It ain't your normal 3 cylinder

    Toyota showing efficiency is not just about engine size

    I bought my 2017 Toyota Aygo around 9 months ago and I absolutely fell in love with this car. Not just for the sporty color scheme and the blacked out rims, but because it was small and SUPER fuel efficient. Originally I thought the mega fuel efficiency was purely down to the fact that it is powered by a 1 liter 3-cylinder engine and the fact that it is compact car. However, when I was buying the car, the sales person told me that the reason for it's high fuel efficiency is down to a clever trick by Toyota in having the engine run an Atkinson cycle instead of a pure Otto Cycle.

    Otto Cycle vs. Atkinson Cycle

    The Otto Cycle is the classic engine cycle for a spark ignition combustion engine, found in most petrol powered cars. There are four events, the inlet valve opening and the air rushing in, the compression stroke where the air gets squeezed inside the fixed volume of the cylinder, the power stroke where the energy of the gas is released in a "bang" and is turned into mechanical work, and finally the exhaust stroke where the exhaust gases are released to atmosphere. In a normal otto cycle the valve timings will be set such that the amount of fresh charge lost between the intake and compression stroke is minimal and the amount of fresh charge lost between the exhaust and intake stroke is minimal as well. Allowing the compression ratio to be in the ball park of 9-10:1. However in the late 1800s, James Atkinson invented the Atkinson engine that would maximize efficiency with a slight cost to power density. The modern variation of this theory is achieved by effectively reducing the compression ratio but keeping the expansion ratio the same throughout the cycle. This is achieved through holding the intake valve open longer to allow that reverse flow of fresh back out into the intake manifold. Thereby effectively reducing the compression ratio of the cycle without altering the expansion ratio (making the compression ratio smaller than the expansion ratio). The end goal of greater fuel efficiency is achieved through all of this by allowing the pressure of the combustion chamber to be as close to atmospheric pressure at the end of the power stroke. When this is achieved this means that all of the of the available energy has been extracted from the combustion process, i.e. better fuel efficiency. However, this does come at the expense of the power production of the engine, and believe me that shows when travelling up a hill.

    Is this the future?

    For all of the die hard petrol engine supporters out there this may well be the future, with the addition of a hybrid element here or there. From what I have experienced with my Aygo it is an extremely happy medium. You're not spending extortionate amounts of money on petrol every month because of the astonishing 60+mpg this car achieves. You still have that petrol smell, feel, and soul of driving and internal combustion engine car, something which feels slightly lost with a very sterile feeling electric car. Lastly, when you rev the heck out of it it sounds like a mini V6. What more could you want?

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    Comments (1)

    • All summed up nicely with the formula:






      and C is the Constant.

      It starts to become fun when P,V and T are all changing with respect to time.

        4 days ago


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