<div class="htmlview paragraph">Increased thermal efficiency for spark ignition engines is studied experimentally using computer simulation of a three phase heat release model. Various compression ratios, combined with advanced multiple-direct-injection techniques, are researched in an effort to extend the effectiveness of current engine designs beyond their present knock-limited compression ratio. A new ideal engine cycle is presented to demonstrate the conditions appropriate for use of 88-98 octane fuel and spark ignition with an 18:1 compression ratio. The value of isothermal combustion to achieve over 42% brake thermal efficiency is presented.</div>