How does Atkinson Cycle work? | What is an Atkinson Cycle Engine?

Increasingly, automobile manufacturers are leveraging the Atkinson cycle for engine combustion, particularly in hybrid vehicles. This method of combustion offers significant benefits in terms of fuel efficiency, a feature highly desirable in the hybrid vehicle market. This article explains the Atkinson cycle working, types, and applications. 

What is an Atkinson Cycle?

The Atkinson cycle engine is a type of IC engine. An Atkinson cycle is mainly designed to improve the Otto cycle engine’s efficiency by one particular improvement. In simple words, this cycle is invented to take advantage of the power-generating portion of the cycle while minimizing fuel consumption during the suction stroke.

 In 1882, an engineer James Atkinson invented the Atkinson cycle. 

A modified version of the Atkinson cycle is employed in several contemporary car engines. Previously, the Atkinson cycle was exclusively associated with hybrid electric vehicles like the early models of the Toyota Prius.

However, recent hybrid models and even some non-hybrid vehicles now incorporate engines with adjustable valve timing. This feature allows these engines to operate part-time in the Atkinson cycle, providing excellent fuel efficiency during those periods. Meanwhile, when the engine reverts to the traditional Otto cycle operation, it can deliver standard power density.

Atkinson Cycle Working

An Atkinson cycle works in the same way as an Otto cycle works. However, it is a modified form of the Otto cycle.

Atkinson Cycle Working
PV diagram of Atkinson Cycle

An Atkinson cycle works in the following way:  

  • Adiabatic compression (Compression Stroke): First of all, the piston helps the engine to suck the air-fuel mixture from outside into the cylinder. After that, the piston moves up (from BDC to TDC) to compress the air-fuel mixture. During this upward motion (from 1 to 2) of the piston, it increases the temperature and pressure of the mixture by compressing it. However, during this process, the enthalpy of the air-fuel mixture remains constant while volume decreases (as shown in below given diagram).
  • Isochoric compression (ignition phase): As the piston and compressed air-fuel mixture reaches at point 2, an external heat source provides heat to the compressed mixture. Due to this ignition, the air-fuel mixture’s enthalpy and pressure increase. However, no changes occur in the volume during this phase.  This phase is the same as the isochoric compression phase of the Otto cycle. During this process, the piston moves more and compresses the mixture without changing its volume. 
  • Adiabatic expansion (power stroke) – After the ignition process, the power stroke starts. In this stage, the gas (combusted air-fuel mixture) performs work on the piston and moves it from TDC to BDC. Due to the piston’s downward motion, the gas expands, and the piston’s downward motion rotates the crankshaft, which further rotates the flywheel. In this process, the gas volume increases while pressure reduces. This process is known as Isentropic because there is no change in the enthalpy.  
  • Isobaric exhaust (exhaust stroke): The objective of the latest Atkinson cycle is to permit the pressure inside the cylinder equal to atmospheric pressure at the expansion stroke’s end to be. In this cycle, the decompression doesn’t occur in the cylinder like the Otto cycle. After this exhaust stroke, the piston moves toward TDC from BDC, and the whole cycle repeats. 

For a better understanding, watch the following video:

Read Also: Working of Otto Cycle

Types of Atkinson Cycle Engine

The Atkinson cycle engine has the following major types:

1) Atkinson Differential Engine

Firstly, the Atkinson cycle was implemented in 1882.

Atkinson Differential Engine

In contrast to the advanced version, this cycle was implemented as an opposed piston engine (the Atkinson differential engine). In such a case, a single crankshaft was linked with two opposing pistons via a non-linear connecting rod.

For the half revolution, one piston remains uniform while the other piston approaches it and returns. While, in the next half revolution, the second piston goes in uniform condition, but the first piston approaches it and returns.

Therefore, for each revolution, one piston of the Atkinson differential engine delivered the compression and power strokes while the other piston delivered the exhaust and suction strokes.

Read More: Different Types of Internal Combustion (IC) Engines

2) Atkinson Cycle Engine

In 1887, Atkinson invented another engine that was called as Atkinson cycle engine. It had a cam, poppet valves, and an eccentric arm to generate four strokes of the piston for each turn of the crankshaft. The Atkinson cycle of this engine had a shorter suction stroke and compression stroke than the power stroke and discharge stroke.

Atkinson Cycle Engine

The cycle engines were manufactured and sold by UK engine manufacturers for many years. Atkinson was also given production permission to other companies.

3) Atkinson Utilite Engine

Atkinson designed 3rd engine that was known as the Utilite Engine. The 2nd engine of Atkinson was effective, but it had a linkage that was very hard to balance for quick working. Therefore, Atkinson felt that he should perform modifications to make his cycle more suitable as higher-speed engines.

After improvements in the previous model, Atkinson designed a new model that was “Utilite Engine.” In this new design, he eliminated the connecting rod and created a more balanced, more traditional engine that could run at a speed of approximately 600 rpm and generate power at every revolution. His “Cycle engine” had a proportionally longer expansion stroke and a shorter compression stroke.

