The Rankine cycle is widely used used commercially to generate power in stationary power plants, and is under consideration as a potential waste heat recovery system for use in both light-duty (BMW, earlier post; Honda, earlier post) and heavy-duty (earlier post) applications.

A generic Rankine cycle system consists of a pump, evaporator, expander, and condenser. The working fluid is pumped to the evaporator where it is vaporized by heat. The vapor flows to the expander where it can be used to generate electricity, then comes back to the condenser, where it cools back to a liquid to begin the cycle again.

Hussain and colleague David Brigham developed a transient numerical model capable of capturing the main effects of this cycle using different designs under different conditions.

Inputs into the model include exhaust mass flow rate and temperature downstream of the catalytic converter, ORC component size and geometry. Outputs include mechanical power, electric power, and system backpressure. The Ford team used an organic fluid rather than ater as the working fluid because of a number of advantages, including a very low freezing point.

The evaporator was a shell-and-tube type design with exhaust gas passing through the tubes. High side and low side pressures were assumed to be fixed; what varies is the mass flow rate of the working fluid depending on the amount of heat that is available.

They defined the Power Factor (PF) as the power generated by the ORC system divided by the vehicle accessory load need; i.e., a PF of 1 would mean that the power generated by the ORC waste heat recovery system fully met the accessory load demand, with a concomitant improvement in fuel economy.

Using the simplest ORC design (exhaust gas alone as the source of thermal input—i.e., no use of coolant fluid for preheating due to increased system complexity to support that), Hussain and Brigham found that for a 2.5-liter conventional engine in the non-hybrid Escape, the PF was 0.9 on the highway cycle and 0.513 on the city cycle. For the Atkinson-cycle 2.5L engine used in the Escape Hybrid, the PF was 0.98 on the highway, and about 0.32 in the city. In general, the power output during EPA Highway drive cycle is much higher than EPA City due to higher exhaust mass flow rate and temperature.

The ORC for a light-duty vehicle can generate enough electricity to partially offset and in some case almost fully offset accessory load on the vehicle. That obviously depends on the type of vehicle and the drive cycle. On the EPA highway drive cycle, it comes very close to meeting the electric load; on EPA city it is less, about one-third to one-half. For the hybrid it’s a little worse because the engine turns off, there is no exhaust gas. On the highway the hybrid is a little better because it is a little heavier vehicle, there is a little higher engine work, higher exhaust energy.

—Quazi Hussain

The Ford team also found that, contrary to their concerns, the backpressure with ORC waste heat recovery under this particular design was a little lower than the baseline system. The reason, they concluded it that in this particular design, the exhaust gases cool tremendously, resulting in a pressure drop. This lower backpressure will further boost the fuel economy gained by the electricity produced by the Rankine bottoming cycle.