AME 436 Energy and Propulsion

1
AME 436Energy and Propulsion

• Lecture 5
• Unsteady-flow (reciprocating) engines 1
• Basic operating principles,
• design performance parameters

2
Outline

• Classification of unsteady-flow engines
• Basic operating principles
• Premixed-charge (gasoline) 4-stroke
• Premixed-charge (gasoline) 2-stroke
• Premixed-charge (gasoline) rotary or Wankel
• Nonpremixed-charge (Diesel) 4-stroke
• Nonpremixed-charge (Diesel) 2-stroke
• Design and performance parameters
• Compression ratio, displacement, bore, stroke,
  etc.
• Power, torque, work, Mean Effective Pressure
• Thermal efficiency
• Volumetric efficiency
• Emissions

3
Classification of unsteady-flow engines

• Most important distinction premixed-charge vs.
  nonpremixed-charge
• Premixed-charge frequently called Otto cycle,
  gasoline or spark ignition engine but most
  important distinction is that the fuel and air
  are mixed before or during the compression
  process and a premixed flame is ignited (usually
  by a spark, occasionally by a glow plug (e.g.
  model airplane engines), occasionally homogeneous
  ignition (Homogenous Charge Compression Ignition
  (HCCI), under development)
• Nonpremixed-charge frequently called Diesel
  or compression ignition but key point is that
  only air is compressed (not fuel-air mixture) and
  fuel is injected into combustion chamber after
  air is compressed
• Either premixed or nonpremixed-charge can be
  2-stroke or 4-stroke, and can be piston/cylinder
  type or rotary (Wankel) type
• Why is premixed-charge vs. nonpremixed-charge the
  most important distinction? Because it affects
• Choice of fuels and ignition system
• Choice of compression ratio (gasoline - lower,
  diesel - higher)
• Tradeoff between maximum power (gasoline) and
  efficiency (diesel)
• Relative amounts of pollutant formation (gasoline
  engines have lower NOx particulates diesels
  have lower CO UHC)

4
Classification of unsteady-flow engines
5
4-stroke premixed-charge engine

• Animation http//auto.howstuffworks.com/engine3.h
  tm

Intake (piston moving down, intake valve open,
exhaust valve closed)
Exhaust (piston moving up, intake valve closed,
exhaust valve open)
Compression (piston moving up, both valves closed)
Expansion (piston moving down, both valves closed)
6
4-stroke premixed-charge engine

• Source http//auto.howstuffworks.com/engine3.htm

7
2-stroke premixed-charge engine

• Most designs have fuel-air mixture flowing first
  INTO CRANKCASE (?)
• Fuel-air mixture must contain lubricating oil
• On down-stroke of piston
• Exhaust ports are exposed exhaust gas flows
  out, crankcase is pressurized
• Reed valve prevents fuel-air mixture from flowing
  back out intake manifold
• Intake ports are exposed, fresh fuel-air mixture
  flows into intake ports
• On up-stroke of piston
• Intake exhaust ports are covered
• Fuel-air mixture is compressed in cylinder
• Spark combustion occurs near top of piston
  travel
• Work output occurs during 1st half of down-stroke

8
2-stroke premixed-charge engine

• Source http//science.howstuffworks.com/two-stro
  ke2.htm

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2-stroke premixed-charge engine

• 2-strokes gives 2x as much power since only 1
  crankshaft revolution needed for 1 complete cycle
  (vs. 2 revolutions for 4-strokes)
• Since intake exhaust ports are open at same
  time, some fuel-air mixture flows directly out
  exhaust some exhaust gas gets mixed with fresh
  gas
• Since oil must be mixed with fuel, oil gets
  burned
• As a result of these factors, thermal efficiency
  is lower, emissions are higher, and performance
  is near-optimal for a narrower range of engine
  speeds compared to 4-strokes

10
Rotary or Wankel engine

• Uses non-cylindrical combustion chamber
• Provides one complete cycle per engine revolution
  without short circuit flow of 2-strokes (but
  still need some oil in fuel)
• Simpler, fewer moving parts, higher RPM possible
• Very fuel-flexible - can incorporate catalyst in
  combustion chamber since fresh gas is moved into
  chamber rather than being continually exposed to
  it (as in piston engine) - same design can use
  gasoline, Diesel, methanol..
• Very difficult to seal BOTH vertices and flat
  sides of rotor!
• Seal longevity a problem also
• Large surface area to volume ratio means more
  heat losses

