Engine lubrication
The engine lubrication serves to reduce friction between moving engine parts of internal combustion engines and to dissipate heat from the bearing points and from the piston by means of lubricating oil . In addition, possible wear particles, combustion residues and other contamination should be flushed into the oil filter. Various methods are used to implement the lubrication systems.
Consumption lubrication
Mixture lubrication
With mixed lubrication , the lubricating oil is added to the fuel in the prescribed mixing ratio (1: 25/1: 50 etc.). The oil contained in the two-stroke mixture reaches the lubrication points and is burned with it. Mixture lubrication is still used today in small two-stroke engines (such as in lawnmowers, power saws or mopeds). This system was used in passenger cars until the end of the 1980s - the two-stroke engines from the GDR such as the Trabant , Wartburg or Barkas are probably best known .
The advantage is the simple preparation of the mixture (just add oil to the fuel and shake a little). The disadvantage is the separation of the two components in the event of long idle times, which cannot be completely prevented even by special additives added to the lubricating oil. Segregation can be remedied by shaking or stirring again; However, if this is forgotten or overlooked, start-up and concentricity problems and even engine damage can result.
Fresh oil lubrication
In this system, which is also used in two-stroke engines - also known as separate lubrication - the separately carried lubricant is fed to the engine by a pump and then also burned ( automatic fresh oil system , for example used in the DKW F 102 ). The advantages are the more precise and demand-dependent dosing compared to mixed lubrication, whereby the lubrication properties can always be kept constant, oil consumption is reduced and the exhaust gas quality is improved. The somewhat higher system costs are a disadvantage.
Fresh oil lubrication is also used for cylinder lubrication in large two-stroke marine diesel engines . The crankshaft and the other lubrication points are lubricated with wet sump lubrication (see below).
Centrifugal lubrication
This very simple lubrication system is often used with four-stroke small engines (often lawnmowers and aggregate engines ). In engines with a vertical crankshaft, there are several “blades” on a separate gear wheel (often combined with the speed controller). In engines with a horizontal crankshaft, however, there is a "finger" on the connecting rod bearing . The "blades" or the "fingers" beat through the oil filling and hurl the oil through the crankcase , which ensures sufficient lubrication by the resulting oil mist. The disadvantage is that no oil filter can be used here and that only roller bearings can be used for the bearing points in the crankshaft drive (especially crankshaft and connecting rod bearings) , which has manufacturing disadvantages (split crankshafts or overhung cranks (only on one side)).
Circulating lubrication
Wet sump lubrication
Wet sump lubrication, mainly referred to as pressure circulation lubrication in the German-speaking world, is used as a lubrication system in the vast majority of four-stroke engines used in automobiles and motorcycles . Here the lubricant supply is carried in the oil pan , which closes the crankcase at the bottom. The oil pump sucks the oil from the oil pan, conveys it through the oil filter and from there through channels to the lubrication points. The connecting rod bearings are supplied with oil through holes in the crankshaft. The oil escapes from the bearings of the crankshaft and the connecting rod and reaches the cylinder liner and the upper connecting rod bearings as an oil mist . The rotary motion of the crankshaft ensures additional swirling of the oil mist in the crankcase and thus the cooling of the pistons and the maintenance of the oil film on the piston running path of the cylinder, provided that there are no separate oil spray nozzles for cooling the piston crown. The oil that drips from the crank drive and runs back from the cylinder head collects again in the oil pan.
Dry sump lubrication
Dry sump lubrication is used for four-stroke engines; Similar to fresh oil lubrication, the lubricant is carried in a separate container (oil tank) and transported to the lubrication points by a pressure pump (first oil pump). However, the dripping lubricating oil is sucked out of the oil sump with a second pump (suction pump) and returned to the oil container. This return pump basically has a higher delivery rate than the pressure pump in order to reliably pump the lubricating oil back from the crankcase into the oil reservoir in every operating situation. Aircraft engines with cylinders or cylinder banks pointing downwards (for example radial engines or V or in-line engines in a hanging arrangement) are usually built with dry sump lubrication. In contrast to wet sump lubrication, the oil level must usually be read off the dipstick in the oil tank when the engine is idling. Some aircraft engines, such as B. the Rotax 912 that use blow-by gases from the combustion, in order to press the oil from the crankcase back into the oil reservoir. The suction pump can then be omitted here.
- Advantages of dry sump lubrication
- For engines that are exposed to strong changes in position or acceleration during operation ( off-road vehicles , aircraft ), more reliable engine lubrication can be achieved through the defined level in the appropriately designed oil tank, since air is not sucked in.
- Larger quantities of oil can be used, which improves the cooling performance and extends the oil change intervals.
- The size of the oil pan does not depend on the amount of engine oil used and the position of the oil collection container can be selected as desired. This enables better use of space and weight distribution.
- Disadvantages of dry sump lubrication
- With at least one additional oil pump and the separate oil tank, dry sump lubrication is more complex than wet sump lubrication. Therefore it is mostly only used in airplanes, flat sports cars, motorcycles or off-road vehicles.
- The two oil pumps make the lubrication system more complex and add an additional fault possibility, but there is no redundancy : If one oil pump (regardless of which one) fails, the lubricating oil circuit breaks down.
literature
- Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag GmbH, Braunschweig, 2000, ISBN 3-14-221500-X
- Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2001, ISBN 3-8085-2067-1
- Wilfried Staudt: Handbuch Fahrzeugtechnik Volume 1. 1. Edition, Bildungsverlag EINS, Troisdorf, 2005, ISBN 3-427-04520-X