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.