Overhead valve

The OHV valve timing (english overhead valves, overhead valves ') refers to a structure of 4-stroke reciprocating piston engine in which the intake and exhaust valves are located in the cylinder head. While using the direction of movement of the valve opening in relation to the piston movement defines the DIN standard 1940, the terms "top -driven " and "bottom -controlled motor ", but it contains no definition for the OHV valve control. The valve is controlled via pushrods and rocker arms.

From the term itself, the position of the camshaft is not clear. In general, the term used for OHV engine in which the camshaft in the crankcase - is placed - in other words " lying down ." Engines with camshaft in the cylinder head hot OHC (or DOHC ) or CIH engine.

• 3.1 disadvantages compared to SV
• 3.2 disadvantages compared to OHC

Historical

The OHV design has been known since the early 20th century. But were primarily for cost reasons until the end of the 1930s, many four-stroke petrol engines where maximum power was not required, as a " side -valve engine " designed with vertical valves (SV- valve control).

Only towards the end of the 1950s with their progress in production and material quality, coupled with higher demands on performance of the last SV engines were (eg, 1957 in the Mercedes 180 and 1962 in the Taunus 12M of Ford) replaced by OHV engines.

Structurally, the transition from SV to OHV engine relatively easy to accomplish, since the camshaft retained its position in the crankcase. Based on the old SI construction from 1936 was developed by Fiat 1948 same on the outside looking new engine with overhead valve control for the Fiat Topolino 500. First published in 1955 Fiat 600 it was replaced by a new design that was used with larger displacement in several Fiat models, most recently in Panda until 1986.

OHV engines were built by all automobile manufacturers. The most well-known design is the boxer engine of the VW Beetle. Other representatives of the OHV type are, for example, since 1959 as " Kent " ( four-cylinder inline engine ) and " Essex " (V- six-cylinder engine, 1967 ) known petrol engines from Ford A modified V-6 design was until 1985 in Ford Granada and Scorpio successor until 1994; of the "Valencia " type advanced four-cylinder even built until 2001 in the Ford Fiesta. too, the first in the Opel Kadett A 1962 used OHV engine was up in the 1990s in different models of General Motors Group ( inter alia by Opel and Vauxhall ) used.

Benefits

Advantages over SV

The basic advantage of OHV design is the better combustion chamber shape compared to an engine with side valves (SV- motor). In addition, aerodynamically favorable forms of inlet and outlet channels arise. This leads to a significant performance increase over SV engines. Therefore, OHV engines were first used in racing and in the aircraft engine.

Disadvantages

Disadvantages compared to SV

Disadvantage compared to the SV engine are the higher construction cost (cost) and also the louder mechanical noise of the engine. Because of the long bumpers and the different thermal expansion of the materials of the cylinder block and bumpers careful adjustment of the valve clearance is required. Nevertheless, the tickernde running noise of the valve control, especially on a cold engine is hard to prevent. For small engines particularly relevant are the higher due to the larger number of components manufacturing costs. Therefore, SV motors can in this area continues to this day, so that there are at lawn mowers depending on the price range parallel models with SV engine and models with OHV engine.

Disadvantages compared to OHC

The disadvantage compared to a OHC engine is the greater inertia of the valvetrain. Due to the higher number of moving parts, the engines emit at higher speeds to their limits, since the provision of the valves no longer fast enough on the valve springs.

Mixed form CIH

A hybrid of OHV and OHC design is CIH ( Camshaft in Head ), a type used inter alia by Opel between about 1965 to about 1994, when the camshaft laterally in the cylinder head - so far overhead - is placed and the takes place control of the valves without bumpers with short tappets and rocker arms. This design made ​​possible by lower oscillating masses slightly higher speeds than OHV engines with long bumpers. The 6- cylinder versions (as in Captain / Admiral / Diplomat ) as later 4- cylinder from 1.9 l displacement with self-adjusting hydraulic tappets (4- cylinder engine for the first time in the Opel Rekord D 1.9 SH, built in 1972 ) were fitted, which the control of the valve clearance was not necessary. In the course of this action the timing chain was provided with a hydraulic chain tensioner, which reduces the maintenance effort in these motors.

Controlled above / below controlled

These terms were in 1940, " Internal Combustion Engines" of DIN eV in March 1958 introduced in the revision of the terminology in the German Standard DIN standard work. Then lie in a top -controlled engine valve head and gas path above a level which lies on the piston located at the top dead center, the bottom of the motor -driven, they are at or below this level. If other control devices - instead of valves - used these definitions shall apply mutatis mutandis.

Accordingly, the valves controlled hot top - regardless of the position of the camshaft - or down controlled.

The formulations above / below relate to an ordinary installation position of the engine: the crankshaft is located below the piston - even with radial engines - the opposite direction facing above.

The current (as of April 2013) edition of the standard was issued in December 1976.