<div class="htmlview paragraph">This paper describes the theory, mechanical execution, and development of an automatic, hydraulically-actuated piston that provides a practical method of obtaining a variable compression ratio engine. When applied to compression-ignition engines, an increase in output of 50% (0.5 to 0.75 bhp/cu in.) has been achieved without a corresponding increase in maximum combustion pressure. By providing a high compression ratio for starting and light load conditions, the engine has demonstrated substantial improvements in cold starting ability as well as improved potential for multifuel operation.</div>