Internal combustion engine pistons

The piston is a component of the internal combustion engine. The main function of the piston is to transform the pressure generated by the burning air-fuel mixture into force, acting on the crankshaft. Passenger vehicles have aluminium alloy pistons while commercial vehicles can also have steel and cast iron pistons.

The piston is part of the crankshift drive (also called crank mechanism), which is made up from the following components:

  • piston
  • piston rings
  • connecting rod
  • crankshaft
Engine crankshaft drive (crank mechanism)

Image: Engine crankshaft drive (crank mechanism) Credit: Rheinmetall

where:

  1. piston
  2. connecting rod
  3. crankshaft
  4. flywheel

There are also secondary engine functions fulfilled by the piston:

  • contributes to heat dissipation generated during combustion
  • ensures the sealing of the combustion chamber, preventing gas leakages from it and oil penetration into the combustion chamber
  • guides the movement of the connecting rod
  • ensures to the continuous change of gases in the combustion chamber
  • generates the variable volume in the combustion chamber
Kolbenschmidt pistons

Image: Kolbenschmidt pistons
Credit: Kolbenschmidt

The shape of the piston depends mainly on the type of combustion engine. Gasoline (petrol) engine pistons tend to be lighter and shorter compared with the diesel engine pistons. Piston geometry has many subtleties, due to the complexity of the its working environment, but we the main parts of a piston are:

  • piston head, also called top or crown: is the upper part of the piston which comes into contact with the gas pressure withing the combustion chamber
  • ring belt: is the upper-middle part of the piston when the piston rings are located
  • pin boss: is the lower-middle part of the piston which contains the piston pin
  • piston skirt: is the area under the ring belt

Piston pin and skirt axes

Image: Piston pin and skirt axes

Main piston parts

Image: Main piston parts
Credit: [3]

where:

  1. piston top
  2. top land
  3. ring belt
  4. control struts
  5. pin retaining clip
  6. pin boss
  7. piston pin
  8. piston rings
  9. piston skirt

The piston is connected to the connecting rod through the piston pin (7). The pin allows the piston to rotate around the pin axis. The pin is kept in place in the piston by the pin retaining clip (5).

After the piston crown comes to ring belt (also called ring zone) (3). Most of the pistons have three ring grooves, in which piston rings are mounted. The top ring is called the compression ring, the middle on is the scraper ring and the bottom one is the oil control ring. The compression ring needs to seal the combustion chamber in order to prevent the inside gases to escape into the engine block. The oil control ring scraps the oil from the cylinder wall, when the piston is on the power or exhaust stroke. The middle ring has a combined role of assuring compression in the cylinder and scrapping excess oil from the cylinder walls.

The piston skirt (8) keeps the piston balanced inside the cylinder. It is usually covered with a low friction material to reduce the friction losses. The piston pin bore or boss (6) is hosting the piston pin (7), which connects the piston to the connecting rod.

Geometric characteristics of the piston

Pistons need to work properly in a wide range of temperatures, the range being -30 °C to up to 300-400 °C. In the same time needs to be light enough in order to have low inertia and allow high engine speeds. There are a couple of geometric characteristics of the piston which are presented below.

Piston ovality

Due to the combustion process, the temperature inside the engine cylinders reaches hundreds of degrees Celsius. The piston is one of the main components which absorbs part of the generated heat and dissipates it into the engine oil. Because the piston pin axis contains more material than the skirt axis, the thermal expansion along the pin axis is a little bit higher than the thermal expansion along the skirt axis. For this reason the piston is oval, the diameter along the pin axis is between 0.3-0.8 % smaller than the diameter along the skirt axis [6].

Piston ovality

Image: Piston ovality

Piston conical shape

The shape of the piston is not a perfect cylinder. At low temperature, the clearance between the piston and the engine cylinder is higher compared with high temperatures. Also, the clearance is not constant along the piston length, it is smaller around the piston top compared with the piston skirt region. This is to allow more thermal expansion of the piston head, since it contains more volume of metal.

