SV valve control
The SV valve control (side valves ) or side valve arrangement refers to a design in four-stroke engines in which the valves are arranged next to the cylinders . The designation upright valves is also used for this design, in the Anglo-Saxon language the designation flathead dominates, which expresses the particularly low design of the cylinder head. A characteristic feature of an SV engine is that the cylinder head does not contain any parts of the valve train; At first glance, SV engines can therefore be confused with two-stroke engines .
A T-head engine is when the valves are facing each other - on both sides of the piston. In an L-head engine, on the other hand, the valves are close together - on the same side of the piston; T-head engines have two underlying camshafts.
Picture explanations
The picture above shows a Norton engine of the most popular type 16H. The British Army used it with several hundred thousand machines built during World War II. The valve stems are hidden behind the silver housing cap with the word “Norton” on the cylinder. The little dark lever is the decompression lever , or more correctly, a valve lifter: It allows the exhaust valve to be held open by hand to make the start process easier.
The middle picture shows the open valve train of the BMW R 42 motorcycle from the 1920s. The camshaft, located in the middle above the crankshaft, actuates the open valves via tappets (above the cylinder, on the left above the exhaust pipe). The valve springs are also exposed. Because of the smaller cylinder displacement (250 cm³ each), no decompression device is required to start the engine.
The picture below shows a cylinder with pistons and valves. The valves are next to each other and can therefore be operated by just one camshaft located below.
Historical
The need for gas exchange control of cycle - motors is as old as the steam engine. However, the valve system of the steam engine is not suitable for controlling flammable gases. Therefore, Nikolaus August Otto had to build something different for the four-stroke engine: a gas exchange with valves that allow the working space to be filled with fresh gases in a controlled manner and the working space to be emptied of the burnt gases. In the early days of engine technology, it was common to only control the exhaust valve, more precisely: to open it against the closing pressure of a spring, while a spring-loaded "automatic" valve enabled fresh gas to enter: the " sniffer valve ". It quickly became apparent, however, that the force that was required to overcome the spring force of the sniffer valve during suction was a considerable obstacle to good performance. So one went over to the controlled inlet valve, which logically connected a gas duct "from bottom to top" to the cylinder head from the initial rising flow spray nozzle carburettors according to the Maybach patent with an inlet valve that has the sealing plate "above" and is actuated by the bottom Cam on the camshaft that is driven closely by the crankshaft. This is the SV working principle.
Due to the disadvantages (see below), more efficient combustion chamber designs were sought for sports use, which required the valves to be relocated "upwards" in the cylinder head and more complex actuation, be it via bumpers ( OHV ) or be it with overhead camshaft ( OHC ). However, these types of construction are significantly more complex, which is why they were reserved for sports and the more affluent public for about three decades.
An interim solution was the attempt to arrange the valves with their plates against each other in the combustion chamber, usually in a line with the exhaust valve at the bottom as in the SV arrangement and the intake valve opposite it in the cylinder head, and there with a single bumper and deflection above Rocker arm is operated. Hence the English name for this: "Inlet over Exhaust", inlet (valve) above outlet (valve), IOE .
SV engines were used in vehicles from 1900 through the late 1950s. The last German passenger car with such an engine was the Taunus 12M by Ford , which was produced until 1962. In vehicle construction, with a few exceptions, side-controlled engines were replaced by OHV and OHC engines. Well-known examples of the SV design are the Ford V8 from 1932 to 1952 (with 90 hp initially) and most motorcycles from the 1930s and 1940s. Even today, the SV valve train is installed in some sidecar motorcycle models from Russian ( Dnepr , Ural ) and Chinese (Donghai, Chang Jiang) production: These motorcycles are further developed copies of the 750 BMW motorcycles of the 1930s ( BMW R 71 ).
Advantages and disadvantages
advantages
Since the valves of an SV engine are only actuated by the camshaft via short tappets and the camshaft is in turn driven by a simple spur gear set or a short timing chain , the valve train has only a few individual parts. If the crankshaft and other components are mounted with roller bearings, no separate pump is required for the oil circulation, because all moving parts are located in the crankcase and are therefore adequately lubricated. SV engines are compact because the cylinder head does not contain any parts of the valve train. The production of the motors is easy due to the smaller number of parts. Both criteria make this type of construction popular today for small motors for power generators , irrigation pumps , lawn mowers and (more rarely) chainsaws .
Thanks to the slow combustion of the fuel mixture in the cylinder, as well as the smaller number of moving parts, SV motors are characterized by a smoother run than "overhead control" (OHV / OHC) motors. Because the intake and exhaust gases are routed with little flow, this type of engine has little torque at higher speeds, that is, a flatter power curve , which tends to require gearboxes with a smaller number of gears. Thus, both a cheaper solution for manual gear shifting is possible, as well as being well suited for automatic transmissions.
Although the knowledge of OHV and OHC engines was available long before the Second World War, it was mainly SV engines that were built. The reason was the lower expected damage caused by a valve torn off due to poor material quality. As a rule, this could not fall into the cylinder and damage the piston crown there.
disadvantage
Due to the larger sealing surface between the cylinder and the cylinder head, the risk of sealing problems is higher than with engines with overhead valves. The elongated, jagged shape of the combustion chamber is also quite far from the spherical ideal. The result is that the gas mixture takes longer to burn. This results in low maximum speeds, relatively low liter performance and higher fuel consumption with relatively poor exhaust gas values, namely due to carbon monoxide and hydrocarbons (C m H n ), as well as soot formation . With Ricardo heads these disadvantages are reduced, the combustion chamber is more compact because the cylinder head is provided with a step ("pinch edge") above the piston. To lubricate the valve stems, if lubrication is otherwise not possible, top oil must be added to the fuel , which also contributes to the increased C m H n proportion of pollutants in the exhaust gas. The piston crown is not cooled directly by the gasoline-air mixture flowing in. The piston is therefore exposed to higher thermal loads, which also limits the potential for increased performance. Also because of the required minimum height above the valves, there are limits to the increase in performance and efficiency through higher compression .
