turbochargers.pdf

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A Practical Approach to High Performance
Turbochargers
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Turbocharger Exploded View................................
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Turbocharger Troubleshooting Chart ................................
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Common Terms
Common Terms ................................
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Selecting a Turbocharger Compr
Selecting a Turbocharger Compressor
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Camshafts for Turbocharged Engines
Camshafts for Turbocharged Engines ................................
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Turbocharger Exploded View
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Turbocharger Troubleshooting Chart
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Common Terms
Adiabatic Efficiency
A smaller A/R on the turbine will spool the
turbo faster, but become more restrictive at higher
rpm. If you use a large turbine A/R ratio for top-end
performance, the turbo will take longer to spool up.
Turbine A/R is critical to performance. Street
engines work best if they have low-end boost,
meaning a conservative A/R ratio on the turbine.
On the compressor side, you want to keep
the rpm in or near the peak efficiency island as
much as possible. The A/R ratio has an effect on
where this point is. There are a lot of compressor
maps available, so choosing a compressor housing
and trim is just a matter of matching it to your flow
needs.
A 100% adiabatic efficiency means that
there is no gain or loss of heat during compression.
Most turbochargers will have a 65-75% adiabatic
efficiency. Some narrow range turbo's can get
higher; these types of turbo's generally work well in
engines that operate over a narrow rpm range. In
general the wide range turbo's don't have as good
peak efficiency, but have better average efficiency
and work better on engine that operate over a wide
rpm range.
Pressure Ratio
Charge-Air-Cooler
This is the inlet pressure compared to the
outlet pressure of the turbocharger's compressor.
For single stage turbo's, the inlet pressure will
usually be atmospheric (14.7 psi) and the outlet will
be atmospheric + boost pressure. The inlet
pressure can be, and usually is slightly below
atmospheric. This is due to any restriction in the air
cleaner and intake plumbing up to the turbo.
For staged turbo's the inlet pressure will be
the outlet pressure of the turbo before it +
atmospheric, and the outlet will be inlet pressure +
additional boost from that turbo. Staged turbo’s are
common in high boost applications like tractor
pulling engines.
Also known as an intercooler and is nothing
more than a heat exchanger. When intake air is
compressed by a turbocharger it is also heated. Hot
intake air is not good for power and will increase the
chance of detonation. A charge-air-cooler reduces
the intake temperature; it absorbs some of the heat
out of the charge. With less heat, you'll need less
boost pressure to get the desired power and
decrease the chance of detonation. Anything that
reduces the intake temperature is a big plus in a
supercharged engine.
Boost
Density Ratio
Usually measured in pounds per square
inch, it is the pressure the turbocharger makes in the
intake manifold. One of the ways to increase airflow
through a passage is to increase the pressure
differential across the passage. By boosting the
intake manifold pressure, airflow into the engine will
increase, making more power potential.
Turbochargers compress the air to make it denser,
this is what allows more oxygen in the engine and
give the potential to make more power. The density
of the inlet air compared to the density of the outlet
air is the density ratio.
Turbine
Waste Gate
The turbine side of the turbocharger is what
converts the energy of the exhaust into mechanical
energy to turn the compressor. It consists of the
turbine housing and turbine wheel.
The waste gate is a valve that allows the
exhaust gasses to bypass the turbine. Most waste
gates rely on boost pressure to open them, although
some are controlled electronically. The most
common ones you’ll see today are activated by a
spring-loaded diaphragm. The spring holds the gate
closed, when there is enough boost pressure behind
the diaphragm to over come spring force, the waste
gate opens.
The simplest of boost controllers simply
bleed of boost pressure to the waste gate. You can
A/R Ratio
The A/R ratio is the area compared to the
radius of the compressor or turbine housing. Larger
A/R ratios will flow more.
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install a “Tee” fitting in the waste gate actuator hose
with a valve that bleeds boost pressure back to the
air cleaner. The more the valve is opened, the high
boost pressure will be.
Boost Threshold
Unlike turbo lag, which is the delay of boost,
boost threshold is the lowest possible rpm at which
there can be noticeable boost. A low boost
threshold is important when accelerating from very
low rpm, but at higher rpm, lag is the delay that you
feel when you go from light to hard throttle settings.
Turbo Lag
A turbocharger uses a centrifugal
compressor, which needs rpm to make boost, and it
is driven off the exhaust pressure, so it cannot make
instant boost. It is especially hard to make boost at
low rpm. The turbo takes time to accelerate before
full boost comes in; it is this delay that is known as
turbo lag. To limit lag, it is important to make the
rotating parts of the turbocharger as light as
possible. Larger turbo's for high boost applications
will also have more lag than smaller turbo's, due to
the increase in centrifugal mass. Impeller design,
and the whole engine combo also have a large
effect on the amount of lag. Turbo lag is often
confused with the term boost threshold, but they are
not the same thing, lag is nothing more than the
delay from when the throttle is opened to the time
noticeable boost is achieved.
Turbo Cool down
A turbocharger is cooled by engine oil, and
in many cases, engine coolant as well. Turbo's get
very hot when making boost, when you shut the
engine down the oil and coolant stop flowing. If you
shut the engine down when the turbo is hot, the oil
can burn and build up in the unit (known as "coking")
and eventually cause it to leak oil (this is the most
common turbocharger problem). Oil coking can also
starve the turbo for oil by blocking the passages. It
is a good idea to let the engine idle for at least 2
minutes after any time you ran under boost. This will
cool the turbo down and help prevent coking.
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Selecting a Turbocharger Compressor
Engine Air Flow Requirements
In order to select a turbocharger, you must
know how much air it must flow to reach your goal.
