Advantages of super long stroke are:
- Low quality fuel can be burnt as more time is available for fuel combustion.
- Slow rpm is possible so large diameter propeller can be used and hence propeller efficiency increases.
- More time for heat dissipation to cooling water.
- Better scavenging: Scavenging is the process in which the exhaust products in the cylinder are replaced by fresh air. As the stroke-to-bore ratio increases, so fresh air has to travel larger distance between the intake ports at one end of the cylinder and the exhaust ports at the other end. This increased distance results in higher scavenging efficiency and, as a result, lower pumping work because less fresh air is lost via charge short circuiting.
- Higher compression ratio can be achieved and hence reduced exhaust temperature
- reduced fouling, smoke, NOx
- Increased thermal efficiency: Simple geometric relationships show that an engine cylinder with longer stroke-to-bore ratio will have a smaller surface area exposed to the combustion chamber gasses compared to a cylinder with shorter stroke-to-bore ratio. The smaller area leads directly to reduced in-cylinder heat transfer, increased energy transfer to the crankshaft and, therefore, higher efficiency.
- Power to weight ratio is more.
Classification of engine based on stroke/bore ratio
Short stroke : 2.6-3.2
Long stroke : 3.2-4.0
Super-Long stroke : 4.0-4.7
Ultra-Long stroke : >4.7