Fisher & Paykel Appliances

Modelling the performance of a front loading washing machine.
Analysis and Modelling of Soil Removal and Gentleness of Wash Performance.

Fisher and Paykel Appliances

The clothes washing process removes dirt particles and grease-like products by a synergistic combination of chemical action, thermal action and mechanical action.  More action of one type can compensate, at least to some degree, for less action of another type (ex: if detergent is increased, wash time can be decreased.).

Mechanical action is composed of soil removal through friction within and between garments, friction between garments and the machine and fluid flow through fibres and garments.  Chemical action is soil removal performed by the detergent and thermal action accounts for the effects that temperature introduces (ex: increased detergent chemical action and fibre diameter with higher temperatures; garment stiffening with cold temperatures.)

In the washing process customers are primarily concerned with soil removal1 from their garments, but they are also concerned with the “wear and tear” that the garments are subjected to.  This is referred to as gentleness of action2.  Gentleness of action results from the rubbing actions described above.

Wash Performance (WP) is the combination of the soil removal and gentleness of action.  Many wash parameters can be changed to increase soil removal but will simultaneously decrease gentleness of action.  Trade-offs need to be made and balances struck so that the overall wash performance will be acceptable to the user.

Wash performance testing of a particular washing machine is performed under controlled conditions, including:  Machine dimensions, water temperature and volumes, detergent concentration, load size, wash program parameters (tumble pattern; time; tumble speed).

Swatches of special fabrics are attached to garments to provide estimates of soil removal and gentleness of action.   Test results are used by F&P designers and developers to make design decisions.  Parameters may be changed and testing repeated until acceptable performance is achieved.

There are many variables involved in the washing machine (see a basic list in table 1, below). 

Table 1: Some variables affecting wash performance

Name  Unit  Description 
Drum radius  mm 
Load size  kg  mass of load (dry) 
Load size fraction     = {mass of test load [kg])1 (Maximum volume capacity of drum[kg]) 
Total wash time  mm:ss  time during wash portion of cycle during which the drum is  rotating 
tumble speed  rpm  speed at which drum rotates 
Wash water volume  Litre total amount of water added to machine to wash the clothes  (excludes rinse water) 
Wash liquor concentration  ml/l or g/l = {detergent amount [ml or g]) / (wash water volume [l]) 
number of drum revolutions  sum of drum revolutions in clockwise and anti-clockwise directions during wash portion of cycle (excludes any sensing) 
pause time  mm:ss time the drum spends stationary between clockwise and anti-clockwise movement 
running time  mm:ss time the drum spends rotating in either clockwise or anti-clockwise directions between two pauses 
tumble pattern    (x_y_z) where x = running time clockwise, y= pause time, z= running time anti-clockwise.
NOTE: it would be more complete to  state it as (x_y_z_y)

Desired Outcomes:

While physical testing will always be part of our development regime, we would like to improve our designs and reduce our development time by utilizing mathematical modelling. We would thus like to have a mathematical model that quantitatively estimates the effect on soil removal and gentleness of action that a parameter change would have. The model could be based on fundamental physics, empirical statistics, dimensional analysis or some other method.

Glossary of Terms:

  • Soil type: Description of the dirt present that the washing process is expected to remove. Ex: body oil and skin; charcoal; protein; wine;

  • Gentleness of action: How much wear and tear a load is subjected to in a wash cycle. Quantified by the surface area of fabric swatches that has disturbed thread weaves at the end of a cycle. Commonly referred to as "fray".

  • Soil removal: how much dirt is taken away from a load during a wash cycle. Quantified by the colour change in commercially-prepared pre-soiled fabric swatches from a wash cycle.

1Soil Removal

This is measured by the colour change of “standardised dirt” cloth swatches after a wash cycle when compared to a “clean” standard swatch.  The Soil Removal is defined as two standard deviations less than the average colour change.

SR= Cmean – 2s

2Gentleness of Action

This is measured by the amount of fray seen on 20 or so, 1dm squares of coarse-weave cloth after the wash cycle.

Figure 1: Soil removal swatches, before and after wash cycle (top was heavy cycle; bottom was delicate cycle)

Figure 2: Gentleness of action (fray) swatches- before and after wash cycle