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ORNL computational study finds 15% bio-oil mixture in two-stroke marine engines can cut NOx 13% with efficiency parity

A team at Oak Ridge National Laboratory (ORNL) has developed a new computational model of a two-stroke scaled marine engine, with reduced chemical mechanisms for diesel, biodiesel, bio-oil, and polycyclic aromatic hydrocarbons (PAH). In a new open-access study published in the journal Fuel, the ORNL researchers used their model to assess the emissions of NOx and PAHs (as a surrogate for soot) of these fuel blends.

More than 90% of all the world’s goods are transported on large ocean-going vessels; the marine shipping sector is also among the last to reduce criteria pollutants and is responsible for up to 3% of global CO2 emissions. The sector is now facing multiple regulatory actions: reductions in fuel sulfur content, CO2 emissions, and NOx emissions, as well as expected upcoming limits on black carbon emissions.

Decarbonizing the maritime sector requires the development of new fuel sources that do not compete with other transportation fuels in the global market. Because of the extremely large physical size of the internal combustion engines present in shipping vessels, experimental development of the engine-fuel system is often cost prohibitive. This work aims to develop a computational model of a scaled marine engine. The scaled model is representative of a custom-built 1:10 scale research engine commissioned for lubricant research at Oak Ridge National Laboratory.

—Chuahy et al.

Bio-oil—also called pyrolysis oil—is a liquid fuel produced through vapor condensation of fast pyrolysis of biomass. Its complex composition is uncertain depending on the source of the biomass and the operating conditions of the thermal degradation process.

For the study, the surrogate is based on the bio-oil produced by Dynamotive Energy Systems. For this work, the bio-oil surrogate was further reduced to a six-component mixture reducing computational cost.

The scaled model is representative of the ExxonMobil Enterprise research engine, a custom-built 1:10 scale, uniflow scavenged, two-stroke crosshead marine diesel research engine commissioned for marine lubricant research at ORNL, which is the source of validation data.

Among the major conclusions of the study:

  • A 15% bio-oil mixture can be used with engine efficiency parity, a 13% reduction in NOx, and a 45% reduction in soot precursors. Changes in NOx emissions and efficiency for higher bio-oil blends were shown to be manageable with changes in injection duration and injection timing.

  • Mixtures of up to 40% bio-oil were tolerated under these simulations. Mixtures of 50% bio-oil resulted in misfires for the baseline injection timings. It is likely that more advanced injection timings may enable higher bio-oil mixtures, as advanced timing simulations for 40% bio-oil did not result in overmixing and still preserved features of a mixing rate-limited combustion process.

  • Addition of bio-oil at a constant injection pressure results in a lengthening of the combustion process due to longer ignition delay times and longer injection durations. However, its impacts can be managed by changing the injection duration and the injection timing.

  • Diesel ignition and combustion happen in the periphery of the fuel jet and without significant interference from the other fuel injector. However, bio-oil mixture combustion showed an intricate connection between the ignition sites of the first injection and ignition of the second injection. Swirl motion was shown to be an important parameter in promoting the combustion of the first injection due to hot gas transport from the second injection combustion. Manipulation of the timing between injections will likely have a strong effect on the combustion of bio-oil mixtures.

This research was funded by the Bioenergy Technologies Office of the US Department of Energy.


  • Flavio D.F. Chuahy, Charles E.A. Finney, Brian C. Kaul, Michael D. Kass (2022) “Computational exploration of bio-oil blend effects on large two-stroke marine engines,” Fuel, Volume 322, doi: 10.1016/j.fuel.2022.123977


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