The Onan L Series engine family consists of three, four and six cylinder naturally aspirated models, as well as four and six cylinder turbocharged models. They are designed as in-line, water cooled engines capable of continuous operation up to 3600 rpm. These models employ an indirect injection combustion system and are currently available with power ratings from 19 kW (25 hp) to 90 kW (120 hp). This paper describes the basic engine design and highlights features which enable the L Series engines to offer broad application flexibility while maintaining durability, reliability, serviceability and performance. Specific examples of successful applications are given to illustrate how design features satisfy application requirements. The performance and mechanical development programs, including laboratory and field testing are discussed to exemplify the design validation methodology.