The impact of exhaust gas recirculation calibration on emissions and efficiency with GTL and GTLB30 fuels on a heavy-duty diesel engine

Abstract

A gas-to-liquid (GTL) fuel and its blend with 30% fatty acid methyl esters (GTLB30), as well as reference diesel have been investigated on a modified single-cylinder heavy-duty compression-ignition engine over a large part of the engine operation map. The experimental results indicate that GTL and GTLB30 demonstrate comparable in-cylinder pressure and rates of heat release (ROHR) trends to diesel throughout the operating map. In general, the ROHR of GTL is close to diesel, albeit with a heightened sensitivity to exhaust gas recirculation (EGR) variations. In this work, these variations were 15% above and below baseline values. Meanwhile, the ROHR of GTLB30 is less sensitive to EGR variations because of its inherent higher oxygen content, but this comes at a small penalty in fuel injection duration due to its reduced LHV. Generally, Both GTL and GTLB30 exhibit shorter ignition delay and burn duration compared to diesel. Additionally, GTL achieves higher gross indicated efficiency (GIE) with a weighted average increase of 2.04% over diesel across all baseline engine operation map, while GTLB30 has slightly lower GIE than diesel in most cases, with a weighted average decrease of 0.99%. In terms of emissions, GTL and GTLB30 follow a similar trend with remarkable low particulate matter (PM) emissions, especially at higher EGR ratios (above 35%). Using baseline EGR rates, both GTL and GTLB30 demonstrate significant reductions in engine-out PM emissions, with a weighted average reduction of 34.5% for GTL and 71.5% for GTLB30 compared to diesel across the entire engine operation map. Moreover, GTL illustrates significant potential in breaking the PM-NOx trade-off relationship, with a 16.1% weighted average reduction in NOx emissions compared to diesel at the baseline EGR calibration. Conversely, the NOx emissions of GTLB30 show a weighted average increase of 8.3% (again at the baseline EGR calibration), which can be reduced with increasing EGR. Furthermore, GTLB30 exhibits minimal CO emissions, without any sensitivity to changes in EGR, while both GTL and GTLB30 benefit from a THC emission reduction. Notably, the CO and THC emissions of GTL and GTLB30 show possible compliance in most operating points with Euro V regulation without after-treatment. Finally, NOx reduction across all operating points is possible while respecting Euro VI PM limits for both GTL and GTLB30 by utilizing higher EGR levels at A30, A50, B50, and B70 versus lower EGR values for B30 and C30. The exact magnitude of these EGR levels per operating point are given in a bar-chart based on a non-linear curve fitting procedure, resulting in a weighted average NOx reduction of 38.3% and 68.7% across the tested speed-load range for GTL and GTLB30, respectively.