Ricardo have successfully applied their lean burn gasoline engine technology to spark ignited natural gas engines for industrial applications. An open chamber combustion system using the patented ‘Nebula’ chamber, designed as a simple conversion of a swirling direct injection diesel engine, has been tested as a part of the Ricardo internally funded research programme with very promising results. The tests with a 170 × 170 mm single cylinder research engine have shown that the Nebula gas engine provides fast combustion without excessive cyclic variation up to an air:fuel ratio of 26.5:1 or 1.67 excess air ratio. The test results achieved confirm the potential of the Ricardo Nebula combustion chamber as a lean burn combustion system. Many existing emissions standards were met with good fuel consumption, and the stringent West German and Swiss NOx limits were met at 1200 rev/min without penalties in thermal efficiency through excessive ignition retard.