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I was looking at two cars that friends have, and I was comparing the specs, and don't understand what is going on.

One is a 2017 7 seater Citroën C4, which has a 1.2L 3-cylinder petrol engine, producing 131hp (97kW). The car weighs 1372kg with a claimed top speed of 125mph

Another is a 2022 Yaris Hybrid, which has a 1.5L 4-cylinder petrol engine + electric, but only produces 100hp (74kW). The car is lighter at 1160kg, with a claimed top speed of 100mph.

What gives with the power? How can a 5 year newer engine that is 300cc bigger and has an extra cylinder produce LESS power than the older engine? Given high school/degree level combustion engine physics it should just get more powerful the bigger you make the engine? If the age was the other way around I'd just put it down to quality/design, but this feels like there is a missing variable here, but what is it? Is it something to do with fuel efficiency trade off?

If the idea is that we increase efficiency but reduce power, why does this result in a bigger engine. My understanding was that to maximise efficiency we want to compress the air-fuel mix as much as possible, so that it does maximum work when it expands? Or looking at the Carnot diagram, so that we get the biggest area we can - so does this mean the idea is to burn the same amount of fuel in a bigger cylinder will make a more efficient engine? Or is the idea to simply run an engine at lower revs, getting more power from each cycle?

Is there some significance in 3-cylinder vs 4-cylinder? Again, I would have assumed more means more power.

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  • $\begingroup$ There are some 7 litre v8's that produce less power. Have you considered the emissions instead of the Clarkson indicators of POWER and Top Speed? $\endgroup$
    – Solar Mike
    Commented Dec 2, 2022 at 9:54

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The main difference is the combustion cycle that is used.

Presumably the Citroen uses the Otto cycle which is the most common for intenal combustion engines. The main benefit is that for the same cylinder volume (L) produces more power.

On the other hand, the Hybrid Yaris uses the Atkinson cycle, (which is another 4 stroke cycle) that improves efficiency (i.e. manages to convert more thermal energy to energy transfer to the wheel - ultimately).

So basically, its a tradeoff between maximum power output (i.e. the acceleration of the car) and efficiency (mileage per gallon).


PV diagram of Otto and atkinson Figure: P-V diagram for Otto and atkinson combustion cycles source:car-moteo.info

In the diagram above, it is possible to get a feel of the difference. Essentially the

  • the area enclosed by each polygon (black and red correspondingly) is the total energy that is retrieved as energy for motion (either rotational and/or electrical in the case of the Hybrid yaris).
  • The area below the bottom line is the energy wasted.

This diagram shows that a higher percentage of the power is retrieved

The simplest example, to highlight that difference is race cars, which basically want to maximize output, and the efficiency optimization is a secondary priority.

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