Study of water and hydrogen addition on the pyrolysis and combustion characteristics of isooctane

Abstract

Water injection and hydrogen combustion technologies have gained widespread attention in recent years. Combining H2O and H2 injection could bring more desirable combustion based on the needs of different engine conditions. To fully explore what effects the simultaneous injection of H2O and H2 would have on the combustion process of isooctane, three different subtests were performed in this study: investigation on chemical reaction mechanism, the laminar flame speed measurements and real engine tests. Firstly, H2O and H2 were added to the JSR (Jet Stirred Reactor) reactor to analyze the synergistic effect of H2O and H2 on the pyrolysis process of isooctane. The experimental results showed that the addition of H2O and H2 leads to an increase of OH and H radicals, which increases the concentrations of C1-C3 species and decreases the concentrations of C4 species. To further investigate the effect of H2O and H2 on the radical reaction of isooctane combustion process, this study then measured the laminar flame speed of IC8H18/H2/O2/H2O gas mixture by using constant volume combustion bomb, and found that water injection would reduce the laminar flame speed, and H2 would increase the laminar flame speed accordingly. Sensitivity analysis showed that it was more sensitive to reactions R1, R97 and R12, and was more sensitive to H2O than to H2. Finally, real engine tests were performed on a modified 1.5L gasoline equipped with port H2O and H2 injection systems at 1300 rpm with 30% throttle opening. The results showed that the use of H2O and H2 permitted the manipulation of engine combustion speed. Specifically, H2O injection led to a decrease in the peak cylinder pressure; and the addition of H2 increased the cylinder pressure again·H2O injection resulted in increased ignition delay and slower flame spread, slowing the engine's burn rate when needed. Using H2 addition led to a short ignition delay and accelerated combustion velocity, which counteracted the negative impacts of H2O injection and elevated the engine combustion speed to optimize efficiency. In summary, this paper revealed the complementary coordination between H2O and H2 through experimental and simulation studies. It will provide theoretical guidance for the development of H2O injection with H2 technology for gasoline engines.