3. COMBUSTION CHAMBER
A combustion chamber is the part of an engine in which
fuel is burned.
The combustion chamber is the area inside the engine
where the fuel/air mixture is compressed and then
ignited.
It is generally formed on one side by the shape cast into
the cylinder head,
and on the other side by the top of the piston.
4. The combustion chamber is recessed in the cylinder head
and commonly contains a single intake valve and a single
exhaust valve.
When the piston is at top-dead-center the chamber is at its
smallest dimension, and this is the time
when the fuel/air mixture is at its most unstable condition
and ready to be ignited.
The better the combustion chamber is designed the better
the engine "breathes;"
that is, the more efficient the overall flow of air through
the engine
5. The hot gases produced by the combustion occupy a far
greater volume than the original fuel,
Thus creating an increase in pressure within the limited
volume of the chamber.
This pressure can be used to do work,
For example, to move a piston on a crankshaft or a
turbine disc in a gas turbine.
The energy can also be used to produce thrust when
directed out of a nozzle as in a rocket engine.
6. The shape of the chamber has a marked effect on power
output, efficiency and harmful emissions
The intake valve/port is usually placed to give the
mixture a pronounced "swirl" (the term is preferred to
turbulence which implies movement without overall
pattern) above the rising piston,
improving mixing and combustion.
The shape of the piston top also affects the amount of
swirl.
7. Finally, the spark plug must be situated in a position
from which the flame front can reach all parts of the
chamber at the desired point,
usually around 15 degrees after top dead centre
Various shapes of combustion chamber have been used,
8. Combustion chamber types.
Hemispherical or Pentroof Combustion
Chamber
Wedge-Shaped Chambers
Bathtub or Heart-shaped Combustion Chamber
11. Hemispherical or Pentroof Combustion
Chamber
Hemispherical combustion chamber is one half of
a sphere cast into the bottom of the cylinder head.
The valves are placed at the outside of the bore
area,
and at a specific angle from the crankshaft
centerline.
Hemispherical chambers generally have a central
spark plug, which offers excellent octane tolerance.
12. This creates a more efficient cross-flow movement of the
charge during valve overlap and limits thermal transfer
from the exhaust valve to the fresh charge.
This design offers the best surface-to-volume ratio and
also creates a very short direct exhaust port, essential in
limiting heat rejection into the coolant.
An additional benefit is the distance between the intake
and exhaust valves, which further limits heat transfer.
15. Wedge-Shaped Chamber
This type of chamber resembles an inclined basin
recessed into the deck of the head.
The spark plug is located on the thick side of the wedge
and is usually positioned midway between the valves.
The relatively steep walls in such chamber design force
the air/fuel flow path and deflect .
and force it to move in a downward spiral around the
cylinder axis.
16. During the compression stroke, the compressed air/fuel
area reduces
To such an extent that the trapped mixture is violently
thrust from the thin to the thick end of the chamber.
This builds up significant kinetic energy, which when
ignited contributes to overall power.
19. Bathtub combustion chamber
This is formed in the cylinder head and has two vertical
valves and a side- mounted spark plug.
Mixture in the squish’ area between the piston and the
flat face of the head provides some turbulence within the
mixture,
Air is pushed into combustion space provided air
movement known as squish.
which helps it burn smoothly.
20. Some bathtub chambers have a shaped
projection between the valves,
giving the chamber a heart-shaped plan view.
This swirls the incoming mixture, helping to
atomize it further.