Engines Exposed: Flat, V, or Inline?

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Pagani Zonda Engine

Source: Pagani

I have been a fan of The Simpsons since I was quite young. One of my favorite sequences is in the episode “Reality Bites” in which Homer buys Snake’s car. When he brings it to Moe’s Tavern to show it off to his friends, Moe proceeds to ask him a number of questions about the car:

Moe: Geez, this hot rod is souped up six ways from Sunday! Never had you figured for a gearhead, Homer.
Homer: Oh yeah, I’m a real expert.
Moe: What is that, a six barrel Holley carb?
Homer: You betcha!
Moe: Edelbrock intakes?
Homer: Nothing but.
Moe: Myohoff lifters?
Homer: Oh, yeah.
Moe: I made that last one up.
Homer: I see.

For many, mechanics do seem to speak another language. I had my first encounter with this when I realized that a V8 engine was describing more than just the number of cylinders. I thought that any engine with eight cylinders was a V8. As it turns out, the V means something, and I’m here to explain it to you.

The vast majority of cars are powered by reciprocating internal-combustion engines. This means that the piston moves back and forth inside a cylinder, which is known as reciprocating motion. It was quickly determined that adding cylinders to engines meant there was the possibility for more power.

The question then arose, “Where should the second cylinder be built relative to the first?” From a machining standpoint, putting the cylinders side-by-side makes the most sense as it allows for the block (the piece of metal into which the cylinders are machined) to be one piece. However, engineers, never ones to simply accept the simplest option, have explored a variety of alternatives. Over the past century, three distinct styles of reciprocating engine have emerged.

Source: BMW

Source: BMW

1. Inline (or straight) engines

The inline engine arranges all of the cylinders in a row, which makes the block straight. The main advantages of this engine setup are that it is simple and smooth. BMW used “Straight-six” engines for a long time, and they were well known for being exceptionally smooth. The reason for this is that inline engines with an even number of cylinders are able to balance the forces inside the engine. For each piston moving upward, there is another piston moving downward at the same time.

The main drawback to inline engines is that they take up a larger amount of space. As cylinder sizes increase, the overall length of the block is forced to grow as well. In an era when keeping cars small is an important goal, six and eight cylinder inline engines have fallen out of favor; however, inline-four cylinder engines are extremely popular, and are used in many modern cars. They are often small enough that they can be mounted transversely (parallel to the axles), which provides more space for the passengers. It should be mentioned that an inline engine mounted at an angle is called a “slant” engine, such as the Chrysler slant-six or the BMW slant-six shown above.

2015-porsche-918-spyder-engine

Source: Porsche

2. V-style engines

A “V” engine is so named because the cylinder banks are literally positioned in the shape of the letter V. A V8 engine has four cylinders on each bank of the engine (as shown in the Porsche 918 4.6 liter V8 above), a V10 has five on each bank, etc. The major benefit of this engine style is that it allows for more cylinders in a smaller engine package. “V” style engines are also the basis for the “W” style engines that are used by Volkswagen, which puts two banks of cylinders in each side of the “V.”

The challenge with “V” style engines is that they need to be balanced properly. That requires a bit of math given the firing order and construction of this type of engine; luckily, we have had a lot of trial and error and have determined that V6 engines like the angle between their banks to be set at 60 degrees, V8 engines prefer 90 degrees, and V10 engines like 72 degrees. V12 and V16 engines, in a fascinating bit of engineering, are basically two inline engines that are mated at the crankshaft so they are inherently balanced due to the same smoothness that makes inline engines so appealing. “V” style engines require two separate cylinder heads, which adds to the complexity and cost of these engines.

Source: Subaru

Source: Subaru

3. Boxer (or flat) engines

The Boxer engine was developed by Karl Benz in 1897 and uses a 180-degree angle between the two cylinder banks — meaning that they lay flat and oppose each other. Consequently, the engine balances itself because the pistons move in a synchronized way that balances the opposing forces. Boxer engines have been used by a wide variety of manufacturers over the years, but are primarily found in Subaru and Porsche cars at this point. The flat orientation of the cylinders means that the center of gravity is lower, which can benefit handling.

If you ask the general public, they will tell you that the major challenge of Boxer engines is that they tend to be noisier than other engines. However, this isn’t really true: Each firing of a flat engine produces no more sound than it would if the cylinders were in a different configuration. The reason for the noisy reputation is that the sound is different due to the firing order of the engine and the exhaust system used on the car. It tends to pulse more than straight or V style engines, which is exaggerated due to the large percentage of cars with flat engines (Subaru WRXs in particular) that have aggressive aftermarket exhaust systems.

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