Combustion Fundamentals

Combustion Review Notes for M.Sc. Renewable Energy EG801ME The following review notes are comprised of the “model questions” provided by the course professor. Questions referring directly to heat engines are not answered in this section. Please refer to the notes of “Heat Engine” for these answers. For the purpose of studying combustion for final examination, the answers to these questions are sufficient in depth and breadth. 1. Define combustion and give example of combustion reaction. Combustion is a process in which a fuel is oxidized. For the purpose of our class, the fuel is either H2, C, or a hydrocarbon CXHY and the oxidant is O2 present in atmospheric air. Example:

CH4 + 2O2 → CO2 + 2H2O

As shown above, when a hydrocarbon is oxidized, the major products are CO2 and H2O. To quote another source referring to the practical applications of combustion, it is a “chemical reaction where a fuel is oxidized and a large amount of heat is released” 2. What is meant by electro negativity? How is it related to fuel and oxides? Electro negativity refers to the ability to gain electrons in a chemical process. Oxidizers have a high electro negativity and they gain electrons from the fuel in the combustion process. In any combustion process the reactant of high electro negativity is considered to be the oxidizer and the reactant of low electro negativity is considered to be the fuel. 3. Describe stoichiometric or theoretical air. This is the quantity of air that will supply the quantity of O2 that will completely burn the fuel, with no excess fuel or excess O2 in the products of the reaction. Example:

CH4 + 2(O2 + 3.76N2) → CO2 + 2H2O + 7.52N2

4. Differentiate between the air fuel ratio on a mass basis and air fuel ratio on a molar basis Air fuel ratio on a molar basis can be taken directly from the coefficients in the balanced chemical reaction.

Air fuel ratio on a mass basis must be calculated using the molar air fuel ratio and the molar masses.

5. Calculate the stoichiometric air fuel ratio on a mass basis and on a molar basis for the combustion of: Butane (C4H10) Octane (C8H18) Methane (CH4) Ethane (C2H6) Propane (C3H8) Pentane (C5H12) Methane (CH4) CH4 + 2(O2 + 3.76N2) → CO2 + 2H2O + 7.52N2

Air Fuel ratio on a mass basis may also be solved with the following equation:

Butane (C4H10) Octane (C8H18) Ethane (C2H6) Propane (C3H8) Pentane (C5H12)

AFmolar AFmolar AFmolar AFmolar AFmolar

= = = = =

30.94 59.50 16.66 23.80 38.08

AFmass = 15.47 AFmass = 15.14 AFmass = 16.10 AFmass = 15.68 AFmass = 15.34

6. Differentiate between auto ignition temperature and flash point of a fuel. Auto Ignition Temperature: The temperature at which a fuel (for practical purposes, in atmospheric air) will spontaneously combust without an ignition source present. Flash Point: The temperature at which a fuel (for practical purposes, in atmospheric air) will sustain continuous combustion once an ignition source is introduced. 7. Describe the scope of combustion in modern times. Combustion is the most widely used source of heat energy worldwide. Combustion is used to provide cooking heat for meals in almost all situations, with electrical resistance heating and solar cookers only recently taking a small percent of that work. Combustion is used as a means of disposal of waste. Whether it is super-combustion of toxic wastes in the United States of America or small trash fires in the streets of Kathmandu, Nepal, combustion is used world wide as a means of waste disposal. Combustion is also used for heat in industrial processes such as steel refineries. Combustion is also used to create large amounts of electricity when it is used to create steam that turns steam turbines

connected to electrical generators. With the invention of internal combustion engines, combustion is used to supply a significant amount of mechanical energy for transportation in the form of cars, trains, ships, airplanes, and other vehicles. Until recently, the heavy reliance on combustion has dominated the shape of innovation in the industrialized world so that it is used to provide much of our needed mechanical, electrical, and fluid energy requirements. From cooking fire wood in a pre-industrialized society, to massive freight ships used in the industrialized world, combustion is on eof the most important processes for people’s survival and standard of living. 8. What is the difference between deflagration and detonation flame? Deflagration moves slower than the speed of sound. Detonation moves faster than the speed of sound. 9. What is meant by minimum ignition energy? Minimum ignition energy is the smallest amount of energy required to raise the combustion reactants to a temperature at which they will ignite. In the example of a spark ignition engine, the spark is used to raise the temperature of a small amount of reactants to the ignition temperature and then combustion spreads to all the reactants over time. In this case, not all the reactants are heated to the ignition temperature at the same time. 10. What is meant by flammability limits? How does it vary with increase in the initial temperature of the mixture? The flammability limits are the limits of fuel/air ratio that will result in continuous combustion for a given temperature. The graph below shows the “constant temperature cups” for flame height at a varying fuel/air ration. Where the cups have lowest flame height are the “flammability limits”, with the two low points representing the lean and rich mixture limits. It is also evident that an increase in temperature increases the size of the cup and increases the flammability range. Both leaner and richer mixtures will be able to sustain combustion at higher temperatures, as shown in the graph on the next page. Note: The flammability limits also increase with increasing pressure.

