High-pressure ammonia spray combustion in lean premixed hydrogen mixture

High-pressure direct-injection (HPDI) of ammonia into engine cylinder could provide superior potentials than low-pressure premixed mode in terms of unburned ammonia and nitrous oxide (N2O) emissions, but direct injection of another high reactivity fuel is usually needed to trigger the ammonia spray combustion, complicating cylinder head injector arrangement. To this end, we propose a novel concept of HPDI of ammonia into a premixed lean hydrogen mixture, in which the spark-ignited hydrogen flame enables and enhances the ammonia spray combustion via hydrogen entrainment. This study investigates the combustion and emission characteristics of the HPDI ammonia in the lean premixed hydrogen mixture using a constant volume combustion vessel by varying ambient pressure, hydrogen concentration and injection timing. For the hydrogen-air mixture with the excess air ratio (λ) of 3.0 and ambient pressure of 2.5 MPa, near-complete combustion of the HPDI ammonia occurs with low N2O emissions below 10 ppm. When λ of the hydrogen mixture is increased to 5.0, the ammonia combustion efficiency decreases to around 40 %. Decreasing the ambient pressure enhances the combustion efficiency. When the ambient pressure decreases to 1.0 MPa, the combustion efficiency could increase to 90 % with the early injection case, but with higher NOx and N2O emissions.