11
Rotary or Wankel engine

• Source http//auto.howstuffworks.com/rotary-engi
  ne4.htm

12
Rotary or Wankel engine

• Source http//auto.howstuffworks.com/rotary-engi
  ne4.htm

13
4-stroke Diesel engine

• Conceptually similar to 4-stroke gasoline, but
  only air is compressed (not fuel-air mixture) and
  fuel is injected into combustion chamber after
  air is compressed
• Source http//auto.howstuffworks.com/diesel.htm

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2-stroke Diesel engine

• Used in large engines, e.g. locomotives
• More differences between 2-stroke gasoline vs.
  diesel engines than 4-stroke gasoline vs. diesel
• Air comes in directly through intake ports, not
  via crankcase
• Must be turbocharged or supercharged to provide
  pressure to force air into cylinder
• No oil mixed with air - crankcase has lubrication
  like 4-stroke
• Exhaust valves rather than ports - not necessary
  to have intake exhaust paths open at same time
• Because only air, not fuel/air mixture enters
  through intake ports, short circuit of intake
  gas out to exhaust is not a problem
• Because of the previous 3 points, 2-stroke
  diesels have far fewer environmental problems
  than 2-stroke gasoline engines

15
2-stroke Diesel engine

• Why cant gasoline engines use this concept?
  They can in principle but fuel must be injected
  fuelair fully mixed after the intake ports are
  covered but before spark is fired
• Also, difficult to control ratio of
  fuel/air/exhaust residual precisely since intake
  exhaust paths are open at same time - ratio of
  fuel to (air exhaust) critical to
  premixed-charge engine performance (combustion in
  non-premixed charge engines always occurs at
  stoichiometric surfaces anyway, so not an issue)
• Some companies have tried to make 2-stroke
  premixed-charge engines operating this way, e.g.
  http//www.orbeng.com.au/, but these engines have
  found only limited application

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Engine design performance parameters

• See Heywood Chapter 2 for more details
• Compression ratio (rc)
• Vd displacement volume volume of cylinder
  swept by piston (this is what auto manufacturers
  report, e.g. 5.2 liter engine means 5.2 liters is
  combined displacement volume of ALL cylinders
• Vc clearance volume volume of cylinder NOT
  swept by piston
• Bore (B) cylinder diameter
• Stroke (L) distance between maximum excursions
  of piston
• Displacment volume of 1 cylinder pB2L/4 if B
  L (typical), 5.2 liter, 8-cylinder engine, B
  9.4 cm
• Power Angular speed (N) x Torque (?) 2pN?

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Classification of unsteady-flow engines
18
Engine design performance parameters

• Engine performance is specified in both in terms
  of power and engine torque - which is more
  important?
• Wheel torque engine torque x gear ratio tells
  you whether you can climb the hill
• Gear ratio in transmission typically 31 or 41
  in 1st gear, 11 in highest gear gear ratio in
  differential typically 31
• Ratio of engine revolutions to wheel revolutions
  varies from 121 in lowest gear to 31 in highest
  gear
• Power tells you how fast you can climb the hill
• Torque can be increased by transmission (e.g. 21
  gear ratio ideally multiplies torque by 2)
• Power cant be increased by transmission in fact
  because of friction and other losses, power will
  decrease in transmission
• Power really tells how fast you can accelerate or
  how much weight you can pull up a hill, but power
  to torque ratio N tells you what gear ratios
  youll need to do the job

19
Engine design performance parameters

• Indicated work - work done for one cycle as
  determined by the cylinder P-V diagram work
  acting on piston face
• Note its called indicated power because
  historically (before oscilloscopes) the P and V
  were recorded by a pen moving in the x direction
  as V changed and moving in the y direction as P
  changed. The P-V plot was recorded on a card and
  the area inside the P-V was the indicated work
  (usually measured by cutting out the P-V and
  weighting that part of the card!)
• Net indicated work Wi,net ? PdV over whole
  cycle net area inside P-V diagram
• Indicated work consists of 2 parts
• Gross indicated work Wi,gross - work done during
  power cycle
• Pumping work Wi,p - work done during
  intake/exhaust pumping cycle
• Wi.net Wi,gross - Wi,pump
• Indicated power Wi,xN/n, where x could be net,
  gross, pumping and n 2 for 4-stroke engine, n
  1 for 2 stroke engine (since 4-stroke needs 2
  complete revolutions of engine for one complete
  thermodynamic cycle as seen on P-V diagram
  whereas 2-stroke needs only 1 revolution)

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Engine design performance parameters

• Animation gross net indicated work, pumping
  work

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Engine design performance parameters

