Diesel engines are set to remain the prime mover for heavy-duty, long-haul and marine transport for the coming decades. Incremental powertrain improvement and electrification form only part of the solution, towards the ambitious Carbon Dioxide (CO~2~) reduction targets formed by the 2015 Paris Agreement. There is an immediate need for high technology readiness level drop-in, carbon-neutral fuels that can be used in state of the art compression-ignition engines. Hydrotreated Vegetable Oil (HVO), has reached a substantial market share as an automotive or marine diesel substitute, especially in the Scandinavian countries. Furthermore, it has proven to reduce the well-to-wheel CO~2~ footprint and emissions on stock engines. However, the full potential of this fuel, enabled by clean chemical structure, improved viscosity, and high cetane number, is still not fully exploited.

The present work explores the optimization potential enabled by advanced common-rail engines while operating on HVO. Single-cylinder engine research is focused on adjusting the injection strategy and the external Exhaust Gas Recirculation (EGR), towards optimum performance-emissions trade-off with HVO. The engine is operated at a fixed rotational speed of 2,000 rpm and under constant load (indicated mean effective pressure at 0.5 MPa). Split fuel injection strategy is applied with fixed main injection timing and pilot injection varied both in terms of timing and quantity. The engine tests are conducted without super-charging, under non-EGR conditions, and at a 27% EGR rate.

Within the constraints of a single, representative operating point, the results highlight that at the factory map-based injection strategy, HVO offers soot emissions below 0.015 g/kWh, which is a 50% reduction when compared to diesel fuel. Nitrous Oxides (NO~X~) emissions at the same conditions are however 10% higher than for diesel fuel, which is correlated with higher peak in-cylinder pressures and temperatures. Advancing the pilot HVO injection enabled the reduction of NO~X~ emissions to the level of the diesel baseline, while trading-off on particulate mass (PM), yet keeping the overall advantage of HVO in terms of soot emissions. Interestingly, the two tested fuels exhibited very different responses to EGR. Generally, at 27% EGR, HVO produces twice as much PM as diesel. The heat release analysis provides an explanation of this sensitivity to EGR. Namely, the higher cetane number of HVO causes faster auto-ignition, resulting in less premixed combustion, hence producing more soot. Generally, HVO offers more complete combustion than diesel fuel. Regardless of the pilot fuel injection strategy, the emission of Carbon Monoxide (CO) is reduced by approximately 50% for both EGR and non-EGR conditions. The effect of HVO on emissions of unburned hydrocarbons is also advantageous - both in terms of total indicated-specific values, as well as unlegislated aldehydes and aromatics.

The general conclusion is that, within the constraints of the optimized conventional diesel combustion (CDC) mode, HVO performs better than diesel in terms of both indicated efficiency (up to 1 percentage point higher) and overall emissions. This statement is valid only for light-EGR strategies and the advantages hinder towards higher recirculation rates.


Combustion of Hydrotreated Vegetable Oil in a Diesel Engine: Sensitivity to Split Injection Strategy and Exhaust Gas Recirculation

Division / Sponsors: 
Internal Combustion Engine Division (ICED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2020/volume/84034

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