The applied physics of three-cylinder engines
Three cylinders: It almost doesn’t seem enough, and yet the BMW i8, and plenty of other cars, are motivated by three-cylinder power. What makes them tick?
An inline three is what you get when you take a radial arm drop saw to an inline six, lop it in half, and plug up the holes.
There are some things to like about them in engineering terms. There are three crank pins at 120 degrees in separation, and there’s evenly spaced exhaust pulsing, so a three-into-one exhaust manifold gives you excellent scavenging off the bat (which is important for even filling and combustion in every cylinder).
There’s even firing and perfect phase balance on the reciprocating masses, but they have a lot of planar imbalance effects, and that means heavy counterweights are required unless you want to drive a rolling vibrator. Which could be OK, depending on the mood you’re in. Three cylinders is a total good news/bad news story - they have excellent secondary imbalance properties, so they’re smoother than an inline four at high revs, but the big counterweights mean it’s almost impossible to make them sporty…
...which is why they’re popular in cars designed for men who have had their wedding vegetables removed surgically, often by an ex-wife with a rusty teaspoon.
Also good for the ex-wife with said rusty spoon and a certain veangeful gleam in her eye. You know the cars I mean. Nasty, spiteful, bite-size chunks of excrement like the Audi A3 1.0-litre. Zero sex appeal with four wheels and half a real engine.
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The biggest problem with a three-cylinder engine is: 240 degrees of crank rotation between firing pulses. Firing pulses last about 120 degrees. So you get a push on the crank for 120 degrees, then deafening silence for 120 degrees, repeat. For eternity.
Thus, in the domain of turning versus burning, three cylinders - at least the four-stroke ones - are only ever going to give you 50 per cent. Which is why their natural habitat is the cheap, shitty econobox.
Putting a three-cylinder engine in a car is a decision only an accountant could ever ratify.
There has of course been an outpouring of indignation like this in respect of my inline five report:
“Amarok -v6 not inline 6. Also according to VW spec sheet the v6 produces 165kw peak power from 2500 to 4500 rpm , not just at 4500 as John quotes. So the argument that it has to spin harder to produce its peak power is incorrect.” - Alan
The Amarok is definitely a V6. My bad, and I apologise to you without reservation for incorrectly calling it an inline six. It’s definitely a V6. Hopefully I can get away with crucifiction, for a first offence.
I get my engine data from Redbook.com.au - because it’s a one-stop shop/database for every model, new and used, ever sold in Shitsville. The specs are presented in a unified way that makes comparisons between vehicles quick and efficient.
Redbook is usually pretty good - but in this case they were wrong. Therefore, so was I. According to Redbook, Amarok an inline six. I’ve let them know.
Alan’s statement about power in the Volkswagen Amarok V6 is accurate too - that’s certainly what they claim in the PDF brochure. (About the peak power of 165 kilowatts being available from 2500-4500 rpm.)
However, I’d humbly submit to you that it appears to me, Volkswagen are lying sacks of shit on this because: That’s not physically possible.
The problem is that those two specs are mutually exclusive. You can’t have both because: Physics.
Kilowatts equals Newton-metres times revs divided by 9549 in the metric universe here in Shitsville. Therefore, you cannot make, simultaneously, 165 kilowatts and 550 Newton-metres at 2500 revs. That’s just indefensible bullshit.
The Volkswagen brochure might be better than Isaac Newton at physics, but I doubt it. For any engine to make 165 kilowatts at 2500 revs, it needs to deliver 630 Newton-metres of torque at that point. (Unlikely.)
Far more likely that it’s 550 Newton Metres at 2500 revs - making the power output 143 kilowatts at 2500. Either way, the specs are just another brick in the wall of indefensibly inaccurate Volkswagen bullshit.
