Why does Toyota use Atkinson cycle engines?

Until the late 20th Century, most production vehicles fitted with a spark ignition internal combustion engine adopted a traditional Otto-type four-stroke piston cycle.

As the name suggests, these four-stroke engines have four separate phases within a single cycle – intake, compression, combustion and exhaust – and each phase corresponds to a full piston stroke inside the cylinder.

The cycle begins with a downward piston stroke, which draws a mixture of air and vapourised fuel through the intake valve opening into the combustion chamber. The upward stroke of the returning piston compresses this mixture to around one-tenth of its volume, at which point it is ignited by the spark plug. This explosion drives the piston downward in a stroke that gives the engine its thrust. The final return stroke of the cycle evacuates the spent gases via the exhaust valve so that the process can begin again.

This relatively straightforward thermodynamic cycle has equal compression and expansion ratios in each cylinder. But while it produces a satisfactory output from any given displacement, the Otto cycle is not the most fuel-efficient way of generating power.

In 1882, a British engineer named James Atkinson developed and patented a modified four-stroke cycle that used a variable length piston stroke and delayed intake valve closing to increase efficiency.

Though effective and incredibly clever, this design didn’t stack up financially for mass production. Complex mechanical linkages were needed to achieve different stroke lengths from a single revolution of the crankshaft, while the benefits in efficiency could only be achieved at the cost of some power. Because of these issues, Atkinson’s ingenious design was largely forgotten about for the best part of 100 years.

Atkinson 01

However, with the introduction of variable valve timing technology, Toyota engineers realised that they could generate the most vital, fuel-saving quality of the Atkinson cycle – the delayed closing of the intake valve during the compression stroke – through non-mechanical means; hydraulic actuation of the camshaft position.

This revelation allowed Toyota to build the world’s first Otto cycle engine with a simulated Atkinson-type valve action to significantly improve fuel efficiency. Known internally as the 1NR-FXE, the 1.5-litre four-cylinder petrol engine was installed in the 1997 Toyota Prius and paired with the fantastic new Toyota Hybrid System (now known as Hybrid Synergy Drive).

Atkinson 02

Using an electric motor to assist the petrol engine addressed the shortfall of power characteristic of the Atkinson cycle, but also provided an independent source of motivation in order that the engine could be shut off when possible. After all, the most effective method of fuel-saving is not to have the engine running in the first place!

So successful has this pairing of a simulated Atkinson cycle engine with a hybrid electric powertrain that every Toyota and Lexus petrol-electric hybrid model produced since the first-generation Prius has employed the same fuel-efficient architecture.


  1. Hi Sirs! I am going to receive (for my wife…) my new brand car: Yaris Hybrid! I have photovoltaic pannels: is there any way to recharge bactery through plug in? May be there is a couple of wires…
    Thanks for a positive answer!
    Best Regards

    1. Hello Marco
      Thanks for your post.
      Great to hear that your new Yaris Hybrid is on the way. To answer your question though, you would not be able to recharge the batteries by plugging in and we would not recommend modifying your car in any way. The Yaris hybrid is able to charge the batteries itself using various sources which saves you the trouble of having to do this. Hope this helps but let us know if you have any other questions.

      1. In relation to the question from Marco, I feel that Toyota is being rather short sighted in NOT allowing such a desirable user convenience as some kind of external power charging option?
        I don’t know what kind of power (Voltage and current) is provided by the hybrid generator to the battery system, but perhaps it could be possible for some Toyota specific equipment to be incorporated in conjunction with the owners PV system to facilitate ‘home’ charging?
        Regards Mark

      2. Hi Mark,
        Thanks for your feedback.
        We appreciate that plugging in isn’t a feasible option for everyone at the moment, which is where our hybrids come in handy.
        However, if you’re looking for a plug-in hybrid, you can look at our range here – https://www.toyota.co.uk/plug-in-hybrid.

      1. You may or may not be correct, but to visualise what happens when the spark hits the compressed fuel / air mixture the word ‘explosion’ is far more descriptive and understandable.

    1. Hi there,
      Thank you for your post. Great spot! We have indeed forgotten about the two-stroke engines. Thank you for your feedback we have now amended this post.

  2. Will the gearbox still be a CVT? I have a Yaris and the only bugbear is the automatic gearbox which tends to rev high before settling down which makes it quite noisy.

  3. I’ve just ordered a RAV4, which obviously utilises the Atkinson cycle design you have so brilliantly developed further. I am so excited, having had very positive experiences of how the car drives and handles during my test drives.

    1. Hi Jon,
      Thanks for your comment, and congratulations on your new RAV4!
      We hope you’ll love it when it arrives.
      Which grade did you opt for?

      1. Hello, I’m intendind to mount an LPG system in my RAV 4 Hybrid (2016 model). Do you know If the RAV4 atkinson motor can work well with the lpg system?

      2. Hi Stefan,
        We would not recommend this, as this may invalidate your warranty.
        Please contact your nearest Toyota Centre to see what other options are available.

  4. After nearly 70 years of driving I have never driven such a magnificent piece of machinery. I drive an AURIS hybred.

    1. Thanks for your kind words, Tony. It’s great to hear that your Auris is serving you so well!
      We wish you many more happy miles.

  5. Hi, i have an auris hybrid, 2016 model that i use for my private hire taxi business i have covered 101000 miles trouble free so far and have regular comments from customers of how smooth and comfortable thr car is, i agree with them and are very happy with it!!!

  6. Hi Toyota,
    I have tweaked my motorcycle engine to run on Otto as well as Atkinson cycle when switched to, without making any changes to the original engine, it’s valve setting, crank linkage or the cylinder head. Also, a wide range of variable compression ratio is achievable too.


  7. I was skeptical of Toyotas decision introducing 100 year old concept and more complexity to vehicles. But the figures stack up, Toyota is reliable and after having driven one I ordered one! Very well integrated machines!

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