If you ask your average diesel bro what his
favorite engine is, you're gonna get a mixed bag of answers between Cummins, Powerstroke,
and Duramax, with one of those engines being in inline-six and two of them being a V8.
But, if you were to ask the same question to somebody in the commercial truck world, you
get a wide array of answers including Detroit, Paccar, Caterpillar, Volvo, Mack, and so on. And
that got me thinking, why do commercial trucks, why do semi trucks use inline six engines
almost exclusively? In the world of gas cars, you have a wide array of configurations inline
fours, inline sixes, v6, v8, v10, and so on. But, in that commercial world, in the world of semi
trucks, it's pretty much all inline sixes, there's really no other configuration that's being
made today. So, sit back and get comfy because i'm gonna tell you exactly why inline sixes are the
superior engine platform in the commercial truck world, in the diesel world, and why they're also
a great engine platform for gasoline-powered cars. To start this out, I think we should look at
some of the more popular engines being made, 2019, 2020, or you know at least some sort of
recent data so that we're on the same page as far as what's popular and what is not popular.
This set of data actually comes from Fleetseek and it covers the 2019 to 2020 truck models and
basically what engine was the most popular in heavy duty trucks. As you can see on this
graph, Detroit is the most popular in 2020, followed by Paccar, Cummins, Volvo, Mack, and
Caterpillar. 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 an inline six, Mack has their MP line of engines which are in
inline six, and then Caterpillar who doesn't actually make on the road truck engines anymore
but they used to and they were pretty much all 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 for why the inline
six configuration 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. The first reason
for commercial trucks using this configuration is because there are no size constraints or at least
very limited size constraints in these big trucks. Really, the big reason that you'll
find V configuration engines in cars, is because cars have to have a relatively short
hood and small engine bay. That means you cannot have a very long engine. In terms of gas cars,
the outlier on this is BMW, who still uses a lot of inline six engines, even in some of their
smaller applications. But, for the most part, most manufacturers either use a V6, V8, or inline four.
With a V8, for example, your engine is really only four cylinders long in length. So, it's roughly
half the length of an inline eight engine and the exact same thing applies with a V6 engine 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 my next point, which is
torque and engine speed. Now, we've talked about this in other videos, 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 that the inline six configuration is inherently better at producing
torque or inherently produces more torque than any other configuration such as a V6 or V8. And I
think it's worth noting, that that's not actually really true. Now, the different configurations
allow different usage between bore and stroke and the relationship between those, which allows
each engine configuration to make more power. But, the configuration itself isn't inherently
going to produce more or less torque. To recap on some of the info from our bore versus stroke
video, bore is the diameter of the cylinder and stroke is the movement from bottom dead center
to top dead center. Typically, 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 very simple
and if you wanted to increase displacement and ultimately increase power output, you would
just increase bore or stroke. The easiest way to explain the torque measurement is to visualize 100
pounds on the end of a one foot wrench. That would equal 100 pound feet on the center of the axis
of the nut that the wrench is putting force on. If you change this to a two foot wrench, you'd
now be talking about 200 pound-feet of torque. With this simple visual, you can see how
increasing stroke would ultimately increase torque, since you're increasing the throw of the
crankshaft, with 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 pounds, and sometimes even more. So
for them, horsepower is irrelevant in a sense. Sure, it's 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 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 137 millimeters and a stroke of 169 millimeters. With
that longer stroke, the engine will inherently make a ton of torque, but it will be limited
in terms of peak RPM, which in this context, really doesn't matter at all. Most of these trucks
spend their time between 1000 and 2000 RPMs, so a low rev limit isn't an issue for them at
all. In fact, keeping RPMs low is one of the many things that allows these commercial engines to
rack up millions of miles of use without failure. An engine built 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 a very simple design and minimal parts. That brings us to the next
major point, which is that the crank pins 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 crank pin 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 crank pin basically have double the work to do, because the
crank pin is dealing with two pistons, rather than one. On top of that, the rod bearings aren't
shared on an inline engine, which means that the bearings have a much larger surface area to work
with and provide lubrication with. That brings us 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 engine has less
parts and less complicated parts than a comparable V engine. There's only one head, which means
that there's less valve train parts to break and a simpler gear train. There's bigger bearings for
improved lubrication. There's no need to focus on rotating assembly weight, because the rev limit is
super low. There's only one exhaust manifold and so on. To put it simply, pretty much everything
is simpler on an inline six engine, as compared to a V6 or V8 engine. And that can translate over to
gas inline six engines as well. Regardless though, an inline six just simply has less parts
to break, which means improved reliability, which is paramount when you're getting paid by the
mile and if your truck breaks down. your income stops. The next major reason for using an inline
six engine is that they are 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 that this creates a positive net force, which creates a vibration in the vertical plane.
We can see this by multiplying the number of cylinders by the crank throw, which gives us
720 degrees. In order to balance this out, counterweights and balancing shafts are used. On
the other hand, you can really break an inline six engine into two three-cylinder engines, with a
120 degree crank throw. An inline three with a 120 degree crank throw, has a total of 360 degrees,
which means there is inherently minimal vibration because the frequency is the same as 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 the third piston moving upwards. Take this and double it up to an
inline six, and it's simple to understand why the inline six configuration is inherently balanced,
as compared to other configurations. Now, the last major point that 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 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, is 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. Of course, there
are other reasons for using the inline six engine, but everything we discussed in this
video is pretty much every single major point and i'm sure that there will be truckers and
technicians, especially diesel technicians down in the comments, you know, letting us know some
of the other key points that make these engines so reliable, so efficient, and so good for this
specific use. And if you guys enjoyed this video, be sure to smash the thumbs up button, really
helps me out. Drop a comment down below if you think there's anything I forgot to add or
anything I might have accidentally messed up in this video. Drop a comment down below letting
me know. While you're down there, get subscribed, check out some of the other videos on the
channel, and I'll see you guys in the next one.