Piston clearance

Image: Piston clearance (conical shape)

Piston thermal expansion

Image: Piston thermal expansion (if cylindrical shape)

Piston pin offset

The piston movement inside the cylinder has 3 degrees of freedom, 1 primary and 2 secondary:

  • along the vertical axis of the cylinder, between the top dead centre (TDC) and bottom dead centre (BDC) (primary, y-axis)
  • around the pin axis (secondary, α – angle)
  • along the skirt axis (secondary, x – axis)

The primary movement is generating torque at the crankshaft, this is desired from the mechanical point of view. The secondary movements happen due to the combination of several factors: connecting rod bidirectional movement and clearance between piston and cylinder. Both secondary movements cause friction with the cylinder walls and also noise, vibration (piston slap).

Piston thrust and pin offset

Image: Piston thrust and pin offset

When the crankshaft rotation is clockwise, the left side of the cylinder is called the thrust side (TS) and the opposite side is known as the anti-thrust side (ATS). Piston impacts can occur on either side of the cylinder. Piston slap excites the engine block and manifests itself in the form of surface vibrations, which are eventually radiated as noise in the vicinity of the engine [9]. Another inconvenient is that when the piston moves through the TDC and BTC, increased load is generated onto the crankshaft, because the piston is aligned with the crankshaft centre of rotation.

The piston pin offset is the misalignment between the centre of the piston pin bore and the centre of the crankshaft. By having it in the design, it improves the noise characteristics of the engine due to piston slap at TDC. This is a major NVH (noise vibration and harshness) concern to production engineers who want to eliminate alarming noises anywhere they can. The second reason is to improve engine output by reducing internal friction on the TS and ATS.

The pin offset reduces the mechanical stress that is caused in the connection rod, when it reaches TDC or BDC, because the connecting rod does not have to slam the piston in opposite direction at the end of the stroke. This offset is causing the rod to travel on an arc path at TDC and BDC.

Mechanical loads on the piston

The piston is the component of the internal combustion engine (ICE) which has to sustain the most mechanical and thermal stress. Due to the piston, the power of the ICE is limited. In case of very high thermal or mechanical stress, the piston is the first component to fail (compared to engine block, valves, cylinder head). This is because the piston must be a compromise between mass and mechanical and thermal stress resistance.

The cyclical loading of the piston due to [6]:

  • the gas force from the cylinder pressure
  • the inertia force from the oscillating motion of the piston, and
  • the lateral force from the support of the gas force by the inclined connecting rod, and the inertia force of the oscillating connecting rod

determines the mechanical load.

There vertical forces acting on the piston consists of: pressure forces, generated by the expanding gases, and inertia forces, generated by the piston’s own mass [10].

\[F_{p}=F_{gas}+F_{ineria}\]

The inertia forces are much smaller than the pressure forces and have highest intensity when the piston changes direction, at TDC and BDC.

Piston von Mises stress and mechanical deformation

Image: Piston von Mises stress and mechanical deformation
Credit: [7]

Piston vertical forces function of crankshaft angle

Image: Piston vertical forces function of crankshaft angle
Credit: [7]

The above piston forces are calculated using advanced Finite Element Analysis techniques for a aluminium piston used in light vehicles with diesel engine [7].

The combustion process has different characteristics for diesel and gasoline ICE. In a diesel engine, the peak gas pressure, during combustion, can reach 150 – 160 bar. In a gasoline engine, the maximum pressure is below 100 bar. Because of higher pressure, diesel pistons have to withstand higher mechanical stress.

In order to work without any failure in such harsh conditions, diesel engine pistons are designed heavier, sturdier with more mass. The drawback is higher inertia, higher dynamic forces so lower maximum engine speed. One reason for which diesel engines have lower maximum speed (approx. 4500 rpm), compared to gasoline engines (approx. 6500 rpm), is heavier mechanical components (pistons, connecting rods, crankshaft, etc.).

Thermal loads on the piston

The piston crown comes in direct contact with the burning gases, within the combustion chamber, so it’s exposed to high thermal and mechanical stress. Depending on the type of the engine (diesel or gasoline) and the type of fuel injection (direct or indirect injection), the piston crown can be flat or can contain a bowl.

