OHV valve control
The Overhead valves ( English o ver h ead v Alves , "overhead valves," German "overhead valves") is a design of the valve gear of four-stroke - Reciprocating - internal combustion engines in which the intake and exhaust valves in the cylinder head are arranged. The valves are operated by a camshaft below via tappets , push rods and rocker arms. Some two-stroke diesel engines with direct current scavenging, such as low -speed marine diesel engines , also have hanging outlet valves, but these can also be hydraulically operated.
With the help of the direction of movement of the valve opening in relation to the piston movement, DIN standard 1940 defines the terms "top-controlled" and "bottom-controlled motor", but it does not contain any definition for OHV valve control. The position of the camshaft is not apparent from the term itself . As a rule, the term OHV is used for engines in which the camshaft is placed in the crankcase - ie "below".
If the camshaft is in the cylinder head, one speaks of an OHC valve control (overhead camshaft, overhead camshaft), with two camshafts of DOHC (double overhead camshaft). DOHC does not automatically mean four valves per combustion chamber ( multi-valve technology ); In principle, the two overhead camshafts can also operate one valve per combustion chamber. As an OHC variant, Opel manufactured so-called " CIH engines " (camshaft in head, camshaft in the cylinder head) from around 1965 to around 1994 .
history
Élie-Victor Buchet applied for a British patent (No. 16,084) on August 5, 1899 for a cylinder head with overhead valves. Engines based on this system were produced by Société Buchet in the early 20th century. Mainly for cost reasons, however, until the end of the 1930s, many four-stroke petrol engines , which did not require maximum output, were designed with upright valves as "side valves".
After the Second World War, in the course of mass motorization, higher demands were made on performance in connection with low fuel consumption. It was not until 1960 that the last side valve engines in German automobile manufacture - for example in 1957 in the Mercedes-Benz 180 and 1962 in the Taunus 12M of the Ford works in Cologne - were replaced by OHV engines.
In terms of design, the transition from the SV to the OHV engine was relatively easy to accomplish, as the camshaft retained its position in the crankcase. Based on the old SV design from 1936, Fiat developed a new, outwardly similar-looking engine with OHV control for the Fiat 500 Topolino in 1948 . It was only in the Fiat 600 , which appeared in 1955, that it was replaced by a new design, which was used with increased displacement in several Fiat models, most recently in the Panda until 1986.
OHV engines were built by most automobile manufacturers. Probably the most famous construction is the boxer engine of the VW Beetle . Other representatives of this type are z. For example, the gasoline engines from Ford, known since 1959 as "Kent" (four-cylinder in- line engine ) and "Essex" (six-cylinder V-engine , from 1967) . A modified V-6 design was used in the Ford Granada until 1985 and in the successor Scorpio until 1994 ; the four-cylinder, which had been further developed into the “Valencia” type, was even installed in the Ford Fiesta until 2001 . The OHV engine first used in the Opel Kadett A from 1962 was also used in various models of the General Motors group (including Opel and Vauxhall ) until the 1990s .
advantages
Advantages over SV
The fundamental advantage of the OHV design is the more favorable shape of the combustion chamber compared to an engine with vertical valves (SV engine). In addition, flow-technically more favorable shapes of the inlet and outlet channels result. This leads to a considerable increase in performance compared to SV engines. Therefore, OHV engines were first used in racing and aircraft engine construction.
disadvantage
Disadvantages compared to SV
Disadvantages compared to the SV engine are the higher construction costs (costs) and the noise generated by the valve train. Because of the long bumpers and the different thermal expansion of the cylinder block and bumpers materials, careful adjustment of the valve clearance is required. Nevertheless, the ticking noise of the valve control can hardly be prevented, especially when the engine is cold. In the case of small motors, the higher manufacturing costs due to the larger number of components are particularly relevant. This is why SV motors can remain in this area to this day, so that lawnmowers, depending on the price range, have parallel models with SV and OHV motors.
Disadvantages compared to OHC
The disadvantage compared to an OHC engine is the greater inertia of the valve train. Due to the higher mass of the moving parts, the achievable nominal speeds reach their limits more quickly.
Mixed form CIH
A hybrid of OHV and OHC construction is CIH ( Camshaft in Head ), a construction used by Opel from approx. 1965 to approx. 1994, in which the camshaft is placed on the side of the cylinder head - insofar as it is on top - and the valves without bumpers short tappets and rocker arms operated. Due to the lower oscillating masses, this design enables slightly higher speeds than with OHV engines with long bumpers. The 6-cylinder variants (as in the Kapitän / Admiral / Diplomat) were equipped with self-adjusting hydraulic valve lifters (4-cylinder for the first time in the Opel Rekord D engine 1.9SH, built in 1972) , like later 4-cylinders from 1.9 l displacement there was no need to check the valve clearance . In the course of this measure, the timing chain was also provided with a hydraulic chain tensioner , thus reducing the maintenance effort for these engines.
Steered above / steered below
These terms were used when the terminology standard DIN 1940 "Internal combustion engines" was revised by DIN e. V. introduced into German standards in March 1958. According to this, in the case of an overhead motor, the valve disk and gas paths are above a level that rests on the piston located in the top dead center; in the case of the motor under control, they are in or below this plane. If other control devices - instead of valves - are used, these definitions apply accordingly.
Accordingly, the valves are also called top-controlled - regardless of the position of the camshaft - or bottom-controlled.
The formulations above / below refer to a normal installation position of the engine: The crankshaft is below the pistons - also in radial engines - the opposite direction points upwards.
The current (as of April 2013) edition of this standard was issued in December 1976.
See also
Individual evidence
- ↑ British Patent 16,084 (1899) on espacenet.com
- ↑ See 1901 catalog, reproduced by The 1901 Buchet Powered Truffault Quadricycle on theoldmotor.com