Methods of Measuring Friction

Exact calculation of the friction losses involves a great deal of work. There are various ways of determining the friction, although the majority of these exhibit significant inaccuracies. The following methods are commonly used for calculating the friction:

- The rundown method: Here the engine is switched off after stabilization at an operating point, and the change in speed is measured as a function of time. The friction moment or mean friction pressure is then calculated using the moments of inertia of the moving masses.

- The shutoff method: On multiple-cylinder engines, the fuel supply to one of the cylinders is shut off, and this cylinder is then dragged along by the other working cylinders. The friction loss can be determined from the change in effective engine power before and after the fuel shutoff.

- The Willans lines: The fuel consumption of an engine is plotted on the Y axis against the mean effective pressure p_me for various engine speeds. The intersections with the negative p_me axis are then determined by linear extrapolation of the values down to fuel consumption zero; these can be roughly regarded as the mean friction pressures at the respective engine speeds.

- The motoring method: The engine is motored on a test rig by an external motor. The motoring power required to drive the engine is regarded as the friction loss. With this method either the engine can be motored at operating temperature and measured immediately after shutting off the fuel supply or it can be conditioned via external thermostat installations.

- The strip method: Strip measurement is a special form of motoring that is used to measure the friction losses of the various engine components, such as, the friction of the engine, the valve train, and the auxiliary drives. The designation derives from the method where the engine is dismantled (stripped) step-by-step on a motoring test rig. The friction losses of the individual components are determined from the difference between the measured values with and without these components. The total friction of the engine is obtained by addition of the values for the individual components.

- The indication method: This method can be used to determine the friction of an engine in motoring mode. Integration of the measured cylinder pressure over a working cycle gives the indicated work which, referred to the swept volume, gives the indicated mean pressure p_mi. If the mean effective pressure p_me calculated from the torque measured at the drive shaft is subtracted from this, we obtain the mean friction pressure p_wr.

- Special measuring method: Apart from the friction measuring methods described above, there are a large number of other methods for determining, for example, the friction of individual components during operation. Torque measuring flanges can be used to carry out measurements on components driven by shafts. For the piston group there are various facilities for measuring the piston frictional force.

A crucial aspect for the precision and reproducibility of the individual methods and, hence, for the comparability of various measurements is strict compliance with the boundary conditions. For all these measurement methods, for example, the lubricating oil and coolant temperatures of the engine have to be set to less than ± 1 K. This is generally possible using only high-precision external thermostat installations.

Of the possibilities described for determining p_mr, the first three are subject to significant inaccuracies from the principle of the method alone and are therefore suitable only for the identification of trends.

With the motoring method, the problem is that the inertia moment of a complete engine includes not only the mechanical engine friction and the drive power of the auxiliary drives but also the charge cycle losses and that without additional indication no distinction can be made between the friction and the charge cycle losses. However, since the charge cycle losses react very sensitively to changes in ambient conditions on the test rig or to minor differences in the intake and the exhaust systems, the comparability of different engines is rather restricted with this method.

With the strip method, the boundary conditions can be set very accurately using external systems so that a good reproducibility and comparability of the results can be achieved. Characteristic for the strip method is the fact that the engine is always driven via the output shaft. This has the advantage over other measuring methods that the boundary conditions for the components under consideration are as close as possible to the conditions in the engine proper and a good transferability of the results is guaranteed.

At the same time, this results in the limitation to the application of the strip method for determining the friction losses of any particular parts of the rotating engine: A functional (in the sense of the motoring operation) configuration of the engine must be possible with and without simultaneous movement of the parts under consideration. As a consequence, this means that the friction values measured for a component always also include the friction attributable to the drive; these are also eliminated when the components are removed. For example, the determination of the friction in the valve train also includes the friction generated in the timing belt or timing chain. This is also expedient in that the power losses can be allocated to the component in question and the load and any dynamics affect the level of the power losses.

The indication method demands a higher measuring complexity in order to obtain reliable results. A great influence comes from the fact that with multiple-cylinder engines, the individual cylinders can exhibit significant differences in their mean pressure. For this reason, a pressure measurement on all the cylinders at the same time is necessary.

This causes considerable measurement complexity in practice. Furthermore, the complexity is increased by the fact that even minor errors in the TDC positioning and deviations in the pressure measurement from the calibration curve of the pressure sensors cause a significant difference in the p_mi value, and errors in the torque measurement distort the p_me value. Very great demands, therefore, have to be made on the accuracy of the indication and the torque measurement, as the result of the subtraction (of the mean friction pressure) is more than one power smaller than the initial parameters, so that the percentage errors are multiplied by a factor of ten.

Even minor deviations in the determination of the TDC of the piston therefore influence the calculation of the mean indicated pressure and, thus, also of the mean friction pressure. Fundamental studies have shown that an error of only 0.1° in the TDC position of the crankshaft can affect the calculated mean friction pressure by more than 10%, depending on the engine load.

A direct comparison of the different measurement methods is not possible, since the different boundary conditions influence the measurement results. This is illustrated as an example of a diesel engine with direct injection in Fig. 9-2. The fluid temperatures have been kept the same for the complete series of tests: 90°C oil temperature in the main gallery and 90°C coolant outlet temperature. A good correlation over the whole engine speed range is obtained between the results of the strip measurements and the motoring measurement (the charge cycle losses were determined by indication and deducted).

Fig. 9-2. Comparisons of different measuring methods on a car diesel engine with direct injection

The different friction values discovered with the motored engine are attributable to the following influences:
- The lubricant film temperatures in the engine are higher in spite of the same temperature in the main oil gallery.
- The combustion results in higher temperatures at the piston group and cylinder barrel.
- The lateral piston forces change due to the gas pressure.
- The load conditions of the injection pump change.