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Proton Holdings today held a successful test firing of its new GDI engine at the Ricardo works in the UK. So, what is GDI and TGDI, and what does it mean in terms of engine design?

GDI stands for Gasoline Direct Injection, and the T denotes Turbo. In terms of Proton’s engine development, this means a move away from multi-point injection (MPI) where the injectors shoot fuel into the inlet tract to the more efficient GDI, with injectors firing directly into combustion chamber.

It differs from other forms of fuel injection in that fuel is delivered at a much higher pressure directly into the combustion chamber. The advantage of this is fuel is able to be more accurately controlled in terms of volume and timing, resulting in more hp from a given engine capacity.

The development of GDI is not new, having made its first appearance in aircraft engines over 120 years ago. The 1990s saw a rebirth of GDI, with Mitsubishi including it into its 4G93 engine. Other automakers followed suit through the 2000s, with BMW even trying out a low-pressure GDI in its V12.

According to Proton, its GDI engine will give a fuel savings of 25% over the previous VVT engine in the Iriz. Typically, a GDI engine will work in one of three modes. In normal running, the ECU will set the GDI to as close to the stoichiometric ratio as possible, which in theoretical terms is 14.7:1 of air to petrol by mass.

While perfect combustion is never achievable in the real-world, the engineering push towards this goal will result in better fuel efficiency and controlled emissions. For Proton’s new engines, this means working towards the forthcoming Euro 6C standard.

At low engine speeds, where the engine is turning at or near idle, the ECU sets the engine up for an ultra lean burn. While Proton didn’t say anything about how far they were going to push the limit, some engines from other manufacturers go as far as 65% lean, right on the verge of detonation.

For wide-open throttle, GDI goes the other way, enriching the mixture to ensure that power is available, and a little left over for cooling. In TGDI applications, the fuel-air mixture can be optimised to suit boost pressure, and any changes in barometric pressure – which turbos can be sensitive to – that might affect the engine’s performance.

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The GDI path has the benefit of putting the fuel mixture in optimum position for the spark to propagate the flame front in the most efficient manner. This means the maximum amount of fuel is burnt during the combustion cycle for the maximum amount of power for piston speed. While MPI can do this, it is to a much lesser degree, and efficiency is subject to the porting of the inlet.

So, if that’s the way it works, and the benefits, what are the drawbacks? One is complexity. Instead of having injectors in the manifold or inlet tract, the injectors are shooting fuel directly into the combustion chamber.

This means one injector per cylinder, sometimes two, depending on design. The cylinder head has to be redesigned to accommodate this. The other is that fuel delivery is under much higher pressure. The fuel delivery system has to be upgraded to suit, and this also means high pressure hoses and fittings.

Also, GDI injectors are also more sensitive to contaminated fuel, or fuel containing too high a percentage of after-market additives in the wrong ratio. Sludging of the injectors and carbon build-up is a real possibility, since the injector orifices are much finer than in MPI.

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Because fuel is no longer sprayed on the back of intake valves, it is possible for build-up to occur on the intake ports. Stratification of the fuel mixture could also lead to carbon build-up on the chamber walls, with carbon contaminants blocking injectors and lodging in catalytic convertors, causing local hot spots and leading to the failure of the convertor.

Some engine designers get around the problem of stratification and carbon build-up by using both GDI and port injection. The ECU in such engines, such as in the Toyota D-4S and VW’s EA888, is programmed to use either, or both, injectors, depending on load and engine state.

All this costs money, and Proton says it spent RM600 million to develop the new range of engines. This complexity is somewhat off-set by the inclusion of diagnostic software to troubleshoot errors in the GDI.

For Proton, the numbers it is looking for in the 1.5 TGDi are 180 hp and 250 Nm, already much higher than MPI 1.6 CFE, 138 hp, 205 Nm and 25% more efficient. In the short-term, GDI engines promise better fuel efficiency and more power per litre of fuel, while in the long-term, the reduced emissions will benefit the environment.