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Turbocompounding the Opposed-Piston 2-Stroke Engine
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 06, 2021 by SAE International in United States
Event: SAE WCX Digital Summit
The paper presents analytical research conducted into the level of fuel consumption improvement that can be expected from turbocompounding a medium-duty opposed-piston 2-stroke engine, which is part of a hybridized vehicle propulsion system. It draws on a successful earlier study which showed a non-compounded opposed-piston engine to be clearly superior to other forms of 2-stroke engine, such as the widely-adopted uniflow-scavenged poppet valve configuration. Electrical power transmission is proposed as the method of providing the necessary variable-speed drive to transmit excess turbine power to the system energy storage medium. The work employs one-dimensional engine simulation on a single-cylinder basis, using indicated fuel consumption as the reportable metric, coupled to positive or negative power flow to the engine from the compounder; this is a variation on an approach successfully used in earlier work. Here it shows the sensitivities of the overall system to cylinder pressure, the compressor and turbine efficiencies, and also provides a means to investigate the effect of the power transmission efficiency on the overall benefit. Reheating the air before the turbine is also investigated as a means of providing a “burst” performance facility, albeit at the expense of extra fuel consumption. Friction and its effect on the turbocompounding benefit is discussed in relation to the magnitude of the fuel consumption increase with reheat. The concept of electrical power transmission between the crankshafts of the engine is introduced, with reference to its effect on friction and how this provides a means of realizing a variable compression ratio strategy for the engine. Overall, this study is part of a research stream underway at the University of Bath termed ISOTOPE (an integrated system for optimization of the thermodynamics of opposed-piston engines).