Toyota Motor Corp. says it has developed a 4-stroke, direct-injected (DI) gasoline engine that will be fitted in a production subcompact car (Japanese market) by the end of this year.
In conjunction with the new DI engine is another crucial item: a production-ready 3-way exhaust catalyst capable of oxidizing the oxides of nitrogen (NOx)-rich exhaust inherent to lean-burn engines.
The new engine, designated D-4 by Toyota, is a 2L DOHC inline 4-cyl. Compression ratio is 10:1, and the company says the engine operates efficiently on regular unleaded gasoline.
The D-4 incorporates a highly optimized combustion chamber: The piston crowns are fitted with a lipped, cup-like formation that focuses the injected air/fuel mixture tightly around the spark plug for maximum burn.
Intake air is drawn through helical ports that create a high degree of horizontal swirl, which Toyota says combines with the lipped combustion chamber to not only maintain highly stable combustion -- a quintessential problem for lean-burners -- but also to stratify the air/fuel mixture.
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.
Toyota says 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.
Lean-burn engines are known for their characteristic high levels of NOx emissions. The company says the new D-4 uses a high degree of exhaust gas recirculation (EGR) -- as much as 40% when the engine operates in the lean-combustion mode -- and the natural EGR-like operation of the VVT-i system to drastically reduce NOx production.
The new 3-way catalyst uses a storage/reduction design to purify occluded NOx at the stoichiometric level -- when the engine operates in lean modes, excess NOx is stored until it can be combined/purified with exhaust gases produced during stoichiometric operation.
Toyota also says the new engine relies on powerful, precise engine-management software to accurately tailor fuel-injection timing and duration. At high loads, such as during heavy acceleration, fuel is injected earlier in the compression stroke, creating a homogeneous mixture and optimum power. Crossover points between homogeneous and stratified mixtures produce semi-stratified air/fuel mixture for "smooth torque transition."
Torque production in the low- and middle-speed ranges, says the company, is improved by 10% over conventional engines, while fuel economy in the Japanese 10-15 urban fuel economy test is improved by a significant 30% or better. Moreover, Toyota claims the quicker response presented by the DI design improves 0 to 62 mph (0-100 km/h) and passing acceleration times by 10%.
Toyota says the new D-4 will first be coupled with an automatic transmission to produce "a very low fuel-consumption model within the year."
