Mitsubishi is currently the leader of GDI (Gasoline Direct Injection) technology. It has already applied GDI in different engines, from 1.5-litre four to 4.5-litre V8. Now most of its production engines are GDI-equipped.Mitsubishi claimed GDI consumes 20 to 35% less fuel, generates 20% less CO2 emission and 10% more power than conventional engines. How can it be so magical ? The following paragraphs will tell you its secret.
Theory of GDi
Gasoline direct injection technology is one of the branches of "Lean Burn Technology". What it differs with Lean Burn is the adoption of directly fuel injection system.
Direct fuel injection has been used in diesel engines for many years, but not in petrol engine until recently. Inherently, direct injection has two advantages :
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In conventional engines, fuel injectors, even in MPi (multi-point injection) designs, the injected fuel pulverise in the intake port (near intake valves) before entering the combustion chambers. Why not directly inject into the cylinder ? because it is impossible to spread the fuel uniformally in everywhere. On the contrary, inject into the main entrance (intake port) assures all air mix with fuel in the same rate.How can Mitsubishi applied direct injection without such problem? Let us look at the following diagrams:
Main injection takes place as the piston approaches top dead centre on the compression stroke, shortly before ignition. As seen in the above pictures, the concave-section piston concentrates more fuel around the spark plug, this allows successful ignition without misfire even when the air / fuel mixture is very lean. This explain why GDI can operate under fuel / air ratio of 1 : 40 under light load, which is even leaner than Lean Burn Engines. As a result, more complete burning is achieved.More Power
Mitsubishi GDI engine has an extraordinarily high compression ratio of 12.5 : 1, this is perhaps the highest record for production petrol engine. The result is higher power output.
How can it prevent combustion knock under such pressure ? The secret is the pre-injection process. During compression, the heated air is cooled by the fuel spray, thus knocking becomes less easy to occur.
NOx emission
One of the few drawbacks of GDI engine is the higher NOx pollutant level. Luckily, a newly developed catalytic convertor deal comfortably with it. Nevertheless, USA and many developing countries cannot be benefited by it because their high-sulphur petrol will damage the catalyst.
Theory of GDi
Gasoline direct injection technology is one of the branches of "Lean Burn Technology". What it differs with Lean Burn is the adoption of directly fuel injection system.
Direct fuel injection has been used in diesel engines for many years, but not in petrol engine until recently. Inherently, direct injection has two advantages :
- Since the fuel is injected under high pressure directly into the combustion chamber, just before ignition by the spark plug, this allows the precise control of charge stratification vital to ignite ultra-lean air / fuel mixtures.
- Direct injection also dispenses with the need for a throttle, so eliminating the pumping loss associated with drawing air around a conventional engine's butterfly valve.
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In conventional engines, fuel injectors, even in MPi (multi-point injection) designs, the injected fuel pulverise in the intake port (near intake valves) before entering the combustion chambers. Why not directly inject into the cylinder ? because it is impossible to spread the fuel uniformally in everywhere. On the contrary, inject into the main entrance (intake port) assures all air mix with fuel in the same rate.How can Mitsubishi applied direct injection without such problem? Let us look at the following diagrams:
- Unlike conventional engines, GDI uses upright straight intake port, accompany with a concave-section piston surface, swirl air flow will be generated during compression stroke. When fuel directly injects into the combustion chamber, the swirl helps mixing air with fuel.The fuel injector is another new feature. It pumps out the fuel at higher pressure, enables better pulverisation and more uniformal spread.
Fuel injection takes place in two phases. During intake stroke, some amount of fuel is "pre-injected" into the combustion chamber, cools the incoming air thus improve volumetric efficiency, and ensuring an even fuel / air mixture in everywhere.
Mitsubishi GDI engine has an extraordinarily high compression ratio of 12.5 : 1, this is perhaps the highest record for production petrol engine. The result is higher power output.
How can it prevent combustion knock under such pressure ? The secret is the pre-injection process. During compression, the heated air is cooled by the fuel spray, thus knocking becomes less easy to occur.
NOx emission
One of the few drawbacks of GDI engine is the higher NOx pollutant level. Luckily, a newly developed catalytic convertor deal comfortably with it. Nevertheless, USA and many developing countries cannot be benefited by it because their high-sulphur petrol will damage the catalyst.
