The relationship between octane rating engines compression ratio and octane number is an important aspect of ASTM D2699 & ASTM D2700.
The octane rating engines are equipped with a variable compression cylinder assembly. Compression ratio (C.R.) is the ratio of the volume of the combustion chamber, with the piston at bottom dead center (BDC) to the volume with the piston at top dead center (TDC). The octane rating engines C.R. is variable from 4:1 to 18:1 by changing the relative position in the clamping sleeve.
Octane rating engines are equipped with a C.R. changer motor. Turn the worm shaft, the worm wheel is rotated within the clamping sleeve and internal threads to screw the cylinder either up or down. Thus changes the volume of the combustion chamber. Turning the worm shaft in a counterclockwise direction lowers the cylinder in the clamping sleeve and increases C.R. Turning the worm shaft clockwise raises the cylinder and decreases C.R. Rotation of the worm shaft in the clamping sleeve is accomplished either by an electric C.R. changer motor.
It is necessary to establish a uniform measure of the C.R. or the cylinder height position. The dial indicator has remained as one method of making the measurement. Indexing to a prescribed reading is now accomplished by adjusting the cylinder height to produce a specific compression pressure, when the octane rating engine is motored.
Using digital counter to indicate the tenths turns of worm shaft was used as an alternative cylinder height measuring method. When the C.R. changer motor came into popular use. The digital counter is also set to a prescribed reading, by adjusting the cylinder height to produce a specific compression pressure, when the octane rating engine is motored.
In ASTM D2699 & D2700, guide tables of both digital counter reading and dial indicator reading versus octane number are incorporated into each of the octane test methods.