How It Works
The turbocharger is bolted to the exhaust manifold of the engine. The exhaust
from the cylinders spins the turbine, which works like a gas turbine engine. The
turbine is connected by a shaft to the compressor, which is located between the
air filter and the intake manifold. The compressor pressurizes the air going
into the pistons.
Figure 1. Diagram of how a turbocharger is plumbed in a car.
The exhaust from the cylinders passes through the turbine blades, causing it
to spin. The more exhaust that goes through the blades, the faster they spin.
Figure 2. Inside a turbocharger
On the other end of the shaft that the turbine is attached to, the compressor
pumps air into the cylinders. The compressor is a type of centrifugal pump; it
draws air in at the center of its blades and flings it outward as it spins.
In order to handle speeds of up to 150,000 RPM, the turbine shaft has to be
supported very carefully. Most bearings would explode at speeds like this, so
most turbochargers use a fluid bearing. This type of bearing supports the shaft
on a thin layer of oil that is constantly pumped around the shaft. This serves
two purposes: It cools the shaft and some of the other turbocharger parts; and
it allows the shaft to spin without much friction.
Turbine power available to drive the compressor can be varied in
two ways: 1) the Area to Radius (A/R) ratio of the turbine housing can be
changed to alter turbine inlet pressure; and 2) the turbine wheel trim can be
specified to effect an increase or decrease in turbine pressure for a given
turbine housing A/R. (Toysports)
Compressing air
Compressing air makes it hotter, and less dense. Now as the whole point of
compressing the air is to get more of it in the cylinders, this is bad. For this
reason almost all turbo cars cool the air down after the turbo by means of an
intercooler. Interestingly a common misconception is that the turbo heats up the
air because it is physically connected to the hot exhaust. In fact the vast
majority of the heat comes from compression.
It's often said that the boost pressure alone determines the power increase.
Not true. It is the AMOUNT of air that you can get in the cylinders that
determines the power the engine will produce. All the energy to drive the car is
locked up in the fuel. A certain amount of fuel needs a certain amount of air to
burn - if you can fit more air in, you can burn more fuel.
The pressure of the air is linked to it's temperature, amount and volume:
pressure x volume = amount x temperature (well with a constant stuck
in there as well but you get the idea). Since the volume of your engine isn't
going to be changing, it's easy to see that the amount of air you can fit in is
proportional to the pressure (bigger pressure = more air) and inversely
proportional to the temperature (lower temperature = more air).
But enough equations, all you need to know is that a high pressure AND a low
temperature are ideal for getting a large amount of air into the engine.
Wastegates
If the turbo was as simple as I have described above, it would just compress
the air more and more until something went pop! In fact it is kept to a
predetermined boost level by the wastegate. This is a port in the exhaust stream
just before the turbo which can be opened to let gases bypass the exhaust
turbine and therefore keep the amount of boost produced to a set level. On the
turbos found on our cars this wastegate is "Internal" i.e. it is built
into the turbine housing (bigger turbos need external w/g, see below). Attached
to the intake side is an actuator, which at a preset boost level pushes a rod
which opens the wastegate in the exhaust side.
Without the wastegate working properly and allowing enough gas to bypass the
exhaust turbine, a phenomenon called "overboosting" occurs where the boost level
can run dangerously high. Sometimes fitting a very large bore high flow exhaust
can cause this with the stock wastegate.
Turbo performance and efficiency
You hear many numbers thrown around describing turbos, but perhaps the most
misleading are the hp numbers. "This is a 500hp turbo" doesn't really mean
anything. You can guess from it that it's a pretty big beast, but there's no way
you can guarantee 500hp from it. You will also hear turbos described by the size
of impeller wheels, exhaust turbine wheels, A/R ratios etc.
A hefty flow of (relatively) cold, dense air is the aim with a turbo.
Generally the bigger the turbo the easier this is. This is because the larger
turbo is a more efficient compressor of air at the sort of values we are looking
at (say approx 7 - 30 psi, which is approx 0.5 to 2 bar). The shaft of a turbo
spins very fast (typically > 100,000 rpm), and the bigger turbo's shaft can spin
slower to flow the same amount of air.
It's a widely believed myth about turbo sizing that (say) 15 psi on one turbo
is 'equal' to 15 psi on another. This isn't true. A large turbo will be able to
flow a lot more air than a smaller one at any given pressure. And it's amount of
air that counts.
Most manufacturers are cagey about releasing flow figures for their turbos.
This makes it difficult to compare different manufacturers models. It is useful
if the manufacturer provides a compressor map, this is the best way to describe
a turbo and see it's characteristics.
Terms
Compressor Size:
When hybrids are talked about, numbers like "57 trim" are often used. The 57
bit is the visible size in mm of the impeller (called "compressor inducer") that
you see when you look down the intake. The actual wheel extends beyond this into
the housing (this size called "compressor exducer"). For comparison, the 'trim'
of a standard ST205 turbo is 47mm, whereas the trim of a good all-round hybrid
without too much lag might be 50 - 54 mm.
A/R:
A/R is a number often used to describe turbos. It is basically the area (A)
of the cross section of the intake pipe divided by the radius (R) of the
internal bit of the impeller wheel. A 'big' A/R turbo (say > 1.0) will flow LOADS
of air, at the expense of lots of lag.(ST205.net)
Ceramic turbos:
All turbos have something called "lag", which everyone has no doubt heard of.
This occurs because once you open the throttle it takes a while for the exhaust
gas stream to get sufficiently big to spool up the turbine. The bigger the
turbo, the greater the lag.
On some models Toyota has attempted to address the problem of lag by making
the exhaust turbine of the turbo out of lightweight ceramics rather than steel
(simple light wheels turn easier). This is what is meant by "ceramic turbo".
One downside of this is that the ceramic parts have been known to shatter at
high speeds.
Clipping:
Clipping is the technique of cutting away some of
the material on the fins of the impeller wheel of the turbocharger. In other
words, to 'clip' a turbo is to make the fins in the exhaust path smaller. The
cut is usually done at an angle of between 10 and 20 degrees - the bigger the
angle, the more material is removed from the fins.
This may seem like a dumb thing to do, since smaller fins mean that the exhaust
gases will impart less force to the turbine wheel and consequently increase
turbo lag. This is true, but the benefit of clipping is found in the high RPM
range of the motor. At higher RPMs, the turbo may have already surpassed the
required user-set boost levels and is not contributing to engine power.
Since the impeller wheel in the exhaust stream partially blocks the exhaust gas
flow (by design), it can act as a significant restriction at high RPMs, when the
exhaust flow rate is highest. Clipping the turbo reduces this restriction and
allows more air to flow past the turbo wheel at high RPMs, thereby improving
airflow through the engine and increasing top-end response. (93
R1 @ nopistons)
This site is meant to give information related to the 1993
(o)Mazda RX-7 Twin Turbo. Anything from rotary engines to wiring diagrams and turbo upgrades to tuning info, this site has it all! efini 93 rx7 13b anfini Turbo RX-7 Turbo RX7 turbo rx7 rx7tt rx-7tt As well as the Rotary Engine Pickup Truck aka REPU repu
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