The thermal load from the gas temperatures in the combustion process is also a cyclical load on the piston. It acts primarily during the expansion stroke on the combustion chamber side of the piston. In the other strokes, depending on the operating principle, the thermal load on the piston is reduced, interrupted, or even has a cooling effect during gas exchange. In general, heat transfer from the hot combustion gases to the piston occurs primarily by convection, and only a slight portion results from radiation.

Piston operating temperatures

Image: Piston operating temperatures
Credit: [3]

The heat generated during combustion is partially absorbed by the piston. Most of the heat is transferred through the ring area of the piston (around 70%). The piston skirt evacuates 25% of the heat and the rest is transferred further to the piston pin, connecting rod and oil. Higher engine speed means higher piston temperature. This happens because the accumulated heat doesn’t have time to dissipate between two consecutive combustion cycles. In the same time, higher engine load means higher piston temperature, because there is more air-fuel mixture burning, which generates more heat.

Temperature distribution in a gasoline engine piston

Image: Temperature distribution in a gasoline engine piston
Credit: [6]

Temperature distribution in a diesel engine piston with cooling channel

Image: Temperature distribution in a diesel engine piston with cooling channel
Credit: [6]

Piston thermal loading

Image: Piston thermal loading
Credit: [7]

In relation to the expansion stroke, the duration over which the thermal load from combustion acts is very short. Therefore, only a very small portion of the component mass of the piston, near the surface on the combustion side, follows the cyclical temperature fluctuations. Nearly the entire mass of the piston, therefore, reaches a quasi-static temperature, which can, however, have significant local variations.

Piston cooling

As specific power output increases in modern combustion engines, the pistons are subjected to increasing thermal loads. Efficient piston cooling is therefore required more frequently in order to ensure operational safety.

2009 Ecotec 2.0L I-4 VVT DI Turbo (LNF) Piston Head and Oil Jet

Image: 2009 Ecotec 2.0L I-4 VVT DI Turbo (LNF) Piston Head and Oil Jet
Credit: GM

The piston temperature can be lowered by circulating oil through the piston mid region. This can be accomplished by using oil jet devices mounted on the engine block, which inject engine oil, through an orifice, when the piston is close to bottom dead center (BDC).

Tenneco Powertrain has designed a new steel piston for diesel engines it has designed with a “sealed-for-life” coolant chamber in the crown, allowing the pistons to operate safely at crown temperatures more than 100°C higher than current limits.

EnviroKool piston cooling technology

Image: EnviroKool piston cooling technology
Credit: Tenneco

To form the EnviroKool crown, an integral cooling gallery is created within the piston using friction welding, and then filled with high-temperature oil and an inert gas. This chamber is permanently sealed with a welded plug. According Tenneco Powertrain, EnviroKool technology makes it possible to overcome the temperature limitations of conventional open galleries that use lubricating oil as a heat-transfer medium.

Types of pistons

The geometry of the piston is constrained due to the cubic capacity of the ICE. Therefore, the main way to increase the mechanical and thermal resistance of the piston is by increasing its mass. This is not recommended because a piston with high mass, has high inertia which translates in high dynamic forces, especially during high engine speed. The resistance of the piston can be improved by geometry optimisation but there will be always a compromise between mass and mechanical and thermal resistance.

At a first look, the piston seems a simple component, but its geometry is quite complex:

Diesel piston technical description

Image: Diesel piston technical description
Credit: Kolbenschmidt

Gasoline piston technical description

Image: Gasoline piston technical description
Credit: Kolbenschmidt

Legend:

  1. bowl diameter
  2. piston crown
  3. combustion chamber (bowl)
  4. piston crown edge
  5. piston top land
  6. compression ring groove
  7. ring land
  8. groove base
  9. recessed ring land
  10. groove sides
  11. oil scraper ring groove
  12. oil return bore
  13. piston pin boss
  14. retention for groove distance
  15. groove for retainer ring
  16. piston boss distance
  17. piston boss distance
  18. stepped edge
  19. piston diameter 90 °C against the piston pin bore
  20. piston pin bore
  21. bowl depth
  22. skirt
  23. ring zone
  24. piston compression height
  25. piston length
  26. oil cooler duct
  27. ring carrier
  28. bolt bush
  29. diameter measuring window
  30. crown camber

As you can see there are significant differences between diesel and gasoline pistons.

