Why Do Semi Trucks Use Inline-Six Engines?

If you ask your average diesel bro what is his favorite truck is, you’re going to get a mixed bag of Cummins, Duramax, and Powerstroke answers, with two of those engine platforms being a V8 and one of them as an inline-six. But, if you ask a similar question to someone in the world of commercial trucks, you’ll get a Detroit, Cummins, Paccar, Volvo, Caterpillar, Mack, and so on, with almost engines being an inline-six.

That got me thinking, why are the large majority of commercial trucks using an inline-six engine. I started my automotive life in the world of gas cars, where there’s a massive mix of engine configurations, but commercial engines don’t seem to have that diversification in terms of the engine’s configuration.

Who’s Offering Engines?

For Detroit, they’re offering their DD engine line of inline-six engines, Paccar has their MX and PX line of engines which are inline-six, Cummins has a few different lines of engines with the ISX15 being their flagship engine which is an inline-six, Volvo has their D11 and D13 engines which are inline-six, Mack has their MP line of engines which are inline-six, and then Caterpillar who doesn’t actually make on-the-road truck engines anymore, but they used to and they were inline-six.

So, as you can see, pretty much everything on the market is an inline-six. You have to go pretty far back to see anything other than an inline-six being popular. The reasons why the inline-six has completely taken over as the go-to engine configuration for on-the-road commercial trucks are actually fairly simple, but let’s hop into them one by one.

Size Constraints

The first reason for commercial trucks using this configuration is because there are no size constraints. Really, the biggest reason that you’ll V configuration engines in cars, is because cars have to have relatively short hoods and engine bays. That means you can’t have a very long engine. In terms of gas cars, the outlier on this is BMW, which uses a lot of inline-six engines even in some of their smallest applications.

With a V8, for example, your engine is really only four-cylinder long in length, so it’s roughly half the length of an inline-eight engine, and the exact same thing applies with V6 engines and inline-six engines. If you have size constraints, a V engine can often be the best solution, but commercial trucks really don’t have any size constraints. These trucks are big, with massive engine bays that have to house engines up to 15 or 16 liters in size.

With that in mind, it’s easy to fit a massive inline-six engine in an equally massive truck. With very minimal size constraints, you can basically run any configuration you want, so an inline-six isn’t a size issue like it is on non-commercial trucks and cars, but that still begs the question, why choose an inline-six over all the other available configurations, and that brings me to me next point which is torque and engine speed.

Torque and Engine Speed

We’ve talked about this in other articles, but the relationship between bore and stroke has a big impact on your engine’s performance. I’d like to highlight that there are a lot of people who claim the inline-six configuration is inherently significantly better at producing torque than a V engine, but in this context, the configuration of the engine has little to do with torque production, rather it’s how each configuration is able to make use of its size for bore and stroke.

To recap on some of the info from our bore vs stroke article, bore is the diameter of the cylinder and stroke is the movement from bottom dead center to top dead center.

Typically both bore and stroke are measured in millimeters, but for many American engines, you’ll see these measured in inches. So, mathematically bore and stroke are really simple. If you wanted to increase displacement and ultimately increase power, you would just increase bore or stroke.

The easiest way to explain torque measurement is to visualize 100lbs on the end of a 1ft wrench. This would equal 100 lb-ft on the center of the axis. If you changed this to a two-foot wrench, now you’d be talking about 200lb-ft.

With this simple visualization, you can see how increasing stroke would ultimately increase torque, since you’re increasing the throw of the crankshaft, which is the pivot point, which is the center of the crankshaft, further away from where the piston is pushing down on the crankshaft.

Horsepower is just a function of torque times engine speed, whereas torque is a measurement of rotational force. In the case of these commercial trucks, they’re hauling massive loads up to 70, 80, 90k lbs, sometimes even more. So for them, horsepower is irrelevant in a sense. Sure it’s still a useful measurement of performance output, but ultimately these trucks need huge amounts of torque.

