Transient operation of internal combustion engines with Rankine waste heat recovery systems

Prior papers have shown the potentials of gasoline-like internal combustion engines fitted with Rankine cycle systems to deliver Diesel-like steady state fuel conversion efficiencies recovering the exhaust and the coolant waste heat with off-the-shelf components. In addition to the pros of the technology significantly increasing steady state efficiencies - up to 5% in absolute values and much more in relative values - these papers also mentioned the cons of the technology, increased backpressures, increased weight, more complex packaging, more complex control, troublesome transient operation, and finally the cold start issues that prevent the uptake of the technology. This paper further explores the option to use Rankine cycle systems to improve the fuel economy of vehicles under normal driving conditions. A single Rankine cycle system is integrated here with the engine design. A latest turbocharged 1.6 L direct injection engine has the coolant circuit modified to serve as pre-heater for the Rankine cycle fluid. This fluid is then vaporised and superheated in the boiler/superheater coaxial to the exhaust pipe located downstream of the turbocharger turbine and the closed coupled catalytic converter. The exhaust ports are insulated to reduce the heat losses. The pump of the Rankine cycle system is electrically operated. The expander of the Rankine cycle system drives a generator to recharge the traction battery pack. The thermal engine is connected to the transmission through an electric clutch and a motor/generator that permits to supplement/replace the thermal engine energy supply, recover the braking energy and start/stop the thermal engine. The integrated Rankine cycle system is intended to permit short warming-up profiles, reduced heat losses and reduced weight and packaging issues, delivering significant benefits during cold start driving cycles as the NEDC in addition to the long term, constant load and speed extra urban driving. (c) 2012 Elsevier Ltd. All rights reserved.