Combustion Engineering

FUEL -A composition of chemical elements which, in

rapid chemical union with oxygen, produce combustion.

COMBUSTION -The rapid chemical union with the

oxygen of an element whose exothermic heat of reaction is
sufficiently great and whose rate of reaction is such that
quantities of heat at high

COMBINED STATE
Hydrocarbon (consisting of the so-called volatile matter)

HEATING VALUE -Also known as Calorific Value of fuel

is defined as the amount of heat energy liberated or released
by the complete combustion of a unit mass or volume of a
fuel and when the products of combustion are cooled to the
original temperature.
-For solid and liquid fuels, the heating value is expressed in
kJ/kg, kCal/kg, or BTU/lb.
For gaseous fuels, kJ/m3, kCalm3, or BTU/ft3.

HEATING VALUE FOR SOLID FUELS

By computation based on the ultimate analysis of the solid
fuels; and By computation using proximate analysis.
Combustion Engineering
AIR-FUEL RATIO
THEORETICAL MASS OF AIR
The amount of air required for complete combustion of C,
POWER CYCLES
H, and S. It does not depend on how much material is
actually but what can be burned.

• HEAT ADDED, 𝑄A
𝑸A=𝑻HΔ𝑺
• HEAT REJECTED, 𝑄R
• NET WORK/WORK DONE, 𝑊𝑁𝐸𝑇
• 𝑾𝑵𝑬𝑻=𝑸𝑨−𝑸𝑹

COMBUSTION OF HYDROCARBONS
The basic chemical equations of combustion are as follows:
Fuel + Air= Products of Combustion
𝐶n𝐻n+𝑎𝑂2 +3.76aN2→𝑏𝐶𝑂2+𝑐𝐻2𝑂+3.76aN2
Combustion Engineering
Part 1: Fuels and Combustion 11. There are 20 kg of flue gases formed per kg of
fuel oil burned in the combustion of a fuel oil
1.A certain gasoline gives a hydrometer reading C12H26. What is the excess air in percent?
of 66.5°𝐵𝑒 at 5.6℃. Calculate the higher heating
value of this gasoline in kJ/kg. 12. A diesel power plant uses fuel with heating
value of 43,000 kJ/kg. What is the density of the
2.Fuel oil in a day tank for use of an industrial fuel at 25°C?
boiler is tested with hydrometer. The hydrometer
reading indicates a 𝑆𝐺 = 0.924 when the 13. A petrol has the following analysis 85.5%
temperature of the oil in the tank is 35℃. carbon, 14.4% hydrogen, and 0.10% sulfur.
Calculate the higher heating value of the fuel in Calculate the volume of air at 1.0 bar and 15°C
kJ/kg. required for perfect combustion of 1 kg of the
fuel. Note: Rair = 0.287 kj/kgK
3. Liquid Octane fuel burned with ideal
proportion of air. Calculate the ideal air-fuel ratio 14. A cylindrical tank 4 m long and 3 m diameter
by weight and the higher heating value. is used for oil storage.Calculate how many days
can the tank supply the engine if the stored oil is
4. A bituminous has the following compositions: at 27°API and the engine's fuel consumption is 60
Carbon – 71.5 % ,Hydrogen – 5.0 % ,Oxygen – kg/hr?
7.0 % , Nitrogen – 1.3 % , Sulfur – 3.6 % , Ash –
8.2 % , Water – 4.4 %.Calculate the higher 15. A diesel engine consumed 945 liters of fuel
heating value of coal. per day at 30°C. If the fuel was purchased at
15.5°C and 30°AP| at P29.00/li, then calculate the
5. It is required to find the theoretical volume of cost of fuel to operate the engine per day.
air at 20℃ and 100 𝑘𝑃𝑎 absolute pressure to burn
one kilogram of coal. The ultimate analysis of 16. Find the heating value of a certain fuel with a
coal as fired is as follows: SG = 0.997.

