valve per cylinder. The separate specially shaped (twin intake port design) intake ports are the same size, but only one port receives fuel from an injector. This creates two vertical vortices of identical size, strength and rotational speed within the combustion chamber during the intake stroke: one vortex of air, the other of an air/fuel mixture. The two vortices also remain independent layers throughout most of the compression stroke. Near the end of the compression stroke, the layers collapse into uniform minute turbulences, which effectively promote lean-burn characteristics. More importantly, ignition occurs in the initial stages of breakdown of the separate layers while substantial amounts of each layer still exist. Because the spark plug is located closer to the vortex consisting of air/fuel mixture, ignition arises in an area of the pentroof-design combustion chamber where fuel density is higher. The flame then spreads through the combustion chamber via the small turbulences. This provides stable combustion even at normal ignition-energy levels, thereby realizing lean-burn. The engine computer stores optimum air fuel ratios for all engine-operating conditions—from lean (for normal operation) to richest (for heavy acceleration) and all points in between. Full-range oxygen sensors (used for the first time) provide essential information that allows the computers to properly regulate fuel delivery. Diesel engines All diesel engines can be considered to be lean-burning with respect to the total volume, however the fuel and air is not well mixed before the combustion. Most of the combustion occurs in rich zones around small droplets of fuel. Locally rich combustion is a source of particulate matter (PM) emissions. See also Engine knocking Hydrogen fuel enhancement Footnotes Citations References "Advanced Technology Vehicle Modeling in PERE, EPA, Office of Transportation and Air Quality" Engine technology