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We owe much of the development of the turbocharger to the aircraft
industry. The piston engine aircraft developers were dealing with a severe performance
problem in the early 1920’s. As an aircraft went higher, the loss in air density
caused engine performance to drop appreciably. Aircraft were limited to about 20,000 feet
of altitude due to the engine performance loss. The answer to the performance problem was
to force air into the engine to offset the loss in air density at altitude. Supercharging
was born. |
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A supercharger is an air compressor driven by gears, belts or chains
from the engine crankshaft. The development of the supercharger was not just a simple
"throw it together". There is a monument on top of Pikes Peak dedicated to the
men that spent a winter at 14,000 feet testing centrifugally supercharged aircraft
engines. The work was done by GE under the direction of Dr. Sanford Moss. This was picked
up in the mid 20's by the American racing fraternity for the 122 CID (cubic inch
displacement) class by Fred and Augie Duesenberg and brought to full development by Harry
Miller in his 91 CID class cars. |
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The superchargers were successful in restoring engine performance at
altitude and, by the 1930’s, superchargers were beginning to appear on automobiles.
Bentley, Morris Garage (MG) , Bugatti, Mercedes, Auburn, Cord and dozens of others made
successful supercharged touring vehicles and race vehicles. Positive displacement
superchargers were the rule in Europe while centrifugal superchargers were typical in
American practice. |
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The aircraft industry continued to push the development of superchargers
up into the mid 1930’s. The 10 year period prior to World War II saw engine power
double and then triple. The Pratt and Whitney Wasp and Super Wasp engines are examples of
this amazing power increase. At about this time, someone recognized that the energy in the
hot exhaust would drive a turbine. The turbine could be used to drive a compressor mounted
on the same shaft. This was the beginning of the "Turbo Supercharger". Sometime
in the 1950’s, the "super" was dropped out of the center and the name
became simply "turbocharger". Turbo superchargers were used on the B-17’s
in service as early as 1935 with turbine wheels nearly a foot across, exhausting directly
into the airflow under the wing. |
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As boost levels began to climb, engine failures became more frequent.
The problem was detonation. As air is compressed, it heats up. Anyone with a home air
compressor can easily prove this to themselves by carefully touching the piping where the
air is discharged into the storage tank. |
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This temperature increase ratio is related to the ratio of boost
(absolute discharge pressure divided by absolute inlet pressure) to the .285 power. |
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The increased air temperature into the engine made detonation much more
likely to happen. |
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The solution was to add a heat exchanger in between the turbocharger
discharge and the intake manifold. Again, the technology was pioneered by the aircraft
industry with Lockheed and others routinely building intercooled aircraft by the late
1930’s. All high performance aircraft of World War Two were both turbo supercharged
and Intercooled. |
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Most Intercoolers were of the familiar "core" type mounted
just below the engine. Other innovative solutions were tried. The P-38 Lightning was an
excellent example of this; the leading edge of the wing was used as an Intercooler. If, as
happened on early models, the engine backfired it tended to remove the turbocharger from
the engine and bubbled the leading edge resulting in simultaneous losses of power and
lift! The problem was so severe, and coupled to the fact that the leading edge
Intercoolers were easily damaged in combat, that by mid 1941 Lockheed had reverted to the
familiar "core" type Intercooler. |
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If Turbocharging is so great, why wasn’t it pursued in automobiles
until recently? The development of turbocharging for aircraft was, in most cases, easier
than the implementation on automobile engines. Aircraft engines typically run a steady
state throttle process while automobile engines are operated with rapidly changing
throttle conditions. The problem was being able to cost effectively control mixture. |
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When used with a carburetor, the turbocharger can either be placed in
front of the carburetor, "Blow Through", or behind the carburetor, "Pull
Through". Both methods have advantages and disadvantages. |
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The "Blow Through" arrangement meant that the carburetor was
under pressure. The carburetor had to be specially built to avoid float collapse as well
as to keep from blowing the fuel out of the float bowl. Under changing boost (throttle)
conditions, the carburetor would go from essentially zero pressure to full pressure within
seconds. The advantage of this method was that the Intercooler was positioned in front of
the carburetor so that only "dry" air was supplied to the Intercooler. This was
the typical method used for aircraft. The disadvantages to this method were the high cost
and maintenance of the carburetor and the exceedingly high temperature of the air fed into
the carburetor. The high air temperature meant that boost levels were often restricted
simply because the parts in the carburetor would not tolerate the heat. |
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The "Pull Through" arrangement used a standard carburetor.
