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Otto Engine Design

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Design Constraints:

Based on the ideal Otto cycle, a single cylinder, spark ignition, four stroke engine is to be designed. The engine should run at 3600 rpm and produce 18 horsepower. The intake air conditions are as follows: The temperature of the air is a 77°F, the pressure is 1 atmosphere, and the relative humidity is 71%. The heat loss to the surroundings from combustion is assumed to be 22% of the total potential energy released. The bore and the stroke is assumed to be the same size. Also, a compression ratio of at least 8.5:1 is required. The engine is to run on natural gas.

The composition of the natural gas analyzed is:

Natural Gas Composition


Percent by Volume:











Natural Gas as a Fuel:

Natural gas has many uses, one of them being engine fuel. Natural gas is one of the only alternative fuels to oil that can be used to power large transportation vehicles.


There are some benefits to using natural gas as an engine fuel. Natural gas burns approximately 65 percent cleaner than coal and 25 percent cleaner than oil per kilowatt hour. Comparing natural gas to diesel fuel: Natural gas emits 99% less sulfur dioxide, 80% less carbon monoxide, and 25% less carbon dioxide. Another benefit to using natural gas as a fuel is cost. On an energy-equivalent basis, abundant supplies of natural gas are available at less than half the price of a barrel of oil and when used for transportation purposes, natural gas is approximately 20 to 30 percent more affordable than gasoline and diesel. There is also enough domestic natural gas to last 100 years.        



Natural gas is colorless, odorless, tasteless, and extremely flammable. Detecting a leak of natural gas can be extremely difficult making natural gas dangerous.  Natural gas contains carbon monoxide which is deadly if inhaled; it is amongst the top of the list for carbon monoxide related deaths. Natural gas may have fewer pollutants than other fossil fuels; it is still a fossil fuel and still emits pollutants. Also, being a fossil fuel means it is a non renewable source of energy.

Design Process:

  1. Balance molecular equation for 100% air
  2. Balance molecular equation for 2% excess air
  3. Balance molecular equation with 71% humidity
  4. Solve for specific heat ratio K
  5. Solve for temperature 2 using polytropic equation for ideal gas
  6. Find the heat loss from combustion
  7. Solve for temperature 3
  8. Solve for temperature 4
  9. Solve for compression work, )[pic 1]
  10. Solve for expansion work, )[pic 2]
  11. Calculate thermal efficiency of the cycle
  12. Calculate the total mass inside the system
  13. Find the volume of the cylinder
  14. Calculate the bore and the stroke of the cylinder
  15. Find the Mean Effective Pressure of the system
  16. Calculate Indicated Specific Fuel Consumption
  17. Determine the estimated hourly operating cost at full power

Summary of Results


[pic 3]

Compression Ratio:

[pic 4]

Polytropic Constant:

[pic 5]

Thermal Efficiency:

[pic 6]

Heat Loss:

24802.63[pic 7][pic 8]

Compression Work:

[pic 9]

Expansion Work:

[pic 10]

Mass flow rate:

[pic 11]

Volume of cylinder:

[pic 12]

Bore and Stroke:

[pic 13]

Mean Effective Pressure:

[pic 14]

Indicated Specific Fuel Consumption:

[pic 15]


Original Balance

[pic 16]

Balance with 2% Excess Air

[pic 17]

Balance with 77 F, 71% Humidity

Humidity Ratio: [pic 18]

So, the water in the balance:

[pic 19]

[pic 20]

 [pic 21]

Total Mole in Reactants from Balanced Equation

[pic 22]

Total Mole in Products from Balanced Equation

[pic 23]

[pic 24]

[pic 25]

[pic 26]

[pic 27]

[pic 28]

[pic 29]

[pic 30]

[pic 31]

[pic 32]

[pic 33]

                [pic 34]

[pic 35]

        [pic 36]

[pic 37]

[pic 38]

24802.63[pic 39][pic 40]

[pic 41]

[pic 42]

-[pic 43]

Using Table A-23E to interpolate, Find [pic 44]

[pic 45]

[pic 46]

[pic 47]

[pic 48]

[pic 49]

[pic 50]

[pic 51]

[pic 52]

[pic 53]

[pic 54]

[pic 55]

[pic 56]

[pic 57]

[pic 58]

[pic 59]


[pic 60]



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