Diesel engine pistons must withstand higher pressures and temperatures, therefore they are bigger, bulkier and heavier. They can be manufactured from aluminium alloys, steel or a combination of both. The diesel piston contains part of the combustion chamber in the piston head. Due to the shape of the cross-section of the piston head, the diesel engine piston is also called omega head piston.

Gasoline (petrol) engine pistons are lighter, designed for higher engine speeds. They are manufactured from aluminium alloys and usually have a flat head. Gasoline engines with direct injection (DI) have special heads in order to direct the fuel stream in a tumble motion.

Bellow you can see some high definition pictures of diesel and gasoline (petrol) engines.

LS9 6.2L V-8 SC piston (aluminium, gasoline/petrol engine with indirect injection)

Image: LS9 6.2L V-8 SC piston (aluminium, gasoline/petrol engine with indirect injection)
Credit: GM

Ecotec 2.0L I-4 VVT DI Turbo (LNF) piston (aluminium, gasoline/petrol engine with direct injection)

Image: Ecotec 2.0L I-4 VVT DI Turbo (LNF) piston (aluminium, gasoline/petrol engine with direct injection)
Credit: GM

Car diesel engine piston with rings (aluminium, diesel)

Image: Car diesel engine piston with rings (aluminium, diesel)
Credit: Kolbenschmidt

Monosteel piston (steel, diesel)

Image: Monosteel piston (steel, diesel)
Credit: Tenneco

Pistons materials

Most of the pistons for automotive applications are made from aluminium alloys. This is because aluminium is light, has enough mechanical resistance and good thermal conductivity. There are heavy-duty applications, commercial vehicles, which are using steel pistons, which are more resistant to higher pressures and temperatures in the combustion chamber.

Aluminium pistons are produced from cast or forged, high-temperature resistant aluminium silicon alloys. There are three basic types of aluminium piston alloys. The standard piston alloy is a eutectic Al-12%Si alloy containing in addition approx. 1% each of Cu, Ni and Mg [3].

Main aluminium alloys for pistons are [3]:

  • eutectic alloy (AlSi12CuMgNi): cast or forged
  • hypereutectic alloy (AlSi18CuMgNi): cast or forged
  • special eutectic alloy (AlSi12Cu4Ni2Mg): cast only

Because the aluminium alloy has lower strength than cast iron, thicker sections have to be used so not all the advantage of the light weight of this material is realised. Moreover, because of its higher coefficient of thermal expansion, larger running clearances have to be allowed for aluminium pistons. On the other hand, the thermal conductivity of aluminium is about three times that of iron. This, together with the greater thicknesses of the sections used, enables aluminium pistons to run at temperatures about 200°C lower than cast-iron ones [8].

In some applications, the strength and wear resistance of aluminium alloy pistons in not enough to meet the load demand, therefore ferrous materials are used (e.g. cast iron, steel). There are several methods of using ferrous metals in piston manufacturing:

  • as local reinforcement, ferrous metal insertions (e.g., ring carriers)
  • as extend parts of composite pistons (e.g., piston crown, bolts)
  • pistons constructed entirely of cast iron or forged steel
Pistons for heavy-duty engine - cross-section

Image: Composite piston for heavy-duty engine – cross-section
Credit: [8]

Composite design piston for diesel marine engines

Image: Composite design piston for diesel marine engines
Credit: Warstila

There are two types of ferrous materials used for pistons or pistons components [6]:

  • cast iron:
    • austenitic cast iron for ring carriers
    • cast iron with spheroidal graphite for pistons and piston skirts
  • steel
    • chromium-molybdenum alloy (42CrMo4)
    • chromium-molybdenum-nickel alloy (34CrNiMo6)
    • molibden-vanadium alloy (38MnVS6)

Cast iron materials generally have a carbon content of > 2%. Pistons in highly stressed diesel engines and other highly loaded components in engines and machine design are predominantly made of M-S70 spherolithic cast iron. This material is used, for example, for single-piece pistons and piston skirts in composite pistons [6].