You first need to figure the cubic feet per minute of
air flowing through the engine at maximum rpm. The
formula to figure this out for a 4-stroke engine is:
Temperature Rise
A compressor will raise the temperature of
air as it compresses it. As temperature increases,
the volume of air also increases. There is an ideal
temperature rise, which is a temperature rise
equivalent to the amount of work that it takes to
compress the air. The formula to figure the ideal
outlet temperature is:
(CID × RPM) ÷ 3456 = CFM
For a 2-stroke engine it is:
(CID × RPM) ÷ 1728 = CFM
T 2 = T 1 (P 2 ÷ P 1 ) 0.283
Lets assume that you are Turbocharging a
350 cubic inch engine that will redline at 6000 rpm.
The formula will look like this:
Where:
T 2 = Outlet Temperature °R
T 1 = Inlet Temperature °R
°R = °F + 460
P 1 = Inlet Pressure Absolute
P 2 = Outlet Pressure Absolute
(350 × 6000) ÷ 3456 = 607.6 CFM
The engine will flow 607.6 CFM of air
assuming a 100% volumetric efficiency. Most street
engines will have an 80-90% VE, so the CFM will
need to be adjusted. Lets assume our 350 has an
85% VE. When will then need to take that into
account as well. The complete formula would look
like this:
Lets assume that the inlet temperature is
75° F and we're going to want 10 psi of boost
pressure. To figure T 1 in °R, you will do this:
T 1 = 75 + 460 = 535°R
(CID × RPM x VE%) ÷3456 = CFM
The P 1 inlet pressure will be atmospheric in
our case and the P 2 outlet pressure will be 10 psi
above atmospheric. Atmospheric pressure is 14.7
psi, so the inlet pressure will be 14.7 psi, to figure
the outlet pressure add the boost pressure to the
inlet pressure.
For our 350, it would look like this:
(350 × 6000 x 0.85) ÷ 3456 = 516.5 CFM
Our 350 will actually flow 516.5 CFM with an
85% VE. That is the first step; to know how much
volume the turbocharger will need to flow
P 2 = 14.7 + 10 = 24.7 psi
For our example, we now have everything
we need to figure out the ideal outlet temperature.
We must plug this info into out formula to figure out
T 2 :
Pressure Ratio
The pressure ratio is simply the pressure in
compared to the pressure out of the turbocharger.
The pressure in is usually atmospheric pressure, but
may be slightly lower if the intake system before the
turbo is restrictive, the inlet pressure could be higher
than atmospheric if there is more than 1
turbocharger in series. In that case the inlet
pressure will be the outlet pressure of the turbo
before it. If we want 10 psi of boost with
atmospheric pressure as the inlet pressure, the
formula would look like this:
T 1 = 75
P 1 = 14.7
P 2 = 24.7
The formula will now look like this:
T 2 = 535 (24.7 ÷ 14.7) 0.283 = 620 °R
You then need to subtract 460 to get °F, so
simply do this:
(10 + 14.7) ÷ 14.7 = 1.68:1 pressure ratio
620 - 460 = 160 °F Ideal Outlet Temperature
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This is an ideal temperature rise of 85 °F. If
our compressor had a 100% adiabatic efficiency,
this is what we’d expect outlet temperature to be.
Since it will not have a 100% adiabatic efficiency, we
need to do some more figuring.
flow. To compare the inlet to outlet airflow, you must
know the density ratio. To figure out this ratio, use
this formula:
(Inlet °R ÷ Outlet °R) × (Outlet Pressure ÷ Inlet
Pressure) = Density Ratio
Adiabatic Efficiency
We have everything we need to figure this
out. For our 350 example the formula will look like
this:
The above formula assumes a 100%
adiabatic efficiency (AE), no loss or gain of heat.
The actual temperature rise will certainly be higher
than that. How much higher will depend on the
adiabatic efficiency of the compressor, usually 60-
75%. To figure the actual outlet temperature, you
need this formula:
(535 ÷ 656) × (24.7 ÷ 14.7) = 1.37 Density Ratio
Compressor Inlet Airflow
Using all the above information, you can
figure out what the actual inlet flow in CFM. To do
this, use this formula:
IOTR ÷ AE = AOTR
Where:
IOTR = Ideal Outlet Temperature Rise
AE = Adiabatic Efficiency
AOTR = Actual Outlet Temperature Rise
Outlet CFM × Density Ratio = Actual Inlet CFM
Using the same 350 in our examples, it
would look like this:
Lets assume the compressor we are looking
at has a 70% adiabatic efficiency at the pressure
ratio and flow range we're dealing with. The outlet
temperature will then be 30% higher than ideal. So
at 70% it using our example, we'd need to do this:
516.5 CFM × 1.37 = 707.6 CFM Inlet Air Flow
That is about a 37% increase in airflow and
the potential for 37% more horsepower. When
comparing to a compressor flow map that is in
Pounds per Minute (lbs/min), multiply CFM by 0.069
to convert CFM to lbs/min.
85 ÷ 0.7 = 121 °F Actual Outlet Temperature Rise
Now we must add the temperature rise to
the inlet temperature:
707.6 CFM × 0.069 = 48.8 lbs/min
Now you can use these formulas along with
flow maps to select a compressor to match your
engine. You should play with a few adiabatic
efficiency numbers and pressure ratios to get good
results. For twin turbo's, remember that each turbo
will only flow 1/2 the total airflow.
75 + 121 = 196 °F Actual Outlet Temperature
Density Ratio
As air is heated it expands and becomes
less dense. This makes an increase in volume and
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