System Partitioning: Three Important ‘Tests’

Ensuring System Elements Are Not Trivially Simple or Inordinately Complex
An important activity of the systems engineer is to understand the system well enough that it can be partitioned into subsystems and components. When partitioning a system, there are three “tests” that can be used to “ensure that each element is neither trivially simple nor inordinately complex and has wide application[1].” In this post, we explain the three ‘tests’ and apply them to an automobile system.

The three tests are:

1. Significance: Each functional element must perform a significant and distinct function that usually engages multiple simple functions.

2. Singularity: Each of the functions belongs within the technical view of one engineering specialist discipline.

3. Commonality: The function performed by the element can be seen in a wide range of systems.

Can These Ideas Be Applied To Partitioning Passenger Automobiles?
As an example, consider a passenger automobile. It can be partitioned into four principal subsystems which are the powertrain, chassis (also called vehicle frame-work), control system, and suspension. Together, these four sub-systems constitute the passenger vehicle system (neglecting auxiliary functions).

Figure 1. Vehicle model showing some of the subsystems and components[2].

This is shown in Figure 1 which illustrates a passenger vehicle subsystem. Each of the four subsystems were analyzed to verify that they are neither trivial nor inordinately complex by application of the principles of significance, singularity, and commonality tests. Each of the four subsystems is significant since each performs a separate function and uses multiple simple functions (components).

In addition, each subsystem is singular in that the subsystem falls (within reason) into one engineering specialist discipline. For instance, the powertrain is regarded by General Motors as a separate engineering position [3]. Also, consider that the suspension system falls within the engineering specialty of Vehicle Dynamics Engineer [4].

The other test is that of commonality and each of the four subsystems can be seen in other systems. For instance, the powertrain subsystem is used in heavy equipment systems such as tractors and in large vehicle systems such as buses. For these reasons, the four subsystems pass the tests of significance, singularity, and commonality.

Wraping It Up
System partitioning is an important function of systems engineering. The example in this post analyzed an automobile and its four subystems. It showed how the three ‘tests’ ensure the subsystems are neither trivially simple nor inordinately complex.

[1] A. Kossiakoff, et. al., Systems Engineering: Principles and Practice (Hoboken, NJ: John Wiley & Sons, 2011): 46.
[2] G. Rill, “Vehicle modeling by subsystems,” Journal of The Brazilian Society of Mechanical Sciences and Engineering, Vol. 28, No. 4, Oct/Dec 2006, pp. 430-442.
[3] This is evidenced by a recent job posting by General Motors for a “Powertrain Engine Engineer”
[4] See for instance the job posting by PACCAR for a “Vehicle Dynamics Engineer” position.

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