The iron alloys designated as steels generally have a carbon content of less than 2%. When heated, they transform completely into malleable (suitable for forging) austenite. The iron alloys are therefore excellent for hot forming, such as rolling or forging.

Piston technologies

There are several advanced piston technologies, each having the purpose of increasing the mechanical and/or thermal resistance, lowering the friction coefficient or lowering the overall mass (keeping in the same time the mechanical and thermal properties).

Below you can find examples of modern pistons, manufactured by Kolbenschmidt, each with distinctive technologies.

Kolbenschmidt diesel piston

Image: Diesel piston with cooling channel, bolt bush and ring carrier
Credit: Kolbenschmidt

Kolbenschmidt articulated piston

Image: Diesel articulated piston with forged upper steel section and aluminium skirt
Credit: Kolbenschmidt

Kolbenschmidt gasoline LiteKS piston

Image: Gasoline engine piston in weight-optimised LiteKS® design with ring carrier
Credit: Kolbenschmidt

Kolbenschmidt with cast-in ring carriers of cast iron

Image: Cast-in ring carriers of cast iron increase the durability of the first ring groove of diesel pistons many times over. Kolbenschmidt is a leader in the development of Alfin ring-carrier bonding
Credit: Kolbenschmidt

Kolbenschmidt piston with hard-anodised ring grooves

Image: Hard-anodised ring grooves prevent wear and micro welding in pistons for petrol engines
Credit: Kolbenschmidt

Kolbenschmidt piston with graphite coating

Image: KS Kolbenschmidt pistons feature special LofriKS®, NanofriKS® or graphite coatings on the piston skirt. These reduce friction inside the engine and ensure good emergency running properties. LofriKS® coatings are also employed for acoustic reasons. Their use minimises piston slap noises. NanofriKS® is a further development of the tried and tested LofriKS® coating, and additionally contains nanoparticles of titanium oxide, to increase the coating’s wear resistance and durability
Credit: Kolbenschmidt

Kolbenschmidt Ferrocoat piston

Image: Iron-coated piston skirts (Ferrocoat®) guarantee reliable operation when used in aluminium silicon cylinder surfaces (Alusil®)
Credit: Kolbenschmidt

Kolbenschmidt Hi-SpeKS piston

Image: Specially shaped piston pin bores (Hi-SpeKS®) raise the dynamic load capacity of the piston pin bed, thereby increasing the piston’s durability
Credit: Kolbenschmidt

Below you can find examples of modern pistons, manufactured by Tenneco Powertrain (former Federal Mogul), each with distinctive technologies.

Elastothermic piston

Image: Elastothermic® piston (aluminium piston for gasoline/petrol light vehicle)

Features:
– gallery cooled piston improves power and fuel consumption of downsized gasoline engines
– elastothermic cooling gallery reduces the piston crown temperature by about 30°C
– decreased temperature of the first ring groove by about 50°C, consequently reducing carbon deposition and groove & ring wear for long life low oil consumption and blow by
– reduced risk of uncontrolled combustion such as low speed pre-ignition

Credit: Tenneco Powertrain (Federal Mogul)

Aluminium diesel pistons

Image: Aluminium diesel pistons

Features:
– optimized gallery locations for maximum cooling can result in up to a 10 % lower bowl rim temperature
– advanced side-casting techniques significantly improve structural stability (even with thin wall designs)
– restructuring of the combustion-bowl rim and bowl base can provide up to a 100 % increase in fatigue life

Credit: Tenneco Powertrain (Federal Mogul)

Monosteel diesel pistons

Image: Monosteel diesel pistons (steel piston for diesel heavy-duty vehicle or industrial application)

The Monosteel® piston provides the strength and cooling performance to meet the most arduous engine requirements in heavy-duty and industrial engine markets including the new generation of engine firing pressures required for Euro VI on-highway regulations and beyond.