• Brake work (Wb) or brake power (Pb) work power
  that appears at the shaft at the back of the
  engine
• Historically called brake because a mechanical
  brake like that on your car wheels was used in
  laboratory to simulate the road load that would
  be placed on an engine in a vehicle)
• Whats the difference between brake and indicated
  work or power? FRICTION
• Gross Indicated work brake work friction work
  (Wf)
• Wi,g Wb Wf
• Note that this definition of friction work
  includes not only the rubbing friction but also
  the pumping work I prefer
• Wi,g Wb Wf Wp
• which separates rubbing friction (which cannot
  be seen on a P-V diagram) from pumping friction
  (which IS seen on the P-V)
• The latter definition makes friction the
  difference between your actual (brake) work/power
  output and the work seen on the P-V
• Note the friction work also includes work/power
  needed to drive the cooling fan, water pump, oil
  pump, generator, air conditioner,
• Moral - know which definition youre using

22
Engine design performance parameters

• Mechanical efficiency (brake power) /
  (indicated power) - measure of importance of
  friction loss
• Thermal efficiency (?th) (what you get / what
  you pay for) (power ouput) / (fuel heating
  value input)
• Specific fuel consumption (sfc)
  (mdotfuel)/(Power)
• units usually pounds of fuel per horsepower-hour
  (yuk!)
• Note also

23
Engine design performance parameters

• Volumetric efficiency (?v) (mass of air
  actually drawn into cylinder) / (mass of air that
  ideally could be drawn into cylinder)
• where ?air is at ambient conditions
  Pambient/RTambient
• Volumetric efficiency indicates how well the
  engine breathes - what lowers ?v below 100?
• Pressure drops in intake system (e.g. throttling)
  intake valves
• Temperature rise due to heating of air as it
  flows through intake system
• Volume occupied by fuel
• Non-ideal valve timing
• Choking (air flow reaching speed of sound) in
  part of intake system having smallest area
  (passing intake valves)
• See figure on p. 217 of Heywood for good summary
  of all these effects

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Engine design performance parameters

• Mean effective pressure (MEP)
• Power could be brake, indicated, friction or
  pumping power, leading to BMEP, IMEP, FMEP, PMEP
• Note Power Torque x 2pN, thus Brake torque
  BMEPVd/2pn
• I like to think of MEP as the first moment of
  pressure with respect to cylinder volume, or
  average pressure, with volume as the weighting
  function for the averaging process
• Useful for 2 reasons
• Since its proportional to power or work, we can
  add and subtract pressures just like we would
  power or work
• (More important) It normalizes out the effects of
  engine size (Vd), speed (N) and 2-stroke vs.
  4-stroke (n), so it provides a way of comparing
  different engines and operating conditions
• Typical 4-stroke engine, IMEP 120 lb/in2 8
  atm - how to get more? Turbocharge - increase
  Pintake above 1 atm, more fuel air stuffed into
  cylinder, more heat release, more power

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Engine design performance parameters

• Pumping power (pumping work)(N)/n
  (?P)(?V)(N)/n
• (Pexhaust - Pintake)VdN/n
• but PMEP (pumping power)n/(VdN), thus PMEP
  (Pexhaust - Pintake)
• (wasnt that easy?) (this assumes pumping loop
  is a rectangle)
• Estimate of IMEP
• Typical engine ?th,i,g 30, ?v 85, f 0.068
  (at stoichiometric), QR 4.5 x 107 J/kg, R 287
  J/kg-K, Tintake 300K
• ? IMEPg / Pintake 9.1
• In reality, we have to be more careful about
  accounting for the exhaust residual and the fact
  that its properties are very different from the
  fresh gas, but this doesnt change the results
  much

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Engine design performance parameters

• Emissions performance usually reported in grams
  of pollutant emitted per brake horsepower-hour
  (yuk!) or grams per kilowatt hour (slightly less
  yuk), e.g.
• Brake Specific NOx (BSNOx) mdotNOx / (Brake
  power)
• One can also think of this as (mass/time) /
  (energy/time) mass / energy grams of
  pollutant per Joule of work done
• but Environmental Protection Agency standards
  (for passenger vehicles) are in terms of grams
  per mile, not brake power hour, thus smaller cars
  can have larger BSNOx (or BSCO, BSHC, etc.)
  because (presumably) less horsepower (thus less
  fuel) is needed to move the car a certain number
  of miles in a certain time
• Larger vehicles (and stationary engines for power
  generation) are regulated based on brake specific
  emissions directly