Improving Efficiency and Power Density of Hydrogen Engines of Positive Ignition Single-Fuel and Compression Ignition Dual-Fuel designs
Authors:   Prince Mohammad Bin Fahad University, SAUDI ARABIA, Boretti Alberto 
Published: April 2022

The latest advances in power density and efficiency of internal combustion engines (ICEs), both diesel direct injection (DI) compression ignition (CI), and gasoline positive ignition (PI), DI and jet ignition (JI), ICEs, are transferrable to single fuel or dual fuel hydrogen engines (H2-ICEs). H2-ICEs have the potential to deliver peak efficiency about 50%, peak power efficiency above 46%, and cycle average efficiency above 35% when fitted to hybrid powertrains. The major hurdles to achieve these performances are the development of hydrogen injection system. Dual fuel DI diesel-hydrogen CI ICEs have the potentials to deliver extremely low emissions of the regulated pollutants, NOx, unburned hydrocarbons (HC), CO and particulate matter (PM), and CO2 emission. Hydrogen PI ICEs with DI and JI have the potentials to deliver extremely low emissions of the regulated pollutant NOx, practically zero emissions of regulated pollutant PM, and practically zero emission of CO2 (while a small amount of lubricating oil may end up in the combustion chamber, but this amount is negligible). The PI ICEs allow much higher power densities, being the combustion event controlled by the turbulent mixing rather than vaporization, mixing and diffusion times. Thus, combustion duration is about independent of the engine speed in PI ICEs but increasing with the engine speed in CI ICEs. While combustion of hydrogen only in a CI engine is still troublesome, needing added research and development, much simpler is the use of hydrogen and diesel in a dual fuel CI engine accepting the DI of both diesel and hydrogen. While the dual-fuel DI diesel-hydrogen CI ICEs are a workable short-term replacement of the traditional hydrocarbon fueled PI ICEs, as cryogenic H2-ICES may share infrastructure and fuel system technology with LNG-ICEs, the hydrogen PI ICEs with DI and JI are an even better environmentally-friendly proposal while offering better performances. Simulations are presented for a dual duel diesel-hydrogen engine featuring two injectors per cylinder, one for the diesel and one for the hydrogen. The coupling of a pilot/pre-injection of diesel with a main injector of hydrogen is straightforward. Mixed modes of combustion are possible injecting part of the hydrogen before, and a part after the diesel injection ignition. The engine has a super turbocharging, where the turbocharger shaft is connected to the crankshaft by gears and a continuously variable transmission. Steady-state maps are presented in detail. Even if this aspect is not investigated, thanks to the super turbocharging, the engine has excellent transient behaviors, in decelerations (energy recovery) as well as accelerations (no turbo-lag). Dual fuel CI ICEs have also the advantage of a hydrogen city driving delivering extremely low emissions of pollutants as well as CO2, and a diesel interstate and rural areas driving delivering still relatively low emissions of pollutants as well as CO2.