Decompression (combustion engine)
In internal combustion engines, decompression is the term used to describe the process of reducing the resistance caused by the compression work during the compression stroke by temporarily releasing the compression. Its primary purpose is to make it easier to start the engine.
Use and functionality
Decompression devices are mainly used on engines without an electric starter to start the starting process with muscle power, e.g. B. by a kick starter , a crank , pedals or a reversing starter . The piston or pistons of an engine with larger individual cubic capacities are relatively difficult to move from the stand over the dead center of the compression stroke when starting, depending on the compression and cubic capacity , since the compression work to be performed creates a strong counterforce that enables a minimum angular speed to be reached Crankshaft at which the engine starts is difficult. In order to facilitate this acceleration of the crankshaft to the starting speed, this drag resistance is reduced during the first revolutions by a targeted leak in the working chamber of the cylinder , so that the crank drive and the other rotating masses can absorb momentum. Such devices are also used in engines with electric starters, in particular large-volume one or two cylinders in the motorcycle sector, since the lower moment of resistance means that the starter itself and the starter battery can be designed to be smaller and therefore more weight-saving.
Often single-cylinder internal combustion engines of older motor vehicles ( oldtimers ), mainly motorcycles and mopeds, occasionally also small automobiles or tractors with single-cylinder engines, are equipped with decompressions. Such devices are also found in modern lawnmower motors , stationary motors and hand-held power tools such as power saws .
Decompression devices can be manual or automatic. The targeted pressure loss in two-stroke engines is usually achieved by a small valve screwed into the cylinder cover, which, when actuated by a Bowden cable, allows the compressed working gas to escape into the ambient air. Semi-automatic valves, which are opened manually at the push of a button before the start and automatically closed again by the working pressure of the first ignitions, are common in work machines. In valve-controlled four-stroke engines, on the other hand, the actuation mechanism of an exhaust valve is usually acted on, so that it does not close at all at times ("valve lifter") or an additional, small opening stroke is inserted during the compression stroke to blow off part of the compressed gas into the exhaust.
Decompression lever
The decompression lever (“deco lever”, “deco”) actuates the decompression valve in the cylinder head , especially in single-cylinder two-stroke engines, or opens one or more exhaust valves in four -stroke engines ; in this case it is also referred to as the valve lifter lever. The operating lever is spring-loaded; you pull or push it to start. When the engine has gained momentum via the kick starter or the pedals without being hindered by compression, the lever is released so that the engine can start working as compression begins.
In the DIN standard DIN 73005 "Control levers and wheel installation in motorcycles", May 1940 edition, a position on the left handlebar end between the clutch lever and the ignition (adjustment) lever was provided for the lifter control lever. DIN 71901, sheet 2 "Levers and handles for motorcycle and motorcycle handlebars; arrangement and connection dimensions at handlebar ends", October 1951 edition, dimensioned the attachment for the "lifting lever C according to DIN 71902", and in DIN 71902, sheet 3 "Lever for motorcycles - and motorcycle handlebars; external cable lifting lever ", design details and dimensions for the lever were specified; the effective lever arm should then be about 65 mm long and the path of the cable pull should be 15 mm at full lever deflection. Zinc was planned as the material. The first edition DIN 71902 FI; Sheet 3, dated May 1943, had specified further details.
Automatic decompression
More recent are solutions to automate the effects of decompression. On motorcycles, this can be done semi-automatically so that after actuating a lever for one or two crankshaft revolutions (the number of revolutions that is achieved with the full actuation of the kick starter) the valve lifter is active, but the valve automatically closes completely at the next TDC passage and the engine ignites. The valve lifter of the single-cylinder engine of the Yamaha SRX600 motorcycle model is linked to the kick starter .
Automatic decompressions with centrifugal force control usually work in such a way that a small auxiliary cam is arranged per cylinder in the base circle of the slide path of an exhaust cam. This consists of a steel bolt arranged parallel to the axis of rotation of the camshaft , which lies in a tangentially intersecting hole in the cam. The bolt has a flat spot on one side which, when facing the slide, has no effect on the valve timing. If the bolt is rotated relative to the camshaft, assigns its rounding of the cam profile produces and acts via a cup tappet , towing or rocker arm to the valve. A spring-loaded centrifugal weight is usually located in the camshaft's timing chain sprocket or directly on the cam, which turns the auxiliary cam pin into the deactivated position (= no decompression) when a certain minimum speed is reached (still a little below idle speed). When the engine is stationary and at the starter speed, however, the cam is in the active decompression position. Examples include the two-cylinder V-engine of the motorcycle model Suzuki TL1000 , who built since 2012 BMW "Wasserboxer" the R-models with 1200 cm 3 , the current KTM EXC four-stroke. The GX series from Honda, which are small single-cylinder four-stroke engines for work equipment, also use such a system. On the Ducati 1199 Panigale motorcycle , a similar mechanical solution is used to change the profile of the opening cams of the Desmodromic system when starting , so that the associated valves are kept open longer.
Decompression brake
Another application of decompression devices is the decompression brake (also known as “Jacobs Brake” or “Jake Brake” ) to increase the engine braking power of diesel engines in heavy commercial vehicles when pushing.
Web links
- Ducati Panigale decompression - working principle , Youtube video, in which the function of the automatic decompression on a Ducati Superquattro engine is explained , accessed on June 4, 2016
Individual evidence
- ↑ Description on the homepage of the power tool manufacturer Dolmar , accessed on June 11, 2016
- ↑ Two-cylinder boxer engine 1200 cm³ (air-water cooled), description on bmw-motorrad.de ( memento of the original from June 4, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , accessed June 4, 2016
