A combustion chamber designed to increase the burning rate of lean air-fuel mixtures is described. The chamber utilizes squish motion to generate a series of jets which significantly increase the turbulence levels in the chamber during the early phase of combustion. The fuel economy and exhaust emissions resulting from the new chamber design are compared to a conventional bowl-in- piston design over a wide range of air-fuel ratios. The new chamber design results in an increase in the combustion rate as shown by a reduction in the MBT spark advance during lean operation. The faster burning rate leads to a reduction in the brake specific fuel economy of some 5 percent compared to the conventional chamber design. Emissions measurements show an extension of the lean limit of operation and a reduction in hydrocarbon emissions during lean operation. The extended lean limit also enables reduced emissions of nitrogen oxide compounds to be achieved.