Piston seizure
As Kolbenfresser (in Austria even piston seizure, rubbing marks or Verreiber ) are in reciprocating engines and - compressors damage to the moving piston in the area of the contact surface to the cylinder respectively.
causes
Inadequate lubrication of the running surfaces of the piston and cylinder results in dry friction , which increases the temperature of the piston and cylinder significantly. This initiates a friction weld . The piston and the running surface of the cylinder ( cylinder liner ) connect firmly to one another and the machine is suddenly brought to a standstill. The causes for this range from increased temperatures for various reasons to insufficient or incorrect lubrication and assembly or even construction errors . Most piston seizures occur in internal combustion engines .
When scuffing , changes in the tribological system increase the temperature of the piston. Since the piston heats up faster than the surrounding cooled cylinder, it expands more and begins to block when the clearance approaches zero. This leads to "seizing" (complete blockage of the piston in the cylinder) within a very short time due to the welding of the piston material to the cylinder wall at the top or bottom dead center when the relative speed from piston to cylinder approaches zero. The tendency towards piston seizure is also increased by the most commonly used material combination aluminum (piston) and gray cast iron (cylinder), as aluminum has a higher coefficient of thermal expansion .
lubricant
Loss of lubricant due to too high speeds, too high engine load, excess fuel (which dissolves the oil film), sludge formation in the oil, clogged oil spray nozzles, insufficient oil or too much oil (since the crankshaft can hit the oil, causing air bubbles to form in the oil and the The lubricity of the oil is severely impaired) causes a local cooling or lubrication deficiency on the piston.
The reciprocating engine's piston , which moves up and down , generally needs to be lubricated and cooled with oil . The frictional heat should be dissipated and the coefficient of friction (see friction ) should be minimized. In the case of two-stroke engines , the oil required for lubrication is usually mixed with the gasoline ( two-stroke mixture ). Since the engine is only supplied with the idle gas mixture when coasting (when driving downhill) ( throttle valve closed), there may be a lack of lubrication. This does not occur if the idle gas is set as low as possible beforehand. In this way, the oil contained in the fuel / air mixture can maintain the lubrication of the engine. If the amount of air for the mixture sucked in during overrun operation is too small for combustion, the temperature remains low and lubrication is guaranteed. In the vast majority of four-stroke engines , lubrication is carried out by means of an oil pump that delivers the oil to the lubrication points.
cooling
Failure of cooling due to loss of coolant or blockage of the heat exchangers are known problems. The increase in temperature can lead to the oil film tearing off. The result is excessive friction and overheating of the piston. Due to the thermal expansion, this results in an excessive increase in the diameter of the piston in the cylinder bore.
foreign body
Foreign bodies lead to abrasive wear in the piston (e.g. due to heavy contamination of the intake air) due to increased friction, which increases the tendency for the components to seize. A defective, broken piston ring can lead to increased friction and ultimately piston seizure. Poor combustion or foreign matter in the fuel can lead to deposits on the piston and cylinder running surface, which can tear off the oil film and thus trigger piston seizure. Deformation of the piston due to large foreign bodies in the event of engine damage (valve damage) or burnout can also lead to piston seizure.
Delimitation of similar damage
Preliminary stages of piston seizure have the same causes. The difference is that the engine can still be turned.
Butt jam
A precursor of the piston-eater is the Kolbenklemmer : Here, the piston sits fast not completely when the engine load is sufficiently reduced rapidly and cools the engine.
Rule of conduct to prevent piston seizure: Even if the engine should start again after a piston jamming, it should under no circumstances continue to be operated, but should be switched off again as soon as possible. Otherwise the piston seizure can follow after only a short period of operation, which would ultimately destroy the engine.
Notes and consequences
A sudden drop in engine performance during operation, often combined with abnormal noises, can be an indication of imminent damage. Likewise, before the damage, the performance of an old engine can improve for no apparent reason - that too is an alarm signal.
When a piston is blocked, all moving parts of the engine are suddenly braked. Their kinetic energy acts on the blocked piston, whereby very high forces arise due to the low elasticity and the short braking distance. The connecting rod cross-section fails due to excessive tensile or compressive forces and the connecting rod tears off. The crankshaft and the shaft bearings can also be deformed . The damage that occurs is almost always irreparable. The torn off connecting rod or parts thereof can also penetrate the crankcase.
Repair
A piston jammed or seized piston must be repaired in a specialist engine repair workshop . To do this, the cylinder is usually bored open and then honed . The cylinder surface is finely machined and smoothed. The defective piston usually has to be replaced. If the diameter is increased when the cylinder is being repaired, an oversized piston must be used. Practically all engine manufacturers have these in their range. New piston rings are also used with the new piston. An exception here are newer aluminum engine blocks, which are coated with highly abrasion-resistant materials on the cylinder surface. Since the material applied in the plasma or laser coating process is only a few 1/100 mm thick, they cannot be reworked. For this reason, there are no oversized pistons. Cylinder liners made of steel or gray cast iron are mostly shrunk in here and the original pistons are retained. For example, the first generation of five-cylinder engines with pump-nozzle injection from the VW group with the engine code AX were striking. In these, the plasma coating increasingly came off the engine block. At the time, repairs were impossible and the entire engine block had to be replaced.
Piston seizure on a motorcycle
In the case of a piston seizure, the drive axle locks up , and so does the rear wheel on a motorcycle ; there is a great risk of falling. If you pull the clutch quickly, the connection between the wheel and the blocked piston is interrupted and a fall can possibly be avoided.
Related topics
A similar effect to the piston seizure is used in friction welding to intentionally join two metal parts.
For more damage see: engine damage
literature
- Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2001, ISBN 3-8085-2067-1 .
