High vehicle fuel efficiency is achieved by reducing fuel consumption
during driving both in the city and on the highway. During driving in the
city, the system accumulates the energy derived from vehicle motion
during braking, and uses it to assist in vehicle propulsion at a later
time. The energy can be stored either in a compressed-air reservoir or in
an electric battery. During cruising on the highway, the engine operates
in a two-stage gas-expansion cycle. The engine has primary and secondary
cylinders. Only primary cylinders operate in internal-combustion mode.
After expansion in primary cylinders, combustion gas is subjected to a
second stage of expansion in secondary cylinders. This substantially
improves the engine efficiency. Whenever heavy engine load is needed, all
cylinders operate in internal-combustion mode.