Before we talk about the design tradeoffs, we
need to talk about a some of the possible problems with turbochargers that the designers must take into account.
Too Much Boost
With air being pumped into the cylinders under pressure by the turbocharger,
and then being further compressed by the piston (see How Engines Work for a demonstration), there is more danger of knock.
Knocking happens because as you compress air, the temperature of the air increases. The temperature may increase enough to
ignite the fuel before the spark plug fires. Cars with turbochargers often need to run on higher octane fuel to avoid knock.
If the boost pressure is really high, the compression ratio of the engine may have to be reduced to avoid knocking.
One of the main problems with turbochargers is that they do not provide an
immediate power boost when you step on the gas. It takes a second for the turbine to get up to speed before boost is produced.
This results in a feeling of lag when you step on the gas, and then the car lunges ahead when the turbo gets moving.
One way to improve turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows
the turbine and compressor to accelerate quickly, and start providing boost earlier.
Small vs. Large Turbocharger
One sure way to reduce the inertia of the turbine and compressor
is to make it smaller. A small turbocharger will provide boost more quickly and at lower engine speeds, but may not be able
to provide much boost at higher engine speeds when a really large volume of air is going into the engine. It is also in danger
of spinning too fast at higher engine speeds, when lots of exhaust is passing through the turbine.
A large turbocharger can provide lots of boost at high engine speeds, but may have bad turbo lag because of how long it
takes to accelerate its heavier turbine and compressor.
In the next section, we'll take a look at some of the tricks used to overcome these challenges.
Optional Turbo Features
Most automotive turbochargers have a wastegate, which
allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too fast at high engine speeds. The
wastegate is a valve that allows the exhaust to bypass the turbine blades. The wastegate senses the boost pressure. If the
pressure gets too high, it could be an indicator that the turbine is spinning too fast, so the wastegate bypasses some of
the exhaust around the turbine blades, allowing the blades to slow down.
Some turbochargers use ball bearings instead of fluid bearings to support
the turbine shaft. But these are not your regular ball bearings -- they are super precise bearings made of advanced materials
to handle the speeds and temperatures of the turbocharger. They allow the turbine shaft to spin with less friction than the
fluid bearings used in most turbochargers. They also allow a slightly smaller, lighter shaft to be used. This helps the turbocharger
accelerate more quickly, further reducing turbo lag.
Ceramic Turbine Blades
Ceramic turbine blades are lighter than the steel blades used
in most turbochargers. Again, this allows the turbine to spin up to speed faster, which reduces turbo lag.
Some engines use two turbochargers of different sizes. The smaller
one spins up to speed very quickly, reducing lag, while the bigger one takes over at higher engine speeds to provide more
When air is compressed, it heats up; and when air heats up, it expands.
So some of the pressure increase from a turbocharger is the result of heating the air before it goes into the engine. In order
to increase the power of the engine, the goal is to get more air molecules into the cylinder, not necessarily more air pressure.
An intercooler or charge air cooler is an additional component that looks something like a radiator, except air passes
through the inside as well as the outside of the intercooler. The intake air passes through sealed passageways inside the
cooler, while cooler air from outside is blown across fins by the engine cooling fan.
The intercooler further increases the power of the engine by cooling the pressurized air coming out of the compressor before
it goes into the engine. This means that if the turbocharger is operating at 7 PSI boost, the intercooled system will put
in 7 PSI of cooler air, which is denser and contains more air molecules than warmer air.
Please read this disclaimer.
In no way can we be held liable for any damage or injury caused by using or following any information contained
in this web site.
We do suggest that anyone attempting to perform any modifications to their vehicle make sure they take the
proper precautions to ensure their own safety as well as the safety of others involved.
Check your local and state laws concerning emission's standards to ensure you are not violating any laws if you choose
to use information in this site.