Additional swirl is promoted with special swirl-inducing high pressure injectors. The end result is a fuel-rich mixture in the direct vicinity of the spark plug, with extremely lean air/fuel ratios near the cylinder walls The D-4 can operate stably with air/fuel ratios as high as 50: 1. compared with the company's existing indirect- injected lean-burn engine that operates with maximum air/fuel ratios of roughly 24: 1 Toyota says a coincidental advantage of the highly stratified charge employed in the new engine is a cooler air charge due to increased fuel vaporization the cooler charge increases volumetric efficiency, and the engine also is fitted with the company's WT-i(Variable Valve Timing intelligent) system to precisely control each cylinder's intake eventAdditional swirl is promoted with special swirl-inducing high￾pressure injectors. The end result is a fuel-rich mixture in the direct vicinity of the spark plug, with extremely lean air/fuel ratios near the cylinder walls. The D-4 can operate stably with air/fuel ratios as high as 50:1, compared with the company's existing indirect￾injected lean-burn engine that operates with maximum air/fuel ratios of roughly 24:1. Toyota says a coincidental advantage of the highly stratified charge employed in the new engine is a cooler air charge, due to increased fuel vaporization. The cooler charge increases volumetric efficiency, and the engine also is fitted with the company's VVT-i (Variable Valve Timing, intelligent) system to precisely control each cylinder's intake event