In this mode, at speeds up to 120 km/h, the Mitsubishi GDI engine achieves complete combustion with an air-fuel ratio of 40:1. Conventional engines: 14.7:1 Lean-burn engines: 22:1 Mitsubishi GDI engine: 40:1. Late in the compression stroke, the High-Pressure Swirl Injector sprays fuel in a tight, swirling spiral towards the piston top. This spiral and the "clockwise tumble" airflow keep the fuel spray concentrated around the sparkplug. When the sparkplug ignites, a powerful firestorm is controlled by the spherical cavity. No fuel is wasted. The result: a 20% improvement in overall fuel economy. Ultra-Lean Combustion Mode 1. Piston moves downward. / 2. Piston reaches bottom and begins compression stroke. 3.Fuel is injected late in compression stroke. / 4. Sparkplug ignites mixture.