Requirements and types of SI combustion chambers

Requirements

• High power output and thermal efficiency without knock (a high compression ratio possible).
• Compact shape with a small surface-to-volume ratio, so heat loss is small.
• Short flame travel: spark plug centrally placed or near the exhaust valve.
• High turbulence (squish) for fast, complete burning, but not so much that heat loss rises.
• Smooth pressure rise and low emissions.
• Cool exhaust valve and last-burning end gas near a cold surface.
• Good volumetric efficiency: large valves and smooth gas passages.

Types

1. T-head

 [ IV ]  [ EV ]
   \__piston__/

Valves on opposite sides, needing two camshafts; long flame travel, high knock tendency. Obsolete.

2. L-head (side-valve)

   plug
 ___|___
|  head | 
|  [V][V]  valves at the side
|__piston_|

Simple, one camshaft, easy to build and good turbulence from the squish area. But the chamber is not compact, flame travel is long and the compression ratio is low (about 6.5).

3. I-head (overhead valve)

   IV  plug  EV
    \   |   /
   ___\_|_/___
  |  piston   |

Valves in the head above the piston. Compact chamber, short flame path, high compression ratio (8-10) and large valve area. Pushrod or overhead cam drives are more complex. Most modern engines use it.

4. Hemispherical and pent-roof chamber (modern) with the plug in the centre, four valves per cylinder and central injection give the shortest flame travel and high efficiency.