Atkinson Utilite Engine

The operation of the Utilite engine is very similar to that of a standard 2-stroke, except that the exhaust port is in the center of the stroke.

In this design, a cam-functioned valve (which doesn’t open until the piston approaches the stroke end) in the power stroke to stop the backflow of the pressure as the piston passes through the discharge valve. This discharge valve opens when the piston reaches near the bottom of the stroke.

When the piston returns for a compressed stroke, the exhaust valve remains open, and fresh air fills the combustion chamber and expels exhaust gases until the discharge port is enclosed with the piston.

As the discharge or exhaust port closes, the piston starts to pressurize the remaining air in the combustion chamber.

A small fuel pump injects fuel during the compression process. The source of ignition was hot tubes like other Atkinson engines. This construction provides a 2-stroke engine with long power and short compression strokes.

Read Also: Working of Rankine Cycle

Why do we need to reduce the compression ratio for the Atkinson cycle?

In the Otto cycle, after the combustion process, the force exerted on the piston during the power stroke increases so that when the piston reaches BDC, the exhaust valve opens, and useless heat discharges from the combustion chamber.

Therefore, this cycle uses to reduce the compression ratio for more expansion during the expansion stroke so that the entire force generated due to the combustion process can be used on the piston before the piston reaches BDC.

This means that the Atkinson cycle always has a lower/equivalent performance than the Otto cycle. However, the Otto cycle has lower thermal efficiency than the Atkinson cycle.

Efficiency of the Atkinson Cycle

The efficiency of the Atkinson cycle can be calculated by the below-given formula:

efficiency of the Atkinson cycle

In the above equation, the work done by the system is calculated as:

W = Work done by the system = QS – QR

In the above equation:

QR = Heat rejected by the system

QS = Heat added to the system

Work done by the Atkinson cycle

Expansion Ratio (e) = V4’/V3


Compression Ratio (r) = V1/V2 

For the isentropic compression processes (1 to 2):

isotropic process

For the Isochoric compression process (2 to 3):

Isochoric compression of Atkinson cycle

Adiabatic expansion (3 to 4):

Adiabatic expansion

After putting the values of the T4, T2, T3 in the Efficiency formula. After putting these values, the final efficiency formula will be:

final efficiency of Atkinson cycle

Advantages of the Atkinson Cycle 

  • Improved Fuel Efficiency: One of the major advantages of the Atkinson cycle is its exceptional fuel efficiency than traditional Otto cycle engines.
  • Lower Heat Rejection: The Atkinson cycle expels less heat in the exhaust which maximum heat is utilized to improve overall efficiency.
  • Best for Hybrid Vehicles: The reduced power output of the Atkinson cycle than Otto cycle engine is less of a disadvantage in hybrid cars, which may utilize an electric motor to supply extra power when required. This feature of the Atkinson cycle makes it best suitable for hybrid cars.
  • Low Emissions: The Atkinson cycle engine is more efficient. Therefore, it usually generates fewer emissions compared to conventional IC engines.
  • Lower Pressure: The lower peak pressure while the combustion process is occurring, may cause less stress on engine parts that help to improve engine service life.

Disadvantages of Atkinson Cycle

  • The Atkinson cycle engine consumes less fuel but also generates less power.
  • This cycle has a lower compression ratio.

Difference between Atkinson Cycle and Otto Cycle

Atkinson CycleOtto Cycle
An Atkinson cycle has higher efficiency than the Otto cycle.An Otto cycle is less efficient.
It consumes less fuel; therefore, it generates low power.This engine has high power because it has a high compression ratio.
This engine consumes low fuel.It consumes high fuel.
It has a higher expansion ratio than the compression ratio of the Otto cycle.The Otto cycle has a higher compression ratio than the compression ratio of the Atkinson cycle.

Read More: Working of Otto Cycle

FAQ Section

Why is Atkinson cycle more efficient?

The Atkinson cycle is more efficient because it has a large expansion ratio than the compression ratio.  It delays the closing of the suction valve until the piston completes 20% to 30% of the upstroke during the compression stroke.

What is the biggest engine in the world?

Wärtsilä RT-flex96C is the biggest engine in the world. The Wärtsilä RT-flex 96C is a turbocharged 2-stroke piston diesel engine that was developed to drive large container boats. 

What cars use the Atkinson cycle?

  • Mazda Tribute electric/Mercury Mariner/ Ford Escape 
  • Chrysler Pacifica plug-in hybrid model minivan
  • Ford C-Max plug-in hybrid and hybrid model
  • Chevrolet Volt

What is the difference between Atkinson and Otto cycle Engines?

The main difference between Otto and Atkinson cycle engines is the timing of the intake valve and exhaust valve. In an Atkinson cycle engine, the intake valve stays open longer than it does in an Otto cycle engine.

Read More
  1. Types of Engines
  2. Working of Otto Cycle
  3. Working of Rankine Cycle
  4. Different types of IC Engines

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