Inline 5-cylinder engines

The Straight-five engine is not very common in motor industry. only Audi (2.2 and 2.3-litre), Honda (Acura TL), Volvo (2.0-litre, 2.3 turbo and 2.4-litre), Fiat group (2.0 and 2.4-litre Super Fire series) and Mercedes diesels adopted such designs. 

However, straight-five engine has its own advantages. 

Firstly, it bridges the gap between 4 and 6-cylinder engines, thus may offer the best cylinder capacity for optimised efficiency; 

Secondly, compared with 4-cylinder engines, it saves one balancer shaft; 

Thirdly, compared with Straight 6-cylinder engines, it is short enough to be fitted transversely into the engine compartment of front-wheel-drive cars, driving directly the inline gearbox. 

Lastly, it can be derived from a modular design consisting of 4 and 6-cylinder inline engines, not only saving development cost but also eliminating the investment of a new production line. 

Fiat, Mercedes and Volvo’s 5-pots, for example, are made as modular engines.

The inline-5 engine fires once every 720° / 5 = 144° crank angle. As a result, the crankshaft design is as shown in below. Firing order is 1-3-5-4-2.
 

 

Mathematical analysis proves that both its resultant first order force and second order force are balanced. Therefore it doesn’t need the twin-balancer shafts as in a big 4-cylinder engine. 

However, it generates end-to-end vibration like 3-cylinder engines, because piston 1 is not in the same position as piston 5, and piston 2 is not in the same position as piston 4. 

Therefore both ends of the engine will vibrate up and down with respect to the engine center.

Solution - single balancer shaft

Obviously, the solution is the same as 3-pot engines, that is, employ a balancer shaft on which there are counter weights moving in the opposite direction to the pistons. The balancer shaft is driven by the engine at the same speed as the crankshaft.

Is that enough to make 5-cylinder engine as smooth as 6-cylinder? no. For packaging reasons, the balancer shaft cannot be placed in the most optimised position, that is, right above or below the crankshaft. Therefore it has to be offset to either side of the engine, resulting in incomplete cancellation of vibration.