Inline 6-cylinder engines
As shown in the picture, straight-6 engine is simply two 3-cylinder engines mated symmetrically together, thus piston 1 is always in the same position as piston 6, piston 2 the same as piston 5 .... in other words, the engine is balanced end-to-end and requires no balancer shaft, unlike 3-cylinder engines.
What about vertical / transverse forces? like 3-cylinder engines, the vertical and transverse forces generated by individual cylinders, no matter first order or second order, are completely balanced by one another. The resultant vibration is nearly zero, thus inline-6 is virtually a perfect configuration.
Inline-6 is not the only configuration can deliver near perfect refinement, but it is the most compact one among them. All boxer engines are perfectly balanced, but they are two wide and require duplicate of blocks, heads and valve gears. V12 engines also achieve perfect balance, but obviously out of the reach of most mass production cars. Automotive engineers knew that long ago, that’s why you can see most of the best classic engines were inline-6, such as Rolls-Royce Silver Ghost, Bentley Speed Six, Mercedes SSK, many Bugattis, Jaguar XK-series and BMW’s various models.
V6 engines
V6 engines, excluding Volkswagen’s 15° VR6 (to be discussed later), are not just made from splitting inline-6 into two banks arranged in V-shape. A V6 has a very different crankshaft - only 4 main bearings instead of 7. In other words, between two adjacent bearings there are crank throws for 2 cylinders, one from bank A and another from bank B. While V8 engines have those 2 cylinders shared the same crank pin, V6 engine has to split the crank pin into two pieces, with a splay angle between those pins (30° splay angle for 90° V6; 60° splay angle for 60° V6). These are shown in below.
For better balance, most V6s are arranged such that the banks are placed at either 60° or 90° to each other. In this way, the movement of cylinders in bank A matches those in bank B, thus there is no vibration generated between banks. Besides, like 3-cylinder engines, there is no vertical and transverse vibration.
However, both 60° or 90° V6s have somewhat end-to-end vibration like 3-cylinder engines, especially is for 90° V6. (sorry, I don’t have the theory) It needs a counter-rotating single balancer shaft, at crank speed, to suppress the vibration. The balancer shaft is located inside the V-valley, so it is not space engaging. On the other hand, 90° V6 has a decisive advantage in production point of view - it can be machined in V8’s production line because both of them are 90°. (unlike V6, V8 can only be optimised at 90°) This save a lot of production cost. An example is Mercedes’ supersmooth 2.4 to 3.2-litre V6s, which share the same architecture with V8s but added with a balancer shaft.
60° V6 is smoother to the extent that, with adequate design engine mount, most of them could be made nearly as smooth as inline-6 engines without the need of balancer shaft. It is also narrower, so easier to be packaged into a FWD cars, mounted transversely.
60° V6 versus Inline-6
As space efficiency becomes more and more important, most car makers favour V6. The most influential V6 was perhaps Alfa Romeo’s 2.5-litre 60° V6 used in the GTV6. It established a reputation for V6 that it can be compact, powerful and smooth. An equivalent inline-6 would have never fit the small and sloping engine compartment of that car. Compare the shape of BMW with the Alfa and you’ll know the packaging advantage of V6s.
Straight-six engines are nearly impossible to be used in front-wheel drive cars as well. Even a car as wide as Volvo S80 has to introduce the world’s shortest gearbox in order to make space for the 2.9-litre straight-six mounted transversely in the engine compartment.
Longitudinal mounted inline-6 doesn’t have such problems, but it engages too much space in north-south direction, thus engage some space which would have contributed to cockpit room.
However, BMW is still loyal to inline-6 engines. Ultimately, inline-6 engine is more efficient yet smoother. V6 has more energy loss because it duplicates valve gears and camshafts (which increase frictional loss), while the use of 2 cylinder banks leads to more heat loss. In terms of production cost, although V6 has 3 fewer main bearings, it has more valve gears - which is getting more and more costly these days, with the introduction of twin-cam, hydraulic tappets / finger follower and variable valve timing. Inline-6 is going to be cheaper than equivalent V6.
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