Introduction to Engine Knocking

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What is knocking

• In an engine , when the fuel remaining after
  normal combustion ignites explosively by high
  pressure and temperature produced from normal
  combustion ,rather than spark from spark plug ,
  it causes knocking or detonation.

This explosive release of energy is what causes
the characteristic "knocking. It creates the
characteristic metallic "pinging" Sound.
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Causes of knocking

• The main driver for knock is ignition timing. As
  the ignition is advanced - the spark is fired
  earlier in the engine operating cycle - the
  pressure and temperature during the combustion
  event become higher, thus making knock more
  likely.
• Happens when fuel burn ignites irregularly and
  prematurely
• Usage of fuel with low octane rating
• Poor design of engine and due to their own
  structural problem

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Diagramatic explanation
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NORMAL COMBUSTION

• Under ideal conditions the common internal
  combustion engine burns the fuel/air mixture in
  the cylinder in an orderly and controlled
  fashion. The combustion is started by the spark
  plug allowing required time for the combustion
  process to develop peak pressure at the ideal
  time for maximum recovery of work from the
  expanding gases.

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• The pressure due to expanding gases in combustion
  chamber rises smoothly to a peak, as nearly all
  the available fuel is consumed, then pressure
  falls as the piston descends. Maximum
  cylinder pressure is achieved a few crankshaft
  degrees after the piston passes TDC, so that the
  increasing pressure can give the piston a hard
  push when its speed and mechanical advantage on
  the crank shaft gives the best recovery of force
  from the expanding gases.

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ABNORMAL COMBUSTION (DETONATION)

• When unburned fuel/air mixture beyond the
  boundary of the flame front is subjected to a
  combination of heat and pressure for a certain
  duration, detonation may occur. Detonation is
  mainly the instantaneous, explosive ignition of
  at least one pocket of fuel/air mixture outside
  of the flame front. A local shockwave is created
  around each pocket and the cylinder pressure may
  rise sharply beyond its design limits. If
  detonation is allowed to persist under extreme
  conditions or over many engine cycles, engine
  parts can be damaged or destroyed.

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• If the shockwaves created due to knocking have
  enough intensity and/or over a long enough period
  of time , these shockwaves and excessive
  pressure and temperature can cause engine damage
  including (going from minimal to destructive).
• Erosion and pitting of the piston and/or cylinder
  liners ,resulting in small wear particles which
  may enter the engine lubrication/oil system ,plug
  the catalysts, and/or possibly block valves from
  closing completely. The wear may not be unlike
  water pump cavitations' damage.
• Actual holes punched or burned in the pistons.

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• Reducing air and coolant temperatures is always
  good for suppressing knock (colder air is also
  more dense, allowing the engine to take in a
  greater mass of air.)
• The strategy for reducing knock - and so allowing
  more ignition advance (and hence more torque) -
  is to use a fuel with a higher octane rating.
  Higher octane fuels contain more components such
  as benzene that are resistant to self-ignition,
  and less of components such as n- heptane that
  self-ignite relatively easily.

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• There are two ways to meet the demand of modern
  high-performance engines for fuels of high octane
  numbers.
• The first one is to produce artificial petrol
  that are rich in branched-chain alkanes.
• Another method is to add an anti-knock agent to
  petrol. The most commonly added compound is
  tetraethyl lead (CH3CH2)4Pb. When burned, this
  lead compound produces small particles of lead
  oxide which tend to combine with the free
  radicals produced in the chain reaction of
  combustion. This slows down the reaction and
  makes it smoother.

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• To prevent lead from accumulating in the engine,
  1,2-dibromoethane (CH2BrCH2Br) is added to
  petrol. The formation of lead bromide is volatile
  and is swept away in the car exhaust.
• The exposure of lead in air leads to serious
  effect. Lead is a neurotoxin, or nerve poison. It
  can cause damage in the central nervous system
  (brain and nervous system), especially in young
  children.

