The Atkinson Cycle alters the length of and/or the characteristics of the intake, compression and power strokes in the
internal combustion engine to improve fuel efficiency.
What it does is reduce the amount of charge (air/fuel mixture) entering on the intake stroke but allows the power stroke its
normal range of downward movement.
This means the piston still travels its maximum range down the cylinder on the power stroke but uses less fuel and compression
to do so.
The Atkinson cycle or Atkinson engine was invented by British engineer James Atkinson in 1882 and patented by him in several
countries in the following years.
The original design of an Atkinson engine used a uniquely designed crankshaft and additional linkages between the crankshaft and
the piston connecting rod.
This allowed the the engine to complete the four strokes in one rotation of the crankshaft and allowed the intake/compression
strokes to be shorter than the power/exhaust strokes.
The result is a more fuel efficient engine but also one that's down on horsepower compared to a
comparable regular otto cycle engine especially at low revs/speeds.
The original Atkinson engine was also bulky and complicated for its size/power output because of the additional links and is not
used in cars today.
When the term Atkinson cycle is used in a modern day vehicle its usually a somewhat conventional engine that uses an altered
version of variable valve timing to achieve the effect of reduced intake/compression strokes.
The valve timing keeps the intake valve open during part of the upward compression stroke and that forces some of the intake charge
back into the intake manifold before the valve closes and this essentially has the effect of shorter intake/compression strokes.
The valve timing takes care of the bulky/complicated issue but the power disadvantage is still present so its impractical to use
an Atkinson cycle engine in the average car.
They seem to well suited to the modern day electric-hybrid where the assistance of the motor
makes up for the power disadvantage while keeping the vehicle just as fuel efficient.
A wide range of hybrids from four cylinder economy engines to large displacement engines.
From the Ford C-Max and Yaris hybrid to hybrid crosssovers from Mercedes Benz and Lexus use Atkinson