Engineering Explained: The Pros And Cons Of Different Engine Types

The most common engine types - the four-cylinder, the boxer-four, straight-six, V6 and V8 - have their own pros and cons. Here's everything you need to know in one handy guide...

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BMW - Engineering Explained: The Pros And Cons Of Different Engine Types - Blog

What makes more power, a 4.0-litre V6 engine or a 4.0-litre V8? The answer isn’t so simple. When discussing various engines, the layout isn’t the biggest contributing factor to how much power it makes. With a bit of ingenuity (and you know, cash), a four cylinder engine can make just as much power as a V12. So what makes manufacturers choose different engine layouts? Here are the advantages and disadvantages of each layout.

1. Four-cylinder inline four

BMW - Engineering Explained: The Pros And Cons Of Different Engine Types - Blog

Let’s start with one of the most common engines, the inline four cylinder. There’s a reason it’s common, largely because it’s so simple: one cylinder bank, one cylinder head and one valve train. Here’s all you need to know:

Advantages:

  • The four-cylinder, inline four is small and compact, meaning it easily fits in nearly any engine bay.
  • It’s also lightweight, and with only one exhaust manifold, weight is further reduced.
  • With only one cylinder head, there are fewer moving parts than engines with multiple cylinder banks. This means less energy is lost which reduces the probability of malfunctions.
  • Primary forces are balanced because the outside two pistons move in the opposite direction of the inside two pistons (see picture above).
  • Four-cylinder engines are easy to work on; the cylinder head is the highest point which makes spark plug jobs and valve train access very easy.
  • Four-cylinder engines require lower manufacturing costs.

Disadvantages:

  • Secondary forces are not balanced, which ultimately limits the size of the engine.
  • Inline fours will rarely exceed 2.5 litres to 3.0 litres.
  • Larger four cylinder engines will often require balancing shafts to cancel the vibration caused by the secondary imbalance.
  • High centre of gravity compared to some layouts (H4).
  • Not as rigid as some layouts (V6, V8).

Here’s a quick video explanation of the four-cylinder engine:

2. Horizontally-opposed

BMW - Engineering Explained: The Pros And Cons Of Different Engine Types - Blog

From a performance standpoint, there aren’t many options as attractive as an engine with horizontally-opposed cylinders. The boxer four isn’t nearly as common as the other engines on this list, but from an engineering standpoint it’s a logical choice for your race car.

Advantages:

  • Primary and secondary forces are well balanced. This is a smooth engine.
  • This allows for less weight on the crankshaft, resulting in less power lost to rotational inertia.
  • Low centre of gravity allows for better handling.

Disadvantages:

  • Packaging size: these are very wide engines.
  • Flat engines were once used in Formula 1 for their performance advantages, but due to their width they obstructed airflow and are no longer used.
  • Complexity - two cylinder heads/valve trains.
  • Rocking couple (plane imbalances) due to offset pistons to allow for the connecting rods to connect with the crankshaft.
  • Maintenance can be challenging if packaging is tight.

3. Straight-six

BMW - Engineering Explained: The Pros And Cons Of Different Engine Types - Blog

An engineer’s object of affection, the straight-six is the result of tacking on two more cylinders to an inline four engine. BMW loves them, and it’s the layout of one of the most well-known boost-ready engines, the 2JZ. So what’s so special about the straight-six?

Advantages:

  • The straight-six is Inherently balanced.
  • The layout combined with its firing order leads to essentially the smoothest engine out there.
  • V12s and Flat-12s are the next step in further reducing vibration, as they are two I6s matched together.
  • Lower manufacturing cost - single cylinder block with all the cylinders in one orientation.
  • Simple design, easy to work on much like the I4.

Disadvantages:

  • Packaging can be difficult due to the length.
  • Not ideal for FWD vehicles.
  • High center of gravity (vs flat engines).
  • Lower rigidity than V engines as it’s long and narrow.

Here’s a quick video explanation of the straight-six:

4. V6

BMW - Engineering Explained: The Pros And Cons Of Different Engine Types - Blog

Now cut that straight-six in half and match the two cylinder banks to a common crank. The V6 is a common layout when there are six spark plugs involved. It’s also the current layout for Formula 1 engines. Why use it?

Advantages:

  • They’re compact and can easily be used for both FWD and RWD vehicles.
  • Allows for greater displacement than four-cylinder engines, typically meaning more power.
  • Rigid design.
  • Formula 1 chose to use V6s rather than I4s for the 2014 season because they wanted to use the engine as a stressed member of the car.

Disadvantages:

  • Two cylinder heads means added cost, complexity, and weight.
  • Additional rotational inertia and friction (more moving parts).
  • High centre of gravity vs flat engines.
  • Cost is often greater than inline.
  • Secondary imbalance requires additional weight on the crankshaft.
  • Two exhaust manifolds means additional weight.

5. V8

BMW - Engineering Explained: The Pros And Cons Of Different Engine Types - Blog

When you add a cylinder to each bank of the V6, you get an icon in both American muscle and European exotics - the V8. It can produce a refined whine, or a shuddering burble. So what makes this layout such a popular choice?

Advantages:

  • Packaging size (short in length).
  • Good balance, depending on the crankshaft type and firing order (flatplane vs crossplane).
  • Rigid design.
  • Allows for high displacement.

Disadvantages:

  • Like a V6, the V8 engine’s weight can be high.
  • Additional rotational inertia and friction (more moving parts).
  • Cost and complexity will be higher.
  • Higher centre of gravity vs flat engines.
  • Engine weight is usually increased.
  • Packaging is large, typically restricted to RWD/AWD vehicles.

Let us know below which engine type you are currently running and what you like and loathe about it.