Why are big-rig trucks always/frequently powered by an inline-6 engine?

Reciprocating-piston internal combustion engines have a particular sound that goes along with the number and configuration of cylinders. A twin, inline 4, inline-6, V6, V8, V10, and V12 all sound different from each other.

Judging from the sound, it seems that the vast majority of big-rigs on the road these days. Why so? The cabover layout was abandoned years ago in favor of the more aerodynamic configurations seen today, but the tradeoff is increased overall length. A different engine configuration could meet power/torque requirements while providing a shorter overall length. My guess is that the smaller cylinders of a V8 engine with equal total displacement would reduce efficiency, and the larger cylinders of an inline-4, while perhaps even increasing efficiency, would cause excessive torque pulsation (requiring a massive starter motor and a massive flywheel).

But a V6 could provide exactly the same bore/stroke while providing a shorter overall package than an inline 6. In fact, the V6 is a very popular engine in passenger cars. So why is it not favored over the inline-6 for big rigs?

More reliable and simpler engine design? Just a guess.

at least in north america, they pretty much are all inline-6 turbodiesels, anywhere from 10 to 16 liters displacement. There have been other configurations in the past; Caterpillar had the 3208 and 3408 V8s and 3412 V12, Mack had the E9 V8, International and Cummins had a couple of V8s each, and the two-stroke Detroit Diesels were used in inline-6, V6, V8, and V12 setups.

the best explanation I’ve heard for why they’ve settled on inline-sixes is:

  1. they’re powerful enough for the job, and are inherently balanced unlike a four cylinder,

  2. they can be packaged to sit lower between the frame rails, where a vee engine can’t,

  3. they’re the easiest type to do an in-frame rebuild on. meaning, they can do a total overhaul without having to physically pull the whole engine out of the truck.

I don’t know anything about engines, but thanks for this thread; I’ve been doing a lot more interstate driving now (for nearly a decade prior I was only driving in the city) and I just noticed recently there are no more cabover semis (they were extremely common when I was a kid). I’ve been idly wondering what that was about, where they went, why they went away. Your Wikipedia link seems to explain that it was more about that there used to be laws about the length of trucks, which were mostly repealed. Length regulations also being the reason cabovers are still the “usual” configuration in many other countries. Anyway, cool; now I can stop speculating on that when I should be thinking about driving :slight_smile:

oh, plus a 60° V6 would have a weaker crankshaft than an inline six. in order to be even close to balanced, a 60° V6 uses a flying pin crankshaft with two offset crankpins between each main bearing. they’re inherently weaker where one crankpin is next to another. in an I6, each cylinder has its own crankpin, and in most cases each one is separated by a main bearing.

you could make a V6 with a stronger crankshaft, where opposing cylinders share a crankpin (like a V8,) but it would require a 120° vee angle which means the engine would be wide and have to basically sit on top of the frame.

V-6 engines vibrate too much.

They also have two connecting rods between each pair of main bearings, which may increase wear.

The “cabover” design is almost universal in Europe due to strict length limits. Operators want the maximum load space within the rules. 26 pallets (4’x2’) has been the norm, but we are now experimenting with 15.6 metre trailers that can take 30 (that’s on the floor - these trailers are frequently double stacked up to the maximum 44 tonne (97,000lbs gross weight). Driver comfort is not the first consideration, although modern trucks are fairly comfortable.

There are a good few V8s in operation but fuel economy is poor and maintenance costly compared to the standard straight six. Youtube video

The inline 6 is inherently balanced, so there’s no additional engineering required to damp out vibrations or enable high RPMs, which is most emphatically NOT true of a V6- they’re all sorts of unbalanced, and require balance shafts and other mechanical gew-gaws to make them run smoothly.

So inline 6 engines can be inherently simpler than other designs. They’re used in trucks, cars and even ships, because that inherent balance makes them really easy to scale up and manufacture.


Yes, straight sixes have perfect primary and secondary balance.

One piston in each pair is always on its power stroke.

Ford’s F-150, for many years the best-selling truck, had a straight six for a long time.
ETA: ninja’d by bump. Or is it bumped by a ninja?

Reading that page, this paragraph gives a good explanation.

it’s also why Ferrari’s flat 12 in the Testarossa was simply a 180° vee; there was no need to make it an actual boxer.

An inline engine offers good access for repair and accessories on its sides plus each cylinder may be supported by two main bearings, important for a diesel. Before the small block Chevy V8 was developed in 1955, the GMC 302 C.I. inline six truck engine was popular in racing because of its 7-bearing crankshaft.

This is one of those mature technologies where, for many applications, the proportions of an I6 just work really well for the displacement (power capability) required. And they can be balanced perfectly for smoothness.