Turbocharged vs. Turbo-Compounded Ethanol Engine: Fuel-Air Equivalence Ratio Impact 2009-36-0050

The quest for high efficient internal combustion engines has intensified in the last years due to, among other reasons, increasing fuel costs and the pressure to reduce environmental deterioration. One of the possible alternatives capable of providing the sought efficiency gains is the recovery of the energy wasted in the exhaust gases and turbo-compounding is one obvious option. Compression ignition engines are usually the target of turbo-compounding, however, the rising interest for alternative fuels could result in the use of turbo-compounding for spark ignition engines as well. Due to its different flame propagation mechanism, spark ignition engines may force operation at higher fuel-air ratios, eventually creating a quite distinctive operational behavior.

This paper evaluates the impact of fuel-air ratio on losses and efficiencies of a 12-liter, 6-cylinder intercooled turbocharged engine and its turbo-compounded version, both running on ethanol, applying a method described by Winterbone et al [15–18], named turbine work function, as an alternative to the 2^nd law of Thermodynamics.

Engine performance simulations showed turbo-compounded configuration featuring an optimum fuel-air ratio due to a trade-off between combustion rate and pumping losses, on one hand, and exhaust energy recovery on the other, trend not detected in the turbocharged configuration. This trade-off may impose a fundamental limitation on turbo-compounded spark ignition engines.