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Common Disadvantages


Conventional engine designs contain several disadvantages that designers have endeavored to overcome since the beginning of internal combustion (IC) engines.  Some of these are friction, low mechanical efficiency, and less than ideal volumetric function.

Volumetrically, cylinder charge has always fallen short of desirable.  As a piston travels down in the cylinder chamber it draws in air due to the expanding volume creating a negative pressure (less than atmospheric).  The disadvantage occurs when the piston reaches BDC (bottom dead center) and begins moving back up the cylinder chamber before the pressure equalizes.  To combat this short fill, the intake valve is left open while the piston travels back up in the cylinder; thus, producing a greater charge due to the momentum of the moving air mass.

Additionally, common valve timing opens the intake valve before the exhaust stroke is completed.  This creates a condition where both intake and exhaust valves are open at the same time.  During this time the piston forces exhaust gases into the intake manifold creating a reversal of air flow.  This reversal must be overcome during the intake stroke creating additional losses of efficiency.

Mechanically, all conventional IC engines transmit energy through compound angles created by the piston, connecting rod, and crankshaft torque arm.  Each angle causes a change in the force vector as energy moves from the piston to the crank.  Every change in direction splits the energy producing losses in the form of friction, heat, and imbalanced momentum.


Four Stroke vs. Two Stroke


Generally, four-stroke engines are more volumetrically efficient than two-stroke engines but have greater friction losses due to the greater number of degrees between power strokes.



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