Turbochargers, the Thermo Cycle, and Air Handling

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Turbochargers, the Thermo Cycle, and Air Handling

• Or
• How can we get more air in this thing!

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Why do we turbocharge?

• Remember power is a function of Pbme
• The only way to raise Pbme is to extend the
  addition of heat (injection of fuel), and
  increase ß, the fuel cutoff ratio
• But we cannot take the A/F ratio above 14.91
  (generally F, fuel equivalence ratio will be lt
  0.7 which implies A/F of 21.31)
• Sooo WE NEED MORE AIR!

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Why do we turbocharge?

• Does the turbo itself increase the engine power
  or efficiency?
• No. We still have to add more fuel for that
• The turbo does perform the required air pumping
  function without having to utilize Brake Power.
  This energy flow would otherwise go out with the
  exhaust
• Exceptions? Some attempts to add turbo power to
  the crankshaft. Difficult
• So how much power is available to the turbo?

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The Turbo Energy in the Thermo Cycle
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Assume a very long expansion stroke where the
gasses can expand all the way to atmospheric
pressure
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Blowdown Work in the cycle. Could be
available to the turbo
Exhaust Pumping Work available to the turbo
Pressure
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Exhaust Stroke Pressure
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Intake manifold Press
Atmospheric Pressure
Volume
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Types of Turbochargers

• Pulse Turbocharger
• Idealized as a small turbine immediately outside
  of the exhaust valve
• Pressure rises to P5 and some of the blowdown
  area is available
• Constant Pressure Turbocharger
• Forget the blowdown
• Use a large exhaust manifold and maintain it at
  exhaust stroke pressure

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Practical Turbos for Off-road Equipment
P5 (end of normal expansion)

• Our turbos try to make use of both blowdown and
  exhaust pumping energy
• Wish to avoid windmilling of the turbine when
  pressure drops

Exhaust Stroke Pressure
Turbo Inlet Pressure
Exhaust Opens
Crank Angle
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Objective for Pulse Overlap
Turbo Inlet Pressure
Exhaust Opens
Exhaust Opens
Exhaust Opens
Exhaust Opens
Crank Angle
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Manifold Sizing and Reflected Pulses

• Want to minimize the volume of the intake
  manifold to allow the highest possible pressure
  rise at the turbine as each exhaust valve opens
• As an exhaust pulse reaches the turbine, part of
  it is reflected back down the manifold
• If the pulse reaches another OPEN exhaust valve
  during the valve overlap it will force gasses
  back into the cylinder impeding the scavenging
  process

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Valve Spiral
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How Turbochargers are Implemented

• Lets define the theoretical air consumption of a
  Diesel
• De is engine volume (L)
• Ne is engine speed (rpm)
• ?a is air density (kg/m3)
• How does theoretical consumption compare to
  actual air flow?

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How Turbochargers are Implemented

• Now we define the Air-Delivery Ratio
• (I may refer to it as Volumetric Efficiency out
  of habit )
• Air Delivery Ratio or Volumetric efficiency will
  be symbolized with ev or ?v
• ev or ?v is the ratio of actual airflow to
  theoretical airflow

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How Turbochargers are Implemented

• A naturally aspirated engine will typically have
  ?v of about 0.85 when running loaded
• This means the cylinder has 85 of the air in it
  that it would have if allowed to fill completely
  at atmospheric pressure
• Using a turbocharger will increase the air
  delivery ratio and the air handling capacity

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Higher ?v or ev with Turbo

• The new air delivery ratio can be estimated from
  the following equation
• P2 and P1 refer to the absolute intake pressure
  (after turbo) and ambient air pressure before the
  turbo

• Similarly, T2 and T1 refer to absolute
  temperatures after and before the turbocharger

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P1 , T1
P4 , T4
P2 , T2
P3 , T3
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Pressure Ratio and Temperature Ratio

• Pressure ratio across the compressor, ?pc is
  defined as
• T2 is greater than T1 because the air has been
  compressed passing through the turbocharger
• Temperature ratio is
• Where ?c is compressor efficiency ( I use ? ...
  book uses e )

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Turbine and Compressor Maps
?pc
?pt
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Turbine Flowrates

• Both maps use mass flowrate through the device on
  one axis
• Mass flow through the compressor is the Airflow
  rate, mc
• Mass flow through the exhaust turbine is the
  Airflow rate PLUS the fuel flowrate
• mt mc(1 A/F)

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Selecting a Turbocharger.. Steps

• Determine the power you wish to attain
• Calculate the Pbme to make sure youre not going
  to blow it apart. lt1250 kPa OK?
• Calculate mf using known BSFC for the N.A.
  engine.
• Calculate ma ( mf x A/F)
• Note you must choose the fuel equivalence ratio
  and implicitly A/F for this step

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Selecting a Turbocharger.. Steps

• Select a compressor and a point on the compressor
  map (1st guess.)
• Now we have to find out where we reeeaally be on
  the compressor map
• Look at the following equation (8.7)
• Note that on the right we have actual airflow
  over theoretical, which is the air delivery ratio
  ev

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Selecting a Turbocharger.. Steps

• Assume a value of compressor efficiency, ?c
• Iterate a solution to equation 8.7 (prev slide)
  for ?pc
• Check on the map at ?pc and ma and see what value
  of ?c results. If its NOT what you assumed,
  select a value closer to this one for ?c and try
  iterate for ?pc again
• After a couple of iterations youll have a
  solution

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Now the Exhaust Turbine

• Youve found an OK operating point for the
  compressor
• The power to run the compressor MUST come from
  the turbine
• Also, they run at the same speed
• So we need a turbine solution that matches the
  compressor in terms of power output and speed

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Mechanical Efficiency

• Mechanical efficiency of the turbocharger relates
  how well it transmits shaft power from turbine to
  compressor
• What determines this?
• Primarily viscous friction in the oil of the
  journal bearing between the turbine and the
  compressor

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Characteristic Value Equations

• The previous equations can be reworked to two
  characteristic value equations
• One is related to power available
• One is related to power required

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Variable definitions

• In the previous equations
• u -(k-1)/k
• k is the specific heat ratio (1.4 for air) but
  modified to reflect real operating temps
• et is turbine efficiency as a decimal
• em is turbo mechanical efficiency
• Cpc is constant pressure specifc heat of ambient
  air
• Cpt is constant pressure spec heat of heated air

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• When you find a solution that equates these you
  have a point where the engine and turbo should
  operate
• Whew!
• Now read the paragraph on the top of page 174.
  Its hard to put that in a powerpoint slide
• I can, however show the figure

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Some typos in your text

• Equation 8.9A there should be a slash / between
  F and A
• Equation 8.9B the P in the denominator of the
  last term should be a ?
• In the paragraph below these, mechanical
  efficiency is approx 0.98, not turbine efficiency
• Some errors in the example on 175 also

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Time for an Example

• Example is done in a word processor format rather
  than Powerpoint
• If you are viewing this on the web there should
  be a separate link to the example

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Intercoolers

• Air-to-water or Air-to Air
• Cooling this air is a constant pressure process
  (turbo keeps packin it in as we chill it)
• Another slight gain in ?v or ev
• Note all temps are in absolute units

• ?v of turbod engine
• Gain from the intercooler

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Intercoolers

• Benefit of intercooler is two part
• 1st we gain a higher air-delivery ratio so we can
  fuel to a somewhat higher level and gain some
  additional power
• 2nd, the temperature of the air in the intake
  manifold is considerably lower. This means the
  air temp throughout the combustion cycle
  (cylinder temps) will also be lower, which is
  easier on our components and may generate less NOx