Currently reading: Under the skin: Why the straight-six engine is on the way back
Unlike a four, straight-six engines don’t need balancer shafts to be virtually vibration free
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3 mins read
28 September 2020

There's something about a straight-six engine, with that intoxicating soundtrack and silky smooth power delivery. Yet for a time, it looked as though they would disappear from the radar to be replaced by vee engines. BMW even substituted a V8 for the M3’s sublime 3.2-litre six for a time. Now manufacturers are embracing the inline six once again and, ironically, one of the reasons for that is why they gave way to V6s in the first place: packaging.

First, though, why is the straight six such a desirable configuration for an internal combustion engine, regardless of the fuel type? The clue lies in that silky smoothness. Vibration is caused by reciprocating parts, like pistons, and things that rotate, like the flywheel or crankshaft. Engine balance is fiendishly complex but the two main types of vibration are primary ones, which have an even frequency in sync with the engine revs, and secondary. Primary vibrations are caused by the inertia of the pistons moving up and down at a manic rate and the force of combustion. Secondary vibrations are more complex, uneven and essentially caused by the internal geometry of an engine, which means pistons accelerate faster in the top half of the stroke than the bottom half.

The configuration of the engine makes a big difference to both types of imbalance. In an inline four with pistons connected to the crankshaft at 180deg intervals, when pistons one and four are travelling upwards then two and three are moving downwards. The power strokes don’t overlap and there’s a primary imbalance and a high secondary imbalance. In a straight-six engine, pistons are connected to the crankshaft at 120deg intervals, power strokes overlap and each piston has a twin moving in the opposite direction, cancelling out imbalance. So although there’s still some secondary imbalance, primary balance is perfect.

Although lacking the perfect primary balance of straight sixes, 60deg V6s became popular in mainstream cars mainly for packaging reasons. It’s relatively straightforward to mount them transversely for front drive, and when mounted in line, a shorter V6 makes more room for crash structures. Some car makers based V6s on cut-down 90deg V8s but these, too, lacked the inherent smoothness of a straight six. But as turbocharging became more prevalent, packaging considerations gave inline sixes the edge over vee engines.

Inline engines have a hot and a cold side, with an exhaust manifold on one side taking hot gases away and the fuel injection system on the other. Mounted longitudinally, there’s more space on the hot side for single and increasingly common bi-turbo set-ups, e-boosters and managing under-bonnet heat. There’s also more room for fast-emerging technologies such as the belt-driven starter-generators of mild hybrids, which are bulkier than the alternators they replace. Apart from the inherent advantage of smoothness, straight sixes are more economical to make and have a simpler design, with one cylinder head instead of two, fewer camshafts and less complex induction and exhaust systems.

Stick 'em up - quick!

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28 September 2020

I can see that an in line six is superior to a V6 for the reasons given in the article. But surely the reason why the straight six is now less common has more to do with the superior efficiency of a four given the latter's inherently lower internal friction and lower manufacturing cost. Also the loss of smoothness can be compensated to a large extent by balancer shafts. 

That said, straight sixes always sound good  and it's good to see them being continued. 

28 September 2020

...I'd also love to know what Patrick Fuller thinks.

28 September 2020

One of the very very few good trends in cars.

28 September 2020

Well yes I suppose there's no reason that straight sixes could't come back into fashion,maybe we could could see a modern equivalent of the unit used in the Triumph Vitesse,realistically the configuration is only fit for RWD usage. Not that it's hasn't been tried,who remembers the Chevrolet Epica a former Daewoo product that was sold in Blighty with a two litre straight six,there were very few takers for that even though the engine was designed by Porsche 

28 September 2020
ianp55 wrote:

there's no reason that straight sixes couldn't come back into fashion, maybe we could could see a modern equivalent of the unit used in the Triumph Vitesse, realistically the configuration is only fit for RWD usage. 

The original Triumph Vitesse had a tiny six cylinder of only 1600cc before it was enlarged to 2 litres!  BMC did manage to shoe-horn its straight 6 into the FWD 2200 Landcrab and its successor, the Princess.

Straight sixes have always hummed in a refined manner, while in-line fours just boomed and buzzed until improved NVH techniques improved them somewhat.  V6s sounded almost as good but they never seemed to be quite as smooth and I suspect that they didn't breathe very well as they always seemed to be rather thirsty on fuel.  Cross-plane crank V8s (not those horrible flat plane Ferrari things) still sound the best though!

28 September 2020

In reality with modern turbo engines being so powerful, the modern straight sixes are replacements for larger V8 engines, with the previous V6s being replaced by turbo 4s.

What's missing is less powerful straight 6 engines, for close wanting smoothness rather than massive power.

28 September 2020

A straight six (and derivatives like a v12) are balanced in primary, secondary and tertiary forces.  You can do the same with a bent eight, but you need balancer shafts etc.  A while since uni, but think the comment about secondary imbalance remaining is wrong.

28 September 2020
Yes, it's like the last hurrah of the dinasours, they want to go out in style.

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