Automotive Fuel Efficiency
A simplified analysis indicates that automotive fuel economy is improved by
the choice of engine cylinder dimensions. Better fuel economy can be obtained
if the dimension of the piston stroke is greater than the diameter of the cylinder
bore.
With four-stroke, naturally aspirated, internal combustion engines, power is
proportional to piston area and piston speed. Power is work per unit time. Work
is force (pressure times piston area) times distance (piston stroke). Pressure
(in the cylinder) is determined by volumetric and combustion efficiencies. Mechanical
efficiency (friction losses) affects the effectiveness of the cylinder pressure.
A longer piston stroke improves all three efficiencies.
Air flow into the cylinder is determined by piston speed. A longer stroke gives
the same piston speed at a lower engine speed. A lower engine speed reduces
rotational friction. Combustion efficiency is determined by compression ratio
and chamber shape. For a given compression ratio, a longer stroke can yield
a better chamber shape.
Consequently, when purchasing a vehicle, odds are better fuel economy will be
obtained at the same performance (same total piston area), if the dimension
of the piston stroke is longer than the diameter of the piston bore.
This simplified analysis is consistent with a more complex analysis and experience.