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• The addition of tetra-ethyl-lead (TEL) a soluble
  salt added to gasoline was common until it was
  discontinued for reasons of toxic pollution.
• Lead dust added to the intake charge will also
  reduce knock with various hydrocarbon fuels.
  Manganese compounds are also used to reduce knock
  with petrol fuel.
• . Steam will suppress knock even though no added
  cooling is supplied.
• Certain chemical changes must first occur for
  knock to happen, hence fuels with certain
  structures tend to knock easier than others.
  Branched chain paraffins tend to resist knock
  while straight chain paraffins knock easy.

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• Gasoline mixtures that are rich in straight-chain
  alkanes such as heptanes ignite very readily and
  explode rapidly. This causes 'knocking' and
  inefficient combustion.
• Combustion of branch-chain alkanes like
  2,2,4-trimethylpentane (iso -octane) is much
  smoother and more controlled.

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Advantages of unleaded petrol

• Many countries are now phasing out the use of
  leaded petrol.
• The production of unleaded petrol involves
  cracking, isomerisation and reforming reactions
  to generate high-octane petrol.
• Unleaded petrol also contains an important
  additive methyl tertiary butyl ether (MTBE,
  octane rating 118).

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• Hence, gasoline mixtures rich in branch-chain
  alkanes are more efficient in burning and less
  likely to cause knocking

Fuels with high octane numbers burn smoothly and
efficiently in high performance engines.
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Petrol Engines and Knocking
The isomerisation plant produces branching chain
alkanes which improve the octane number of fuels.
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• We measure the problem using the octane number.
  The higher the number, the better the fuel. A
  fuel with a low octane number is likely to cause
  knocking. One with an octane number over 100
  should be fine.
• There are two main ways of increasing the octane
  number of petrol (i.e. improving it).
• Use alkanes with branching chains rather than
  straight chains.
• Use aromatic alkanes (with rings)
• These alkanes are produced on the site using the
  isomeriser and the reformer,blended into the
  petrol at the end is constantly monitored for
  its octane number.

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Octane rating

• Octane rating of a spark ignition engine fuel is
  a measure of the resistance to detonation or
  knocking compared to a mixture of iso -octane
  (2,2,4-tri methyl pentane, an isomer of octane)
  and n- heptane. It is a numerical representation
  of the antiknock properties of motor fuel,
  compared with a standard reference fuel, such as
  isooctane, which has an octane number of 100.
• Octane rating does not relate to the energy
  content of the fuel .It is only a measure of the
  fuel's tendency to burn in a controlled manner,
  rather than exploding in an uncontrolled manner.

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Types of octane ratings

• RON (Research octne number) - The most common
  type of octane rating used worldwide is
  the Research OctaneNumber (RON). RON is
  determined by running the fuel in a test engine
  with a variable compression under controlled
  conditions, and comparing the results with those
  for mixtures of iso-octane and n-heptane.
• MON (Motor octane number) - There is another
  type of octane rating, called Motor Octane
  Number (MON), which is a better measure of how
  the fuel behaves when under load as it is done at
  900 rpm instead of the 600 rpm of the RON . MON
  testing uses a similar test engine to that used
  in RON testing, but with a preheated fuel
  mixture, a higher engine speed, and variable
  ignition timing to further stress the fuel's
  knock resistance.

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• AKI (Anti-knock index) - In most of the
  countries , RON is used. But in the United States
  , Canada and some other countries, the average of
  the RON and the MON is used, which is called
  the Anti-Knock Index (AKI). It may also sometimes
  be called the Road Octane Number (RdON), Pump
  Octane Number (PON), or (RM)/2.
• Depending on the composition of the fuel, the MON
  of a modern gasoline will be about 8 to 10 points
  lower than the RON. So, the octane rating shown
  in the United States is 4 to 5 points lower than
  the rating shown elsewhere in the world for the
  same fuel. 

