LohringMiller
Well-Known Member
- Joined
- Jan 25, 2017
- Messages
- 938
The secret, especially in larger engines, is scavenging and breathing. The secret at the model level is friction reduction, especially in the piston and liner fit. As you scale down an engine the power producing displacement decreases as the cube of the scale. Surface areas decrease as the square of the scale, a much slower reduction. That scaling helps a lot with port and heat dissipating areas, but not with friction. Friction losses depend to some degree on the surface areas of the rubbing parts, a larger fraction of the displacement in model engines.
Larger two strokes' rpm is usually limited by breathing, but model engine rpm is often limited by inertia and the rod big end bearing. Scavenging plays a part, but isn't as critical. The best scavenging comparison I have is the difference between the M&D modified Zenoah cylinder and their cast cylinder. Both are nearly identical except in the transfer passages. The cast cylinder doesn't have an ideal shape, but does have an inner wall. Quickdraws have even better scavenging but may be limited by their piston port intake. Tuned pipes have the biggest influence on scavenging compared to all these other factors. For that reason, I bet relatively poor transfer shapes can be overcome with carefully fitted ABC or AAC piston and liner construction and the right pipe for the application.
Lohring Miller
Larger two strokes' rpm is usually limited by breathing, but model engine rpm is often limited by inertia and the rod big end bearing. Scavenging plays a part, but isn't as critical. The best scavenging comparison I have is the difference between the M&D modified Zenoah cylinder and their cast cylinder. Both are nearly identical except in the transfer passages. The cast cylinder doesn't have an ideal shape, but does have an inner wall. Quickdraws have even better scavenging but may be limited by their piston port intake. Tuned pipes have the biggest influence on scavenging compared to all these other factors. For that reason, I bet relatively poor transfer shapes can be overcome with carefully fitted ABC or AAC piston and liner construction and the right pipe for the application.
Lohring Miller