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ENCYCLOPAEDIC

Refinement of Engine In-Cycle Losses of Parasitic and Errant Dynamic Nature

Home  Research Programme  Surface Tribology  Top Compression Ring  Cylinder Liner  Piston Skirt  Experiments   
Research Programme
 
Research in the Encyclopaedic Project combines predictive models and experimental tests. The analytical research is focused principally on understanding the tribological and mechanical factors that affect the engine performance through increased friction, mainly the functionality of cylinder liners, piston ring-pack and piston skirt.
 

Principles of Tribology

 

  1. Friction occurs as the result of interaction of surfaces opposing their relative or impending motion (Amontons, 1699, De Coulomb, 1785)
  2. Relative motion of surfaces is opposed by the interaction of their asperities which are subject to shear (boundary friction), therefore typical asperity geometry and shear strenght of surfaces determine the level of friction.
  3. Clean and prepared solid surfaces have a high shear strength, therefore present high friction, however, most surfaces have a lower shear strength surface oxided film, thus reduced friction.
  4. The principle of lubrication is to provide a layer of low shear strength material between the loaded slidding surfaces, reducing the resistance to motion (i.e. friction)
  5. Fluids' resistance to motion is far less than the solids (viscous friction). This property of fluids is term viscosity.
  6. The lower the viscosity, the lower the friction. Thus air, gases and vapours are ideal for lubrication.
  7. Low viscositry lubricants have low load carrying capacity, hence increased load reduces the layer thickness (film) of air and gasses and thus results in direct contact of surface.  
  8. Fluids have to be supplied to surfaces and entrained into the contact by their relative motion. Therefore, the inlet geometry and speed of relative motion are critical factors for providing sufficient volume of fluid ahead of a contact, so that higher lubricant films are formed at higher relative velocity of surfaces.
     

The Stribeck figure describes the importance of the service parameter in relation to friction.

 

 

The regime of lubrication of a compression ring during a 4 stroke cycle is a mix of boundary and viscous shear. This depends on the service parameter, conjunctional geometry and surface topography. Piston conjunctions (ring-pack and skirt) are subject to continual changes in load, temperature and sliding velocity, making the task of mitigating friction quite arduous.
 
 
At top and bottom dead centre positions of the compression ring, the regime of lubrication is usually boundary or mixed due to low sliding speed. Friction is mainly due to asperity interactions, but introducing surface texturing can encourage resevoirs of lubricant and film formation by micro-wedge effect. Through the rest of the cycle, with sufficient speed and load, the regime of lubrication is hydrodynamic or iso-viscous elastic. Friction is mainly due to viscous shear of the lubricant film therefore reduction of lubricant viscosity can decrease friction.

 

Click on the above picture to find more about Conjunctional Geometry
 
 In-cylinder lubrication depends on the piston stroke position. The dominant lubrication regime at reversal points is boundary or mixed lubrication regime and at mid stroke is hydrodynamic. Boundary lubrication by its very nature can lead to friction and wear, while hydrodynamic lubrication is very effective in limiting these problems especially if the lubricant film is at least three times thicker than the root mean square roughness of the counterfaces.

To find related publications go to Loughborough University Repository

 

The aim of the project is to develop combined analytical and experimental techniques to identify, predict and quantify sources of friction and mechanical losses in the piston/ring-pack-connecting rod-crank subsystem.  In order to achieve this objective the following steps are necessary:

1. Refine understanding of transient nature of tribology of piston/ring-pack to cylinder liner/bore contact.

2. Find out what factors contribute to frictional/mechanical losses under transient conditions
3. Develop a parametric analysis tool to predict tribodynamic performance in pistons
4. Develop testing facilities for performance evaluation of engines
5. Establish testing procedures to evaluate friction/dynamics/emissions
6. Design and develop proven advanced prototype cylinder liners and associated products
7. Create a knowledge base of project findings
8. Establish links between engine load, speed, surface quality, lubricant and friction
9. Add structural and thermal integrity, inertial dynamics and mechanical losses to the above.
10. Find parameters that minimise parasitic and errant dynamic losses and check for their robustness, cost, efficiency, and environmental suitability