However, the addition of fuel prior to the turbocharger severely impacted turbocharger
life because the air was laced with droplets of fuel. The high temperatures after the
turbocharger typically vaporized all of the fuel. The placement of an Intercooler after
the turbocharger reduced Intercooler performance because of the fuel air mixture. In
addition, the long piping runs with an Intercooler significantly affected the ability of
the carburetor to control fuel mixture. |
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Several Turbocharged cars made it into production in the 60’s and
70’s. The late 60’s Corvair is an example as is the Mustang in 1979. None of
these versions can be called successes. |
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The energy crisis of the late 1970’s fostered a rebirth of the
turbocharger in the automobile industry. The turbocharger promised increased performance
on small engines without a corresponding fuel economy penalty. The marriage of
turbocharging with electronic fuel injection is the reason that turbocharging is now a
viable automotive technology. Quite simply, the quest for increased fuel economy and
emissions control forced automakers to develop electronic fuel injection and fuel
injection eliminated virtually all of the problem areas normally associated with
turbocharging. |
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With fuel injection, the turbo could be supplied with "dry"
air. In addition, the dry air allowed the effective use of intercoolers. All of the
elements were now in place to cost effectively build turbocharged vehicles. |
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A turbocharger is composed of two fan wheels on a common shaft. Just
like in a child’s pinwheel, exhaust air blows across one wheel, the turbine, and
causes the shaft to spin. Just like a common blower fan, the other fan wheel, the
compressor, connected to the shaft spins, pulls in air, and pushes air out. |
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Because the displacement of the engine is a physical dimension, the
discharge flow volume (cfm) of the turbocharger (except when Intercooled) is equal to the
displaced volume of the engine at speed as corrected by volumetric efficiency of the
engine. The inlet cfm through the air cleaner is something more than the engine
displacement. This has led some people to claim that turbochargers increase the cfm of the
engine. In actuality, the cfm of the engine hasn’t changed, but the DENSITY of the
air has. |
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This is an important principle. Turbocharging increases the DENSITY of
the air going to the engine. It doesn’t change engine displacement. Since
turbocharging changes density, the cfm flowrate from the inlet to the compressor to the
cylinder head of the engine is no longer constant. Attempting to talk about the cfm flow
of a turbocharged engine provokes all kinds of inconsistent concepts. The one thing that
is consistent about turbocharging is the MASS FLOWRATE of the air as measured in pounds
per minute. This is why turbocharger maps show lb/min versus pressure ratio rather than
cfm versus psi boost. Trying to work in either cfm or psi boost on a compressor map
generates a map that is only correct at one place and at one time. A mass flow –
pressure ratio map can be applied anywhere on earth at any time. |
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One of the real problems with turbocharging is that the pressurization
of the air generates a significant amount of heat. The heat causes the density of the air
leaving the turbo to be lower than it would be if you could just pressurize the air at
constant temperature. The key to producing consistent high power levels with a
turbocharger is to remove that heat from the engine air stream. We do that with an
INTERCOOLER. |
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An intercooler is simply a heat exchanger. It transfers heat from the
engine air stream to someplace else. That someplace else can be the air flowing around the
car or it could be to a water loop in the car. Eventually even the water loop has to
reject the heat to the air around the car. |
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Intercooling is a DENSITY RECOVERY technique. By removing heat from the
engine air stream, the engine air becomes more dense. The advantages are twofold. First,
the power output of the engine increases because the same boost level now has a larger
lb/minute mass flow rate. Second, the boost level can safely be raised since detonation is
less likely to occur with cooler engine air. |
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The Intercooler can be thought of as a tube within a tube. Let’s
say the inner tube has engine air flowing through it and the outer tube has ambient air
flowing through it. As the engine air "scrubs" along the dividing wall, it
transfers heat to the wall and then to the other air stream. The amount of heat
transferred is dependent on several things. |
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The first item that influences heat transfer is how different the
temperatures are between the two air streams. The larger the temperature difference, the
more easily heat is transferred. For an analog idea, the temperature difference is
equivalent to voltage in a simple DC resistor circuit. The higher the voltage, the larger
the current flow. The same rationale holds for the Intercooler, the higher the temperature
difference, the more heat transferred. As the temperature of the two air streams approach
one another (voltage goes to zero), heat transfer stops. |
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The next item that controls heat transfer is the "wetted" area
of the heat exchanger. In the analogy above, the wetted area is the perimeter of the pipe
multiplied by the length of the pipe. The more area, the more heat transferred. |
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The third thing that controls heat transfer is a difficult concept
called the heat transfer coefficient. This is related to the turbulence of the air or
water across the metal tube wall as well as to the physical properties of air or water.
