The Differences Between Inline Four & Boxer Four Engines

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In this video we are going to be comparing inline four cylinder engines with boxer four 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 engine.

image/text credit: Engineering Explained 

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.

image/text credit: Engineering Explained 

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 one, two, three, and four.

image/text credit: Engineering Explained 

For the boxer engine, the firing interval is one, three, two, four, while on the straight four it’s one, three, four, two, so the order of the last two cylinders firing is switched. On the boxer engine, you’ll notice the pairs of pistons move in and out together.

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This means that the primary forces when the piston reaches 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.

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, however, 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 cylinders cancels out the rocking motion of the other, unlike in a V6 configuration.

VIAEngineering Explained
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