Over-run and cut-off

Over-run and cut-off

During over-run, the mixture must be made more lean to reduce fuel consumption and polluting emission levels (HC + CO).

To achieve this aim, the control unit contains a function known as fuel CUT-OFF that is dependent upon the closure of contact (1) in the idle control motor at a given coolant temperature and rpm. The first two conditions necessary for activating the CUT-OFF function are:

• - coolant temperature in excess of 40° C,
• - engine rpm in excess of 1800/min.

If the above over-run conditions exist, idle contact (1 - throttle closed) earths terminal 3 of the control unit, which cuts off fuel injection via terminal 21.

In addition to fuel cut-off, the control unit activates a specific ignition curve with "over-run" advance settings.

Fuel injection cut-off and specific advance curves remain in operation until the engine reaches a speed of 1500/min. Below this speed, injection pulses are activated to prevent the engine stalling.

When injection is restored, the control unit implements a fuel enrichment strategy in order to re-form the fuel film on the manifold and thus optimise engine efficiency.