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Compression ratio

• One of the strokes in engine working is the
  compression stroke, where the engine compresses a
  cylinder-full of air and gas into a much smaller
  volume before igniting it with a spark plug. So ,
  The octane rating of gasoline actually tells you
  how much the fuel can be compressed before it
  spontaneously ignites by high compression and
  pressure. The amount of compression of air-fuel
  mixture is called the compression ratio of the
  engine i.e. the ratio of initial volume of
  gas-air mixture to the final volume of the same.

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INFERENCE

• I conclude that compression ratio of your engine
  determines the octane rating of the gas or fuel
  you must use in the engine. A fuel with a higher
  octane rating can be run at a higher compression
  ratio without detonating. 
• So, higher the compression ratio , more will be
  the energy output of the engine. That's why
  higher should be the octane rating so that fuel
  can resist more compression before ignition.

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• Now a days the scenario of fuel production
  utilization has been undergoing a tremendous
  change

• The utility of fossil fuels has been increasing
  at a very high rate .
• The utility now a days is in such that the
  existing fuel resources doesn't serve for a much
  longer time. The time for which they will serve
  us are as shown

Resource Consumption (in MTOE) of total energy Reserve (in MTOE) Available for use(years)
Oil ,Liquid petroleum products 3507 35.7 1,51,984 43
Natural Gas 2122 21.2 1,56,706 74
Coal 2342 23.4 5,01,172 214
Total 7971 80.3 8,09,862
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• In addition to this even the flue gasses obtained
  by these fuels create lots of damage to the
  ecosystem. The systems that developed naturally
  over millions of years does take few days to get
  ruined . So we need to start a serious search for
  the alternatives to the existing fuels
• Few things are to be kept in mind
• (1) Cost of production  utilization
• (2)  Eco friendliness
• (3)  Efficiency of fuel produced
• So we need to focus on Green fuels And find

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• What fuel can we use that will not completely
  destroy our atmosphere, and or environment ?
  
• The answer would be unanimously
  ------

GREEN FUELS

• The fuels that are likely to serve our purpose
  are
• (1) Electric (2)
  Hybrid     (3) Natural gas (4)
  Hydrogen    (5) Fuel cells  (6)
  clean diesel (low sulfur)   (7) Biodiesel (B100)
   (8) Ethanol (E85.)

• Each of the mentioned fuels will be weighed out
  according to the amount of pollution they do and
  do not contribute to the atmosphere

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ADVANTAGES -

• The advantages of an electric ran vehicle can be
  summed up in four words
• PLUG IN AND GO ".
• Being that they are operated purely on battery,
  they give out no emissions or other toxins
  usually associated with modes of transportation.
  As far as being green and the lesser evil of most
  fuels, electric is the least damaging fuel to the
  environment.

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• DISADVANTAGES-
• Unlike a gas engine, it takes a lot longer to
  charge an electric car and it can be a hassle
  when making long road trips.
• For numerous reasons such as, not every gas
  station is equipped with compatible equipment to
  fuel electric cars, if your car stalls from a low
  battery it is harder to transport electric, than
  a gallon of gas. 
• In addition, the price of electric fueled
  vehicles is quite expensive and the price to
  convert your mobile can range from
  5,000-10,000. Not to mention your electric bill.

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• A hybrid is a VEHICLE  that uses two sources of
  fuel, some use gas combined with electric, solar
  or water.
• ADVANTAGES-
• Since it is also using other sources of fuel it
  gives got less emissions than an engine solely
  using gasoline.  
• In addition, some hybrid cars you can get up to
  50 per gallon.  This means on long drives or road
  trips you could drive well over 500 miles before
  you would need to stop and get gas.  

