Waste Heat Recovery from Combustion Engines based on the Rankine Cycle
Most of the energy in the fuel burned in modern automotive internal combustion engines is lost as waste
heat without contributing to the vehicle’s propulsion. In principle some of this lost energy could be
captured and used to increase the vehicle’s fuel efficiency by fitting a waste heat recovery system to the
engine. This thesis presents investigations into the design and functioning of waste heat recovery
systems based on Rankine cycle technology for vehicular applications.
To facilitate the design of such systems, the performance of different working fluids and expansion
devices was investigated using a zero-dimensional model of the Rankine cycle. Simulations using this
model indicated that water-based fluids should perform well when recovering waste heat from a high
temperature source such as a combustion engine’s exhaust gas. In addition, evaluations based on
similarity parameters indicated that displacement expanders are optimal in systems having low flow
rates and high expansion pressure ratios, both of which are to be expected in vehicular systems using
water as the working fluid. Organic working fluids allow higher flow rates in the cycle, making the
efficient use of turbines possible.
Data from the simulations using the zero-dimensional model were used to guide the design and
construction of a demonstrator test bench featuring a Rankine cycle-based recovery system that recovers
waste heat from the exhaust gas recirculation system of a heavy duty Diesel engine. The test bench uses
water as the working fluid and a piston expander as the expansion device. The Rankine cycle’s thermal
efficiency was 10%, corresponding to 1-2% of the engine’s power output. To find ways of improving
the system’s performance, one-dimensional models of the expander and the system as a whole were
created and then validated by comparing their output to experimental data obtained with the test bench.
The expander model suggested that reducing the compression ratio would make it possible to reduce the
steam inlet pressure by 30% without affecting the expander’s power output. This hypothesis was then
The expander model was used to rank the relative influence of selected steam boundary conditions and
expander geometry parameters on the performance of a piston expander. The inlet pressure, steam inlet
cut-off timing, expander speed and outlet pressure were found to be the most significant main effects on
expander performance. It was also shown that interaction effects between steam conditions and expander
geometry had considerable influence on both power output and efficiency.
waste heat recovery
VDL, Chalmers Tvärgata 4C, Johanneberg Campus, Göteborg
Opponent: Professor Vincent Lemort, Thermodynamics Laboratory, Université de Liège, Belgium