Doctor Diesel

Hold still now…

Web04I have enjoyed reading your column for about twenty years and long may you continue. I found the article in Issue 322 on selective catalytic reduction typically interesting and informative. On the same page was a letter about electronic parking brakes, from which I inferred that the system uses the regular hydraulic system to apply the brakes on all four wheels. Is this the case? If so, how are the brakes then locked on? An explanation of how the system works in your inimical style would be appreciated. Also, with hill start assist/hill hold systems, how does the system sense when to release the brakes?
Tony Heygate

Quite a tough challenge to answer this in not too many words. Since the first EPB (Electronic Parking Brake) was seen on the 2002 BMW 7 Series, quite a number of different systems have appeared, from various suppliers, such as TRW (the US originators), Continental and Bosch. There are fundamentally two types, with variations within these. The simpler type employs a cable system, just as with normal lever handbrake systems, but with the cable tension applied by an electric motor, which operates the normal rear axle callipers/shoes – be they drums or discs. This route is obviously easier to engineer into systems derived from a traditional handbrake system, on cars where the EPB is possibly only a feature of high-specification variants.

The more advanced systems dispose of the cable and rely totally on electrics to operate an electric motor to actuate separate callipers on the rear wheel brakes. Such truly electronic systems can be integrated fully with the normal foot operated power assisted hydraulic brakes, where the power is nowadays supplied by a vacuum pump rather than intake manifold vacuum, as used to be the case. This means that at speeds beyond a certain point, typically 7 to 10mph, the EPB can be linked to electronically initiate operation of all four hydraulic brakes, and therefore the ABS system, in which case the EPB effectively becomes a full four wheels emergency brake. Various overrides can be built into the systems to prevent EPB brake release if doors are open, seat belts are not used in occupied seats, and also to prevent release if vehicle movement is detected. Release is allowed in an in-gear, ignition on, accelerator applied situation, with an anti-roll back lock-out. In either case, although the variations are probably easier to engineer into the fully electronic system, hill-hold, hold-assist, and similar systems are just integrated into the electronics, with input from wheel speed and other motion sensors, and timers, such that the brakes will either hold on after foot release for a few seconds, or if roll-back is detected by sensors. I hope this outlines the methods employed, although I think after your letter that this might be a suitable subject for longer description in Back to Basics! Many thanks for your interest and support. Best regards,
Doc Diesel

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