It is generally thought that the Standard Motor Company and Ricardo Engineering limited the success of the Standard 23C engine due to the design and dimensions of the entrance to the pre-combustion chambers. This single design element is often blamed for the poor starting performance of these engines.
Many engineers recommend the enlargement of this port, so the purpose of this article is to examine why this should improve the running characteristics of the engine.
Where exactly are we talking about?
We are talking about the hole on the cylinder head which faces the top of the piston (the entrance to the pre-combustion chamber from the cylinder). It is the entrance that is ground out to be enlarged (rather than the chamber itself). Bear in mind where the head gasket goes (ie. don’t grind out any further than where the gasket sits).
Why does this help?
Enlarging this entrance allows the air out of the cylinder (been compressed by the piston) to enter the precombustion chamber more quickly, which improves the cranking speed (as it isn’t trying to push the air through such a small hole). This also allows the pressure in the pre-combustion chamber to build faster and higher than it normally would do. Obviously the diesel engine works by compression ignition, so this increase in pressure makes ignition more likely and hence should improve starting performance. When the ignition occurs there is then a more free passage of the expanding gasses down into the cylinder (as we have now enlarged the entrance/exit to the pre-combustion chaber).
The ongoing development of diesel engines and preference towards direct injection tells us that anything which limits the flow of expanding gasses onto the piston reduces efficiency.
The differences in efficiency will only be small for each cycle, but remember that a 4 stroke engine turning at 2000rpm competes 1000 cycles per minute, so any small ineficiencies are magnified 1000 times and therefore become significant.