Cross-plane or Flat-plane Crankshaft?

One of the most important parts of Internal Combustion Engines (ICE) is the crankshaft. There are different crankshaft configurations according to the type of engine and performance required. With that in mind, this blog post explains the attributes and differences of cross-plane and flat-plane crankshafts for V8 engines using Dymola software as a design, test and validation tool.

INTRODUCTION

The Internal Combustion Engine (ICE) is an engineering masterpiece composed by hundreds of parts working in total synchrony and harmony with the main objective of delivering the right amount of power to get the vehicle in motion.

One of the most important parts of ICEs is the crankshaft. It is present in every engine, from the one used by your daily car to the one used by F1 cars.

In a vehicle, the crankshaft is a shaft located at the bottom of the engine. It converts the reciprocating motion (coming from the pistons) to rotational motion (present in the rotation of the driveline, gearbox and wheels).

Figure 1. Flat-plane crankshaft for a V8 engine
(Grabcad, 2015)
Figure 1. Flat-plane crankshaft for a V8 engine
(Grabcad, 2015)

There are different crankshaft configurations according to the type of engine and performance required. That said, this blog post is going to be focused on cross-plane and flat-plane crankshafts (Fig.1) for V8 engines.

CROSS-PLANE CRANKSHAFT

A crankshaft is represented by an arrangement of 90 degrees between crank throws. This configuration can only be used on 8 and 16 cylinder engines (basically those with multiple of 8), therefore, it is the most common around the globe and widely employed in the American muscle cars world.

PROS

  • Unique engine note
  • Smooth running due to a constant crank rotation as a consequence of having cylinder firing every 90 degrees
  • High torque at low rpms

CONS

  • Heavy counterweights (additional rotational mass)
  • Lower revving limit
  • Larger crankcase

FLAT-PLANE CRANKSHAFT

On the other hand, a flat-plane crankshaft is a crankshaft with an arrangement of 0,180,180 and 0 degrees between crank throws. This configuration is used with less frequency on V8 engines, nevertheless, it is commonly used on in-line 4 cylinders. An interesting fact is that this type of crankshaft is the choice par excellence for European supercar manufacturers such as Ferrari and McLaren due to the following points.

PROS

  • Well balanced (compared to cross-plane cranks) though worse second order vibrations.
  • Light counterweights (compared to the cross-plane cranks)
  • Quicker engine response
  • High revving limit
  • Compact design
  • Better exhaust scavenging (compared to the cross-plane cranks) due to alternate bank firing sequence.

CONS

  • Vibrations as a result of the 180 degrees throw offset angles
  • Lack of torque at low rpms

CRANKSHAFT MODELLING IN DYMOLA

The two previous crankshafts were modelled in Dymola with the aim of highlighting their characteristics and carrying out the respective comparison.

CROSS-PLANE CRANKSHAFT

This crankshaft experiment was modelled employing a 3600cc V8 engine with a configuration of 0, 90, 270 and 180 degrees between crank throws (Fig. 2).

Figure 2. Cross-plane crankshaft experiment
Figure 2. Cross-plane crankshaft experiment

FLAT-PLANE CRANKSHAFT

As for the flat-plane, the past experiment was taken as a base. Then, the angle between the crank throws (throwOffsetAngle) was modified in order to have a configuration of 0, 180, 180 and 0 degrees between crank throws (Fig.3). The valve timing was also revised to be correct for the flat plane crank configuration.

Figure 3. Flat-plane crankshaft experiment
Figure 3. Flat-plane crankshaft experiment

CONCLUSIONS

Both crankshafts are good options for V8 engines, it all comes down to the application for which they are going to be employed. If it is more on the racing side, the flat-plane crankshaft is one step ahead due to its compact design, reduced weight and quicker response. On the other hand, if you prefer engine sound over performance then the cross-plane crankshaft is your choice to go for.

CURRENT INNOVATIONS

Nowadays car manufacturers have deployed new technologies which also help to reduce the disadvantages of flat-plane crankshafts. On one hand, the invention of active engine mounts and engine damping have reduced the transmission of vibrations into the vehicle, while on the other hand, the wide-scale use of conventional turbochargers and new E-Turbos (Fig.4) have increased the amount of V8’s torque delivered at low rpms.

Figure 4. Garret E-Turbo
(Autoblog, 2019)
Figure 4. Garret E-Turbo
(Autoblog, 2019)

REFERENCES

  1. Biermann, R. (2018). Battle Of The V8s: Flat-Plane Vs. Cross-Plane. [online] CarBuzz. Available at: https://carbuzz.com/features/battle-of-the-v8s-flat-plane-vs-cross-plane.
  2. Bosch, R. (2004). Automotive handbook (6th ed.). Bosch.
  3. Grabcad (2015). Crankshaft for V8. [image] Available at: https://grabcad.com/library/crankshaft-for-v8-1.
  4. Autoblog (2019). Garrett E-Turbo marks the evolution of electrically-assisted boost. [image] Available at: https://www.autoblog.com/2019/10/21/garrett-e-turbo-electrically-assisted-turbocharger.

Written by: Jose Miguel Ortiz Sanchez, Project Engineer

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