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Mastering Gas Direct Injection

Friday, July 1, 2016 - 07:00
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Gas Direct Injection (GDI) is sometimes referred to as Direct Injection (DI). Placing a fuel injector directly into the combustion chamber and bumping fuel system pressures into the thousands of PSI makes a lot of difference in both vehicle operation and how we approach diagnostics. GDI engines have compression ratios up to 12:1 (vs. 10:1 for most engines) and can deliver A/F ratios ranging from 14.7:1 to as lean as 40:1. For many years, gas direct injection was an oddity on a very small number of cars dating back decades. It has become so common now, however, that if you purchase a new vehicle there’s a very good chance you’ll own your first GDI-equipped ride. We’ll cover some operational and diagnostic highlights for GM and Ford in this article.

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Injectors – Fast & furious

Electrically, a typical GDI ECM controls the injector by providing a ground along with high-side driver activation. The PCM amplifies the 12-volt supply by using a capacitor to store and discharge a 50-90 volt pulse of stepped up power. GDI injector coils are typically around 1.5 ohms and can be pulsed quite quickly – as fast as 2 ms between pulses. Doing the Ohms law math or simply connecting an amp clamp to an injector circuit will indicate a very quick pulse or series of pulses that are as high as 10 amps. Pulse widths (injector on time) for GDI are much lower (0.4 – 5ms) compared to MPI / SFIs 3.5 – 20 ms. Multiple injection events can occur during the exhaust stoke, intake stroke, compression stroke or even during the power stroke. The number of sprays along with the exact timing of the sprays is determined by the ECM based on engine load, throttle angle delta, coolant/air temperature and RPM, along with the desired spray mode.

Direct injection of course means the injector sprays INSIDE the cylinder. Note the position of the injector straight across the head from the spark plug. What will keep the valves clean? Fuel certainly won't be doing that job as was the case with MPI / SFI!

IMPORTANT SAFETY NOTE: Both diesel common rail and GDI injectors have voltage and current levels that could be injurious or lethal (over 60 volts). Treat them like an orange cable on a hybrid or electric vehicle – don’t touch! When applying a back probe pin, for example, do it with the engine OFF and don’t touch when running.

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