NOx emission control strategies in hydrogen fuelled automobile engines

Compression ignition engines are more dominant in automotive sector than spark ignition engines due to higher torque and better fuel efficiency. However, nitrous oxides and particulate matter are the main emissions from the CI engines. Hydrogen is used as an alternative fuel due to its high diffusivity, high flame speed and wide flammability limits. Elevated levels of nitric oxide emissions are the main concern with the usage of hydrogen as a fuel. In order to meet the requirements of present-day emission standards, many control techniques are developed. This article categorises the available NOx emissions control strategies into pre-intake, in-cylinder and post-combustion techniques. A summary of various methods are presented to control the NOx emissions from literature with hydrogen as a main fuel. Intake temperature, intake pressure, exhaust gas recirculation, O-2 concentration, air fuel ratio, addition of inert gases, water and steam injection are discussed in pre-intake control strategies. In-cylinder control strategies like varying compression ratio, swirl and injection timing are studied. Various post-combustion control strategies like SCR, urea injection, Vanadia sublimation, hydrocarbon SCR, lean NOx trap, SCR lean trap NOx are explained.