A strategy to improve the efficiency of stoichiometric spark ignition engines

A commercial four-stroke spark-ignition engine has been tested at steady conditions, with three different compression ratios, namely; 10, 11.5 and 13. Exhaust Gas Recycle (EGR) has been varied in the range 0% - 20%. Air/fuel ratio has been maintained at stoichiometric by a closed-loop control with Exhaust Gas Oxygen sensor feedback. Significant gains on fuel economy and CO emission index have been achieved at medium and high loads by the simultaneous adoption of EGR and high compression ratios. In these conditions the sum of HC and NOx emission indices attains significant reductions at any load. The tests have shown that EGR allows to avoid knock even at wide open throttle and Maximum Brake Torque timing.