Mass burning rate was varied by using two single-cylinder engines, each fitted with different combustion-chamber geometries. The slower-burning chamber incorporated a side-mounted spark plug, the faster-burning chamber combined a more compact combustion space and a centrally located spark plug.
The faster-burning engine exhibited an improved efficiency-NOx emissions trade-off, primarily due to the engine's higher maximum thermal efficiency and greater tolerance to charge dilution by recirculated exhaust gas.
Faster burning also reduced cyclic variability in combustion, increased the knock-limited thermal efficiency, but offered no advantage in regard to hydrocarbon emissions.
