The homogeneous charge compression ignition (HCCI) engine is one of the modern industrial types of internal combustion engine.
Advantages of HCCI engines include low NOx emissions and low fuel consumption [1,2].
However, the main disadvantage of them is the lack of a direct method for combustion timing control, which is caused to exhibit undesired performance at high load region and severely limits their operating range [3,4].
Using additives can be utilized in order to control combustion in HCCI engines with a change in mass percentages of combustion chamber.
One of the most important additives to the internal combustion engines charge is the exhaust gas recirculation (EGR).
Jung and Iida showed that EGR retards combustion timing and reduces pressure rise rate of HCCI engine.
Nishi et al. studied on the effect of EGR ratio in HCCI combustion using a single-zone code with detailed chemical kinetics mechanism and concluded that with increase in EGR ratio, the maximum temperature and peak values of the low and the high temperature rate of heat release decrease.
Also, maximum pressure rise rate was decreased with the increase in EGR ratio while combustion duration was prolonged.
Putrasari et al. conducted experimental and computational study to investigate the dimethyl ether HCCI auto ignition under EGR.
Based on their results, the combustion duration dominantly depended on the EGR addition.
Bhaduri and et al. showed that EGR can be used as an efficient control parameter for HCCI engines operated with synthetic syngas.
Hydrogen (H2) and syngas are other important additives which have been used for HCCI combustion control.
Voshtani et al. studied the effect of reformer gas (RG) on HCCI combustion of natural gas and indicated that RG affects the combustion process and can control the start of combustion (SOC) time.
Neshat et al. using a multi zone model studied on the effect of RG on combustion of primary reference fuels in the HCCI engine.
Their results showed that RG affects chemical process in combustion chamber and changes combustion timing and exhaust emissions.
Wang et al. theoretically investigated the effects of the reformed fuel on the combustion of the fresh fuel by using n-heptane.
They reported that the reformed fuel may decrease the ignition delay time of the fresh fuel.
Neshat et al. founded that chemical effects of H2 on supercharged n-heptane HCCI combustion is more significant than carbon monoxides (CO).
Also, water vapor is one of the additives that can be used in internal combustion engines.
The idea of water addition to in-cylinder charge, first time was considered for diesel engines.
The effects of water addition on diesel combustion have been studied and concluded that water in diesel engines is added to optimize the mixture formation, improve power generation and emissions reduction.
The idea of adding water to combustion components in HCCI engines has been also considered in order to combustion control and optimization of engine performance and emissions.
The advantages of water compared to EGR as an additive in HCCI engines include the lack of equipment cost, absence of free radicals that accelerates the combustion phenomenon and absence of hydrocarbon particles that increases the pollutants of unburned hydrocarbons (UHC) and CO.
Although the use of water as an additive is more inexpensive and efficient to combustion control, there are a few studies that have focused on it.
Kaneko et al. studied the effect of water addition to in-cylinder charge during low temperature combustion strategies and indicated that water addition expands the engine operating range and reduces the engine exhaust emissions.
Steinhilber et al. reported that water addition decreases the engine exhaust emissions.
Iwashiro et al. injected water into the combustion chamber of a dimethyl ether fueled HCCI engine and showed that water injection improves fuel economy and expands the engine operating range.
Ogawa et al. founded that water addition on HCCI combustion significantly reduces the low-temperature oxidation and the engine exhaust emissions.
Megaritis et al. studied the effect of water blending on bioethanol HCCI combustion and showed that water addition leads to lower the maximum pressure rise rates and further improve emissions [27,28].
Flower et al. showed that adding water to ethanol as a fuel for HCCI engine would increase thermal efficiency and significantly reduce NOx emissions from exhaust.
Christensen and Johansson, experimentally investigated addition of water as additive to three different fuels (natural gas, iso-octane and ethanol) in HCCI engines and showed that addition of water increases the combustion time of all three fuels.
Valero-Marco at al. studied on the potential of water direct injection in a HCCI gasoline engine.
They showed that water injection is an efficient strategy to increase the maximum affordable load.
Due to the lack of adequate and coherent studies on the effect of water on HCCI combustion timing, performance and emissions, extensive studies to investigate the effects of water on the combustion process are inevitable.
The idea of water added on HCCI combustion has been already studied via experimentally; but thermal, chemical and dilutions effects of water on the combustion process are not examined yet.
It should be pointed out that effects of adding water on the important species in the combustion chamber, chemical reaction rates are not achievable experimentally.
The main contribution of the present study is to investigate the effects of water as an additive on performance and emissions of a methane fueled HCCI engine.
Different effects of water addition on combustion timing, engine emissions and performance of engine are discussed.
Effects of water on chemical reactions and species are discussed and the effects of water on radical’s formation is focused.
Different values of water are added to in-cylinder charge and its thermal, dilution and chemical effects are computed separately.
To achieve the goal, the HCCI engine was first simulated using a multi zone model, and then the engine’s capability to provide accurate results could be verified using experimental data and eventually water is added to the in-cylinder charge at IVC and its effects on the processes inside the combustion chamber is studied.
Conclusion
In the present study, water is added to the composition of the combustion chamber and its effects on the performance and pollutants of an HCCI engine fueled with natural gas are investigated. For this reason, a multi zone model coupled to a semi-detailed chemical kinetics mechanism is utilized and five different values of water are added to in-cylinder mixture and its various effects are studied. Also, chemical, thermal and dilution effects of water are calculated based on artificial inert species method. The most important results are:
• Water addition retards the SOC time and reduces combustion chamber pressure and temperature. Thus, water addition, similar to other additives, may help to control the combustion process of a HCCI engine.
• Adding of water up to about 3% increases the engine thermal efficiency and decreases exhaust emissions from it.
• Thermal effect of adding water on start of combustion and emissions formation is more significant than its dilution effect.
• Water addition has no significant chemical effect on combustion process.
• Water addition reduces the reaction rates but does not alter the reaction pathways.
Reference: M.F. Ahari, E. Neshat, Advanced analysis of various effects of water on natural gas HCCI combustion, emissions and chemical procedure using artificial inert species, Energy. (2019) 842–852. doi:10.1016/j.energy.2019.01.059.
