Experimental investigation of the achieving methods and the working characteristics of a near-zero NOx emission turbocharged direct-injection hydrogen engine

•NOx below 20ppm can be regarded as near-zero emission for hydrogen engine.•Most working conditions can achieve near-zero emission after optimization.•Turbocharger and high injection pressure increase the power over 2.5 times.•The maximum brake thermal efficiency reaches 40.4% with near-zero emission. Hydrogen, as clean and renewable energy, is regarded as a promising fuel applied to internal combustion engines. The relatively high level of Nitrogen oxides (NOx) emissions is an important limiting factor to realize the application of hydrogen engine. Previous researches on the NOx emissions reduction were based on the naturally-aspired or supercharger hydrogen engine with medium load and efficiency. In this study, a 2.0 L turbocharged direct-injection engine is used to find the methods of achieving near-zero NOx emission (without any posttreatment equipment) with large power and high thermal efficiency. The effects of the coefficient of excess air (λ), the timing of the start of injection (SOI), and injection pressure on NOx emissions are investigated detailly with the engine speed range of 1000 rpm to 4000 rpm. The target of the NOx emission is set below 20 ppm according to the emission standards. A leaner combustion (λ = 3.29@ 3000 rpm) compared to λ = 2.62@ 2000 rpm is required to achieve near-zero emission. The utilization of the turbocharger and improved injection pressure increase the power and efficiency simultaneously. The maximum brake mean effective pressure (BMEP) can reach 13.3 bar@3000 rpm, and the maximum brake thermal efficiency (BTE) reaches 40.4% @2000 rpm with near-zero NOx emission. Various techniques, such as turbocharging, exhaust gas recirculation, and increased compression ratios, are also compared to qualify the trade-off relationships among the power, efficiency, and emission of the direct-injection hydrogen engine. These conclusions can be used to broaden the working boundary and optimize the fuel economy of turbocharged direct-injection hydrogen engines with near-zero NOx emission.