Evaluating the Potential of a Turbo-Compound System for a Heavy-Duty Natural Gas Engine: A Modelling Study

Combining a low-carbon content fuel, such as natural gas, with a high-efficiency engine can reduce greenhouse gas emissions significantly in hard-to-electrify long-haul trucking applications. Turbo-compounding, where an additional power turbine is installed in the exhaust stream after the turbocharger turbine, can extract useful amounts of energy from diesel engine exhaust at high loads. This work assesses the net benefits of combining turbo-compounding with a high-efficiency, natural gas fuelled heavy-duty engine. The effects on brake specific fuel consumption (BSFC), greenhouse gas emissions, and engine-out emissions of nitrogen oxides (NOx) and methane (CH₄) are considered. The experimentally validated 1D model for a 13L diesel pilot- direct injection of natural gas, heavy-duty engine in GT-SUITE^TM is used to develop a series turbo-compound model. The effects of turbine sizes and flow capacities in fixed-geometry turbocharging and power turbines are evaluated on the engine’s performance, considering the trade-off between power output in the power turbine and turbo-compound losses because of increased back pressure. A parametric analysis is conducted in the 1D model to select the best combination of turbine sizes and the gear ratio between the power turbine’s shaft and engine’s crankshaft to optimize the rotational speed of the power turbine. The results show that the turbocharging turbine’s size has the most significant effect on BSFC. The model results indicate that the most promising combination of turbines may reduce BSFC by 1% to 4% at high loads within the range of 1000 rpm to 1400 rpm, with even larger reductions of 5% to 6% at the peak power conditions around 1600 rpm. At lower loads (below 40%), the BSFC increased from 1% at mid-load to 6% or more at low loads. The net benefits of the turbo-compound system are evaluated in a developed class-8 truck model in GT-SUITE^TM over standard long-haul and regional delivery transient drive cycles with different cargo loads. The truck transient simulation results show that fuel consumption was reduced by 2% to 4% in 35% to 100% cargo loads in drive cycles with more cruising time but did not change substantially in lower cargo loads. More benefits in higher cargo loads are attributed to the shifted engine operating points to the higher loads where the turbo-compound system significantly improves engine system efficiency. The truck simulation results also showed that the turbo-compound system did not change the cumulative engine-out NOx and CH₄ emissions over the studied drive cycles.