Petrol Engine Presentation USA.ppt

Prof. B Dinesh Prabhu P E S C E Mandya
Petrol Engine

Engine Classifications
1.Application.
2.Basic engine design

Engine Classifications…
3. Working cycle
4. Valve or port
design & location.
(c)
(a) Cross,
(b) Loop & (c) Uniflow Scavenging

5. Fuel
6. Method of mixture preparation.
7. Method of ignition
8. Combustion chamber design.
9. Method of load control.
10.Method of cooling.
Engine Classifications…

References:
• Microsoft Encarta
• Britanica Encyclopedia
• Theory & Practice in IC Engines By C F Taylor
• Fundamentals of I C Engines-J B Heywood
• Internal Combustion Engines-Ganesan
• Internal Combustion Engines-M L Mathur & S P
Sharma

Chapter 2
2 Stroke & 4-stroke Cycle Engines
Principles of engine operation (SI & CI)
Scavenging -systems
-theoretical processes,
-parameters
-relative merits & demerits
Valve & Port timing diagrams
Port design
Scavenging pumps
Relative merits & demerits of engines

4-stroke Cycle SI Engine
Principle of Operation
• Cycle of operation is completed in 4-strokes
of the piston or 2 revolutions of the
crankshaft.
• Each stroke consists of 180°, of crankshaft
rotation and hence a cycle consists of
720°of crankshaft rotation.

• The series of operations of
an ideal four-stroke SI engine are as
follows
Suction Stroke
Compression Stroke
Expansion or Power Stroke
Exhaust Stroke

Stroke Valve Position
Suction Stroke
Suction Valve open
Exhaust Valve closed
Compression Stroke Both Valves closed
Expansion or Power
Stroke
Both Valves closed
Exhaust Stroke
Exhaust Valve open
Suction Valve closed

Suction Compression Ign.-Combn. Expansion Exhaust
4-stroke Cycle
SI Engine

Valve timing diagram in
relation to the pressure
volume diagram
• Actual Valve Timing
(a) Mechanical factor
(b) Dynamic factor
- Intake valve timing
- Exhaust valve timing

Actual Valve Timing
(a) Mechanical Factor
cam mechanisms, valve opening & abrupt
closing! mechanical – slow lifting & closing
to avoid noise, wear & bounce
(b) Dynamic Factor
charge inertia, tendency
- Intake valve timing -ram effect!
- Exhaust valve timing -increase in work
required to expel!

injector injector injector injector injector

Two-stroke Cycle Engine
principle of Operation
• 2 & 4 stroke – Difference! - method of filling cylinder
with fresh charge & removing burned gases from
cylinder
• 2 stroke engine suction is accomplished by
air/charge compressed in crankcase/by blower.
• No piston strokes required for suction/exhaust
operations. 2 piston strokes required to complete
cycle, compression and expansion or power stroke.

Types of 2 Stroke Engines
•Based on scavenging method
i)Crankcase & ii) Separately scavenged engine
•Based on scavenging process (air flow)
i)Cross flow scavenging,
ii)Loop scavenging (MAN, Schnuerle, Curtis type)
iii)Uni-flow scavenging (opposed piston, poppet
valve, sleeve valve)
•Based on overall port-timing
i) Symmetrical & ii) Unsymmetrical scavenging

Crankcase-scavenged

Ideal & Actual Indicator
Diagrams for Two-stroke SI Engine

Separately Scavenged Engine
Loop-scavenged 2-stroke Engine

2-stroke
Crankcase-
scavenged,
Supercharged
&
4-stroke Engines
Valve Timing Diagrams

Scavenging process
• The replacement of the products of
combustion in the cylinder from the previous
power stroke with fresh-air charge to be
burned in the next cycle .
• can be divided into four distinct periods
- Pre-blow down,
- Blow down,
- Scavenging &
- Additional-Charging

Parameters
Delivery ratio
Delivery ratio usually varies between 1.2 to 1.5, except
for closed crankcase-scavenged, where it is less
than unity.
Scavenging efficiency
If means all gases existing in the cylinder at
the beginning of scavenging have been swept out
completely.
mass
reference
cycle
per
mixture)
(or
air
delivered
of
mass

del
R
charge
cylinder
trapped
of
mass
retained
mixture)
(or
air
delivered
of
mass

sc

1

sc


Theoretical Scavenging Processes
Perfect
scavenging,
Perfect
mixing and
Complete
short
circuiting

