Anatomy of a Turbocharger System
The Concept of Forced Induction
The turbocharger is a centrifugal air pump driven by the
engine exhaust gas. It forces additional amount of air or air-fuel mixture
into the engine. This increases combustion pressure and engine power.
In most automotive engines, atmospheric pressure is the only
force that pushes air into the intake manifold. These engines are naturally
aspirated (N.A.). The air they 'breathe' is at normal atmospheric pressure.
The amount of fuel that can be burned in the cylinders is limited by the
amount of air that the atmospheric pressure pushes in.
An engine can produce more power at the same speed (rpm) if
more air-fuel mixture is forced into the cylinders. More air-fuel mixture
means higher pressures during the power strokes and higher power output.
Using a pump to force additional air-fuel mixture into the cylinders is called
forced induction. It is one way to improve volumetric efficiency (V.E.). An
engine with forced induction may produce 35 to 60 percent more power than a
similar naturally aspirated engine.
This concise introduction to the Turbo-Charger is quoted from
the textbook "Automotive Engines (8th edition)" by Crouse & Anglin,
published by McGraw-Hill International. In three short, precise paragraph, it
perfectly serves to introduce the Turbo-Charger in this article.
As explained above, an engine will produce more power if more
air-fuel mixture is forced into the combustion chambers. In NA engines, the
maximum possible amount of air that can be fed into the combustion chamber is
the displacement of that cylinder. For a 4-cylinder 1.6l engine, this will be
approximately 400 c.c. of air per cycle. Flowing 400 c.c. of air into a cylinder
of the 1.6l means it is working at 100% volumetric efficiency (V.E.). Engines
with variable valve timing and other racing derived technologies (e.g.
DOHC-VTECs) do achieve V.E. of above 100% but only a little bit more as they are
still ultimately limited by the amount of air normal atmospheric pressure can
push in. But maximum power is derived by igniting an optimal
air-to-fuel mixture. So if we want to get more power, we will need to burn more
fuel. And to burn more fuel, we need more oxygen and that means more air. But
there is no way to feed a lot more than 400 c.c. of air into each cylinder in
N.A. form. However, using a turbocharger, although each cylinder might still be
only 400 c.c. volume, the air is now compressed and there is more oxygen per
volume of air than that at normal atmospheric pressure. Thus in forced
induction, we can feed in more fuel because we have the necessary
amount of oxygen in the air to sustain optimal power production. This then is
the basic operating principle of the Turbocharger.
Turbo-charging is one of the most effective ways to
significantly boost up the power of any engine at all RPMs. While a turbo-kit
would initially seems to involve complicated plumbing and fueling work, in
actual fact once the design principles are understood, it becomes clear and
straight-forward. In this article, I will attempt to introduce and explain the
basic design principles of the turbo-charger application. While I will never
claim to be an expert in this area, what I hope to achieve is to clarify
sufficiently so readers will be able to understand the important components of
the turbo-charger. I will also attempt to look at and explore, very briefly,
several important areas of turbo-charging that are often misunderstood. As
usual, should any reader who are well versed in turbo-charging see any errors in
this article, please feel free to contact the TOV with
The material presented in this article is theoretically based
on the "Automotive Engines" textbook quoted above. My understanding of the
theories in that textbook has been supplemented by my own observations of
several examples of real-life turbo applications in Honda cars.
The most important component of the turbo-charger package is
the turbocharger itself. The photo above is a turbocharger that
has been cut-open to illustrate its construction. The diagram on the right
clearly shows the air and oil flow through the turbocharger.
As explained, the turbocharger is basically an air-compressor -
it sucks in air and compresses it before pushing it out into the engine. An
important part of the turbo-charger is therefore the compressor
itself, identified by the number '1' in the picture. This is a specially
designed rotary blade that when spun will suck air through the opening in the
middle and delivers compressed air out of the surrounding pipe.
In operation, the compressor spins at an extremely high speed,
upwards of 100,000 rpm. It is driven by the turbine, identified by
the number '2' in the picture. The compressor and the turbine are connected to
each other via a specially lubricated shaft (number '3'). When the
turbine is turned, it will turn the compressor via the shaft. The turbine itself
is spun by blowing exhaust gas against it via the inlet. Spent exhaust gas will
be exhaled through the center outlet (and into the exhaust system).
