What's a cubit?

What’s a cubic inch?  (What’s a cubit?).  It is a measure of the size of a piston engine.  Cubic inch displacement is the volume of a cylinder, measured from top dead center to bottom dead center (stroke). Another way of phasing it is displacement of a single cylinder is the cylinder’s stroke times the piston’s area.  And naturally for a multiple cylinder engine, which most of them are, the displacement of the entire engine is the displacement of one cylinder times the number of cylinders. 

   The larger the engine’s displacement, the more power it can produce.  Power comes from burning fuel.  The bigger the displacement the more fuel the engine can burn.  Rule of thumb used to be, that a well designed engine could produce one horsepower per cubic inch of displacement.  Now a days I see plenty of new cars with advertised horsepower considerable larger than one horsepower per cubic inch.  I am not sure I believe those numbers. 

   In recent times the car industry has begun to rate displacement in liters instead of cubic inches.  Handy conversion factor, there are 63 cubic inches to the liter.  A liter is a tad more than a quart.

   Car engines only have to produce full rated horsepower for short periods of time, say the time between when the traffic light goes green until the car reaches the speed limit (or perhaps a little bit more).  Then the throttle is eased back to probably a quarter or less of full rated power.  It doesn’t take much power to keep a car rolling at a steady speed.  If a car engine were operated at full rated power continuously it would break down after a few minutes.  Engines for aircraft and boats need to produce substantial power to keep the aircraft or boat moving.  Air or water drag is far greater than road drag in a car.