Coil ignition has been adopted for the B.R.M. engine in the form of four separate four-cylinder circuits, each with its ignition coil and combined contact-breaker and distributor unit. In addition to best meeting the requirements, this arrangement is of assistance to the designer in positioning the components to give a tidy engine layout and even weight distribution.
Instead of the usual half-engine speed drive, however, the distributor units are driven at only one-quarter engine speed, and utilise double contact-breakers and eight-lobe cams. The reason for this will be apparent from a consideration of the engine speeds to be attained. At a speed of, say, 12,000 engine r.p.m. the .more usual arrangement of a four-lobe cam actuating a single contact-breaker at half-engine speed would result in a cam. speed of 6,000 r.p.m., and at this speed complications due to contact breaker flinging are apt to arise. The arrangement adopted reduces the speed of cam. rotation to 3,000 r.p.m. which overcomes these difficulties. Further, the closed period is increased by the use of double contact-breakers; this is particularly important at the higher speeds in order that the coil can store sufficient energy to maintain its powerful sparking performance.
Each set of ignition equipment thus produces eight sparks for four engine revolutions. A double-ended electrode in the moulded rotor arm, in conjunction with a moulded cap having four electrodes spaced around one-half of the cap, arranges the distribution of two sparks to each cylinder per distributor revolution.
The double contact-breakers, actuated by a hardened steel cam ground to extremely close limits, are arranged so that one pair of contacts makes, and the other breaks, the primary circuit. Condensers of metallised-paper construction are employed, these having the important property of being self-healing.
Lightness in weight and robustness of’ construction to withstand the arduous service conditions are, of course, primary considerations in the design of these distributors. The body and contact breaker base are cast in aluminium: a hardened shaft carrying the cam and rotor arm rotates in ball hearings at either end, the lower bearing being of self-aligning pattern and having an oil seal fitted over it. The distributor unit is flange-mounted and the fixing holes are slotted to enable timing adjustment to be made. The moulded cap is secured firmly in position by means of a strap fixing.
The lightweight ignition coils used are specially designed to give a high standard of performance over the engine speed range.