On combustion instability induced by water condensation in a low-pressure exhaust gas recirculation system for spark-ignition engines

Water condensation in low-pressure exhaust gas recirculation systems is a concern because of a potential impact on components durability, but its implication into the operation of modern spark-ignition engines has not been investigated in detail. In this work, such aspects are evaluated by tests in a turbocharged multi-cylinder engine installed in a climatic test bench. First, a previously developed and validated zero-dimensional model is used to identify conditions where condensation is produced inside the water charge-air cooler, so that the compressor durability and operation is not affected. Then, different engine points are tested with a constant 25% of recirculated rate. Results show that combustion instability can increase up to 6% (absolute) due to droplets accumulation and release inside the water charge-air cooler, confirmed by an endoscope at the WCAC outlet . The evaporation of these droplets during the intake stroke causes charge overleaning in specific cycles, affecting combustion and inducing occasional misfires. Consequently, the fuel consumption and unburned hydrocarbons can be increased up to 4% and 30%, respectively, compared to the results achieved without condensation. The low speed and load condition was clearly the most affected by the sudden dilution given the lower in-cylinder temperatures. The combustion model shows that condensation formation and eventual overleaning phenomenon is increased if low or zero-carbon fuels such as natural gas, methanol, hydrogen or ammonia are used. Results showed an increased temperature for condensation compared to a traditional gasoline, around 10 ∘C for methane and methanol and 20 ∘C for hydrogen and ammonia.