NAZARIN B. NORDIN
nazarin@icam.edu.my
What you will learn:
•
First law of thermodynamics
•
Isothermal process, adiabatic process,
combustion process for petrol/diesel engines
•
Volumetric Efficiency; spark ignition/
compression; ignition process and tests
First Law of Thermodynamics
Conservation of Energy for Thermal
Systems
Joule Equivalent of Heat
•
James Joule showed that mechanical energy
could be converted to heat and arrived at the
conclusion that heat was another form of
energy.
•
He showed that 1 calorie of heat was
equivalent to 4.184 J of work.
1 cal = 4.184 J
Energy
•
Mechanical Energy: KE, PE, E
•
Work is done by energy transfer.
•
Heat is another form of energy.
Need to expand the conservation of energy
principle to accommodate thermal systems.
1
st
Law of Thermodynamics
•
Consider an
example system of
a piston and
cylinder with an
enclosed dilute
gas characterized
by P,V,T & n.
1
st
Law of Thermodynamics
•
What happens to
the gas if the
piston is moved
inwards?
1
st
Law of Thermodynamics
•
If the container is
insulated the
temperature will
rise, the atoms
move faster and the
pressure rises.
•
Is there more
internal energy in
the gas?
1
st
Law of Thermodynamics
•
External agent
did work in
pushing the
piston inward.
•
W = Fd
•
=(PA)
D
x
•
W =P
D
V
D
x
1
st
Law of Thermodynamics
•
Work done on
the gas equals
the change in the
gases internal
energy,
W =
D
U
D
x
1
st
Law of TD
•
Let’s change the
situation:
•
Keep the piston fixed
at its original location.
•
Place the cylinder on a
hot plate.
•
What happens to gas?
Heat flows into the gas.
Atoms move faster,
internal energy
increases.
Q = heat in Joules
D
U = change in internal
energy in Joules.
Q =
D
U
1
st
Law of TD
•
What if we added
heat and pushed
the piston in at
the same time?
F
1
st
Law of TD
•
Work is done on the
gas, heat is added to
the gas and the
internal energy of the
gas increases!
Q = W +
D
U
F
1
st
Law of TD
Some conventions:
For the gases perspective:
•
heat added is positive, heat removed is
negative.
•
Work done on the gas is positive, work done
by the gas is negative.
•
Temperature increase means internal energy
change is positive.
1
st
Law of TD
•
Example: 25 L of gas is enclosed in a
cylinder/piston apparatus at 2 atm of pressure
and 300 K. If 100 kg of mass is placed on the
piston causing the gas to compress to 20 L at
constant pressure. This is done by allowing
heat to flow out of the gas. What is the work
done on the gas? What is the change in
internal energy of the gas? How much heat
flowed out of the gas?
•
P
o
= 202,600 Pa, V
o
= 0.025 m
3
, T
o
= 300 K, P
f
=
202,600 Pa, V
f
=0.020 m
3
, T
f
=
n = PV/RT.
W =

P
D
V
D
U = 3/2 nR
D
T
Q = W +
D
U
W =

P
D
V =

202,600 Pa (0.020
–
0.025)m
3
=1013 J energy added to the gas.
D
U =3/2 nR
D
T=1.5(2.03)(8.31)(

60)=

1518 J
Q = W +
D
U = 1013
–
1518 =

505 J heat out
Performance Factors
Volumetric Efficiency
1a. Indicated Power.
Indicated Power (IP) : Power obtained at the cylinder. Obtained
from the indicator diagram. Given by:
IP = P
i
LANn/60x in Watts
where P
i
is the indicated mean effective pressure, in
N/m
2
,
L is the stroke length, in m
A is the area of cross section of the piston, m
2
,
N is the engine speed in rev/min,
n is the number of cylinders and
x =1 for 2 stroke and 2 for 4 stroke engine.
1b. Brake Power
Brake Power (BP) : Power obtained at the shaft.
Obtained from the engine dynamometer.
Given by:
BP = 2
NT/60 in Watts
where T is the brake torque, in Nm, given by
T = W.L
where W is the load applied on the shaft by the
dynamometer, in N and
L is the length of the arm where the load is
applied, in m
N is the engine speed, in rev/min
1c. Friction Power
Friction Power (FP) : Power dissipated as
friction. Obtained by various methods like
Morse test for multi

cylinder engine, Willan’s
line method for a diesel engine, and
Retardation test and Motoring test for all
types of engines. Given in terms of IP and BP
by:
FP = IP
–
BP in Watts
2. Mean Effective Pressure.
Indicated Mean Effective Pressure (IMEP). This is also denoted by
P
i
and is given by
P
i
= (Net work of cycle)/Swept Volume in N/m
2
The net work of cycle is the area under the P

V diagram.
Brake Mean Effective Pressure (BMEP). This is also denoted by P
b
and is given by
P
b
= 60.BPx/(LANn) N/m
2
This is also the brake power per unit swept volume of the
engine.
Friction Mean Effective Pressure (FMEP). This is also denoted by
P
f
and is given by
P
f
= P
i