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• Hybrid cars are also very affordable where some
  are priced brand new from just 12,000 and up. 
• DISADVANTAGES -
• When a hybrid battery is drained or just stop
  working, it can cost around 5,000 to replace
• In addition, hybrids have a lighter body weight
  to compensate for its heavy engine where in case
  of accident it can be extremely dangerous for the
  driver

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• ADVANTAGES -
• It burns clean as well as releases 70 less
  carbon dioxide than fossil fuel, making it a lot
  cleaner than traditional fuel.Natural gas has a
  higher heating value as well as is less expensive
  than traditional fuels.
• DISADVANTAGES -
• It cannot be recycled, the world can only produce
  so much which in time natural resources can run
  out. In addition, the price to recover remaining
  natural gas due to flow, access etc, is expensive

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•  Hydrogen or water powered vehicles, even though
  they are not yet on the market, or even close to
  being completely developed as far as the other
  fuels mentioned in this report, hypothetically
  there would be many advantages. 
• ADVANTAGES -
• Emissions would be close to none, which is
  beneficial to our environment. Everyone would be
  able to afford  to fuel their vehicle
• DISADVANTAGES -
• It has not been developed for mass production. In
  addition, unemployment would sky rocket due to no
  need for gas stations, or any other gas
  related  occupations. As well as we can only
  assume that, the purchase price of purely watered
  power vehicles would be very pricey.

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• A fuel cell is like a battery in a sense, but
  instead it combines hydrogen and oxygen to
  produce electricity.  Although it is a chemical
  process, it does not involve heat.
• ADVANTAGES -
• It does not involve any moving parts, so
  traditional mechanical complications are
  minimal. Fuel cell emissions only emit water and
  it is very quite during operation.

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• DISADVANTAGES -
• It is expensive to make as it relies on the use
  of platinum . Platinum even though already in
  use throughout the market is a natural resource
  and is bound to run out eventually, especially if
  we increase our dependency on using this rare
  metal.
• In addition, with changing technology mechanics
  will slowly find themselves out of a job as well
  as other vehicle maintenance services

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• Clean Diesel is diesel fuel put through a process
  where pressure is used to remove the majority of
  its sulfur content . It is also called as Ultra
  Low Sulfur Diesel (ULSD).
• ADVANTAGES -
• It reduces 90 of emissions content, compared to
  traditional diesel energy.
• This promotes cleaner air as well as complies
  with EPA(Environmental Protection Agency) quality
  standards  and regulations. 

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• This type of fuel is also compatible with all
  diesel-fueled engines.
• DISADVANTAGES -
• The potential purchase price is more expensive as
  well as current availability is limited .
• In addition, this type of fuel is susceptible to
  eating through engine seals as well as during
  cold weather its liability decreases.

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• Biodiesel Fuel is diesel fuel consisting of oils
  derived from animal fat and recycled cooking
  grease.ADVANTAGES -
• It can be made from home.  Most diesel engines
  can utilize biodiesel fuel as well as it emits
  less air pollutants and greenhouse gas.In
  addition, it is also biodegradable, non-toxic as
  well as is a lot safer to handle compared to
  traditional diesel. 

• DISADVANTAGES-
• The disadvantages of biodiesel fuel is that B100
  gets 10 less fuel economy then tradition diesel
  as well as it also not suitable for really cold
  weather conditions.

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• E85 or ethanol is a mixture consisting of 85
  ethanol and 15 gasoline.ADVANTAGES -
• E85 emits less air pollutants than traditional
  gasoline, also it is resistant to engine fuel
  mixture detonation or in laymen, engine knock.
• E85 is also not as expensive compared to 
  gasoline.

• DISADVANTAGES -
• Only certain cars can run this fuel, such as
  flex-fuel cars, trucks, SUVs.
• In addition, not every gas station carries E85,
  as well as your vehicle will have lower gas
  mileage and  as of now producing, the mixture is
  costly.

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• Relevant to the above explained contexts,
  scenarios and inferences it is evident that we
  can reduce engine knocking and its after effects
  using the green fuels.
• We conclude measures are to be taken at war foot
  lines to improve the quality of these fuels.