precise control of fuel injection, a strong air–fuel swirl created in the combustion chamber, a new linear air–fuel sensor (LAF type O2 sensor) and a lean-burn NOx catalyst to further reduce the resulting NOx emissions that increase under "lean-burn" conditions and meet NOx emissions requirements. This stratified-charge approach to lean-burn combustion means that the air–fuel ratio is not equal throughout the cylinder. Instead, precise control over fuel injection and intake flow dynamics allows a greater concentration of fuel closer to the spark plug tip (richer), which is required for successful ignition and flame spread for complete combustion. The remainder of the cylinders' intake charge is progressively leaner with an overall average air:fuel ratio falling into the lean-burn category of up to 22:1. The older Honda engines that used lean-burn (not all did) accomplished this by having a parallel fuel and intake system that fed a pre-chamber the "ideal" ratio for initial combustion. This burning mixture was then opened to the main chamber where a much larger and leaner mix then ignited to provide sufficient power. During the time this design was in production this system (CVCC, Compound Vortex Controlled Combustion) primarily allowed lower emissions without the need for a catalytic converter. These were carbureted engines and the relative "imprecise" nature of such limited the MPG abilities of the concept that now under MPI (Multi-Port fuel Injection) allows for higher MPG too. The newer Honda stratified charge (lean-burn engines) operate on air–fuel ratios as high as 22:1. The amount of fuel drawn into the engine is much lower than a typical gasoline engine, which operates at 14.7:1—the chemical stoichiometric ideal for complete combustion when averaging gasoline to the petrochemical industries' accepted standard of C8H18. This lean-burn ability by the necessity of the limits of physics, and the chemistry of combustion as it applies to a current gasoline engine must be limited to light load and lower RPM conditions. A "top" speed cut-off point is required since leaner gasoline fuel mixtures burn slower and for power to be produced combustion must be "complete" by the time the exhaust valve opens. Applications 1992–95 Civic VX 1996–2005 Civic Hx 2002–05 Civic Hybrid 2000–06 Insight Manual transmission & Japanese spec Cvt only Toyota lean-burn engines In 1984, Toyota released the 4A-ELU engine. This was the first engine in the world to use a lean-burn combustion control system with a lean mixture sensor, Toyota called "TTC-L"