Investigation of ozone assisted combustion for internal combustion engines applications

In this work, the potential of using ozone for possible applications in both unconventional engines, in particular Homogeneous Charge Compression Ignition (HCCI) engines, and conventional Spark Ignition engines was investigated. For this purpose, simulations were conducted by employing 0-D, 1-D numerical models and Computational Fluid Dynamics (CFD). The iso-octane/air/ozone mixture was the one mainly investigated; however, analyses were also carried out considering the methane/air/ozone mixture. First, the chemical kinetics of ozone-assisted combustion were studied, evaluating the effect of ozone on Laminar Flame Speed (LFS), Ignition Delay Time (IDT), flame structure and reaction paths. Then CFD simulations of both an HCCI engine and a Spark Ignition engine were carried out to evaluate the effect of ozone on performance, fuel economy and specific fuel consumption under different operating conditions (mixture composition, engine speed, spark advance). The main results show that the ozone addition leads to an increase in LFS, however, different behaviour was found for methane and iso-octane. Specifically, for relatively low temperatures (T<540 K) the effect of ozone is similar on the two fuels, while for temperatures higher than about 540 K only in the case of iso-octane a cool flame occurs, leading to a significantly greater increase in LFS than in methane. Furthermore, in the presence of a cool flame and with ozone addition, the increase in pressure leads to an increase in LFS, unlike with methane and unlike without ozone. The formation of a cool flame is due to the chemical reactions in the Low-Temperature Combustion (LTC) regime and the change in reaction pathways enabled by the ozone. The results show that ozone decomposes, producing oxygen atoms that oxidise the fuel, producing OH radicals that continue the oxidation. Ozone decomposition occurs faster the higher the temperature and pressure, with an acceleration of ozone chemical kinetics for temperatures higher than about 540 K. Enabling the LTC regime through ozone leads to a reduction in IDT. Specifically, the results show that as ozone concentration increases IDT decreases, with a greater impact at low temperatures. However, the reduction of IDT with ozone addition is less as ozone concentration increases, especially at low temperatures. These results encourage the use of ozone in HCCI engines where the combustion phase is mainly governed by chemical kinetics. In particular, the HCCI engine simulations suggest that the ozone addition, by reducing the IDT and enabling reactions in the LTC regime, is able to anticipate the mixture auto-ignition, reduce the combustion duration and reduce the cycle-to-cycle variation which characterises these engines especially at low load. Furthermore, the ozone effect is higher in the case with a higher percentage of residual gas, leading to a reduction of the Specific Fuel Consumption (SFC) from 1.54% to 4.96% under some conditions for the case without residual gas and the case with 10% by mass of residual gas. Another interesting factor suggesting the results of the simulations is that the ozone effect could be reduced in the presence of reactive species, especially nitrogen oxide, due to a direct reaction between the two species. However, this effect is limited in an HCCI engine since combustion takes place at low temperatures and the formation of NO_x is relatively low. As regards the ozone applications in Spark Ignition engines, simulation results suggest that the flame propagates faster with ozone addition and this effect is more pronounced the longer the residence time of the mixture. The delay in spark advance or the lower engine speed gives more time to ozone to decompose and enable LTC reactions in both compression and end gas, leading to an increase in flame speed. However, the interaction between the parameters influencing the residence time in the end gases (flame propagation speed, engine speed, spark advance) and those influencing the IDT in the end gases (temperature, pressure, mixture composition) must be carefully investigated, because in some cases mixture auto-ignition can occur, resulting in loss of performance and possible structural damage due to detonation and high maximum pressure rise rates. For this reason, ozone can have interesting applications in Spark Ignition engines operating under lean conditions. Simulation results on LFS show an increase in LFS under lean conditions with ozone addition to higher values than those obtained in stoichiometric conditions without ozone. Furthermore, the reduction of the equivalence ratio leads to an increase in IDT, reducing the risk of mixture auto-ignition. Indeed, the results of CFD simulations in lean conditions show that the flame propagation speed, which without ozone is lower than in the stoichiometric case, benefits more than in the stoichiometric case from the presence of ozone, increasing engine performance: in some conditions with ozone addition, a maximum increase in gross indicated work per cycle of 4% and 14% was obtained for the stoichiometric and lean cases, respectively.