Chapter 3 Engine Classifi cation, Parts Identifi cation 63
When compression is at its maximum, the
spark plug ignites the fuel charge, Figure 3-30C.
When the charge ignites, the rapidly expanding
gases apply pressure to the A-B face of the rotor,
forcing it to continue walking around the fi xed
gear. As it rotates, it transmits the power of the
burning gas to the eccentric shaft rotor journal,
causing the eccentric shaft to spin, Figure 3-30D.
The burning gas will continue to revolve
the rotor until tip B uncovers the exhaust port,
allowing the burning gas to be pushed out the
exhaust port as shown in Figure 3-30E. Follow
rotor face A-B from Figure 3-30A to Figure 3-30E
and note that as the ports are uncovered and the
chamber volume changes, each phase of a four-
stroke cycle is performed.
Atkinson- and Miller-Cycle Engines
In a conventional four-stroke engine, the
intake valve is open during the piston’s intake
stroke, and closes almost as soon as the piston
begins to move upward on the compression
stroke. On an Atkinson-cycle engine, the intake
valve is held open during the fi rst part of the
compression stroke, allowing air to fl ow back
into the intake manifold. This reduces compres-
sion ratio, allowing more effi cient use of the air
in the cylinder.
In an Atkinson-cycle engine, the pres-
sure in the combustion chamber at the end of
the power stroke is equal to atmospheric pres-
sure, ensuring that all the available energy has
been obtained from the combustion process.
This allows more heat energy to be converted
from heat to mechanical energy. This means
that the engine is more effi cient. The Atkinson-
cycle engine gets better fuel mileage, but power
is reduced. Atkinson-cycle engines are used in
some hybrid vehicles.
A Miller-cycle engine is essentially an
Atkinson-cycle engine with a supercharger.
The supercharger pressurizes the intake mani-
fold, keeping the air-fuel mixture from exiting
through the intake valve. This forces more of the
air-fuel mixture into the cylinder. A Miller-cycle
engine produces more power and economy over
a wider RPM range than an Atkinson-cycle
engine of the same size. Figure 3-31 shows a cut-
away of a Miller-cycle engine.
Variable Compression Engines
One automobile manufacturer has devel-
oped an engine that can vary its compression ratio
to minimize fuel consumption while increasing
engine performance. This variable compression
engine is divided into upper and lower portions.
The upper portion moves in relation to the lower
portion. The upper portion, called the monohead,
consists of the cylinder head, intake manifold,
supercharger, exhaust manifold, and integrated
cylinders. One side of the monohead is attached
to the lower portion of the engine through a pivot-
ing connection. The other side of the monohead is
attached to the lower portion of the engine through
a hydraulic actuator. The actuator can tilt the mono-
head upward to reduce compression. Actual move-
ment of the head is slight, only about 4 degrees, but
this is suffi cient to vary compression by 6 points.
Figure 3-32 shows the variable compression
engine at 14:1 compression and at 8:1 compres-
sion. Note the position of the hydraulic actuator
on the right side of each illustration. The hydrau-
lic actuator is operated by an electronic control
system. The control system receives input from
speed, vacuum, airflow, and knock sensors.
A supercharger is used on the variable com-
pression engine. When the supercharger is work-
ing, the control system and hydraulic actuator
raise the monohead to the lowest compression
Figure 3-31.
This Miller-cycle engine design allows the intake valve to
open sooner. A supercharger is used to force the air-fuel
mixture into the cylinders.
Mazda