Side Port IC Engine Design
Posted: 08 Apr 2022, 18:09
THIS IS A SIMILAR POST OF MINE ASKING THIS QUESTION ON ANOTHER FORUM HOPING FOR AN EXPLAINATION.
A question to those who are knowledgeable on design aspects of side port IC model engines, like the venerable Mills 1.3 etc. It's new not SH or a bitsa. Currently on sale. Modern version based on a 70 year old design. To the best of my knowledge and experience(I have numerous examples of each type of engine: Irvine Mills 1.3 & 0.75, Indian Mills 1.3 & 0.75, MP Jet 040 & 1.8, CS Deezil, ED Comp special, etc) the induction is controlled by the piston skirt uncovering the induction inlet hole/s in the liner rear side wall. All my engines bar one, recently acquired (no names no pack drill), do not uncover the liner holes by the piston crown when at BDC. To me this is a serious design/manufacturing flaw as any residual combustion pressure could/would vent into the inlet against the flow of any fresh incoming charge in the inlet tube thus adversely affecting the breathing of the engine.
There are two other failings, to my mind, with this engine, one fixable the other like the above not really fixable. 1/ the front piston skirt obstructs the gas flow into the transfer ports from the crankcase as the piston nears BDC; fixed by cutting away part of the front piston skirt. 2/ the piston crown goes way lower than the bottom edge of the exhaust ports leaving a significant volume that will not be scavenged of exhaust gas; only fix I can see would be is by lowering the cylinder in the crankcase casting, this would also address the inlet port issue.
These issues have been determined by visual examination of the engine in question and comparing it with others I have that employ side port induction. With these other engines the piston crown goes no lower than the bottom edge of the exhaust port; the liner inlet holes are only uncovered by the piston(skirt) on its way up to TDC and then blocked on it's way down to BDC, the holes are never uncovered when the piston is at BDC allowing partial opening at the top edge of the liner inlet holes because the piston crown is too low; some of the other engines have cut away front piston skirt as and when needed by the design.
It's almost as if the wrong liner for the stroke of the engine has been used.
Any comments from those who are knowledgeable on engine design welcomed.
A question to those who are knowledgeable on design aspects of side port IC model engines, like the venerable Mills 1.3 etc. It's new not SH or a bitsa. Currently on sale. Modern version based on a 70 year old design. To the best of my knowledge and experience(I have numerous examples of each type of engine: Irvine Mills 1.3 & 0.75, Indian Mills 1.3 & 0.75, MP Jet 040 & 1.8, CS Deezil, ED Comp special, etc) the induction is controlled by the piston skirt uncovering the induction inlet hole/s in the liner rear side wall. All my engines bar one, recently acquired (no names no pack drill), do not uncover the liner holes by the piston crown when at BDC. To me this is a serious design/manufacturing flaw as any residual combustion pressure could/would vent into the inlet against the flow of any fresh incoming charge in the inlet tube thus adversely affecting the breathing of the engine.
There are two other failings, to my mind, with this engine, one fixable the other like the above not really fixable. 1/ the front piston skirt obstructs the gas flow into the transfer ports from the crankcase as the piston nears BDC; fixed by cutting away part of the front piston skirt. 2/ the piston crown goes way lower than the bottom edge of the exhaust ports leaving a significant volume that will not be scavenged of exhaust gas; only fix I can see would be is by lowering the cylinder in the crankcase casting, this would also address the inlet port issue.
These issues have been determined by visual examination of the engine in question and comparing it with others I have that employ side port induction. With these other engines the piston crown goes no lower than the bottom edge of the exhaust port; the liner inlet holes are only uncovered by the piston(skirt) on its way up to TDC and then blocked on it's way down to BDC, the holes are never uncovered when the piston is at BDC allowing partial opening at the top edge of the liner inlet holes because the piston crown is too low; some of the other engines have cut away front piston skirt as and when needed by the design.
It's almost as if the wrong liner for the stroke of the engine has been used.
Any comments from those who are knowledgeable on engine design welcomed.