11. What is meant by premixed flame? How is it different from diffusion flame? Premixed Flame:

Fuel and oxidizer are mixed together, before combustion begins. Example: Gasoline/Air mixture inside a car engine cylinder

Diffusion Flame:

Fuel and oxidizer are separate before combustion and mixed together at the location of the flame as combustion occurs. Example: The flame on a candle, firewood, lamp, etc.

Premixed Flame

Diffusion Flame

Gasses have a high chemical potential, which breaks down rapidly in the combustion zone.

Gasses do not have a high chemical potential. The fuel may have a small chemical potential.

Preheating zone and reaction zone do not exist simultaneously.

Preheated zone exists on each side of the main reaction zone.

C2, CH & OH radiation from the reaction zones is not of thermal origin.

O2 and OH bands reduce the flame temperature, whereas C2 and CH emission is weak and comes from OH.

There is a high electronic excitement in the reaction zone.

Diffusion flames with O2 at atmospheric pressure do not show electronic excitement in the reaction zone.

There exists non-thermal excitement in the reaction zone from the interaction of oxygen with organic fuel and breakup of unstable intermediates.

No non-thermal excitation occurs, because fuel is decomposed before it reaches the oxygen.

12. What is meant by heat engine and what are its functions? Give examples of heat engines. 13. Define thermal efficiency of a heat engine. 14. Explain the working of a 4-stroke cycle engine and describe the process involved. Explain using P-v & T-s diagrams. 15. Explain the working of 2-strok cycle spark ignition engine with P-v & T-s diagrams. 16. Classify heat engines & differentiate between 2-stroke and 4 stroke engines. 17. Differentiate between SI and CI engines. 18. Compare SI and CI engines with respect to: a) Basic Cycle b) Compression Ratio c) Efficiency d) Speed e) Weight f) Ignition g) Fuel Used 19. Discuss the thermal efficiency of 2-stroke engines and 4-stroke engines and consider the criteria of scavenging. 20. Describe the relative merits and demerits of 2-stroke over 4-stroke engines.

21. Explain the ratings of SI engine and CI engine fuels. SI engine fuels are given an “Octane” rating: During the SI engine process, an undesirable event is when the fuel detonates before the spark occurs and begins deflagration. Also, after the spark begins deflagration and the flame front progresses, some of the products may detonate due to the increased temperature and pressure in the combustion chamber. This detonation causes a shock wave and can damage engine parts. If the detonation occurs early, it will also reduce engine efficiency by applying force during the wrong time in the cycle. This process is often referred to as “knocking” because of a distinct sound the engine makes due to the detonation. Qualitatively, the octane rating of a fuel is a numerical representation of its resistance to detonation, or “knocking”. CI engine fuels are given a “Cetane” rating: During the CI engine process, detonation is also an undesirable process. In this case, the easily detectable symptom of detonation is ignition lag. Qualitatively, the cetane rating of a fuel is a numerical representation of its resistance to ignition lag. 22. Describe Octane and Cetane numbers of fuels. The octane rating for a fuel is the result of a comparison test in a controlled lab environment. The standardized octane ratings are determined by testing the resistance to detonation of volumetric mixtures of iso-octane and n-heptane. The compound isooctane is especially resistant to detonation, and pure iso-octane is arbitrarily given an octane rating of 100. The compound n-heptane is especially susceptible to detonation, and pure n-heptane is arbitrarily given an octane rating of 0. Resistance to detonation testing is performed in a standardized manner for numerous volumetric mixes of isooctane and n-heptane. The octane rating of a fuel is the standardized volumetric mix that displays the same resistance to detonation as the fuel. Example: A fuel with an octane rating of 92 has the same resistance to detonation as a 92% iso-octane and 8% n-heptane volumetric mixture. The cetane rating for a fuel is also the result of a comparison test in a controlled lab environment. The standardized cetane ratings are determined by testing the resistance to ignition lag of volumetric mixtures of cetane and alpha-methane-napthalene. The compound cetane is especially resistant to ignition lag, and pure cetane is arbitrarily given a cetane rating of 100. The compound alpha-methane-napthalene is especially susceptible to ignition lag, and pure alpha-methane-napthalene is arbitrarily given a cetane rating of 0. Resistance to ignition lag testing is performed in a standardized manner for numerous volumetric mixes of cetane and alpha-methane-napthalene. The cetane rating of a fuel is the standardized volumetric mix that displays the same resistance to ignition lag as the fuel. Example: A fuel with a cetane rating of 47 has the same resistance to ignition lag as a 47% cetane and 53% alpha-methane-napthalene volumetric mixture.