Heat release analysis of dual-fuel direct injected ammonia and HTL fuels

• Dual-fuel experimental study of liquid ammonia injections combusted in a Constant Volume Combustion Chamber (40 bar, 923 K initial conditions). • Three different hydrothermal liquefaction-based biofuels and n-heptane as pilot injection. • Apparent heat release analysis for four relative injection timings, and three different ammonia energy shares (88-96%). • Evaluation of ignition delay times reveals earlier heat release for biofuels than for n-heptane. • Earlier pilot ignition improves ammonia heat release. Fastest and highest heat release observed for the least upgraded biofuel. Ammonia (NH 3 ) has the potential to decarbonize combustion engines for intercontinental long-distance shipping as a non-carbonaceous fuel, but its low reactivity necessitates the use of a combustion-promoting strategy. This study experimentally explores a dual-fuel approach using a pilot injection to enhance the ignition and combustion of liquid injected NH 3 mixtures (main injection) under conditions typical for compression ignition engines. Three grades of diesel-type biofuels produced via hydrothermal liquefaction (HTL) of food waste are utilized as high-reactivity pilot fuels. The literature shows, that physical interaction between pilot and main fuel injections is critical for controlling NH 3 heat release. To isolate and control this influence, this study utilizes a constant volume combustion chamber with two automotive direct injectors. The work presented here characterizes the relative injector position and analyses the heat release curves. A set of evaluation criteria, including ignition delay time, is used to quantify the combustion quality. Variations of ammonia energy share (AES) from 88 to 96% and relative injection timings for HTL-fuels (−2.5 ms to 7.5 ms after start of NH 3 injection) are compared to baseline experiments with n-heptane pilots under constant ambient conditions (40 bar, 853 K). Results confirm the importance of spatial and temporal interaction between pilot injection and main fuel. All three HTL-fuel samples showed good combustibility and ignition of NH 3 for AES of 90% and higher, regardless of their processing grade, but depending on relative injection timing. The ignition delay was measured to be shorter for HTL-fuels than for n-heptane. Independent of the AES, the highest combustion efficiency was observed when the pilot injection was delayed by 2.5 ms relative to start of NH 3 injection.