The simplest way to look at this is that the heat transfer coefficient for a water to air
unit is double that of an air to air unit. |
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Heat transfer rate is equivalent to current flow in the electrical
analogy. The product of the area and heat transfer coefficient is equivalent to the
resistance in the electrical circuit. |
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Imagine that a molecule of air is about to be pulled into the air
cleaner of a turbocharged Ford 2.3 liter engine that is spinning at about 5000 rpm and
making 175 Horse Power (stock XR4 5 speed). It’s a nice day outside of the engine
compartment – about 80 degrees and the ocean is just a mile or so down the road.
These conditions are what is called "Standard Temperature and Pressure (STP)".
That’s 80 degrees F and 14.7 psi absolute. |
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As the molecule of air enters into the compressor inlet bell, the
conditions are still essentially STP. The air flow at this point is about 250 cfm. The
molecule of air is caught by the compressor blades that whip around at about 125,000 rpm
and pushed out of the discharge of the compressor housing. The air molecule is now under
15 psi of pressure (boost ratio of 2.02) and its temperature is now almost 250 degrees F.
The cfm flowrate at this point is about 190 cfm but the mass flow of the air in lb/minutes
is unchanged. |
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The air is blown out of the compressor and into the Intercooler. At the
entering point, the condition of the air is for all practical purposes identical to what
it was when the air left the Turbocharger. Inside the Intercooler, the cooler metal of the
unit removes heat from the air. As the air cools, its pressure is essentially unchanged,
but its density again increases. As the air exits the Intercooler, the cfm flow rate has
dropped to about 148 cfm , the displaced volume of the engine cylinders at speed. |
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The turbocharger/intercooler system has widely changing cfm values
through the system. That’s why talking about cfm in this case merely confuses what is
happening. Mass flow has remained constant, but density and therefore cfm has changed
because the temperature and pressure of the air is different in the various parts of the
system. |
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Intercoolers can not remove the heat from the air stream unless they
have someplace to dump the heat. Air to Air intercoolers transfer the heat to the ambient
air directly while air to liquid Intercoolers transfer the heat first to liquid and then
to the ambient atmosphere in a second heat exchanger. |
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At first glance, an air to air unit appears to be simpler and more
capable than the air to liquid unit. In practice however, the liquid to air unit has its
own set of unique advantages. |
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The air to air unit must have air flowing through the unit. Getting
airflow to the intercooler may require fabrication of ducts, hood scoops, or elimination
of air conditioning. |
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The liquid to air unit will require a second loop with another heat
exchanger to finally transfer the heat to the ambient air. The liquid to air system often
has problems with providing enough heat transfer capacity due to the necessity of the
secondary heat reject loop. |
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Given an equal sized heat exchanger core and equal heat reject
temperatures, the liquid to air unit will transfer almost double the heat of the air to
air unit. In addition, the bulk of water stored in the expansion tank provides a
"cushion" to the system. Since turbocharging is a surge demand, the heat
rejection equipment can be smaller if instantaneous demand can be meet with a stored
reserve. Water also has more heat carrying capacity than air. The specific heat of water
is 1.0 BTU/lb while it is only 0.24 BTU/lb for air. |
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The concept of an Intercooler performing better because it
"Flows" more is inherently flawed. From the above discussion on flow in the
system, you can see that the cfm flow rate changes throughout the system and even changes
between the inlet and the outlet of the Intercooler. In addition, the flow through the
Intercooler, even on a 500 hp 2.3 engine, will never approach the 360 cfm measurement
point that is frequently talked about. |
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After having instrumented several intercoolers to measure pressure drop
under various flow conditions, I can say that the pressure drop on ALL currently available
production car intercoolers is insignificant on a 2.3 liter Turbo Ford engine. |
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The best example to illustrate this is the SVO versus TurboCoupe
Intercooler debate. For several years, owners of SVO’s have replaced their
Intercoolers with TurboCoupe Intercoolers. Many have documented performance gains from
doing so. The most common explanation is that the SVO Intercooler flows 280 cfm while the
Turbocoupe Intercooler flows 360 cfm. The logic is that the additional flow capacity of
the TurboCoupe Intercooler made the performance difference. |
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Reality is quite different. The performance gain is real but
"flow" had nothing to do with it. When measured on a flow bench, the pressure
drop of the SVO intercooler when having 280 cfm of STP air pushed through it is only about
0.04 psi. The pressure drop of the TurboCoupe Intercooler under the same conditions is
about 0.03 psi. Pressure drops this low are insignificant because their percentage of
total system pressure is only about 0.1%. The difference between the two units is only
0.03% of the available driving pressure at 15 psi boost. |
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| Well, if pressure drop is not the difference between the units, then what is? |
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The difference is the heat exchanger area of the intercoolers. The
Turbocoupe Intercooler has about 25% more surface area than the SVO Intercooler. While the
Turbocoupe Intercooler won’t cool the air 25% more than the SVO Intercooler due to
the fact that as more heat is transferred, the driving temperatures get smaller, it will
have a significant impact on Intercooler discharge temperature. The cooler the discharge
temperature, the more dense the air and the more power produced. |
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On the grand scale, you can never get an intercooler big enough.
