Engineering Explained: The Pros And Cons Of Inline 5s, V10s And Rotary Engines

It’s not uncommon in the engineering world to do something simply because everyone else is doing it. These next few engine layouts offer unique advantages all while taking the road less travelled. Here's everything you need to know:

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1. Inline 5 Engines

Famously used by both Audi and Volvo, the I5 has actually been produced by a good number of car manufacturers but used in only a few models. It’s the engine for those people who can’t make up their minds between an I4 and an I6. Here are the advantages and disadvantages of each engine configuration.

Advantages

  • Smooth power delivery: unlike an inline four cylinder, the I5 actually has overlapping power strokes (36 degrees of crankshaft rotation overlap).
  • Balanced forces: both primary and secondary forces from reciprocating mass are well balanced vertically, unlike inline four cylinders.
  • Displacement: with more cylinders and balanced forces, the I5 can offer larger displacement than inline four cylinders with less vibration.
  • Packaging: matched against a V6 or I6, the I5 offers superior packaging, allowing it to fit FWD models, but is also flexible for other configurations.
  • Simplicity: like other inline layouts, there is only one cylinder bank, and one cylinder head, allowing for less moving parts and simple maintenance.

Disadvantages

  • Plane imbalance: unlike a four-cylinder engine, the I5 wants to rock front to back as the moments are not cancelled out. This is a result of the piston locations, and requires a balancing shaft to reduce the vibration caused (video below includes more detail).
  • Packaging: longer than an inline four-cylinder.
  • Weight: heavier than an inline four-cylinder.
  • Cost: having a larger block and more moving parts, an I5 costs more than an I4 to produce.

Here’s a quick video explaining the five-cylinder engine:

2. V10 Engines

Famously used in the Dodge Viper and Lexus LFA, though in quite different fashions, the V10 engine bridges the gap between the V8 and the V12 rather well.

Advantages

  • Balanced forces: since a V10 is simply two I5s mated to a common crankshaft, it sees many of the same benefits. This includes the fact that the reciprocating mass forces are balanced.
  • Less reciprocating mass: compared with a V12, a V10 has fewer cylinders and thus can have less reciprocating mass.
  • High revving: Lexus LFA engineers chose to use a V10 rather than a V8, as they were able to get the V10 to rev higher.

Disadvantages

  • Balancing shafts: balancing shafts are required in order to eliminate the vertical moment vibration caused by the plane imbalance, similar to an I5 cylinder.
  • Smoothness: a V12 is inherently balanced and has more overlap between power strokes, making it a smoother engine layout than the V10.
  • Cost: compared to a V8, the V10 is more complex with more moving parts, and consequently is more expensive to make.

Here’s a quick video explanation of a V10 engine:

3. Wankel Engines

Try not to get caught up in the semantics of what to call this engine. Commonly referred to as a rotary engine (even by Mazda, though often this refers to a rotating piston-cylinder-based layout), the Wankel engine was last used in production in the Mazda RX-8. There are no pistons, camshafts, or connecting rods.

Advantages

  • Simplicity: rotary engines can have as few as three main moving parts, versus more than 40+ for piston-cylinder based engines. Fewer moving parts typically leads to better reliability.
  • No reciprocating mass: this allows rotary engines to rev high, and also run very smoothly.
  • Weight: rotary engines are compact and offer great power-to-weight ratios.
  • Power delivery: because of the way a rotor rotates, power delivery lasts for more of the rotation of the crankshaft versus a piston-cylinder engine, resulting in super smooth power delivery.
  • Size: rotary engines are compact, allowing for easy packaging.

Disadvantages

  • Fuel economy: the exhaust often includes unburned fuel, on top of which Wankel engines typically have low compression ratios, resulting in poor fuel efficiency.
  • Emissions: unburned hydrocarbons leaving the exhaust makes it difficult to pass emissions regulations.
  • Rotor sealing: due to the varying temperatures throughout the combustion chamber, the apex seals expand and contract making it difficult to create a good seal, leading to inefficient power production.
  • Oil burning: by design, Mazda Wankel engines burn oil to help maintain the longevity of the apex seals. Not only does this further increase exhaust emissions, but it requires the owner to add oil periodically.

Here’s a video to explain Wankel engines:

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