Week 1 - Intro To Combustion And Fuels

Question 1

Explain combustion (general definition and what happens in combustion?)

Answer 1

• Combustion is a chemical reaction between a fuel and oxygen(usually from air) .. the reactants.
• Molecules of fuel and oxygen have energy stored in chemical bonds between constituent atoms.
• in combustion, atoms rearrange to form new molecules (the combustion products) :
  ~ Products (usually) have lower energy than reactants
  ~ difference in energy is liberated as HEAT … hence combustion is exothermic

Question 2

What is a fuel? Examples

Answer 2

A fuel is something that will burn in oxygen.
Coal is widely used fuel ( mainly carbon) … CO2
Hydrogen may be a widely used clean fuel in the future .. 2H ₂ O
Hydrocarbons also widely used … eg natural gas or methane
( Hydrogen converted to water after combustion)

Question 3

Simple explanation of greenhouse effect due to combustion

Answer 3

carbon dioxide is produced in complete combustion of carbon. Greenhouse effect is the trapping of the suns warmth in the planets lower atmosphere. Some of the infrared radiation passes through the atmosphere but but most is absorbed and re-emitted in all directions by greenhouse gas molecules and clouds.

Question 4

What is detonation?

Answer 4

Detonation is an explosion where the combustion wave spreads as a supersonic shock wave couples to the chemical reaction. In a gas, detonation may propagate to speeds of 1000-2000 m/s. Rapidly raising the pressure

Question 5

What is Deflagrative

Answer 5

‘Ordinary’ combustion is deflagrative, the combustion wave spreads via diffusive heat and mass transfer. Spreads relatively slowly. Much less than the speed of sound

Question 6

Name the strokes in a four stoke engine in order and an example

Answer 6

1. Induction stroke
2. Compression stroke
3. Power stroke
4. Exhaust stroke
   E.g BMW S65 V8 internal combustion engine

Question 7

Describe the induction stroke

Answer 7

• piston moves down the cylinder and draws air/fuel into cylinder through inlet valve.
• cylinder fills with fresh air/fuel

Question 8

Describe compression stroke

Answer 8

• piston rises in cylinder, compressing mixture with both valves closed
• ∴ temp rises
• for spark ignition (SI) : mixture ignited by spark just before min volume.
  For compression ignition (CI) : ignition is spontaneous

Question 9

Describe the power stroke

Answer 9

• the burning gases get hot (heat released from combustion) and expand rapidly
• expanding gases expert force on piston, doing work and pushing it back down the cylinder
• work is transferred from piston to crankshaft
  SI: combustion takes place quickly
  CI: combustion is more progressive

Question 10

Describe Exhaust stroke

Answer 10

• exhaust valve opens
• piston rises and expels combustion products through the exhaust valve

Question 11

Draw the Otto cycle and diesel cycle on PV graphs with labels

Answer 11

Look at page 7 week 1 combustion in notes

Question 12

What do solid fuels comprise of? What are they graded by?

Answer 12

Comprises : ~ carbon ( elemental, HCs and volatiles )
~ hydrogen, nitrogen, sulphur, chlorine, oxygen in organic forms
~ ash (non-combustibles)
~ moisture
Graded by quality : - anthracite: high rank, energy value
- bituminous coal: med rank, higher volatile content
- brown coal or lignite: low rank, high ask and sulphur
- coke : part burnt/ heated for high carbon, no volatiles
- coalition + inter processed special fuels
Grades by size… large pieces to dusty powder

Question 13

What are liquid fuels in modern I.c. Engines a mixture of?

Answer 13

• Hydrocarbons … alkanes,cylanes,aromatics etc. [blend defines fuel characteristics]
• impurities … [implications for pollution]
• additives .. antiknock, anti oxidants, oxygenates [ fine tunes combustion performance]

Question 14

Define two types of rating numbers to classify fuels

Answer 14

Octane number: measures auto ignition I.e. spontaneous ignition of fuel ahead of propogatint flame (petrol engine )…. Compares fuel to a blend of
- iso-octane (octane number of 100)
- n-heptane (octane number of 0)
Cetane number : measures delay in ignition once a rise in pressure occurs (diesel engine fuels) it is a % cetane in mixture that gives a similar performance to fuel under test… compares fuel to a blend of
- cetane (short delay period)
- A-ethylnapthalene ( long delay period )

Question 15

Explain alkAnes

Answer 15

• Have a general formula C(n) H(2n+1)
• alkane form the major component of hydro-carbon fuels
  E.g. methane CH4
• normal alkanes have all the carbon atoms joined by a single bond to produce a saturated hydrocarbon e.g n-octane C8H18 , isomers etc

Question 16

Explain alkEnes

Answer 16

• have general formula CnH2n
• not usually found in crude oil but produced by refining process
• unsaturated hydrocarbons- less than maximum number of hydrogen atoms, with two or more adjacent carbon atoms joined by double bond.
  Eg propene C3H6

Question 17

Explain Cylanes

Answer 17

Have general formula C2 H2n same as alkenes, but have carbon atoms linked in a ring rather than a chain.
E.g cyclopropane C3H6 , Cyclobutane C4H8

Question 18

Explain alkYnes

Answer 18

• have general formula C(2) H(2n-2)
• not usually found in crude oil but produced by refining process
• unsaturated hydrocarbons- less than maximum number of hydrogen atoms, with two or more adjacent carbon atoms joined by triple bond
  Eg proypyne C3 H4

Question 19

Explain alcohols

Answer 19

• General formula C(n) H(2n+1) OH
• substitute an OH group for an atom of h in an alkane
• frequent components of biofuels, but not normally found in fossils
  E.g. methanol, ethanol

Question 20

Explain Aromatics

Answer 20

Have general formula Cn H(2n-6) or Cn H(2n-12)
- ring structure, with one or more six members rings. Equivalent of three double C=C bonds in rings, hence unsaturated hydrocarbons
- substantial component of many fuels
- some can be carcinogenic
E.g. benzene C6 H6

Question 21

Draw a summary table of hydrocarbons in fuels

Answer 21

Look at last page week 1 notes