|Results of the study indicate that a steam hybrid using exhaust waste heat could reduce fuel consumption by up to 31.7%, depending upon drive cycle and vehicle.
A study by researchers at Loughborough University and the University of Sussex, both in the UK, has 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.
The basic concept of the “steam hybrid” system is that energy is recovered from the exhaust in the form of a steam/water mixture. Shaft work is produced as steam is expanded, and is used in one of three ways:
To add torque to the internal combustion engine (ICE) output, thereby reducing fuel consumption and emissions; or
To drive an auxiliary power unit (APU) if the energy is not required by the vehicle (e.g., during braking or idling). Excess energy can be used to generate electricity and charge an electric storage system; or
To provide all the required torque to the drive shaft resulting in zero-emissions driving in inner-city areas.
The study, which is part of a larger investigation of the controllability of energy recovery, modeled a steam hybrid vehicle using two toolboxes: the Quasi-Static Simulation Toolbox (QSS-TB), developed at ETH Zürich, and the Powertrain System Analysis Toolbox (PSAT), developed by Argonne National Laboratory.
For a base vehicle in QSS-TB, the researchers used a VW Golf with a 1.6 liter engine; for PSAT, they used a Honda Civic with a 1.8 liter engine. Both vehicle models were run against the New European Drive Cycle (NEDC); the US FTP-75 Urban Drive Cycle; and the US06 Highway Drive Cycle
Results showed an improvement for both vehicle across all three drive cycles, with the greatest gain in the FTP-75 cycle, and the least with the aggressive US06 cycle.
Subsequent work for the research will include optimized controls for different style of driving, and further work on optimizing the heat exchanger. The design of the heat exchanger will be evaluated in an experimental set-up in which the exhaust gas is generated by a 7.2-liter Caterpillar engine.
In further developing the control architecture, the researchers are using four objectives:
Power/torque. The power or torque demanded from the vehicle driver needs to be met by the ICE, the steam expander, or a combination of the two.
Fuel consumption. Fuel consumption needs to be kept at a minimum.
Steam supply/reserve. The supply of water to the heat exchanger needs to be kept constant to ensure there is a constant steam supply, or at least a reserve of steam for the expander.
Steam quality. The steam supplied to the expander is required to be superheated so that it contains no water droplets which could be potentially damaging to the expander. This will be achieved by raising the temperature of the cold side fluid beyond saturation and ensuring it is superheated at entry to the expander.
The work is supported by the UK Engineering and Physical Sciences Research Council.
BMW and Honda are both investigating the use of waste-heat powered steam systems to enhance fuel economy. BMW’s onboard water/steam-based cogeneration cycle is used to power the vehicle’s accessories, rather than a traction battery pack (earlier post).
Honda is actively 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. (Earlier post.)
Canada-based Clean Power Technologies Inc. (CPTI) is developing a waste-heat powered steam hybrid system—CESAR, Clean Energy Storage and Recovery)—that it claims has shown an up to 40% reduction in vehicle fuel consumption in initial test results. The system is under development by a wholly-owned CPTI subsidiary, Clean Power Technologies Ltd. (CPTL), which is located in East Sussex, UK. The Clean Power Technologies system was developed by Fred Bayley, Professor Emeritus of the University of Sussex.
CESAR uses a heat exchanger to capture waste energy, which is then stored in the form of steam in an accumulator, for on-demand use either in the same primary engine, or in a secondary vapor engine. Power can be produced solely by the secondary vapor engine even after the primary combustion engine has shut down.
The test program using the system included a generic model of an accumulator, supplied by Clean Power’s collaborative partner Doosan Babcock (previously Mitsui Babcock). The CESAR system has been running in parallel with a Caterpillar C18 diesel engine within Clean Power’s test facility in Newhaven, East Sussex since mid-October 2007. Clean Power is now re-designing the second generation of the steam accumulator which will be lighter and more efficient.
In March, CPTI contracted with steam technology specialist Dampflokomotiv-und Maschinenfabrik DLM AG (DLM) to act as a consultant for the further development of the CESAR technology. DLM, a specialist in the development of modern steam traction systems, will provide consultancy, design engineering and stress test related services.
CPTI has entered into a first stage collaboration agreement with Safeway Corporation for the purpose of data collection and to undertake preliminary design work for the steam refrigeration units for the grocery trucks. CPTI has also entered into a collaboration agreement with Voith Turbo Gmbh & Co. KG of Germany to jointly develop a reefer engine.