I get why they are diesel, but why are they all of the inline 6-cylinder configuration? Why not V6 or straight 8 or freakin huge inline 4-cylinders?
In: 715
It’s most efficient to use inline-6 motors.
It’s a balance between engine strength, ease of maintenance and space for other systems.
They have a more simple design, less parts, smoother and produce lower end torque that is important in a truck. They pull heavy loads, so the low end torque allows them to get up and running without having to wind out the engine, which means lower maintenance since they don’t have to be beaten into action.
Inline-6 cylinder engines have a unique characteristic: they can have perfect primary and secondary dynamic balance without needing a balance shaft.
Essentially what this means is that the forces on the engine caused by the pistons and whatnot moving around mostly cancel each other out, and the engine runs very smoothly without needing extra, very heavy parts to dampen the vibrations.
All other engine types (V8, V6, especially I4) have imperfect balance, and V6 and I4 engines have bad enough balance that they need the heavy balance shafts to reduce vibration. I6 engines (and engines made of I6s like V12s) don’t.
As the mass of the moving parts increases, the forces involved increase, so by the time you get to the size of engine needed to move an 18 wheeler, you’re dealing with some truly massive forces. With anything other than an I6, this would mean monstrous vibration or a stupidly heavy balance shaft that would take a lot of engine power to spin.
Thus, for large engine applications, I6s and V12s are an easy choice.
They did use V-6, V-8 and V-12. Back in the 60’s through the early 90’s. IIRC.
Cat made a Big V-8, Cummins made a big V-6 and Detroit made all of them in a 2-stroke. Detroit even made a V-16.
Recently Walmart began testing on a new opposed piston engine. Its a 2-stroke engine, 3 cylinders and 6 pistons.
[https://www.carthrottle.com/post/a-new-106l-two-stroke-diesel-engine-has-3-cylinders-and-6-pistons/](https://www.carthrottle.com/post/a-new-106l-two-stroke-diesel-engine-has-3-cylinders-and-6-pistons/)
Plenty of big V8 diesel trucks still operating here in Australia, but then we have road trains not just a tractor trailer with one trailer, ours have up to three trailers so they need the extra power to pull all that weight.
I work in a mack/volvo manufacturing facility. Fun fact, the 11L and 13L engines Allstars off with the same blocks, pistons, and rods depending on the size, independent of the brand.
You can scale the shit out of an I-6 and still be in perfect balance (as un not shaking itself to bits). The challenge is packaging – they are long engines, they won’t really fit in a modern car transverse application – so you see them in rwd applications.
Gasoline engines typically run at a higher RPM, and most do not have a turbocharger to push air into them through the restrictive intake valves. This is also why it’s common for gas engines to have four smaller valves per cylinder rather than the minimum two valves (one intake and one exhaust).
Since gasoline engines “can” run at a higher RPM, they do, and then they multiply the torque output by using a higher gear-down.
Diesel fuel burns more slowly than gasoline, so the way to get the most power from them is to have a lower engine RPM per wheel revolution, and to have a long stroke, compared to the bore.
A Ford 302 V8 has a roughly similar bore to stroke, while a diesel will will have a longer stroke compared to the bore. A diesel might cruise down the highway between 1800-2000 RM, where a gasoline engine can easily attain 4,000 RPM’s when accelerating.
On the compression stroke, a diesel does not yet have any fuel in the cylinder, and as the air is compressed, it heats up enough to ignite fuel. A diesel will need a high-pressure fuel pump to overcome the high compression in the diesel cylinder, typically 18,000 psi. However, since the fuel is injected at the top of the compression stroke, the fuel “auto ignites” from the heat, and no sparking system ins needed.
Gasoline is much thinner than diesel fuel, so running the fuel injectors at high pressure is difficult, therefore they inject the fuel-mist near the outside of the intake valve (partial vacuum on the intake stroke), and compress the fuel and air at the same time. This means they cannot use high compression, and a car typically might use 10:1, while a diesel might use 15:1
Gasoline burns more completely at a specific ratio (around 14:1 air/gas), but diesel will burn at a wide range of ratios. Air is all around us and free for the taking, so diesels typically use a little more air than the perfect air/fuel ratio to ensure that all of the fuel gets used.
These principles can also help gasoline engines. The V8 302 is fairly well known and analogous of most passenger car engines. However Ford made many of the 300 ci “truck” engine (inline 6-cyl), which had a longer stroke than the 302. It had better low-RPM torque, but it could not run to as high an RPM as the 302.
Even so, the Ford 300 still did not have as high a compression ratio as a diesel, or as under-square of a bore/stroke ratio as a diesel.
If our goal is to make a 300 cubic inch diesel that is as cheap as possible, wouldn’t a 4-cylinder cost less than a 300 ci 6-cylinder? Yes, but…A 4-cylinder 300 ci diesel would vibrate more than a 6, and as others have pointed out, a 6-cylinder inherently has a good primary and secondary harmonic vibration profile, without needing to add a balance shaft (as others here have pointed out).
A lot of stuff is easier on a straight 6. One head. Simpler valve train. They also tend to run smoother and require less balancing than a V configuration.
Typically it’s a simplicity/cost thing. Inline engines are cheaper to manufacture and easier to work on. In fact, an overhaul of an inline engine (especially on a tractor) can typically be performed with the engine in the frame of the vehicle. The inline 6 is inertly balanced where V engines typically aren’t thus they run extremely smoothly and tend to have very long service lives.
Just as a more practical example on simplicity. Since 1996 Ford has invested a ton of R&D resources into overhead valve engines especially with their V8’s. GM on the other hand stuck with the tried and true pushrod style (single camshaft). It’s simple and reliable such to the point when Ford was designing their new premier fleet engine about 5 years ago (the 7.3L Godzilla) they went back to the tried and true pushrod design. Inline 6 engines in the 12L to 16L displacement range have proven to be extremely effective, have lowest cost of ownership, and extremely reliable in the domestic (North American) market.
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