Understanding the Difference Between Inline and Boxer 4-Cylinder Engines

Many if not most people in the world hop in their car, start the ignition, and drive off on their way without a thought about how any of their cars works. For anyone with a technical mind or just naturally curious about how things work, this is probably not what you do. Whether you understand the complexities of all different engine types or know absolutely nothing about engines, this post should help break down some of the differences between an inline 4-cylinder and a boxer 4-cylinder engine.

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As we learned in one of our previous posts, a 4-stroke engine is one where each cylinder goes through a 4 step cycle starting with a power stroke and ending with a compression stroke to start the process over again. This is different from 2-cycle engines that accomplish combustion through exhaust in only 1 stroke, or one full rotation of the crankshaft. If all of this is foreign to you, I might suggest reading our previous post about the many differences before we dig deeper into different types of 4-cycle engines in this post.

Both the inline and boxer engines are based on the same four strokes standard to all 4-cycle engines: intake, power, compression, and exhaust. Both of these engines fire one cylinder for every 180˚ of rotation of the crankshaft as well. The differences in the engines start with the order in which the cylinders fire.

Boxer engines fire off in 1, 3, 2, 4, whereas inline engines fire off in 1, 3, 4, 2 order. For those unclear, this refers to when each of the 4-cylinders fires off or undergoes a power stroke. These firing orders may not seem like a big deal, but when you examine the layout of each engine, we can understand how they affect the balance of the machine as a whole.

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Boxer engines and inline engines both are stable from primary forces as the pistons move in pairs together. However, when we examine secondary forces, which are forces created due to changing accelerations of the pistons as they change directions at the top half of cylinder travel, we start to see some differences. The boxer engine effectively balances out secondary forces because since all of the pistons travel on one plane with the crank in the middle, secondary forces always have a counteracting secondary force from the accompanying piston. For inline 4-cylinder engines, the story is a little different. The pistons in inline 4 engines all travel on the same plane, but the crank is at the end of all 4 pistons. This means that while pistons move in pairs, the secondary forces create from the pistons all point in the same direction. For inline engines, this means that they can have more vibration and inherently be a little more unstable.

So the boxer engine is more stable than the inline engine then right? Well, no. Boxer engines aren’t perfect. Due to the layout of the pistons not being directly inline with their opposing piston, there is a rocking moment created about the crankshaft. With this said, we understand that both types of engines are unstable in certain ways due to different source forces. Surprisingly, if you add 2 cylinders to both engine types making them inline 6-cylinders and boxer 6-cylinder engines respectively, all of the unstableness is handled by the extra pistons. All of the internal forces are canceled out by the respective motion of the extra cylinders.

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Inline engines are used in many production cars, the most common being the Toyota Tacoma. This engine is a 2.7 L inline engine, which is also one of the largest. Most manufacturers will keep inline engines smaller than 3.0 Liters due to the inherent vibrations of inline-4s. Only Porsche and Subaru use boxer engines in their cars. This is in large part due to the low profile that boxer engines allow cars to keep. Because the engine is essentially flat, engine bays height can be kept to a minimum. This “flat” layout also allows the center of gravity of these cars to stay low and reduces body roll.

Even with all of that said, inline engines still have many more applications than boxer engines. Boxer engines are inherently wide, which leaves little room for suspension and steering angle in the car designed. Inline engines are smaller than boxer engines and allow for increased suspension and wheel travel due to their thin nature.

The last main difference in these engines has to do with sound. Anyone who has paid attention to how a Subaru sounds likely realizes they have a distinctive rumble. This has to do with the fact that the exhaust pipes are uneven due to the layout of the boxer engine. This ultimately results in an uneven rumble noise. Inline 4-cylinder engines maintain the same exhaust length for each cylinder, which results in a fairly low-key exhaust note.

These are all the main differences between a boxer and an inline engine. If you’re still lost on the exact mechanics of the engine, I’d recommend again that you go back and check out our previous post about how 4 cycle engines work. If you want to get much of this information in video form, Engineering Explained breaks it down with 3D printed models in the video below.

https://www.youtube.com/watch?v=mgpDTVBeHOw

Trevor is a civil engineer by trade and an accomplished internet blogger with a passion for inspiring everyone with new and exciting technologies. He is also a published children’s book author whose most recent book, ZOOM Go the Vehicles, is aimed at inspiring young kids to have an interest in engineering.


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