7. DIESEL ENGINE
The Combustion Cycle
The four-stroke combustion cycle of the diesel engine is composed
of the intake stroke, compression stroke, power stroke, and the
exhaust stroke.
The diesel engine operates through the motion of compression
pistons in inside the cylinders of the engine.
Pistons inside the cylinders are connected by rods to a
crankshaft. As the pistons move up and down in their cylinders, they
cause the crankshaft to rotate.
The crankshaft’s rotational force is carried by a transmission to a
drive shaft, which turns axles, causing the wheels to rotate.
9. INTAKE STROKE
The air is pushed by atmospheric During
the intake stroke, the intake valve opens as the
piston moves down to allow air into the cylinder.
pressure into the cylinder through the
intake valve port.
10. COMPRESSION STROKE
After the piston reaches its lower limit, it begins
to move upward and as the piston moves up, the
intake valve closes.
The exhaust valve is also closed, so that at this
point in the cycle, the cylinder is sealed.
As the piston moves upward, the air is
compressed and the compression of the air
causes the pressure and the temperature of the
cylinder to increase.
11. POWER STROKE
As the piston reaches maximum compression of the air,
diesel fuel is injected to the combustion chamber filled
with compressed air.
The heat of the compressed air ignites the fuel
spontaneously at the self-ignition temperature of the fuel.
As the cylinder pressure increases, the piston is forced
down into the cylinder.
The power impulse is transmitted down through the
piston, through the connecting rod, and to the
crankshaft, which is rotated due to the force.
12. EXHAUST STROKE
As the piston reaches the bottom of its stroke,
the exhaust valve opens.
As the piston moves up on the exhaust stroke, it
forces the burned gases out of the cylinder
through the exhaust port.
Then as the piston reaches the top of its stroke,
the exhaust valve closes, and the intake valve
opens.
The cycle repeats again with the intake stroke.
14. EFFICIENCY
One positive aspect of the diesel engine is that
they are thermally efficient.
The improved efficiency is caused by the
relatively high compression ratios
The diesel engine is 54% thermally efficient,
while gasoline engines are only 34%.
As a result of diesel engines thermally efficiency,
they are able to achieve better gas mileage
because they produce greater horsepower
output for fuel intake.
15. Piston
Piston is a cylindrical piece of
metal that moves up and down
inside the cylinder
Connected to a connecting rod
The rod connected to the
crankshaft
16. Crankshaft
Turns the piston’s up and
down with in circular motion.
Location varies on engine
type.
In-line engine: located below
the cylinders
V-type engine: at the base of
the V.
Flat-engine: between the
cylinder banks.
17. Other components
Intake valves - open at proper
time to allow air-fuel mixture
enter the cylinder.
Exhaust valves - let out
exhaust (burned exhaust gas).
Oil sump - surrounds the
crankshaft. It contains some
amount of oil, which collects in
the bottom of the sump (the oil
pan).
19. Advantages of a Diesel Engine
Low maintenance, greater efficiency, high power output,
and better fuel economy under all types of loads.
Does not require a spark plug to ignite fuel, it relies on
the spontaneous combustion through the heat of
compression to ignite the diesel fuel.
Because of this type of combustion, a diesel engine are
built more ruggedly and heavily built than the gas
engine.
The ruggedness of the diesel engine gives a two to three
times longer life than the gas engine, which has a life of
around 100,000 miles.
20. Disadvantages
Components of diesel engines are usually heavier that
those of gasoline engines because of the additional
structural strength needed to obtain the higher
compression ratio and power output.
They can emit large amounts of ozone-forming
constituents and particulates.
Because of diesel engines’ weight and compression
ratio, they tend to have lower maximum RPMs than
gasoline engines.
– This makes diesel engines has high torque rather
than high horsepower, and this tends to make cars
with diesel engines slow in terms of acceleration.
21. Disadvantages (cont.)
Diesel fuel is not as readily available at all locations as
gasoline tends to be.
Diesel engines are harder to start in cold weather
conditions, because it is difficult to get the temperature
inside the cylinder up to the self-ignition temperature of
the diesel fuel.
22. Emissions
The most harmful emission from diesel engines are the
nitrogen oxides.
The problem of nitrogen oxide (NO) production in diesel
engines comes from the early rapid burning, which
produces very high-temperature products.
As a result of the pollutants from diesel engines, they are
subject to increasingly stringent regulations that require
continual improvements in the combustion process.
23. Main Differences of Gas and Diesel
The main difference between a gasoline engine and a
diesel engine is that in a diesel engine, there is no spark
plug to ignite the fuel.
Diesel fuel is injected into the cylinder, and it ignites
spontaneously as the heat and pressure of the
compression stroke cause the temperature inside the
cylinder to increase.
The other major difference between the two types of
engines is in the gas mileage.
– As a result of diesel fuel having a higher energy
density than gasoline, diesel engines get higher gas
mileage.
