How a Rotary Wankel engine differs in operation from a standard spark ignition reciprocating engine (SIRE);
In a Rotary Wankel engine, the 4 strokes of the Otto cycle are executed during a single uni-directional rotation of the rotor as opposed to the bidirectional of pistons. Uni-directionaility results in a smoother operation of the engine due to absence of shakes occurring as pistons change direction.
The wankel engine has a triangualar shape yielding 3 combustion chambers formed between the rotor’s edges and the casing. This means there are 3 power strokes per revolution of the rotor that enable the engine to operate at significantly lower rpms and render just 2 rotors sufficient for operation.
Unlike pistons attached directly to the centrally placed crankshaft, Wankel engine's rotor is connected through eccentric gearing with the crankshaft. So, the drive is not direct, the crankshaft rotates typically three times faster than the rotor. Therefore, if a piston has a power stroke per 2 revolutions of the crankshaft, a rotor will, typically, have a power stroke per 1 crankshaft revolution.
(4 Marks)
Why the Rotary Wankel engine is less fuel efficient than a standard SIRE;
Wankel engines usually have a compression ratio of 9:1 compared to 11:1-26:1 of a conventional engine. The low compression results in poor thermodynamic efficiency.
The combustive efficiency is corrupted by the shape of the housing rendering the combustion chamber thin and elongated. The finite flame spread speed results in
incomplete combustion as the flame fails to spread within the duration of the combustion stroke thus leading to low combustive efficiency.
The low thermodynamic and combustive efficiency lead to a lower indicated thermal efficiency than a standard SIRE and, thus, higher specific fuel consumption.
(4 Marks)
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