DOHC engine, the eight-cylinder Tatra (Tatry 77 and 87) and from 1945 Crosley CC with a royal wave driven camshaft. Only towards the end of the 1950 OHC engines were popular in cars. Pioneers were out of Alfa Romeo (DOHC ), BMW and Mercedes- Benz ( OHC with rocker arms ), Lloyd ( Lloyd Alexander) and NSU, which sat on the camshaft drive with a chain. An exception to NSU That is the Ultramax control some motorcycle models and the two-cylinder engines in the car model " NSU Prinz ", there is a pair of push rods to the camshaft drive. Breakthrough in the mass market, however, was achieved only with the drive by a toothed belt. First model with toothed belt driven camshaft was 1962, the glass in 1004, but still no breakthrough in the mass market.

Pros and Cons

Benefits

Engines with overhead camshafts offer because of the low moving mass for valve actuation good conditions for high speeds and can therefore, for a given medium pressure to achieve higher peak performance and a better power to weight ratio than other engines.

Disadvantages

For conversion to overhead camshafts car manufacturers had to design engine blocks and cylinder heads and renew the production line, which is why many manufacturers are always delaying the changeover and for a long time offered their OHV engines.

Unlike OHV engines DOHC engines require greater effort in maintenance and regular replacement of timing belt ( maintenance intervals between 40,000 120,000 km ). The timing chain was by their greater length rather imprecise. A broken timing belt or a broken timing chain often means a serious engine damage: The valves collide with the pistons. However, does not apply to the free rotor, in which the pistons can not hit the valves.

Long timing chains require guiding and tensioning rails; not infrequently there are three or four in circulation. In particular, V-engines with extremely long timing chain rounds are maintenance or control in need ( from the crankshaft up to the first cylinder head, down to the idler wheel, up to the second cylinder head back down to the crankshaft, which are four -chain strand in a V shape ). Slide rails are because of their chain-side plastic overlay wear items that should be changed after approximately 150.000 to 300.000 km.

The complicated management of timing chain or timing belt has long been a reason for the use of OHV design with V-engines in trucks and many American vehicles. Here satisfies a central camshaft that can be driven by means of a short chain and a pair of gears from the crankshaft.