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The Differences Between Inline Four & Boxer Four Engines

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Hello everyone and welcome! In this video we are going to be comparing inline four-cylinder engines with boxer 4-cylinder engines and talking about the individual advantages and disadvantages of each engine layout. We have a 3D printed EJ20 Subaru boxer engine as well as a 22RE Toyota inline four-cylinder engine we’ll start fairly basic and work our way through several different topics for each layout including the four strokes the firing interval and order the vibrations of each layout the packaging differences the cylinder head differences and finally a bit about the sound of each engine. Looking at either layout, they’re both based on the same four strokes intake, compression, power, and exhaust. Both engines fire one cylinder for every 180 degrees of crankshaft rotation but they have slightly different firing orders on each engine. We can see cylinders 1, 2, 3, and 4 for the boxer engine the firing interval is 1, 3, 2, 4 while on the straight four it’s 1, 3, 4, 2. So the order of the last two cylinders firing is switched. Now these layouts matched with the firing interval make for some interesting differences as far as how the engines are balanced on the boxer engine you’ll notice the pair of pistons move in and out together this means that the primary forces when the pistons reach the top of the cylinder as well as when they reach the bottom of the cylinder are canceled out on the inline four-cylinder engine it’s the same story the primary forces cancel out as the pairs of pistons reach the top and bottom at the same time when we get into secondary forces however the engines begin to differ. Secondary forces are created due to the piston traveling faster at the top half of the cylinder then at the bottom half something i’ll include a link to in the video description that breaks it down in great detail what you need to know though is that? when the piston reaches the very top of the cylinder or the very bottom the secondary forces point up we’re out from the piston now with the boxer engine since the pistons point opposite each other these forces are balanced out resulting in a very smooth running engine for the inline 4 all of the forces point in the same direction and thus do not cancel each other out causing the engine to vibrate unless balancing shafts are used the boxer engine isn’t perfect however because the pistons do not perfectly align with each other it creates a rocking moment which makes the engine want to rotate back and forth along the vertical axis. What’s fascinating though, is that if you add two cylinders to either of these designs whether it’s a boxer six or an inline six you can perfectly eliminate all first and second order forces and moments you might think the boxer six would have a rocking motion from the cylinder banks of three but each bank of three cancels out the rocking motion of the other unlike in a V6 configuration looking at the two engines both of these 3D printed engines are at 35% scales you can actually compare them size-wise to one another because of the inherent vibrations of inline four-cylinder engines you won’t tend to see engine size as much larger than about three liters so historically they have existed at much larger sizes one of the largest gasoline four cylinders currently made is in fact a Toyota engine a 2.7 inline 4 used in the Toyota Tacoma only porsche and Subaru currently used flat engines in their vehicles so the size doesn’t tend to be any larger than typical inline 4 cylinders even though it does have better vibration characteristics the other biggest advantage of the boxer engine is the low profile which keeps the center of gravity low and thus reduces the amount of load transfer you have in the car during braking cornering or accelerating which improves grip with a lower center of gravity you can also reduce body roll and choose to use softer springs additionally in the event of a collision it’s easier to position the engine so that it goes underneath the passenger compartment rather than into the passenger compartment for improved safety. Now that’s not to say that the inline four doesn’t have its own size advantages generally, it’s a bit more compact with only one cylinder head and it’s not quite as wide as the boxer engine this leaves more room for suspension geometry and can also allow for a better steering angle since the tires won’t have as much of an interference at full lock moving on to the valvetrain although this particular inline 4 has a single overhead cam you’ll much more commonly see dual overhead like the boxer engine on modern vehicles the big advantage with the inline 4 is that there’s only one cylinder head meaning only one intake and one exhaust camshaft less moving parts and some weight saved. It’s also far far easier to access the cylinder head for service, whether it’s adjusting the valves or replacing spark plugs the I-4 design makes it a much easier task finally we get to the topic of sound and here’s where most people will agree the box rumble is a better sound but it’s not really a true advantage The boxer rumble is a result of unequal length headers and because Subaru is moving away from this exhaust design New ones will generally sound pretty similar to other four-cylinder engines it would in fact be possible to design an inline four-cylinder engine with different length exhaust pipes to create a unique rumble generally associated with Subarus. But it’s not ideal for exhaust scavenging to have unequal pulses it’s not the smartest way to route an exhaust or four-cylinder engine however packaging does not the unequal header length seem appealing when paired with a boxer engine. And a big thank you to Eric Harrell for providing the 3D printed models you can find links in the video description. If you have any questions or comments feel free to leave them below, thanks for watching!

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