Tech insight: Understanding injection


To begin, perhaps a few words about the two main types of fuel injection - pulsed electronic and continuous injection - is called for.

Continuous injection is sometimes referred to as mechanical because fuel metering relies on the mechanical relationship between an airflow sensor and a fuel distributor.

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A typical electronic fuel-injector.Superimposed pictures show the difference between a poor spray pattern, above, before cleaning, and a good spray pattern, below, after cleaning.

Incoming air is measured by an airflow sensor plate, linked to a fuel distributor. This mechanical link meters fuel according to the incoming air flow.

Pressure-activated injectors deliver fuel to the engine. So long as the engine runs, fuel is injected in continuous streams - hence the name. The fuel distributor adjusts fuel pressure to control the volume of fuel delivered under all operating conditions.

In pulsed or electronic fuel injection systems, the incoming air is measured by a sensor that sends a signal to the electronic control unit. In response, the ECU sends fuel to the engine by electrically-operated solenoid valve injectors.

Fuel is injected proportional to engine speed. Delivery is fuel per pulse and according to mixture requirements.

Injectors normally give long and trouble-free service, but injector clogging can happen, depending on how the car is used. Many factors can cause clogging: a car driven long enough to allow complete warm-up and then parked for half an hour or more encourages heat-soak.

This is often overcome by, apart from simply changing driving cycles, fitting a special fan in the engine compartment to cool the injectors once the engine is switched off. There are special injectors available that help carry the heat away from the injector tips. Have a word with your car manufacturer’s spares department to find out what is available.

It’s a catch-22 situation sometimes because, by using a fuel additive to clean the injectors, this could free other deposits that might clog the system. Note that some petrol additives may cure clogging, but form a fluffy carbon build-up on the inlet valves. These may lead to higher fuel consumption as they absorb fuel and, more noticeably in cold running conditions, hesitation and erratic idling.

Seek expert advice before using an additive.

When the engine is running, fuel flow keeps the injector tips cool but once the engine is turned off, it acts as a form of heat sink and for a while temperatures climb especially at the valves and manifolds. This does not affect performance, but the injector tips also get extremely hot and the small amount of fuel at the tip of the injector breaks down and forms a deposit.

The tiny orifice at the tip of the injector thus becomes partially blocked with carbon and restricts fuel flow. If you observe the fuel spray pattern of a dirty injector and then compare it to a clean one, you will see a vast difference and this could mean a 50 percent or so penalty in fuel delivery.

Usually, one injector will become clogged before the others and that one affected cylinder begins to run a lean mixture. Here is where modern technology sometimes does more harm than good. The lambda sensor detects the lean mixture and enriches the mixture for all the cylinders. The result is higher fuel consumption - possibly as much as 25 percent - and erratic idling. - Star Motoring.

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