From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Investigation into the possible sources of external ignition on a vehicle.
| Identifier | ExFiles\Box 3\6\ 06-page162 | |
| Date | 5th May 1934 guessed | |
| - 2 - external ignition. The possible sources we think are:- (1) Loose sparking plug terminals. (2) Ventilation hole to L.T. make and break casing. (3) Space around condenser leaving to L.T. make and break casing. (4) Loose H.T. terminal in coil. (5) Ballast resistance. (6) Loose terminals or unguarded contacts on lamp switch at bottom of steering column. The ventilation holes to the H.T. distributor chamber are guarded by gauze but the two ways in mentioned in items (2) and (3) to the L.T. chamber are not so guarded, nevertheless we hardly think these can be the cause of an ignition of the petrol fumes owing to the cooling effect on the gases passing through the orifices in the casing. With regard to No (4), we think this is not a likely source as most of these terminals we have handled are tight due to the spring in the sheet metal stabber fixed to the end of the conductor. With regard to No (5), the evidence is that this never gets hot enough to start a fire although it becomes sufficiently hot to create fumes if switched on after the engine has been washed down with paraffin. With regard to No (6), we do not think that this is a likely cause but if the evidence on the actual two cars which have fired is not destroyed it would be useful to examine them for loose terminals, and we are also examining the design for unguarded contacts. It is hardly to be supposed however that the headlamps would be switched on even if the dipper switch were inadvertently operated. This brings us back to No (1) as the most possible source of ignition and this raises an important point because we find that the spring wire form of terminal has been standardised on the Bentley chassis and this type of terminal more likely than not leaves the H.T. conductor hanging loosely on the threads of the ignition plug central electrodes, which is very likely to lead to a small external spark quite sufficient to light up sprayed petrol. The evidence is that if an engine be run in the darkness equipped with this form of terminal all the ends of the ignition plugs can be seen to be sparking. We are of the opinion that this type of terminal should not be used on any of our chassis and should be | ||