two stroke engine working and components

1. Two stroke engines

2. Intake for two stroke cycle engine
 in two stroke cycle engine the inlet must be
put at a pressure greater than atmospheric. At
start of intake the cylinder is filled with
exhaust gas at atm pressure. Air under
pressure enters the cylinder and pushes most
of the remaining exhaust residual out of the
still open exhaust port called scavenging

3. Two method are used for putting air
into the cylinder
 through normal intake valves
Through intake slots in the cylinder wall

4.  there are open combustion chamber in the
two stroke cycle engine. It would be extremely
difficult to get proper scavenging in a cylinder
with a divided chamber.
In some automobile air is injected with the
fuel this speeds evaporation and mixing which
is required because of the very short time of
compression stroke
Fuel injection pressure is of 500 to 600Kpa
injection pressure is slightly less at about 500
kpa.

5. Crankcase compression
 in case of two stroke cycle engine using
crankcase compression, lubricating oil must be
added to the inlet air. The crankcase in this
engine cannot be used as the oil reservoir as
with most other engines. Instead the surface
of the engine components are lubricated by
oil vapor carried by the intake air.

6. Scavenging process
 the process of clearing the cylinder after the
expansion stroke is called scavenging. It is the
process of replacement of combustion
product in the cylinder from previous power
stroke with fresh air to be burnt in the next
cycle.

7. SCAVENGING PROCESS
Pre blowdown blowdown scavenging Additional charging

8. Pre blowdown
 on the opening of inlet port, the gasses
expanding in the main cylinder tend to escape
from it and to pre discharge into scavenged air
manifold. This process ends with the opening
of exhaust port.

9. blowdown
 with the opening of exhaust ports, the gasses
existing in the cylinder at the end of expansion
stroke discharged simultaneously into exhaust
manifold. The pressure of main cylinder drops
to a lower value than existing in scavenged air
manifold.

10. Scavenging
 the scavenging air sweeps out all the residual
gasses remaining in the main cylinder at the
end of spontaneous exhaust and replaces
them as completely as possible with fresh
charge.

11. Additional charging
 after the completion of scavenging phase, the
fresh charge continues to flow till the
scavenged ports are open and pressure in the
cylinder rises. The last phase result in better
filling of the cylinder

12. Scavenging parameter
 for the same power generation, more air
input is required in a two stroke engine. This is
because some of the air is lost in the overlap
period of the scavenging process

13. Scavenging systems
• SCAVENGING SYSTEMS
UNIFLOW SCAVENGING LOOP OR REVERSE SCAVENGING CROSS SCAVENGING

14. UNIFLOW SCAVENGING
 it is the most perfect method of scavenging
The fresh charge is admitted at one end of the
cylinder and the exhaust escape at the other
end. The air flow is from end to end, and little
short circuiting between the intake and
exhaust opening is possible.

15. Uniflow scavenging
• 1. In figure one a poppet valve is used to
admit the inlet charge for the exhaust

16. 2.In figure 2 the inlet and exhaust
ports are both controlled by separate
pistons that move in opposite
directions

17. 3. In figure 3 the inlet and exhaust
port are controlled by the combined
motion of piston and sleeve.

18. Loop or reverse scavenging
 the fresh air first sweeps across the piston
top, moves up and then down and finally out
through the exhaust

19. In figure a in the MAN type of loop scavenge
the exhaust and inlet port are on the same
side, the exhaust above the inlet.

20. In figure b the schnuerle type the
port are side by side

21. In figure c The curtis type of
scavenging is similar to the schnuerle
type except that upwardly inlet port
are placed opposite the exhaust ports

22. • With the absence of cams, valves and valve
gear, loop or reverse scavenged engine are
simple and sturdy. They have high resistance
to thermal stresses and are thus much suited
to higher supercharged.

23. Cross scavenging
The inlet and exhaust ports are located on
opposite side of the cylinder
The incoming air is directed upward by the
deflector on the piston and the cylinder head
reverses the direction of flow so that exhaust
gasses are forced through the exhaust ports

25. In this type of arrangement the engine is
structurally simpler than that with the uniflow
scavenging
The main demerit is that scavenging air is not
able to get rid if the layer of exhaust gas near
the wall resulting in poor scavenging.

26. Crankcase scavenging
• In this engine, the charge is compressed in the
crankcase by the underside of the piston
during the expansion stroke. There are three
ports in this engine intake ports at the
crankcase, transfer port and the exhaust port.
The compressed air passes through the
transfer port into the engine cylinder flushing
the products of combustion. This process is
called scavenging, and this type of engine is
called crankcase scavenged engine.

27. The projection on the piston is called the deflector

28. demerits
 this system is uneconomical and inefficient in
operation. This is owing to the fact that the
amount of air which is used for scavenging is less
than the swept volume of the cylinder due to low
volumetric efficiency of the crankcase which
contains a large dead space. Some residual gasses
always remain in the cylinder.
 due to mixing of the oil vapors from the
crankcase with the scavenging air oil
consumption is increased.

29. Scavenging pumps and blowers
• Since the piston cannot carry out the
scavenging process therefore a separate
pumping mechanism called the scavenging
pump is needed to supply scavenging air to
the cylinder.

30. Roots blower
• The Roots blower is a positive
displacement lobe pump which operates by
pumping a fluid with a pair of meshing lobes
resembling a set of stretched gears. Fluid is
trapped in pockets surrounding the lobes and
carried from the intake side to the exhaust.

31. • Rotary vane 1
• Pump body
• Rotary vane 2
• Intake
• Pumping
• Output

32. Centrifugal blower
• In centrifugal blowers, the air is drawn in
through an inlet at the center of the impeller.
Curved vanes on the impeller spin this air
inside the housing, speeding it up as it is
forced outward by centrifugal forces. The
accelerated air on the periphery of the
impeller is able to exit while relatively lower
pressure created at the center works to suck
even more air into the unit