(b) When the piston is near the bottom of its stroke, the ports in
the cylinder liner are exposed. Air, under pressure from the blower, is forced
into the cylinder (intake). At the same time, the exhaust valves open. The
incoming air helps force the exhaust gases out of the engine (exhaust). The
exhaust valves close as the piston starts it compression stroke. Air is
continuously forced into the cylinder until the piston blocks the ports. The
air is compressed and ignited and the piston is forced back down. Every down
stroke is obviously a power stroke. When the piston moves down far enough to
expose the cylinder ports, the sequence begins again.
(3) Control of speed and power. The speed and the power of the diesel
engines are controlled by the quantity of fuel injected into the cylinders.
This is opposed to the common gasoline engine, which controls speed and power
by limiting the amount of air admitted to the carburetor. In the diesel
engine, a varying amount of fuel is mixed with a constant amount of compressed
air inside the cylinder. A full charge of air enters the cylinder on each
intake stroke. Because the quantity of air is constant, the amount of fuel
injected determines speed and power output. As long as the amount of fuel
injected is below the maximum established by the manufacturer who designed the
engine, there is always enough air in the cylinder for complete combustion.
(4) Combustion process. In the diesel engine, there is continuous
combustion during the entire length of the power stroke, and pressure resulting
from combustion remains relatively constant throughout the stroke. In the
gasoline engine, combustion is completed while the piston is at the upper part
of travel. This means that the volume of the mixture stays about the same
during most of the combustion process. When the piston does move down and the
volume increases, there is a little additional combustion to maintain pressure.
Because of these facts, the cycle of the gasoline engine is often referred to
as having Constant Volume Combustion while the diesel cycle is said to have
Constant Pressure Combustion.
b. Turbine Engine. (Figure 115). The turbine engine burns low
performance diesel fuel mixed with compressed and heated air. Air enters the
engine and flows through two compressors. The compressed air is heated by
exhaust gases to aid burning. The heated and compressed air is directed into
the combustion chamber where it mixes with fuel vapor. Continuous burning
occurs once it has been started by an ignition spark. Gases are routed through
the recuperator to heat incoming air and then exhausted.
22
OD0610
Previous Page
