How It Works

Electronic ignition

‘Fuel injection itself has been around for more than a century’

fuzz townshend CCW’s master meChaniC

Enthusiasts often bemoan the loss of tinker-ability on modern classic cars, but the reality is their engines still work on the 150-year-old suck-squeeze-bang-blow principle.

Fuel injection has been around for more than a century. Diesels have been using it since the year dot, doling out accurately measured squirts of atomised fuel at precisely the correct moment.

Earlier petrol injection systems, such as the Bosch in-line pumps in early- to-mid-1960s Mercedes, used technology allied to that of diesel injection, but as the 1970s progressed, mechanical Bosch K-Jetronic became king of the modern engine bay. Solid-state electronics eventually replaced mechanical operation, bringing things pretty much up to date into the modern classic era.

Electronic fuel injection requires a number of sensors to gather information for an electronic control unit (ECU) so that it, in turn, can tell the injectors to supply the correct amount of fuel at precisely the right time, while simultaneously supplying ignition timing information to the coil pack.

First and foremost is the crankshaft position sensor, which feeds information to the ECU so that it can determine basic engine timing and orientation. Basic ignition timing is determined using this information too, although the throttle position sensor feeds additional data that can alter this aspect. Engines with variable valve timing also have a camshaft position sensor, providing information regarding valve orientation.

Throttle bodies feature a throttle butterfly valve, similar to that found in a carburettor and likewise operated by the throttle pedal, which controls air flow into the inlet manifold. Attached to the pivot spindle of this is a throttle position sensor, which feeds data regarding the butterfly’s position to the ECU. A temperature sensor relays information regarding coolant and engine temperature to the ECU, where it is used to determine the amount of fuel supplied by the injectors. This replaces the choke, supplying extra fuel to a cold engine.

In the fuel supply circuit, a lowpressure pump may be used to draw fuel into a swirl pot, which in rudimentary terms is a reservoir of readily available fuel from which an injection-pressure pump can meet the needs of the injectors.

This pump is often situated in the petrol tank, negating the need for a separate swirl pot.

The high-pressure pump supplies fuel direct to the injectors in readiness for when they’re opened by the ECU. Steady pressure is maintained by a regulating valve that opens and returns fuel to the tank when the pressure rises above operating levels. The ECU-controlled injectors spray a given amount of fuel over a given amount of time, depending on temperature, throttle position and crank position.

Other sensors often used include a mass airflow sensor/meter, which determines the volume of air entering the inlet manifold and a Lambda sensor, which analyses exhaust gas oxygen content to enable the ECU to achieve more accurate control of the fuel/air ratio and the waste products of combustion.