Honda is also exploring the use of a Rankine cycle co-generation unit to improve the overall efficiency of a hybrid vehicle by recapturing waste exhaust heat from the internal combustion engine and converting it to electricity to recharge the battery pack. Test results presented in 2008 showed that in 100 kph (62 miles/hour) constant-speed driving, the use of the Rankine cycle improved the thermal efficiency of the engine by 3.8%. In the US highway cycle, the Rankine cycle system regenerated three times as much energy as the vehicle’s regenerative braking system. (Earlier post.)

Researchers at Loughborough University and the University of Sussex, both in the UK, also have concluded that using waste heat from light-duty vehicle engines in a steam power cycle could deliver fuel economy advantages of between 6.3% and 31.7%, depending upon drive cycle, and that high efficiencies can be achieved at practical operating pressures. (Earlier post.)

The basic principle of the Rankine cycle, said Andreas Obieglo, who presented the BMW paper at SAE World Congress, is to bring the working fluid to high pressure, feed it with heat, generate high energy dense steam, and convert it to mechanical energy. In the study presented, the BMW team restricted its evaluation to two basic single loop systems. System A used exhaust gas only as the heat source; System B used exhaust gas and coolant.

The working fluid, which is repeatedly vaporized, expanded, and re-condensed, plays a key role in the capability and cost-effectiveness of a Rankine steam cycle hear recovery system. To optimize the work output for a given temperature gradient, the authors noted, the evaporation enthalpy of the working fluid should be as high as possible. Water has the higher evaporation enthalpy (~2250 kJ/kg), followed by alcohols (methanol ~1100 kJ/kg, ethanol ~820 kJ/kg).

Based on the evaporation enthalpy, one would expect water to be the preferred working fluid for any heat recovery system based on the Rankine steam process. However, in most real world applications the utilization of waste heat is limited by technical restrictions (maximum and/or minimum pressure)

—Ringler et al. (2009)

Based on a quantitative analysis under constrained operating conditions, BMW determined that water delivered the highest thermal efficiency for system A, whereas ethanol is the preferable working fluid for system B (methanol was dismissed a priori due to health risks).

BMW developed a simulation model with the tool Dymola to evaluate the two alternative systems for different engine types. The process for both systems consists of the expander, the pump, the condenser and heat exchangers.

Based on the parametric analysis, BMW found that System B shows a higher potential at typical highway speeds (45-70 mph) for the engine type chosen (4-cylinder, stoichiometric combustion) and operating conditions. Nevertheless, the researchers cautioned, this cannot be interpreted as a general recommendation. Heat source parameters, which are deeply influenced ed by engine type and load profile, as well as operating parameters, which are limited by technical constraints (pressure level, ambient temperature), have significant effects on the net power output.

Based on bench testing, BMW concluded that System B could show additional power outputs between 0.7-2 kW.

This corresponds to an increase in engine performance in the range of 10% close to the road resistance curve for the top gear. Hence the operation of the Rankine cycle system presented leads to a remarkable increase in fuel efficiency. A further important step has been taken in the introduction of waste heat recover system in automotive applications.

—Ringler et al. (2009)

Resources

• J. Ringler, M. Seifert, V. Guyotot and W. Hübner. (2009) Rankine Cycle for Waste Heat Recovery of IC Engines (SAE 2009-01-0174)