Deep Dive: Inline 4 vs. V4 vs. Boxer 4 - What's the Difference? Engine Balance Explained in Detail

In today's video we're doing a very in-depth exploration and comparison of the inline 4, the v4 and the boxer 4 engine. We're exploring and comparing everything from primary and secondary engine balance to firing intervals and rocking couples. We will also see how a boxer 4 is different from a flat non-boxer engine in terms of anatomy and balance. But our deep dive is structured and gradual instead of overwhelming so that you can enjoy the video regardless of your background or prior knowledge. There's also skip points included so that viewers familiar with my engine balance series don't have to watch things that have already been explained in previous videos.
As you might already know, primary engine balance has to do with the mass and thus the inertia of the piston when it changes direction. Now to have an engine with good primary balance we have to balance out the forces associated with the piston's inertia.
The inline four achieves a perfect primary balance by balancing it out piston inertial forces using the inertial forces of other pistons. So when two pistons go up, two pistons go down. This means we have two forces pointing up and two forces pointing down and so they cancel each other out leading to an elimination of primary vibrations.
The boxer 4 also achieves perfect primary balance by using piston masses to balance out other piston masses, it just does it a bit differently compared to the inline four. Instead of having all the pistons in one line, the boxer four splits them into two different banks which are directly opposed to each other which means that the pistons are also opposed to each other
The v4 has perfect primary balance too, but it doesn't rely on piston masses to cancel out other piston masses. Instead it uses the crankshaft counterweight to cancel out the mass of a piston. The reason why a V4 engine can do this is precisely because it's a V engine. If it weren't a V engine than the crank counterweight couldn't be used for this purpose.
The V4 is a lot more „modular“ compared to inline four and boxer four engines. In fact you could say that inline four and boxer four engines are in a way all the same in terms of their basic anatomy. It's modular in the sense that it's essentially two V-twin engines stuck together. This allows you to have any angle between the two crank pins. The angle can be zero like in the Honda RC30, or 180 degrees like in the Honda VFR800, or 70 degrees like in the Ducati Panigale V4. You can also play around with the degree between the cylinder banks. You can go 90 degrees like Ducati, or 70 degrees like the Yamaha Vmax or 65 degrees like the Aprilia RSV4. But no matter what you do, because of the V configuration anatomy, you can't have two pistons be at top dead center at the same time if you put two connecting rods on one crank pin. This means that you're always going to have an uneven firing interval.
Now in the case of the inline four 4 the firing interval is regular or even....but the downside to this is that it can't be anything else. In the case of the v4 the firing interval may always be uneven, but the upside is that we can play with it and create different firing intervals resulting in different engine character, sound and power delivery which can be tailored to match different applications.
When it comes to secondary balance the inline four obviously has the most problems because all the forces point upwards all the time leading to noticeable secondary vibrations. But there's good news and there's bad news when it comes to secondary balance. The good news is that the magnitude is only about one quarter that of primary balance but the bad news is that secondary vibrations occur twice per engine revolution compared to only once for primary vibrations.
The v4 and the boxer 4 have better secondary balance than the inline four but it's still not perfect. In the v4 the pistons don't oppose each other so the secondary forces can't cancel out, but they also can't stack up like in the inline four. The separation between the two bank angles means that the secondary forces from each piston merge into a single resultant force. The magnitude is about 1.4 times that of a single cylinder, and then playing with the offset between the crank pins the v4 essentially "dilutes" secondary vibrations which means that it doesn't need a balance shaft.
Now the boxer 4 seems like it might have a perfect secondary balance because we have forces of equal magnitude but opposite direction. But due to the offset between cylinders the boxer four has a secondary rocking couple. But at least it's better than a flat 4 engine which has a massive primary rocking couple.
A special thank you to my patrons:
Daniel
Daniel Morgan
Pepe
Brian Alvarez
Jack H
Dave Westwood
Joe C
Zwoa Meda Beda
Toma Marini
Nelson
00:00 Primary balance
04:53 Firing interval
10:59 Secondary balance
15:02 Rocking couples
20:20 Which is best?
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