The robust design – constructed of inertia welded, forged steel sections which create large cooling galleries – enable Monosteel pistons to withstand increasing mechanical loads. Monosteel’s evolution includes recent developments for large bore industrial engines, and utilising thin wall lightweight forgings and castings for light vehicle diesel engines.

Product highlights:
– a large, closed structural gallery with superior bowl rim and ring groove cooling, reducing groove distortion and improving oil control and gas sealing
– a profiled, bushingless pin bore
– a full-length skirt for stable piston dynamics, reducing risk of liner cavitation and improving ring sealing
– process allows material flexibility with crown material options to reduce corrosion or oxidation, and/or skirt material options to improve manufacturability.

Credit: Tenneco Powertrain (Federal Mogul)

EcoTough coated pistons

Image: EcoTough® coated pistons (aluminium piston for gasoline/petrol light or heavy-duty vehicle)

The EcoTough® coated piston provides important benefits that help meet customer demands for more efficient engine designs, including reduced fuel consumption and CO2 emissions. It combines low wear and low friction in a single application and reduces fuel consumption by 0.8 % as compared to conventional piston coatings.

Key benefits include:
– compatible with existing and advanced cylinder bore finishes and can be introduced seamlessly in volume engine production as a running change
– composition provides greater thickness than pistons with conventional coatings, providing additional protection
– fulfils stringent environmental standards; contains no toxic solvents
– proprietary, advanced piston skirt coating with solid lubricants and carbon fibers reinforcement, especially designed for challenging gasoline applications
– 10 % friction reduction in Power Cylinder Unit (piston+rings) vs. standard coatings, up to 0.4 % fuel economy improvement/CO2 reduction in european drive cycle tests
– 40 % less wear than standard gasoline coatings, increased robustness in state-of-art boosted gasoline DI engines
– EcoTough® is a patented F-M coating

Credit: Tenneco Powertrain (Federal Mogul)

DuraBowl piston

Image: DuraBowl® piston (aluminium piston for diesel light or heavy-duty vehicle)

DuraBowl® Piston Reinforcement Partial Bowl-Edge Re-melting features:
– extreme refinement of aluminum material structure created by localised re-melting using TIG technology
– up to 4 times improved durability in high specific power engines compared to pistons without bowl re-melting. Allows highly stressed combustion bowl shapes
– F-M DuraBowl® process extends aluminum piston limits in the most challenging applications by increasing the fatigue strength (cycles) of the piston

Credit: Tenneco Powertrain (Federal Mogul)

Elastoval II ultra-lightweight pistons

Image: Elastoval II ultra-lightweight pistons (aluminium piston for gasoline/petrol light vehicle)

Avanced Elastoval® II gasoline piston technology is based on:
– deep undercrown pockets
– inclined side panels
– lightweight pin support design
– thin walls 2.5 mm
– optimized skirt area and flexibility
– F-M S2N high performance alloy

Features and benefits include:
– 15% weight reduction compared with the previous generation gasoline pistons
– allows specific power up to 100 kW/L
– optimised noise and friction performance
Compatible with alfin ring carrier option for increased peak cylinder pressure and robustness against knocking

Credit: Tenneco Powertrain (Federal Mogul)

Frequently asked questions about pistons

What are pistons used for?

Pistons are used in internal combustion engines to transmit force to the connecting rod and crankshaft thus generating engine torque. Pistons convert the gas pressure from the combustion chamber into mechanical force.

What is piston and how it works?

A piston is a component from an internal combustion engine, made from aluminium or steel, used to convert the gas pressure from the combustion chamber into mechanical force, transmitted to the connecting rod and crankshaft.

What is a piston made of?

A piston can be made of non-ferrous material, aluminium (Al), or ferrous material, like cast iron or steel.

What are the two types of piston rings?

The two types of piston rings are: compression rings and oil-control rings.

What are the two major types of pistons engines?