- Carbon – 65.65 %, Hydrogen – 5.87 %, Oxygen 17. Calculate is the specific gravity of a fuel at
– 18.61 % , Sulfur – 1.51 % , Ash – 5.36 %Water 28° Baume
– 3.0 %
Part 2: Power Cycles
6. Calculate the minimum volume of day tank of
28°APl fuel having a fuel consumption of 1 kg/s. 1. A Carnot cycle receives 1000−𝐵𝑇𝑈 of
7. A typical industrial fuel oil, C16H32 with 20% heat while operating between temperature
excess air by weight. Assuming complete limits of 1000°𝑅 and 500°𝑅 . Determine
oxidation of the fuel, calculate the actual air-fuel the cycle efficiency, heat rejected, work of
ratio by weight. the cycle and entropy change during heat
addition.
8. Liquid Octane (C8 H18 ) fuel is burned with 2. A Carnot engine operating between 775−𝐾
ideal proportion of air. Calculate the ideal air-fuel and 305−𝐾 produces 54−𝑘𝐽 of work.
ratio by weight. Determine the efficiency, heat added and
change in entropy during heat addition.
9.Fuel oil in a day tank for use of an industrial 3. Calculate the available energy and the
boiler is tested with hydrometer. The hydrometer reduction in available energy in 𝐵𝑇𝑈/𝑙𝑏
reading indicates a S.G. = 0.924 when the for a Carnot cycle with source temperature
temperature of the oil in the tank is 35°C. of 3460°𝑅, a sink temperature of 520°𝑅
Calculate the higher heating value of the fuel. and an energy supply as heat of 100 𝐵𝑇𝑈
to one pound of a working substance.
10. If the theoretical air-fuel ratio is 15, what is 4. Air enters the compressor of an Ericsson
the approximate higher heating value in kJ/kgfuel cycle at 300°𝐾, 1 bar, with a mass flow
? rate of 5𝑘𝑔𝑠𝑒𝑐. The pressure and
temperature at the inlet to the turbine are
Combustion Engineering
10 𝑏𝑎𝑟 and 1400°𝐾, respectively. 14. Heat Rejected, 𝑄𝑅
Determine the net power developed in kW 15. Net Work of the cycle, 𝑊𝑁𝐸𝑇 and
and the thermal efficiency. 16. Thermal Efficiency of the cycle, e.
5. A Stirling cycle operating with air as
17. An engine operates on the air-standard
working substance has a pressure of 80
Otto cycle. The cycle work is 1000𝑘𝐽𝑘𝑔.
𝑝𝑠𝑖𝑎, a temperature of 250℉, and a
What is the compression ratio of the
specific volume of 3.28 𝑓𝑡3𝑙𝑏 at the
engine if the maximum cycle temperature
beginning of the isothermal heat rejection
is 3173°𝐾 and the temperature at the end
process. The ratio of volumes at the
of isentropic compression is 773°𝐾.
beginning of isothermal heat rejection is 2.
18. An ideal Otto cycle operates with a
The highest temperature involved in the
temperature of 370℃ at the end of the
cycle is 1200℉. Calculate: heat supplied,
compression process, a maximum
heat rejected, and thermal efficiency.
temperature of 1510℃ and a minimum
6. Consider an ideal Stirling cycle engine in
temperature of 20℃. What is the
which the state at the beginning of the
temperature of the beginning of the heat
isothermal compression process is 100
rejection process in ℃?
𝑘𝑃𝑎, 25℃, the compression ratio is 6, and
19. The efficiency of an Otto cycle is 60% and
the maximum temperature in the cycle is
𝑘=1.5. What is the compression ratio?
1100℃. Calculate the thermal efficiency
20. At the beginning of the compression
of the cycle.
7. Air enters the compressor of an Ericsson stroke, an ideal Diesel cycle using air
cycle at 300°𝐾, 1 bar, with a mass flow pressure of 15𝑝𝑠𝑖𝑎, a temperature of 75℉,
rate of 5𝑘𝑔𝑠𝑒𝑐. The pressure and and a specific volume of 13.2 cubic feet
temperature at the inlet to the turbine are per pound. For a compression ratio of 15
10 𝑏𝑎𝑟 and 1400°𝐾, respectively. and a heat addition of 352𝐵𝑇𝑈𝑙𝑏.
Determine the net power developed in kW
Calculate the following:
and the thermal efficiency.
8. An Ericsson cycle uses helium as the 21. Temperature and Pressure at the end of
working fluid. The isothermal compression, at the end of heat addition
compression process begins at 540°𝑅 and and at the end of the expansion, 𝑇2&𝑃2,
17.6𝑝𝑠𝑖𝑎 and 75𝐵𝑇𝑈𝑙𝑏𝑚 of heat rejected. 𝑇3&𝑃3, 𝑇4&𝑃4
Heat addition occurs at 1980°𝑅. 22. Heat Rejected, 𝑄𝑅
Determine the net work produced per unit 23. Net Work of the cycle, 𝑊𝑁𝐸𝑇 and
mass and the thermal efficiency.
9. At the beginning of the compression 24. Thermal Efficiency, 𝑒
stroke, an ideal Otto cycle has an air 25. An ideal dual combustion cycle operates
on 0.60 kg of air. Find the cycle efficiency
pressure of 15𝑝𝑠𝑖𝑎, a temperature of 75℉,
if the compression ratio is 12, pressure
and a specific volume of 13.2 cubic feet ratio is 1.6, cut-off ratio is 1.5, and k=1.3.
per pound. At the end of compression, the 26. An ideal dual combustion cycle operates
specific volume is 1.76𝑓𝑡3𝑙𝑏. The heat on 0.65𝑘𝑔 of air. At the beginning of
supplied to the cycle is 352𝐵𝑇𝑈𝑙𝑏. compression, the air is at 100𝑘𝑃𝑎, and
Calculate the following: 45℃. Determine the volume at the end of
compression if the net work is 300𝑘𝐽 and
10. Compression ratio, 𝑟𝑘
the mean effective pressure is 700𝑘𝑃𝑎.
11. Temperature and Pressure at the end of
compression, 𝑇2&𝑃2
12. Highest temperature and Pressure of the
cycle, 𝑇3&𝑃3
13. Temperature and Pressure at the end of
expansion of the air, 𝑇4&𝑃4