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Advantages of Super Long Stroke Engines

Super long stroke means increased swept volume allowing more mass of air to be drawn into the cylinder which in turn allows more fuel to be burnt, hence increasing power output. Super long stroke increases piston travel time and thereby decreases crank speed so more time for combustion during the stroke. The efficiency of Large dia propellers is best at slower RPM.With super long stroke we can have slower RPM of engine with no change in mean piston
speed of the engine.

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

These engines allow for low quality of fuel to be burnt as there is more time available for fuel combustion. With increase in stroke length we have an advantage of burning dirty fuel/low grade fuel at min rev/low speed with good efficiency. This also results in proper utilization of fuel resulting in reduction of NOx generation.
An engine cylinder with longer stroke-to-bore ratio will have a smaller surface area exposed to the combustion chamber gases 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.

Scavenge air of the cylinder is also affected by the stroke-to-bore ratio in a uniflow-scavenging, two-stroke engine. As the stroke-to-bore ratio increases, the distance the fresh air has to travel between the intake ports at one end of the cylinder and the exhaust ports at the other end correspondingly increases. 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. Again, it improves fuel combustion by ensuring more fresh air.

These engines have a better power to weight ratio and since more heat is converted into useful work, an increased thermal efficiency.


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