Thermodynamics of indirect water injection in internal combustion engines: theoretical assessment of the fresh mixture cooling effect

Abstract

Water injection is a well-known efficient way to improve the performance of internal combustion engines. Amazingly, most of previous studies have yet only assess this process in an experimental manner, depriving us of an understanding of its specific influence on different operating phases of the engine -density of the aspirated fresh mixture, work required by the compression stroke, and so on -but also of the possibility to predict its effects if set up on an existing engine. Thanks to a theoretical framework specifically developed, and similar to the one commonly used for the analysis of air conditioning systems, we start in this paper to untangle in a theoretical manner the different consequences of water injection on internal combustion engines. This first study is specifically focused on the fresh mixture density increase, due to the vaporisation of liquid water in the intake manifold. Results show that, in the best scenarios, we cannot expect to increase the amount of fuel finally aspirated into the cylinders by more than 10%. The methodology presented here, as well as the python software specifically developed, can be of a precious help for the optimisation of such process if applied to existing or future engines.

Internal Combustion Engines Water Injection fed by Exhaust Water Recirculation: a feasibility analysis

“…The fresh mixture is subjected to a temperature drop and then to an increase of its density. The amount of fuel aspirated (for spark ignited engines) or directly injected into the engine cylinder (for compression ignited engines) is thus increased [15]. Power and efficiency increase whereas specific fuel consumption and specific production of carbon dioxide, per kWh produced, decrease.…”

“…Considering the positive role of the water vapour extract from the ambience, represented by the factor AFR • ω 0 , on both values of ψ and WFR max in equations (15)…”

“…The situation is also favourable to gas fed engines, by hydrogen (H 2 ) or by natural gas (represented here the methane CH 4 ), mainly because of the higher hydrogen contents of such fuels (see the values of β in Table 1), but also because of the larger amount of air needed for their proper combustion, that aspirate more ambient humidity in the combustion process. The values of WFR max and ε of equations (16) and (15) for lean combustions of hydrogen [37] and natural gas [38], so with λ = 2, are presented in the lower part of Table 2. The optimal amount of injected water highlighted by some authors, of about 9 for hydrogen fed spark ignited engines by Subramanian, Mallikarjuna, and Ramesh [34] and of about 3 for natural gas ones by Arruga et al [35], is still within the available range of effectiveness of current membranes.…”

“…Values of the maximum Water-Fuel Ratio of equation(16) for two typical values of the water vapour recycling efficiency ε, and of ε of equation(15) corresponding to the minimum and maximum values of WFR found in the literature.…”