Engine capacity explained - 4cyl vs 6cyl vs V8 | Auto Expert John Cadogan

13 Jun 2018 14:07 219
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In-line four-cylinder engines are great. Performance is pretty good for average cars, in the 1.6 to 2.5-litre capacity. They’re easy to build. There’s not too many parts, so there’s minimal machining and assembly, relative to sixes and eights.

Inline fours idle smoothly because the firing pulses don’t make the engine rock. The cranks don’t need real heavy counterweights - so they rev up quickly.So they’re starting to sound perfect, which they’re definitely not.

They’ve got a real inherent problem with a thing called secondary imbalance and this is a huge issue for large capacity inline fours, especially at high rpm.

Up to about two litres in capacity your inline four engine design brainiac can mask that secondary imbalance in a production car using the engine mounts. Over that, you need counter-rotating balance shafts - and it’s really not practical to make a four-cylinder engine bigger than about 2.5 litres for this reason.

There’s no question that the best six in terms of inherent balance is the inline six, and I’d have to say inline sixes done right are just brilliant. BMW’s inline six M3s were awesome.

Unfortunately, inline sixes also tend to be quite long, physically, and therefore hard to package up in an engine bay designed for a four. So if you’ve got a four-cylinder base model whatever, and an inline six on the higher spec, the engine bay is going to be a packaging disaster.

Inline sixes tend to be (basically) two inline threes joined longitudinally, with the back three cylinders phased at 120 degrees to the front three, to deliver even firing pulses. And that’s a problem because you need the front three exhaust ports feeding into one collector and the back three feeding into another and then the two collectors need to feed into one, if you want even exhaust pulses, for efficient scavenging.

So just to prove to you that mechanical engineering is absolutely the applied science of compromise: V6s are far more common. You can forget the cut-down V8 with the rear two cylinders lopped off and a 90-degree ‘V’ - that’s an economically rational abomination.

People think a 60-degree ‘V6’ is ideal. That’s bullshit. The ideal design from a balance perspective is a 120-degree V. Most people don’t know that.

Unfortunately this 120-degree V tends to be too wide to fit in the same engine bay as an inline four, so they went with 60 degrees for compactness and they fudged the 120 by offsetting the crank pins on the quasi-shared cranks by a further 60 degrees.

And unfortunately, there’s not much overlap on those shared pins when you do that. In fact, they’re really more like two separate crank pins with a thin web between them, and that makes the crank in a 60-degree V6 inherently weak - I guess you could call it a design challenge if you wanted to be nice about it.

Above about four litres (atmo four litres) if you’re a carmaker you’d probably go with a V8.

So, just to digress here, the first V8s were essentially two inline fours Siamesed up and inclined at 90 degrees, and sharing the same flat crank. Single-plane crank. Like an inline four. This is a whole ‘good news’/’bad news’ story too.

Unfortunately, although it revved up really fast, it shared the secondary imbalance problems of the inline four - times two - making high-rpm failures hard to prevent.

So they changed to a cross-plane design in the early part of the 20th Century. Cadillac and some other manufacturer co-patented it, if memory serves. When you look end-on at a V8 crank today, the crank pins are arranged in a cross.

So, a couple of things about that: Single-plane designs - the early ones - didn’t really need counterweights on the crank, so they revved up really fast … until they broke into a thousand pieces at high revs.

Cross-plane designs - the current one - need heavy counterweights to counteract the inherent rocking type vibration, so they don’t rev up as fast. But they are more durable at high revs.

And the other thing is the firing pulses. The single plane design was left right left right left right left right. Nice and even.

The cross plane one is left right left left right left right right, right? - so this leads to uneven inlet and exhaust pulses per bank. Stay with me. Change batteries now, if exhausted. It’s a marathon.

So, in your cross-plane V8 road car, like a Mustang or a Corvette - whatever - with four exhaust ports merging into one pipe, per bank, you get uneven filling and scavenging of the cylinders. Because the pulses aren’t even. That prevents truly even combustion across all the cylinders.

And if you want to know why this matters, it’s because that’s what makes the distinctive V8 burbling sound that we petrolheads all react to in such a semi-carnal way. Without that unevenness, the burble goes away.

And I’m tipping nobody wants that.

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