Because the turbocharger spins at speeds of beyond 100,000rpm,
two extremely critical parts of it are the lubrication and the cooling. Both of
these are normally done by engine oil. When operating, engine oil is delivered
to the shaft at high pressure. This serves to 'float' the shaft in a layer of
lubricating engine oil and allows the turbocharger to spin at high speeds with
little or no friction and wear. Specially designed oil passages also permeate
the turbocharger casing and oil flow through these passages together with that
floating the shaft serves to cool the turbocharger. Ensuring proper delivery of
engine oil into the turbocharger is crucial to its operation and reliability.
After extended operation, the turbocharger will glow red hot and we will need to
maintain oil flow through it in order to conduct heat away. The cutaway
turbocharger above has the casing sides painted in yellow and the oil passages
can be clearly seen.
The Structure of a Turbo-Charger System
With this explanation, the construction of a turbo-charger system can be
explained in a clear manner. The most important part is to properly place the
turbine. The turbine is driven by exhaust gas so for optimum placement, it will
be located just after the exhaust ports, where the headers originally are. This
means we need a special manifold to feed exhaust gas from all the cylinders into
the turbine. Then we need to connect the turbine into the exhaust system so the
spent exhaust gas will be discharged.
We now basically need only two more pieces of piping. One is to
fit the air-filter to the compressor part of the turbocharger. Any complicated
piping here is simply to allow us to place the filter into an optimum position
for air-flow. We then need to connect the compressor outlet to the throttle body
so that compressed air will be fed into the engine. The image on the left shows
conceptually the relative location of the turbo-charger in relation to the
cylinder and its intake and exhaust valves as well as the piping that is needed
to connect the turbo-charger to the cylinder head.
The other important connection to the turbocharger is the oil delivery line,
normally a special steel-braided oil hose to tap engine oil from a suitable
location. The oil outlet from the turbine then needs to be connected to the oil
A turbocharger system often includes an
intercooler. The Intercooler is a special cooling coil that is
mounted in between the compressor outlet of the turbo-charger and the throttle
body. When air is compressed, it gets heated up. Hot air contains less oxygen
per volume than cooler air (all other parameters being equal). The intercooler
'intercepts' the compressed air from the turbo-charger and cools it before
sending it out to the throttle body.
However an intercooler has the disadvantage that it introduces
throttle response lag into the system. From light or closed throttle cruising
condition, when the throttle is suddenly floored open, the turbo-charger now
needs to first pressurize the air inside the intercooler before air can be fed
into the engine. This causes a lag between the time when the throttle is floored
open to the time the engine responds. However, the lower air temperature given
by the intercooler allows more power to be generated, as well as allowing more
stable engine operation and consistent power delivery.
To complete the turbo-charger system, we will need various
ancilliary devices to help regulate and control its operation.
The turbocharger will normally include a waste-gate. There are
many types but the most common would be the actuator waste-gate. This is an
air-pressure driven device that opens a flap located on the turbine part of the
housing. When the flap is open, it provides an alternate, low resistance escape
route for the engine exhaust gas into the exhaust system. The flap is open via a
lever that is connected to an air-pressure switch on the compressor. The amount
the flap is open varies directly with air pressures from the compressor. A
spring is used to hold the flap shut against the housing. By adjusting the
spring, it is possible to control at what air pressure the flap will start to
open. In operation, this serves as a method to approximately control the amount
of boost the turbocharger will deliver to the engine. The photo on the right
shows the flap that opens the by-pass 'escape route' from the turbine to the
exhaust system. There are other variations of the waste-gate, but all of them
are designed to control the boost to the engine.
the piping from the compressor to the throttle body called the 'pressure pipe',
there is also often a blow-off valve. The function of the blow-off
valve is to relieve pressure off the pressure pipe when the throttle body
butterfly is closed. When this happens, the turbocharger is still spinning
(often at maximum speed) and pressure build-up inside the pipe will push back
against the compressor blades. This has the effect of slowing the turbocharger
down and will cause a delay in response should the throttle be open again
immediately. The blow-off valve will relieve the pressure build-up by venting
the air out of the pressure pipe. Most turbocharger systems will vent the
pressure back into the air-filter connecting pipe, others will simply vent the
air back out into the atmosphere (there are advantages to doing this). The
mechanism of venting these excessive pressure are also varied, with the most
famous and popular being the Sequential Blow-Off Valve invented by
Taking Care of the MAP Sensor
In stock form, most Honda engines are naturally aspirated.
Honda uses a pressure sensor, the MAP sensor to measure the amount
of air flowing into the engine. MAP sensors measures the air pressure just after
the butterfly valve in the throttle body. The higher the pressure (or more
accurately, the less the vacuum), then more air will be flowing
into the cylinders. In NA form, the maximum possible air-pressure at the
throttle body is therefore normal atmospheric pressure or air at 1.0 bar. The
stock Honda ECU therefore will have PGM-Fi programs that will only work until
1.0 bar MAP pressure value.