Practically speaking, the 80/20 rule holds. You can get 80% of the performance for 20% of
the effort. Under high boost (20 ~ 22 psi) compressor discharge temperature will be close
to 400 degrees F. Getting an Intercooler that will pull this temperature down to about 150
degrees is at the edge of the 80% result window. A unit to do this is the same approximate
size as the Volvo or Saab 9000 Intercooler. |
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There are two places on cars that are documented high static pressure/
high flow areas. The obvious area is just in front of the radiator. A less obvious area is
at the base of the windshield. A third position is in between the radiator and front
crossmember positioned so as to allow vertical flow. |
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A front mounted intercooler has many advantages particularly when
combined with a Cosworth style front grill opening. Obviously air flow is plentiful and
easily obtained. The major difficulty is that the mounting of the Intercooler often
requires relocation of the radiator and may sometimes require that air conditioning be
eliminated. |
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A cowl mounted Intercooler at first seems illogical. However, the next
time you see a NASCAR race car on display, look carefully at how air is directed to the
air cleaner. In all probability, the air cleaner assembly will have a duct that connects
to a cavity at the base of the windshield. For a Merkur, the battery can be relocated to
the trunk and the false bulkhead that separates the cowl area from the engine compartment
can be removed. The Intercooler can then be mounted in the area where the battery was
located. There is excellent air flow, equal to a front mount, through the Intercooler when
it is mounted this way. The disadvantage of this is that the ability to draw outside air
into the interior through the windshield cowl vents is lost. |
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Mounting of a transmission cooler horizontally just behind the radiator
is a common practice. An Intercooler can also be mounted here. The flow path for this
configuration is not the best. Air is normally pulled through the radiator and discharged
under the car. There is a strong possibility that the air flow path will be downward
pulling hot air from the radiator across the Intercooler reducing its effectiveness. |
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Performance of the Intercooler (discharge temperature) needs to be
verified. In my opinion installation of a thermocouple or thermister after the Intercooler
is MANDATORY. While thermisters can be purchased at the local Radio Shack, a mounting will
have to be fabricated. |
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| There are several inexpensive options. Ford used two different thermisters in the EFI systems. |
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Cars equipped with a Vane Air Meter will have a thermister built into
the meter assembly. The thermister can be easily removed and mounted with just a drilled
hole in the throttle body. Attached is a chart showing resistance versus temperature for
this thermister. |
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Many Ford cars were equipped with an Air Charge Temperature Sensor. The
ACT is a thermister built into a holder with an external thread. The ACT can be easily
mounted with a taped hole in the intake manifold. Attached is a chart showing resistance
versus temperature for the ACT. |
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IC performance has so many factors in it that It is difficult to do
ranking. We have to define what engine size, rpm, VE, ambient air temp, location of IC in
car, cooling air flow rate over the IC, boost level, compressor efficiency, etc. I have
developed computer programs and spreadsheets that WILL calculate the performance of a
given IC on a given engine combo. |
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| Given the following, |
| Engine size 2.3 Ambient Air Temp 80 Deg F |
| Ambient air pressure 14.7 psi Engine VE 73% |
| Compressor eff 71% Engine Rpm 5000 rpm |
| Boost pressure 20 psi gage IC coolant flow 50lb/min |
| Calculated compressor discharge temp 293 degrees |
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