The Atkinson cycle was invented by a British scientist, James Atkinson, in the year 1882. Well, it didn’t become a financially viable option for mass production due to its complex mechanical linkages to make it compact and speed-efficient. However, with the evolved technology, R&D has achieved delaying the intake valve timing with the use of hydraulic actuation of the camshaft or with the multi-linkage mechanism. The Atkinson cycle is coming into life again in the wake of much-needed fuel efficiency. 
The Atkinson cycle is nothing but an efficient Otto cycle that is being used on a modern gasoline engine. Wherein the valve timings are being modified to churn out maximum efficiency in compromise with the power output. You will mainly find hybrid cars running on an Atkinson cycle because power from electric motors compensates for the low power output from the engine . The compression ratio is relatively reduced due to the inlet valve being opened up for a slightly longer time.
Well, I won’t dig deep into the linkage mechanism or thermodynamic cycle to make it more complicated. But I will give you a basic and brief explanation of how it works. 1) Intake Stroke- In this stroke, the inlet valve is opened up, drawing an air-fuel mixture into the cylinder wherein the piston moves to the bottom dead center ( BDC ) from the top dead center (TDC). 2) Compression Stroke – The piston moves from BDC to TDC with both valves closed to increase the temperature and pressure of the air-fuel mixture by compressing it. Note- the enthalpy remains constant during this stroke. Well, enthalpy is nothing put the total internal energy and the product of pressure and volume of the system 3) Power Stroke – The compressed air-fuel mixture gets ignited with the use of a spark plug. With both valves closed up, the explosive force pushes the piston down from TDC to BDC, propelling the vehicle with rotation of the crankshaft. 4) Exhaust Stroke- The power stroke sets the momentum, and the piston moves to TDC to push out the burned gases via the exhaust chamber with the inlet valve closed. [caption id="attachment_14744" align="aligncenter" width="800"]
Photo credit-Mech4cars[/caption] The four-stroke engine running on an Otto or Atkinson cycle gets these strokes in common. However, in Atkinson cycle, the compression stroke is shorter, translating into a lesser compression ratio than an Otto cycle. The shorter compression ratio is achieved due to delaying the closing of an inlet valve during the compression stroke. Due to the inlet valve being slightly opened during compression stroke, the work done is comparatively lesser due to reduced piston displacement. The compression stroke is smaller as compared to the expansion stroke. Thus the amount of work done is less, so the fuel burnt is relatively minimal. Moreover, with the intake manifold opened up, the air inside the chamber is also less, thus burning less energy to carry out work done. 
As already mentioned, the downside of this Atkinson cycle is less power. So, this type of engine is mostly used in hybrid cars, where the electric motors give the much-needed juice right from zero rpm to tackle the power crunch. The Atkinson cycle engine combined with electric motors does not just boost the fuel efficiency but also lessens the carbon footprint with fewer emissions than a standard Otto cycle engine.