In turbo-charging, we now have the unusual situation where the
MAP sensor will see air pressure of beyond the 1.0 bar normal atmospheric
pressure. When the MAP sensor feeds such a condition to the ECU, under
stock conditions, this constitutes an error. In stock form, PGM-Fi
sets an error code and goes into back-up mode. PGM-Fi sets the error code not
really just to notify that the MAP sensor signal is out of range, more
importantly, the stock PGM-Fi fuel and ignition maps do not have
data for MAP values beyond 1.0 bar. So, to avoid engine damage, PGM-Fi quickly
goes into its back-up mode. Therefore in turbocharged Hondas, we will need to
work around this design logic of PGM-Fi.
There are a few ways to take care of this. To avoid having
PGM-Fi go into back-up mode, the traditional method is to add special check
valves into the vacuum hose that connects the MAP sensor to the throttle body.
Later Hondas have their MAP sensors mounted directly to the throttle body so the
MAP sensor will then have to be relocated to a remote location. These check
valves prevents air pressure to the MAP sensor from building up beyond normal
atmospheric pressure. The more popular way nowadays is to modify the electronics
such that although the MAP sensor do see boost (anything above normal
atmospheric pressure), the actual signal to the ECU itself is modified so that
it will never indicate a boost condition.
Fuel and Ignition Control
Taking care of the error-code condition of the ECU is just one
small part of the modification needed. Because PGM-Fi does not have fuel and
ignition map values that cater for boost, we will need to ensure that at boost
conditions, the injectors continue to feed sufficient fuel into the engine.
Again, there are several different ways to do this.
The most simple way is the use of an Additional Injector
Controller (AIC). This is an electronic device that works when boost is
detected by the MAP sensor. One or more injectors are mounted such that they
feed additional fuel into the engine. The most popular location is along the
pressure pipe (though some mounts it at the intake manifold plenum and others at
the runners). When boost is detected, the AIC unit injects fuel into the air
stream. Thus the air arriving into the cylinder head is already pre-mixed with a
set amount of fuel so when the original injectors opens, they're effectively
adding fuel into the mixture. By controlling how much fuel is fed by the
additional injectors, the required air-fuel mixture can be quite closely
approximated using this method.
A method popular in the earlier days and still popular now is
to replace the stock ECU with an aftermarket one. The most famous amongst such
devices are HALTEC and MOTEC aftermarket computers,
with the APEXi PowerFC being a more recent introduction. They
basically have fuel and ignition maps that will need to be calibrated not only
for the original NA operating range of MAP sensor values but also for the new
boost range of MAP signals. However, because they replace the original ECU, they
will also alter the operating characteristics of the engine.
Japan's HKS tuning company introduced the concept of the
'piggy-back' computer with their FCON series of computers. An FCON
computer intercepts the wiring harness coming into the ECU. The stock ECU is
connected to the FCON computer instead of directly to the wiring harness. By
this method, FCON is able to manipulate both the input and output signals of the
stock ECU. This innovative device introduces the idea of new fuel and ignition
maps that works in conjunction with the original PGM-Fi maps. The map values in
a HKS FCON computer are used to modify the original NA injector and
ignition timing values of the stock PGM-Fi maps while the boost range of values
are controlled directly by FCON.
Specifically for Hondas only however is the introduction of the
new HONDATA customizable PGM-Fi codes. HONDATA's Boost
Option actually changed the original PGM-Fi code to work with MAP sensor
signal values of beyond 1.0 bar normal atmospheric pressure as well as expanded
the stock fuel and ignition maps to cater for boost conditions. Together with
HKS' FCON computers, it is the most advanced method to take care of fuel and
ignition management for turbo-charged Hondas.
Common optional components of the turbo-charger system are
typically various meters. They are used to monitor various
operating parameters of the engine, e.g. the operating boost of the engine, and
other operating conditions like exhaust gas temperature.
An often indispensable component of the turbo-charger system is
the oil-cooler. When a certain amount of fuel is burned, only a
portion of it gets converted into useful work (driving the pistons and
generating power). Most of it actually gets converted into heat !
