<div class="htmlview paragraph">A new engine concept using direct injection of super heated steam in diesel engines is studied in a Constant Volume Bomb (CVB).</div> <div class="htmlview paragraph">The basic idea behind the Steam Injected Diesel Engine (STID) is to reduce the emissions of both particulates and NOx in addition to increasing the efficiency. By using air cooling and exhaust heat to generate the steam, the STID engine is believed to achieve an increase in the shaft efficiency and a substantial reduction of NOx and particulates emissions.</div> <div class="htmlview paragraph">This paper presents results from studies of the influence of superheated steam injection on the ignition delay. The justification for the study is the large amount of steam injected necessary for the STID engine and the need for studying the effect of superheated steam on engine operation.</div> <div class="htmlview paragraph">The background for the study is the massive steam injection necessary for the STID engine, and a detailed study is required when it comes to its effect on engine operation.</div> <div class="htmlview paragraph">Different ratios of air-to-steam mass and steam-to-fuel are selected in the range present in the Wasa 32 engine used for STID experiment at the Wärtsila factory in Finland.</div> <div class="htmlview paragraph">The pressure development is measured in these test series. The definition for ignition delay employed is the time from the triggering of the diesel injector until the pressure gradient exceeds dp/dt ≥2.</div> <div class="htmlview paragraph">The results indicate a surprisingly moderate dependence of the ignition delay on variations on both air-to-steam ratio and steam-to-fuel ratio in the Constant Volume Bomb (CVB). A small increase is found in the ignition delay as a function of the A/S-ratio.</div> <div class="htmlview paragraph">The steam tested was superheated at approximately 500 °C. This high temperature of the steam may explain the minor influence on the ignition and combustion process.</div> <div class="htmlview paragraph">Increasing the pressure and temperature inside the Constant Volume Rig at the time of diesel injection from 50 bar to 60 bar reduces the average ignition delay with 23 %.</div>