Why can't we use a single big cylinder instead of multiple cylinders in an Engine?

          Pic Credits: Smart Mechanicals
                          

Some of the probable Reasons to avoid single cylinder..

1. Unbalance and vibrations are produced due to high inertia caused when a larger mass is reciprocating at high speeds.

2. The power obtained is intermittent, since there is only one power cycle outof four strokes in the entire engine. In case of Mutli cylinder engine the power produced is continuous since there is atleast one power cycle in one cylinder out of multi cylinder.

3. A heavy flywheel is required to store the energy and  supply it to next 3 cycles, since power is produced in only single cycle.

4. Single cylinder occupy more space to produce same power of that multi cylinders.

5.More starting torque is required to start the engine.

6. Engine Speed: lesser the reciprocating mass - the engine can operate at higher speeds

7. Volumetric efficiency - The amount of air going into a cylinder determines the amount of power that an engine can provide. Turbocharging is what gives engine now the better performance and fuel efficiency. Waste heat is used and the engines can be downsized (reducing friction) with the same power levels. 

Turbocharging a single cylinder engine is extremely difficult as the regular centrifugal turbocharger needs a uniform flow of exhaust gases which is feasible in a multicylinder. Pressure pulses in exhaust will not work in compressing air smoothly and hence you need to only supercharge a single cylinder engine to achieve similar bmep as a multicylinder. 

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