Lean combustion is an effective way for improving the spark-ignited (SI) engine performance. unfortunately, due to the narrow flammability of gasoline, the pure gasoline-fueled engines sometimes suffer partial burning or misfire at very lean conditions. Hydrogen has many excellent combustion properties that can be used to extend the gasoline engine lean bum limit and improve the gasoline engine performance at lean conditions. In this paper, a 1.6 L port fuel injection gasoline engine was modified to be a hybrid hydrogen-gasoline engine (HHGE) fueled with the hydrogen-gasoline mixture by mounting an electronically controlled hydrogen injection system on the intake manifolds while keeping the original gasoline injection system unchanged. A self-developed hybrid electronic control unit (HECU) was used to flexibly adjust injection timings and durations of gasoline and hydrogen. Engine tests were conducted at 1400 rpm and a manifolds absolute pressure (MAP) of 61.5 kPa to investigate the performance of an HHGE at lean burn limits. Three hydrogen volume fractions in the total intake gas of 1%, 3% and 4.5% were adopted. For a specified hydrogen volume fraction, the gasoline flow rate was gradually reduced until the engine reached the lean burn limit at which the coefficient of variation in indicated mean effective pressure (COVimep) was 10%. The test results showed that COVimep at the same excess air ratio was obviously reduced with the increase of hydrogen enrichment level. The excess air ratio at the lean bum limit was extended from 1.45 of the original engine to 2.55 of the 4.5% HHGE. The engine brake thermal efficiency, CO, HC and NO(x) emissions at lean burn limits were also improved for the HHGE. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.