Driver aids
The control room needs to be in touch with the driver in a test vehicle on a chassis dynamometer, either by voice transmission or visually by way of an aid screen which must be easily visible to him.
A major function of such a screen will be to give the driver instructions for carrying out standardized test sequences, for example production test programmes or emissions test sequences. This kind of VDU display is often graphical in form, showing for example the speed demanded and the actual speed achieved. Since in the case of emissions tests, the test profile must follow that prescribed within defined limits, it is usual to include an error checking routine in the software to avoid wasted test time. The screen may also be used to transmit instructions to supplement information conveyed by the two-way voice link.
Fire suppression
For a more general treatment of the subject of fire suppression, see Chapter 4. The risk of a vehicle fire during chassis dynamometer running is quite high, since the air cooling, even with supplementary fans, is likely to be less than that experienced on the road. Underfloor exhaust systems, in particular, can become very hot and could ignite fuel vapour, whatever its source. A fixed fire suppression system is more difficult to design than in an engine test cell because of both the large size of rolling road cells and the more difficult access to the seat of the fire, which may
be within the vehicle body. All vehicle test facilities should be equipped with substantial handheld or hand-
operated fire extinguishers and staff should be trained in their use. One method of fire protection is to fit the test vehicle with a system of the type designed for rally cars which enables the driver or control room to flood the engine compartment with foam extinguishant. Automatic gas-based systems of the type used in some engine test cells are less effective in vehicle cells in view of the greater difficulty in ensuring that all personnel, including the driver, have been evacuated before they are activated. The modern trend is towards water fog suppression systems, which may include discharge nozzles mounted beneath the vehicle and thus near the seat of most potential fires.
There must be a clear and unimpeded escape route for any test driver and the impairment in vision from steam or smoke must be taken into account, particularly in the case of anechoic cells, where the escape door positions should not be camouflaged within the coned surface.
The effect of roll diameter on tyre contact conditions
The subject of tyre rolling resistance has given rise to an extensive literature, which is well summarized in Ref. 1. The bulk of the rolling resistance is the consequence of hysteresis losses in the material of the tyre and this gives rise directly to heating of the tyre. A widely accepted formulation describing the effect of the relative radii of tyre and roll is: