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A constant-volume combustion chamber was used to examine injection of a small quantity of slightly rich fuel/air mixture towards the spark plug around the time of ignition, in an overall very lean mixture rotating at velocities representative of modern spark-ignition engines. The results show that it is possible to achieve 100% ignitability with overall air-fuel ratios in excess of 50 and much faster burn rates than those with initially homogenous mixtures of the same equivalence ratio with high swirl and turbulence. The advantages of this method of local charge stratification have been demonstrated in terms of both pressure measurements and shadowgraphs of the early flame development while the transient characteristics of the injected rich mixture at the spark plug gap were monitored by a fast flame ionization detector.

Lean combustion in spark-ignition engines has long been recognised as a means of reducing both exhaust emissions and fuel consumption. However, problems associated with cycle-by-cycle variations in flame initiation and development limit the range of lean-burn operation. An experimental investigation was undertaken in order to quantify the effects of spark energy released and initial flame kernel growth on the cyclic variability of IMEP and crank angle at which 5% mass fraction was burned in a Honda VTEC-E, stratified-charge, pentroof-type, single-cylinder, optically accessed, spark-ignition engine. Simultaneous CCD images of the flame at the spark plug were acquired from two orthogonal views (one through the piston crown and one through the pentroof) on a cycle-by-cycle basis during the first 40 crank angle degrees after ignition timing, for isooctane port injection at an air to fuel ratio of 22, engine speed of 1500 RPM, 30% volumetric efficiency and 40° crank angle spark advance.

Measurements are presented of droplet characteristics and air velocity in the cylinder of a 0.36 litre four valve engine, equipped with an sohc VTEC-E valve train and port injection. The results show that injection at crank angles, θinj(s), when the inlet valve is open results in most of the liquid volume flux being in the form of droplets with Sauter mean diameter between 20 and 30 mm which strikes the sleeve up to about 2.5 cm below the exhaust valves, thus generating a locally rich cloud there. The amount of liquid phase gasoline passing through the plane 16 mm below the spark plug gap increases with θinj(s) up to 50 CA after intake TDC and this, together with the crank angle of droplet arrival and vapour generation, controls stratification of the gaseous fuel phase. The optimum injection time is when the fuel-rich cloud is generated so that the tumble vortex convects it to the spark plug at the time of ignition.