Eli5: why are car engines never an even number? E.g a 3L engine is always 2967, a 2L is 1984 etc.. why???


Eli5: why are car engines never an even number? E.g a 3L engine is always 2967, a 2L is 1984 etc.. why???

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The engine capacity is basically a measure of a cylindrical volume. This is the area of a circle multiplied by height. The problem is that to manufacture something, we tend to focus on linear dimensions like the bore and the stroke. This makes it easier to design the equipment to make the components.

Unfortunately an area of a circle is pi * radius^2. Since pi is irrational, it is not really feasible to form a circle with a rational linear dimension (like a radius of 88mm) and come up with a “nice” number for the area.

Thus, the volume is not going to be a “nice” number either.

The 3L figure is just rounded for the sake of convenience. No one designs an engine to be exactly 3000 cubic centimeters, and even if they did, they might later on increase the stroke or the bore and it would no longer be 3000 ccs, it might be 3180 ccs, and you’d call it a 3.2 liter.

It breaks down to advertisement laws and the wiggle room they’re allowed. Some are really close and others make you wonder how it’s even legal. As long as they place the actual displacement somewhere in the vehicle, they can get away with it. Another thing is 2.0L sounds a lot better than 1969cc.

I suspect it’s partly due to the way governments assess vehicles for licensing/tax fees with different rates for engine capacity. The thresholds might be at, say 1L, 1.5, 2, 2.5, 3 etc. The manufacturer will design the engine to be under a threshold to make it cheaper to own.

There may also be government fees for pollution (NOx) and CO2 emissions, and the ECU (engine management computer) can be tuned to limit the performance to reduce the cost of ownership.

When I was a teenager I had a motorbike which had a 125cc engine, because that’s all I could legally ride at my age. As it happened, it turned out to be a 150, but nobody ever knew, except me and a mechanic who once rebuilt the engine after a bearing failed.

I got quite confused by the example of a 2L, 1984cc engine not being an “even number” before I read the comments and realised what you were talking about.