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MKW offers insights into the working of GDI systems

Bosch training partners, MKW Motors is offering insights into the workings of gasoline direct injection (GDI) systems.

The aim of the gasoline direct injection (GDI) system is to save up to 25 % of fuel. The efficiency of the spark-ignition engine improves as the fuel-air ratio rises, which is why the engine is ideally run lean. The system is recognised by high-pressure fuel being injected directly into the combustion chamber.

How it works?

Similar to the way in the manifold injection system, the fuel is pressurised by an electric fuel pump to a pre-delivery pressure of 3 … 5 bar and pumped from the tank to the high-pressure pump where it is pressurised to 50 … 120 bar, depending on load and engine speed. The fuel is held in the fuel rail and injected into the combustion chamber by electrically-operated high-pressure injection valves. A pressure sensor measures the fuel pressure in the rail. The pressure level dictated in the program map is set via a pressure-control valve by allowing the surplus delivered fuel to flow back into the low-pressure circuit.

There is a number of injection operating modes; the two fundamental modes are Stratified and Homogenous.

In the lower torque range at speeds of up to approx. 3000 rpm, the engine runs in stratified mode. Fuel is injected on the compression stroke approx 40 – 50° BTDC, the air currents in the combustion chamber transport the mixture cloud to the sparkplug. The result is a homogeneous cloud of ignitable and combustible mixture surrounded by a zone of inert and/or unburned gas without fuel.

In stratified operation the mixture in the combustion chamber is very lean overall typical values of l = 1.6 – 3. Demands for high “speed” and “torque” limit stratified operation. If torque is too high, soot is caused by localised rich areas. In addition, if the speed is too high the charge stratification and the orderly transportation of the mixture to the sparkplug can no longer be maintained due to excessive turbulence.

At high torque and high speed, instead of stratified operation the engine is operated in the homogeneous mode based around normal lambda values of l = 1. Fuel is injected on the intake stroke approx 300° BTDC, the air-fuel mixture formed can dissipate throughout the whole combustion chamber, similar to standard manifold injection system. This mode of operation is necessary when the torque requirement is high as it uses the full volume of the combustion chamber.

Whilst diagnosing GDI high pressure injector valve faults we are able to scope their waveforms quite easily. However before we connect our scope we should think about the voltage and current applied to the injector? GDI injectors operate at a voltage between 50 – 90 Volts and up to 12 – 15 Amps.

Due to the hazardous voltages encountered, BOSCH DO NOT ENDORSE THE VOLTAGE TESTING OF H.P.I.V SIGNALS. Signal evaluation should only be carried out by the use of a current measuring clamp.

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A test which is more difficult with a GDI injector compared to a standard manifold injector is the injector leak test, due to the high fuel pressures involved it is recommended that the leak test be performed without removing the injectors.

Switch the engine off, remove the ignition key, Pull off and seal the crankcase breather hose, Remove the spark plugs and insert the hose of exhaust gas analyser into each cylinder to perform HC measurement.

(A) If the injector is not leaking, the HC value decreases to stabilise at approximately 80 … 200 ppm after 5 … 10 minutes.
(B) If the injector is leaking, the HC value will suddenly increase if a droplet is released during the measurement

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Published on: March 6, 2015

Filled Under: News

Web site: http://mkwmotors.boschauto.co.uk/

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