The Stirling engine is potentially better suited to constant speed/load applications, and could
conceivably have peak efficiency as high as 45 percent,^152 but the high combustion temperatures
result in high NOX emissions without catalytic aftertreatment. Even if an efficiency of 45 percent
were reached, the costs of the hydrogen seals and heater heads cannot be easily reduced. For
these reasons, it appears very unlikely that Stirling engines will be a cost competitive automotive
powerplant, even in constant speed applications.
Waste Heat Recovery.
With spark ignition and compression ignition engines, much of the heat energy of the fuel is lost
to the cooling system, oil, and to the exhaust. Recovery of a portion of this waste heat is an
obvious solution to improve efficiency, but the low temperature of the waste heat makes it very
difficult to recover any energy in a cost-effective way. The coolant and oil temperature are so low
(less than 100oC) that no practical system has been devised to recover this energy. Exhaust heat is
a much better target, but the temperature and quantity of heat fluctuates rapidly is urban driving
conditions.
Recovery of the waste heat has been explored by using a Rankine cycle (steam engine) or by
turbocompounding. The Rankine cycle could convert the water in the cooling system to steam by
using exhaust heat, and expand the steam to provide useful work. Because of the relatively low
temperatures of the exhaust (250oC), the theoretical (or Carnot cycle) efficiency is limited to
about 40 percent--that is, a maximum of 40 percent of this waste heat can be recovered. The
complexity of a heavy steam engine in series with the spark ignition engine, however, has always
outweighed the potential fuel efficiency benefit. Turbocompounding is a simpler heat recovery
method where a turbine (connected to the engine output shaft) recovers the waste pressure energy
in the exhaust. Owing to the low engine load in urban driving and in highway cruise, there is very
little pressure energy to be recovered in a passenger car or light truck engine, but this system can
be useful in heavy-truck diesel engines.
One of the more sophisticated attempts to recover energy was by Toyota. In this application,
the existing cooling system was replaced by a system in which a chlorofluorocarbon working fluid
evaporated into a vapor. Power was recovered from the vapor by means of a scroll expander (that
is, an expander that uses a helical-shaped blade rather than vanes to capture the energy of the
expanding vapor). Theoretical analysis predicted that, at low speed, a fuel economy improvement
of 7 to 8 percent was possible at an ambient temperature of 25°C when such a system was fitted
to a small Toyota with a 1.5 litre engine.^153 In actuality, the system installed by Toyota attained
only a 3 percent benefit, because an unmodified (from production) cylinder head created pressure
losses, and the scroll expander efficiency was also lower than expected.^154 Of course, the waste
heat recovered is sensitive to the ambient temperature, with heat recovery decreasing to near zero
(^152) J. Corey et al., “Design Description of an Automotive Stirling Engine with Competitive Manufacturing Costs,” paper presented at the
Automotive Technology Development Contractor Coordination Meeting U.S. Department of Energy, October 1994. 153
H.Oomori and S. Ogino, “Waste Heat Recovery of Passenger Car Using a Combination of Rankine Bottoming Cycle and Evaporative Engine
Cooling System,” SAE paper 930880, 1993. (^154) Ibid.