Design considerations when creating/choosing a piston design. Written by a mechanical engineer.
The two main requirements of the piston are as follows:
- Contain all the fluids above and below the piston assembly during the cycle.
- Transfer the work done in combustion to the connecting rod with minimal mechanical and thermodynamic losses.
They key thing to note about piston design for absolutely any application regardless of the field is that it is HUGELY dependent upon the task that the engine will be required to fulfill. The pistons for a 1000 horse-power racing car will be designed differently and made from different materials than pistons that are used inside a tractor for example. The shape of the piston crown (see diagram above) can also vary dependent on the type of engine you are dealing with as they can be used to change flow characteristics within the cylinder. The crown of a piston in a diesel engine is likely to look different to the piston of a petrol engine, especially in high performance applications. Despite this all pistons need to obey the following;
Five Key Properties of a Piston:
- Sufficient thermal conductivity
- Low thermal expansion
- High hot strength
- High strength to weight ratio
- High resistance to surface abrasion
A few definitions for the terms that are mentioned above:
Thermal conductivity: The ability of a material to absorb heat without causing damage or significant change to the materials microstructure or properties.Thermal expansion: How much the material will expand when heated.Hot strength: Ability to withstand stress, strain and shear at temperatures higher than room temperature. (Piston temperatures will be covered later in this article).Strength-to-weight ratio: Fairly obvious this one, ideally we want a very light material with very high strength, the lighter and stronger the better.Surface abrasion: The rate at which material wears due to rubbing on the surfaces of it, surface treatments can be added to metals to increase the surface toughness and make them more resistant to this.
The piston is obviously one of THE key components in any engine; it provides the seal, which enables power to transfer to the crankshaft so an effective design is key.
Heat is one of the biggest problems faced when designing a piston, an example of typical road car piston temperatures and their distribution is shown below:
|Fig 2: Typical temperature map for a piston|
So n the crown of the piston the temperatures tend towards around 250 – 300 degrees Celsius and gradually decrease the further from the combustion you go. The second key property I mentioned earlier was that pistons should have a low thermal expansion; this is because at these temperatures pistons will expand, especially on the crown and the top of the skirt. Because of this, pistons are actually tapered, although it is not obvious with the naked eye pistons are wider at the bottom of the skirt than the top to allow for expansion at the crown. If this was not done then when the piston expanded then it could potentially become too wide to fit within the cylinder.
Piston temperatures also vary with speed, which is key to note for higher performance engines, since at 1500 rpm a piston crown can be around 100 degrees Celsius cooler than when the engine is providing 5000 rpm. Piston spray jets may be added to engine in these high performance engines in order to lower the temperature of the crown. These feed of the main oil gallery and usually provide between one half and one litre per minute.
Piston Materials: PROS CONS
Cast Iron Hot strength Mass Hardness Thermal conductivity Thermal expansion
Aluminium Alloys Mass Thermal expansion Strength-to-weight Hot strength Thermal conductivity Hardness
Carbon Fibre Reinforced Mass CostCarbon (CFRC) - Strength-to-weight Operating issues(Research use only) Hot strength Thermal expansion
The operating issues mentioned with the CFRC pistons include hydrocarbon emissions, since the material used for the piston has been observed to absorb fuel particles and then release them in the exhaust causing unwanted pollutants, this is also a knock problem with this technology. However it must be pointed out these pistons are purely experimental at the moment and engineers are experimenting with materials in an attempt to come up with better compounds.
To demonstrate the effects of manufacturing effects on the performance of a piston we will focus on only aluminium pistons, however similar effects follow across most metals.
For spark injection engines, cast aluminium can provide an intricate part at relatively low cost and low weight. Forged aluminium however can provide a finer microstructure and therefore higher strength when compared to casting, albeit more expensive.
Due to the additional pressures occurring in a diesel engine (since the fuel is self igniting) the pistons require local reinforcements. For example, a cast aluminium piston for a diesel engine would be made from a higher temperature alloy than for a petrol engine, and the following reinforcements may be applied:
- Refined casting methods; finer microstructure around the bowl (crown).
- Insertion of high strength cast iron piston ring carriers.
- Insertion of bushes into the pin bores.
That is the end of the main piston design article, further articles going in depth into the design of piston rings, connecting rods and loads of other major components are available in the ‘101 guides’ and ‘vehicle design’ categories found in the right hand side bar. Thank you for reading and please feel free to comment any questions about this subject or any other topic you’d like to see me write a guide to.
By Adam Feneley,
Brunel UniversityMEng Motorsport Engineering (Level 2)Affiliate of the Institute of Mechanical Engineers