The two major types of piston engines are: diesel engine pistons and gasoline (petrol) engine pistons. Function of the material, the two major types of piston are: aluminium piston and steel pistons.

How long should pistons last?

A piston should last for the entire lifetime of the vehicle if the operating conditions are nominal (normal lubrication, regular maintenance of the engine, no excessive load, no excessive temperature). In normal operating conditions a piston should last at least 300000 km going up to 500000 km or more.

What causes holes in pistons?

Usually abnormal high temperatures causes pistons to melt or engine knocking can cause pistons to crack. Faulty injectors can supply excessive fuel into the cylinders which can cause abnormally high combustion temperatures and partially melt the pistons.

How do you know if pistons are damaged?

If a piston is damaged the most likely symptoms are: loss of power due to loss of compression, excessive smoke in the exhaust or abnormal engine noise.

Can you fix a broken piston?

A broken piston can not be fixed, it must be replaced. Piston have very tight geometrical tolerances which most probably can not be met after repair. Also, their mechanical and thermal properties will be altered after repair which will cause further damage. A broke piston can cause significant damage to the engine block, connecting rod, valves, etc. and must immediately be replaced.

Can you drive a car with a bad piston?

You can drive with a bad piston but is not advised. Piston damage can lead to significant failure to engine block, crankshaft, connecting rods, valves, etc. If a damaged piston is not replaced it can lead to total engine failure.

Will piston slap damage my engine?

Piston slap will damage the engine is left unattended. Piston slap for a prolonged time will damage the cylinder lining and the piston itself.

Does piston slap go away when warm?

Piston slap will go partially away when the engine is warm. Piston slap is cause by excess wear of the cylinder liner or the piston itself. When the engine gets warmer the piston has thermal expansion and the clearance between the piston and cylinder will decrease leading to diminished piston slap.

Can I drive with piston slap?

You can drive with piston slap, but is not advised to drive for a long time. Piston slap will cause wear of the piston itself and the cylinder liner. Piston slap can also cause cracks in the piston which can lead to total engine failure if left unattended.

What causes piston skirt wear?

Piston skirt wear is caused by the lack of oil lubrication of the cylinder liner. In normal working condition, the lubrication system splashes oil onto the cylinders in order to avoid direct contact between the piston skirt and the cylinder. If there is a fault in the lubrication system or if the oil level is not sufficient, there will not be enough oil on the cylinder walls and the piston skirt will wear significantly.

References

[1] Klaus Mollenhauer, Helmut Tschoeke, Handbook of Diesel Engines, Springer, 2010.
[2] Hiroshi Yamagata, The science and technology of materials in automotive engines, Woodhead Publishing in Materials, Cambridge, England, 2005.
[3] The Aluminium Automotive Manual, European Aluminium Association, 2011.
[4] Heisler, Heinz, Vehicle and Engine Technology, Society of Automotive Engineers, 1999.
[5] QinZhaoju et al, Diesel engine piston thermo-mechanical coupling simulation and multidisciplinary design optimization, Case Studies in Thermal Engineering, Volume 15, November 2019.
[6] Piston and engine testing, Mahle GmbH, Stuttgart, 2012.
[7] Scott Kenningley and Roman Morgenstern, Thermal and Mechanical Loading in the Combustion Bowl Region of Light Vehicle Diesel AlSiCuNiMg Pistons; Reviewed with Emphasis on Advanced Finite Element Analysis and Instrumented Engine Testing Techniques, Federal Mogul Corporation, SAE Paper 2012-01-1330.
[8] T.K. Garrett et al, The Motor Vehicle, 13th Edition, Butterworth-Heinemann, 2001.
[9] N.Dolatabadi et al, On the identification of piston slap events in internal combustion engines using tribodynamic analysis, Mechanical Systems and Signal Processing, Volumes 58–59, June 2015, Pages 308-324, Elsevier, 2014.
[10] Klaus Mollenhauer and Helmut Tschoeke, Handbook of Diesel Engines, Springer-Verlag Berlin Heidelberg, 2010.

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