• The actual scavenging process is neither one
of perfect scavenging nor perfect mixing. It
probably consists partially of perfect
scavenging, mixing and short-circuiting.
• The first phase of the scavenging process is a
perfect scavenging process which then
gradually changes into a complete mixing
process
Theoretical Scavenging Processes

Classification
Based On Scavenging Process
(a) Cross
Scavenging
Loop
Scavenging…,
(b) M.A.N. Type
(c) Schnuerle
Type
(d) Curtis Type

Uni-flow
Scavenging
(a) Poppet Valve
(b) Opposed
Piston
(c) Sleeve Valve
Classification
Based On Scavenging Process…

Uni-flow
Scavenging
Poppet Valve
Classification
Based On Scavenging Process…

Classification Based On
Scavenging Process…
Reverse Flow
Scavenging

Scavenging Efficiency
• comparison of the
merits of two cycle
engine air scavenging
methods

Comparison Of Different
Scavengingsystems
• scavenging
efficiency, versus
delivery ratio of
different
scavenging
system

Port Design
Depend on following important basic parameters
• Scavenging method
• Shape, inclination & width of ports
• Amount of air/charge delivered
• Scavenging pressure
• Mean inlet velocity –fn. of pr. Ratio, temp. of
scavenging & scavenging factor
• Duration (crank angle) of port opening &
average port height uncovered by piston

Comparison of Two-stroke
SI And CI Engines
• 2-stroke SI engine suffers from
-fuel loss & idling difficulty.
-fuel loss & hence low indicated thermal efficiency
-More Dilution, Short Ckt., Back Firing at low speeds,
less air available for combustion after scavenging
-Fuel injection improves idling, avoids loss of fuel
and also eliminates backfiring as there is no fuel
present in the inlet system.

Advantages & Disadvantages
of Two-stroke Engines
Advantages (over 4-stroke Engines)
• Power developed nearly twice compared
• Work required to overcome the friction of the exhaust and
suction strokes is saved.
• More uniform turning moment - lighter flywheel
• Engines lighter & occupy less space
• Construction is simple - reduces maintenance problems
• Because of scavenging, burnt gases do not remain in the
clearance space as in case of four-stroke engines.

Disadvantages (over 4-stroke Engines)
• High speed - less efficient - reduced volumetric η
• Short ckt.ing increases fuel consumption & reduces the thermal
η
• Effective compression is less
• Liable to cause a heavier consumption of lubricating oil.
• With heavy loads, engines get heated due to excessive heat
produced. Also at light loads, running of engine is not very
smooth because of the increased dilution of charge.
Advantages & Disadvantages
of Two-stroke Engines

The SI engine offers the following
advantages
• Low initial cost.
• Low weight for a given power output.
• Smaller size for a given power output.
• Easy starting.
• Less noise.
• Less objectionable exhaust gas odor and less
smoke.
SI and CI Engine application

• The SI engine finds wide application in automobiles
because passenger comfort and in small airplanes
because of low weight.
• Two stroke petrol engines finds extensive use in
motor cycles, scooters, mopeds, pleasure motor
boats, etc., because of simplicity and low cost.
• The SI engine is also used for light mobile duty like
lawn movers, mobile generating sets, water pumps,
air compressors, etc...
SI engine application

The CI engine offers the following advantages.
• Low specific fuel consumption at both full load and
part load conditions.
• Utilizes less expensive fuels.
• Reduced fire hazard,
• Long operating life.
• Better suited for supercharging.
• Better suited for two-stroke cycle operating, as there
is no loss of fuel in scavenging.
SI and CI Engine Application

• Because of fuel economy the CI engine finds wide
usage in buses, trucks, locomotives, stationary
generating plants, heavy duty equipment such as
bulldozers, tractors and earthmoving machinery.
• Because of the reduced fire hazard the CI engine is
also used for confined installations & marine use.
• The great advantage of CI engine is lower fuel
consumption which counteracts the disadvantage of
higher initial cost, if engine used for long duties.
CI Engine application

Petrol Engine Presentation USA.ppt

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