P
b
N/m
2
3. Efficiencies.
Indicated Thermal Efficiency (
i
) given by
i
= IP/(m
f
. Q
cv
)
m
f
is the mass of fuel taken into the engine in kg/s
Q
cv
is the calorific value of the fuel in J/kg
Brake Thermal Efficiency (
b
) given by
b
= BP/(m
f
. Q
cv
)
Indicated Relative Efficiency (
i,r
) given by
i,r
=
i
/ASE
ASE is the efficiency of the corresponding air standard cycle
Brake Relative Efficiency (
b,r
) given by
b,r
=
b
/ASE
Mechanical Efficiency (
m
) given by
m
= BP/IP = P
b
/P
i
=
b
/
i
=
b,r
/
I,r
Specific Fuel Consumption (sfc or SFC)
This is the fuel consumed per unit power.
Brake Specific Fuel Consumption (bsfc). This is given by
bsfc = m
f
/BP kg/J
if BP is in W and m
f
is in kg/s
bsfc is usually quoted in kg/kWh. This is possible if BP is in kW
and m
f
is in kg/h.
Indicated Specific Fuel Consumption (isfc). This is given by
isfc = m
f
/IP kg/J
if IP is in W and m
f
is in kg/s
isfc is also usually quoted in kg/kWh. This is possible if IP is in
kW and m
f
is in kg/h.
Mechanical Efficiency in terms of the sfc values is given by
m
= isfc/bsfc
Specific Energy Consumption (sec or
SEC).
This is the energy consumed per unit power.
Brake Specific Energy Consumption (bsec). This
is given by
bsec = bsfc.Q
cv
We can similarly define indicated specific energy
consumption (isec) and based on the two
quantities also we can define mechanical
efficiency.
Air Capacity of Four

stroke cycle
Engines
•
The power, P, developed by an engine is given
by
•
Power will depend on air capacity if the
quantity in the bracket is maximized.
•
Plot of power versus air flow rate is normally a
straight line.
Volumetric Efficiency
Indicates air capacity of a 4 stroke engine. Given by
Mi is the mass flow rate of fresh mixture.
N is the engine speed in rev/unit time.
V
s
is the piston displacement (swept volume).
ρ
i
is the inlet density.
Volumetric Efficiency
Can be measured:
At the inlet port
Intake of the engine
Any suitable location in the intake manifold
If measured at the intake of the engine, it is also
called the overall volumetric efficiency.
Volumetric Efficiency Based on Dry
Air
Since there is a linear relationship between
indicated output (power) and air capacity
(airflow rate), it is more appropriate to express
volumetric efficiency in terms of airflow rate
(which is the mass of dry air per unit time).
Since fuel, air and water vapor occupy the same
volume
V
a
= V
f
= V
w
= V
i
Thus we have:
Here ρ
a
is the density of dry air or the mass of dry air per unit
volume of fresh mixture.
Thus, since
Also V
d
= A
p
L
s = 2LN
L is the piston stroke and s is the piston
speed.
Measurement of Volumetric
Efficiency in Engines
The volumetric efficiency of an engine can be
evaluated at any given set of operating
conditions provided and ρ
a
can be
accurately measured.
Measurement of Air Flow
Airflow into the engine can be measured with the
help of a suitable airflow meter. The
fluctuations in the airflow can be reduced with
the help of surge tanks placed between the
engine and the airflow meter.
Measurement of Inlet Air Density
By Dalton’s Law of partial pressures:
p
i
= p
a
+ p
f
+ p
w
In this case p
i
is the total pressure of the fresh mixture,
p
a
is the partial pressure of air in the mixture,
p
f
is the partial pressure of fuel in the mixture,
p
w
is the partial pressure of water vapor in the air.
Since each constituent is assumed to behave as a perfect gas, we
can write
M indicates mass of the substance,
29 is the molecular weight of air,
m
f
is the molecular weight of the fuel, and
18 is the molecular weight of water vapor.
F
i
is the ratio of mass of fuel vapor to that of dry air and h is the
ratio of mass of water vapor to that of dry air at the point where
p
i
and T
i
are measured.
This
indicates
that
the
density
of
air
in
the
mixture
is
equal
to
the
density
of
air
at
p
i
and
T
i
multiplied
by
a
correction
factor,
that
is,
the
quantity
in
the
parentheses
.
The value of h depends on the humidity ratio of the air and is
obtained from psychrometric charts.
For conventional hydrocarbon fuels, the correction factor is
usually around 0.98, which is within experimental error. For
diesel engines and GDI engines, F
i
is zero.
In practice, with spark ignition engines using gasoline and with
diesel engines the volumetric efficiency, neglecting the terms in
the parentheses, is given by
If we do not neglect the terms in the parentheses we get the
following relation for volumetric efficiency:
If
the
humidity
is
high
or
a
low
molecular
weight
fuel
is
used
in
a
carbureted
engine,
the
correction
factor
cannot
be
ignored
.
For
example,
with
methanol
at
stoichiometric
conditions
and
h
=
0
.
02
,
the
correction
factor
is
0
.
85
.
Volumetric Efficiency, Power and
Mean Effective Pressure
Since
and
For
an
engine,
the
mean
effective
pressure,
mep,
is
given
by
Ways to increase power and mep
•
The mean effective pressure may be indicated or
brake, depending on whether η is indicated or brake
thermal efficiency. Thus, the mean effective pressure
is proportional to the product of the inlet density
and volumetric efficiency when the product of the
thermal efficiency, the fuel

air ratio, and the heat of
combustion of the fuel is constant.
•
From the preceding two expressions we can figure
out ways to increase the power and mep of an
engine.
OTTO
CYCLE

THE IDEAL CYCLE FOR
SPARK

IGNITION
ENGINES
•
The Otto cycle is the ideal cycle for spark

ignition reciprocating engines. It is
namedafter
Nikolaus
A. Otto, who built a successful four

stroke engine in 1876 in
Germany using the cycle proposed by Frenchman Beau de
Rochas
in 1862. In most
spark

ignition engines, the piston executes four complete strokes (two mechanical
cycles) within the cylinder, and the crankshaft completes 2 revolutions for each
thermodynamic cycle. These engines are called FOUR

STROKE internal combustion
engines
.
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