Therefore it is unavoidable that more powerful engines will produce more heat,
irregardless of whether it is NA or turbo-charged. Installing a turbo-charger on
a Honda engine will quickly increase its power by a large amount and
consequently the amount of heat it produces. An oil cooler will quickly and
efficiently disperse the heat, keeping the engine running at optimal
Another critical component of the turbo-charger system is the
turbo-timer. This is an electronic timer that is hooked into the
ignition circuit. When the ignition is switched off, the turbo-timer keeps the
engine running for an additional pre-set amount of time, after which the engine
is automatically shut down. As explained earlier, the turbocharger can glow
red-hot under heavy use. By keeping the engine running, the turbo-timer keeps
oil flowing through the turbocharger, ensuring that it is properly cooled down
before shutting down the engine.
A Brief Look at Various Aspects of Turbo-Charging
It is now time to discuss various aspects of turbo-charging.
One extremely key aspect is the target application the system is
installed for. There are two arbitrarily defined approaches to modifying our cars: street-use and race-use approaches. Now, this applies to
turbo-charging too, in fact even more so than other approaches.
Turbo-charger application for street use are often simple
bolt-on mods. Street-use turbo-kits are designed for moderately big power gains
with driveability and reliability. Driveability involves many factors like good
throttle response, all round power (low-end, mid-range, high-rpm), and ease of
driving. Reliability assures us the use of the car every-day without worry about
the engine blowing up or having it spend 1 week in the workshop for every week
that it is driven !
Turbo-Charging for race-use are in many ways directly in
contrast to that for street use. For race-use, we want maximum possible power.
Because racing allows us to keep engine revs more or less within a limited band,
we can frequently tolerate very bad mid and low rpm power, as long as power
within the important rev range is as high as possible. Good idling is certainly
not a consideration at all while race engines typically only need to last for
the duration of a single race - work will be continuously done to improve the
engine in between races anyway !
In practice, this has direct impact on the set-up of the
turbo-charger system. The most important of this is the operating
boost. In a turbo-charged engine, max power is more or less
directly proportional to the boost we run it at. The higher our boost, the
higher the maximum power. But I wrote maximum power for a very good
reason. To sustain high boost, we need a larger turbocharger. The turbocharger
must be big enough to create enough air-flow to sustain the desired boost to the
engine, even more so when we have an intercooler added. When we have a large
turbocharger however, we now have to contend with inertia. Inertia means the
resistance of the turbocharger to spool up quickly.
One important characteristic of turbo-charging is
turbo-lag. The text-book definition of turbo-lag is the delay one
experiences between flooring the throttle to the time the engine responses. With
a big turbocharger, while we will get huge power gains once the compressor
spools up, we get the power only when the compressor spools up. Therefore
one unavoidable characteristic of a high boost system is turbo-lag.
For this reason, a street-use turbo-charger system often use
low to medium boost. For Hondas in basically stock form (no big engine works),
boost are often within the 0.3 to 0.5 bar range. Honda engines, especially the
super high specification VTEC versions uses very high compression ratios and
this conflicts with higher boost levels. Detonation and engine knocking will
become a big problem when boosts are pushed too high.
Running a very high boost set-up is often talked about as a
matter of fact by enthusiasts but the implications are often also casually
ignored. An extremely critical issue related to boost in a turbocharged engines
is the excessive cylinder pressures in very high boost set-ups. When combustion
occurs cylinder pressure is exerted in all directions. This not only pushes the
pistons down but also attempts to push the cylinder head off the block. Very
high boost levels generates very high power but also requires that the engine is
strong enough to handle the stresses involved. This involves extended work like
replacing the piston/conrod/crankshaft assembly, ensuring the block is
strengthened, and many other things. The stock clutch system will not be able to
handle excessive power increases. But clutches strong enough requires excessive
foot pressure to operate while sports or racing clutch also have a very 'grabby'
feel leading to jerky driving. Often overlooked is the transmission's ability to
handle the extra power - the gearbox and even the drive shafts may break under
excessive power. Also important is the rest of the car; brakes, suspension,
indeed the whole chassis must be strong enough to handle the new power.
As can be seen, while turbo-charging will give big increases in
engine power, crucially important is that proper work must be done to ensure the
engine and indeed the whole car itself must be able to handle the new power.
Equally important too is whether the power generated can be effectively utilized
for the car's intended use, and how practical it is to use.
In this article I tried to explain the basic operating
principle and structure of the turbo-charger system in as simple and clear
manner as I can. I also looked at the approaches to turbo-charging as well as
important characteristics of the two main approaches. I hope readers find this
article informative and useful. Again, I have taken all possible means to ensure
this article is accurate. Any error that creeps into this article will be
© Temple of VTEC World