Combustion chamber and piston design - pics

Combustion chamber and piston design – Motorcycle studies 2nd year - 2010

Combustion chamber and piston design

Today’s internal combustion engines are required to be both powerful enough in relation to the machine they are used for, and to withstand very strict pollution regulations.

Two of the major aspects that affect the above requirements are the combustion chamber and piston design.

When a combustion chamber is designed few factors are taking into consideration:

1. High volumetric efficiency (valve head size and lift, valve port shape etc.) – in order to make the most out of the volume of the cylinder (i.e. to get as much mixture as possible for that volume)

2. Maximum thermal efficiency (highest compression rate without having knocks)

3. Minimum heat loss.4. Minimum flame travel with maximum frontal area – in order to

create a fast and efficient burning of the mixture that will prevent unburned fuel to be exhausted.

5. Generated and controlled turbulence to match engine speed.6. Compression squish (force movement of the mixture), that cause

by the piston moving to TDC in the compression stroke - to speed up the mixing of the mixture that is in the chamber.

7. End gas quench area – in order to overcool small portion of mixture, which in turn help to prevent detonation at the end of the combustion.

8. Good scavenging of exhaust gases. 9. Adequate cooling for the spark plug and exhaust valves in order to

prevent hot spots in the combustion chamber that might lead to detonation.

As you can see, the general purpose of the every design, is to make sure that the engine will run at while optimum levels of performance and combustion at all times, meaning that all the mixture will be almost completely burnt while preventing any knockings in the engine.

To achieve the above, there is a need to control the movement of the mixture in the chamber, during its induction and compression.

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This is done in order to first fill the combustion chamber fully and evenly with the fuel/air mixture, and secondly to make sure that all the mixture is burned at all engine speeds.

When the mixture is induced to the chamber, the shape and positioning of the valves are creating a swirl, which is insuring that the mixture is evenly filling the whole chamber.As the mixture fills the chamber, turbulence is also caused in the stream.

Turbulences, which are random dispersed vortices of different size that are accruing in the mixture, are most important, especially in high revving engines, as without them, the combustion will not be able to be fast enough when the engine speed rises.

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All the above will lead to a very efficient combustion process, which also means better performance while still keeping low levels of pollution.

After an optimal design was achieved (in relation to what is expect from the engine), all this power needs to be transferred somehow. In order to achieve this the piston is has to be designed while taking few factors into consideration:

1. To be strong to withstand the explosion forces acting on it.2. To be light so it will be able to work at very high speeds without

loading the crank bearings with every change of direction during strokes.

3. To act as a bearing surface to the side movement that is created by the connecting rod.

4. To conduct heat well - a too hot piston can cause knocking or/and even engine failure.

5. To help the combustion process, by design the crown in such way, that it participates, with the combustion chamber, in mixture distributing.

This is done by using Y alloy (4% copper and 2.5% nickel) which have the best characteristics when it comes to all the above.

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More features which are affecting the piston design:

1. The piston skirt – designed to act as a bearing surface.2. Piston ring belts – groves that are machined into the piston, and

are designed to carry the piston rings. This is has to be done very accurately in order to prevent lost of compression and pass of burned gases to the crank case, and from oil passing into the combustion chamber.

3. Piston webs – casts between the piston crown and the piston pin that are acting as struts that allow the great forces to be transferred to the connecting rod.

Both combustion chamber and piston are designed to give the best performance and fuel consumption, which are in direct relation with the pollution levels an engine is producing.

Today, the new designs are going towards engines that work with leaner mixtures that are improving performance and pollution even more. This is requiring even more complex designs, as the 4-stroke engine of today is pushed to its limits.

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ReferencesHeisler, H. (1995). Advanced Engine Technology. London, UK: Edward Arnold.Heisler, H. (1999). Vehicle and engine technology (2nd Edition ed.). London, UK: Butterworth-Heinemann.Hillier, V. A., & Coombes, P. (2004). Hillier's Fundamentals of Motor Vehicle Technology (Vol. I). Cheltenham, UK: Nelson Thorens Ltd.

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Combustion chamber and piston design - pics