With these inline-six engines, you’ll generally see a very big difference between bore and stroke. If we were to look at a Cummins X15, for example, we’ll see that it has a bore of 137mm and a stroke of 169mm. With that longer stroke, the engine will inherently make a ton of torque but will be limited in terms of peak RPM, which in this context, really doesn’t matter. Most of these trucks spend their time between 1k and 2k RPMs, so a low rev limit isn’t an issue at all for them.

In fact, keeping the RPMs low is one of the many things that allow these commercial engines to rack up millions of miles of use without failure. An engine with a rev limit of 8,000RPM is probably not going to live as long as an engine with a rev limit of 2,000RPM. It just makes sense, your engine is spinning slower and ultimately that means less stress and equally as important, the ability to maintain very simple designs and parts.

Lubrication and Stress

That brings us to the next major point, which is the crankpins aren’t shared on an inline-six engine. What I mean by this, is that with a V engine, you typically have two connecting rods bolted to one crankpin on the crankshaft. So, those two connecting rods have to share limited bearing space. On top of that, the main journals before and after the crankpin, basically have double the work because that crankpin is dealing with two pistons, rather than one.

What this basically means, the rod journal bearings and main journal bearings on a V engine have to deal with more work as compared to an inline engine, where there is only one piston per crankpin, which means the rod and main bearings have less work to do. On top of that, the rod bearings aren’t shared on an inline engine, which means that the bearing has a much larger surface area to work with and provide lubrication with.


That brings me to the next major reason for using an inline-six engine, and that’s simplicity. At the end of the day, a low-revving inline-six has fewer parts and less complicated parts than a standard V engine. There’s online one head, which means there fewer valvetrain parts to break and a simpler geartrain. There are bigger bearings for improved lubrication. There’s no major need to focus on rotating assembly weight because the rev limit is super low. There is only one exhaust manifold.

To put it simply, pretty much everything is simpler on an inline-six engine than a compared V6 or V8, and that can translate over to gas inline-six engines as well. For example, the 4.0L Jeep motor is super reliable and super simple, but on the flip side, you have something like a BMW N54, N55, S55, and so on, that is quite complicated. Regardless though, an inline-six just simply has fewer parts to break which means improved reliability, which is paramount when you’re paid by the mile and if your truck breaks down, your income stops.

Inherently Balanced

The next major reason for using an inline-six engine is that they’re inherently balanced, unlike most engine configurations. If we were to take a V8 engine, for example, the force of the pistons don’t cancel each other out. This is because almost all V8 engines have a crank throw of 90 degrees. With some basic math, you can see this creates a positive net force, which creates a vibration in the vertical plane. We can see this by multiplying the number of cylinders times the crank throw, which gives us 720 degrees. In order to balance this, counterweights and balancing shafts are used.

On the other hand, you can really break an inline-six engine into two 3-cylinder engines with 120-degree crank throws. An inline-three with 120-degree crank throws has a total of 360 degrees, which means there is inherently minimal vibration because of the frequency being the same as the engine’s rotation of 360 degrees.

What I mean by this, is that the inertial force of two pistons moving downwards cancels out of the third piston moving upward. Take this, and double it up to an inline-six, and it’s simple to understand why the inline-six is an inherently balanced engine.

In-Frame Rebuilds

The last major point I’d like to bring up is that an inline-six engine is much easier to work on and fully rebuild without removing the engine from the chassis because the frame rails really aren’t in the way of anything since the engine is pretty much completely vertical. This means that in the case that your engine needs to be rebuilt or fixed, it’ll ultimately have less labor associated with the repair as compared to a V engine.

To summarize all this info, commercial trucks use the inline-six configuration because it’s not a space limitation, allows them to run a big stroke which means more torque, it’s mechanically simpler and more reliable than a V engine, offers improved bearing space and load for the crankshaft and connecting rod bearings, and it’s inherently balanced, which means it doesn’t need balancing shafts.

There are other additional reasons for using an inline-six configuration, but we covered the main points in this article. I’m sure there will be plenty of truckers and technicians down in the comments below who can add in some additional minor benefits and give their thoughts on this configuration.

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