Design of Rankine Cycle Systems to Deliver Fuel Economy Benefits over Cold Start Driving Cycles 2012-01-1713
Prior papers have shown the potentials of gasoline-like internal combustion engines fitted with waste heat recovery systems (WHR) 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 back pressures, 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 liter direct injection engine has the coolant circuit modified to serve as pre-heater for the Rankine cycle fluid. This fluid is then vaporized and superheated in the boiler/super heater 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 new European driving cycle (NEDC) in addition to the long term, constant load and speed extra urban driving. Prior results for the steady engine gas exchange and combustion, the steady waste heat recovery system with organic Rankine cycles (ORC) and the transient engine gas exchange and combustion inclusive of the thermal analysis have been performed with very well known commercial computer aided engineering tools and have already been peer reviewed in the scientific literature. The novelty of the present paper is the better definition of the novel single circuit Turbo steamer system where the operating fluid is water and the engine coolant passages work as pre-heater/boiler and a first very conservative assessment of the transient performance such a system could deliver when integrated in parallel hybrid vehicle architecture.