- In petrol engines, air and fuel are mixed and compressed up to 1/8-1/12 of the original size, while in diesel engines only air is compressed up to 1/14-1/25 of the original size. Petrol engines use spark plugs to ignite the fuel-air mixture, while diesel engines rely on the high temperature of compressed air to ignite injected fuel. - Petrol engines are lighter, cheaper to maintain, and require less compression than diesel engines. However, diesel engines are more thermally efficient, can pull heavier loads, and benefit from cheaper fuel. The type of engine used depends on the specific power and efficiency needs of the vehicle.

1. Internal
Combustion
Fundamentals
Prof. (Dr.) M P Poonia
Director
NITTTR, Chandigarh

2. Four Stroke Engine How it
Works.mp4

3. Spark Ignition vs
Compression Ignition
Spark Plug Injector

4. ENGINE
TERMINOLOGY
VS
VC
D S
When,
D=S, Square Engine
D<S, Under Square
Engine
D>S, Over Square
Engine

5. Compression Ratio
The compression
ratio is defined as
(VS+ VC) / VC

6. Thermal Efficiency vs CR
ɳth = 1 −
1
rɤ−1 Otto Cycle
ɳth
CR

7. Efficient Diesel Engines

8. Kawasaki Vulcan 900
Gasoline
Vehicles
Royal-Enfield

9. Gasoline
Vehicles
Toyota Noah CR = 10.5 Aston Martin CR=11.5
Porsche 911 Carrera
4S, CR=12.5 Mazda's Skyactiv, CR=14

10. Range of CR

11. Displacement
1,298 cc (79.2 cu in)
1,496 cc (91.3 cu in)
1,998 cc (121.9 cu in)
2,488 cc (151.8 cu in)
Cylinder Bore
71.0 mm (2.80 in)
74.5 mm (2.93 in)
83.5 mm (3.29 in)
89.0 mm (3.50 in)
Piston stroke
82.0 mm (3.23 in)
85.8 mm (3.38 in)
91.2 mm (3.59 in)
100.0 mm (3.94 in)
Cylinder Block
AlloyAluminium
Cylinder head
alloy Aluminium
Valve Train
Mazda's Skyactiv, CR=14

14. P
V
pm
patm
Indicated Mean Effective
Pressure (bar)
VS
IMEP

15. Indicated Power (IP)
IP=
𝑷 𝒎
×𝑳×𝑨×𝑵
𝟏𝟐𝟎
IP=
𝑷 𝒎
×𝑽𝑺×𝑵
𝟏𝟐𝟎
Where IP= Indicated Power,
Pm= Mean Effective
Pressure N/m2
L = Stroke Length , m
A = Piston Top Area , m2
N = Engine RPM

16. Actual Diesel and
Otto Engines
Otto
Homogeneous
mixture of fuel and air
is supplied to engine
cylinder
Diesel
Air alone is supplied to the
engine cylinder. Fuel-air
mixture is heterogeneous.
Homogeneous
Charge
Heterogeneous Charge

17. Actual Diesel and
Otto Engines
Otto
Ignition is Initiated by
Means of an
Electric Spark Plug.
Diesel
No spark plug is used. The
High Temperature
of Air After Compression
Ignites Fuel.Spark Plug Diesel Fuel

18. Actual Diesel and
Otto Engines
Otto
Quantity Governed
Diesel
Quality Governed
Diesel Fuel

19. Actual Diesel and
Otto Engines
Otto
Mixture of petrol and
air during suction
stroke
Diesel
Only Air During Suction and
Injection of Diesel
Separately

20. Actual Diesel and
Otto Engines
Otto
Pressure After
Compression Around
10 bar
Diesel
Pressure After Compression
Around 40 bar

21. Heat Addition
Otto Cycle Diesel Cycle

22. Heat Addition
Otto Cycle Diesel Cycle

23. Otto Cycle
 Low Compression Ratio
 Easy starting due to low
compression ratio.
 As the compression ratio
is low, lighter and cheaper
 The running cost of a
petrol engine is high
because of the higher cost
of petrol.

24. Otto Cycle
 The maintenance cost is
less.
 The thermal efficiency is
up to about 26%.
 Overheating trouble is
more due tolow thermal
efficiency.
 These are high speed
engines.
 Employed in light duty
vehicles such as scooters,

25. Diesel Cycle
 Pressure at the end of
compression is about 35
bar.
 The fuel is injected in the
form of fine spray.
 The temperature of air
(about 6000C , 35 bar) is
sufficiently high to ignite
the fuel.

26. Diesel Cycle
 The combustion of fuel takes
place approximately at
constant pressure.
 Compression ratio
approximately from 15 to 25.
 The starting is little difficult
due to high compression
ratio.
 As the compression ratio is
high. The diesel engine are
heavier and costlier.

27. Diesel Cycle
 The running cost of diesel
engine is low because of
the lower cost of diesel.
 The maintenance cost is
more.
 The thermal efficiency is
up to about 30%

28. Diesel Cycle
 Overheating trouble is
less due to high thermal
efficiency
 These are relatively low
speed engines.
 Employed in heavy duty
vehicles like buses, trucks,
,earth moving machines
etc.

29. Summary
 In petrol engines the mixture of air and petrol is
drawn
 In diesel engines only air is drawn in by the falling
piston
 In petrol engine, the mixture is compressed up to
about 1/8th to 1/12th of its original size.
 In diesel engine, only air is compressed up to about
1/14th to 1/25th of its original size.
 In petrol engine, the air and fuel mixture is ignited
using a spark plug

30. Summary
 In diesel engine, fuel is injected at a high pressure
into the hot, compressed air in the cylinder, causing
it to burn and force the piston down. No spark is
required.
 Petrol Engine is lighter in construction hence it
requires lighter flywheel.
 Petrol engines need less maintenance and also
cheaper compared to diesels .

31. Summary
 Diesel engine is more efficient, and diesel as a fuel
is cheaper as compared to petrol.
 Petrol gives more power as compared to diesel
engine but fuel is costly
 Petrol destroys lubrication so a diesel engine would
last longer than a petrol engine.
 Petrol engines are lighter than diesel engines.
 A diesel engine would pull heavy loads easily than
a petrol engine.

32. Summary
 The diesel engine would be steady and carry
heavier loads to longer distances.
 Diesel engines have better fuel efficiency as
compared to petrol due to higher compression ratio.
 Diesel engines give better mileage than petrol
engines.
 Diesel is much cheaper than petrol.

33. Summary
 Diesel engines do not need an ignition system,
which reduces their complexity. But they are more
noisy and may require frequent maintenance as
compared to petrol engines. Also they are more
durable.
 Diesel engines may also need glow plugs in
extreme cold conditions which heat up the cylinder
so that a cold engine can start easily.

34. Thanks