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 FAQ #194
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Turbocharger or Turbo
Exhaust driven air charger.

A turbo charger is a device powered by the pressure of the exhaust gasses.

As it spins up it puts pressurised air into the engine, commonly via an intercooler, and as such gets more air into the engine which gives a bigger explosion with the subsequent increase in power.

It is a very efficient way of getting extra power

The main components of a turbocharger are:

Compressor housing and wheel
Bearing housing
Turbine wheel
Journal bearings
Thrust bearings
Turbine housing
Wastegate
Actuator

Air entering the engine first passes through an exhaust driven compressor. Compressed air results in a larger quantity of air being forced into the engine, creating more power.

The energy used to drive the turbo compressor is extracted from waste exhaust gasses. As exhaust gasses leave the engine they are directed through a wheel placed in the exhaust flow. The gasses drive the turbine wheel around, which is directly connected via a shaft, to the compressor wheel.

Increased exhaust gas drives the turbine wheel faster, this provides the engine more air, producing more power. A limit is met once a pre-determined boost pressure is achieved. At this point the exhaust gas is redirected away from the turbine wheel, thus slowing it down and limiting the maximum boost pressure. This redirection valve is known as the wastegate.

This extraction of energy, from exhaust gas, to improve engine efficiency is the device known as the turbocharger.

Turbochargers were usually seen as power enhancements on performance cars, but today, turbochargers are becoming more regularly used to provide greater torque on small capacity engines. The advantages of using a turbo engine include improved fuel efficiency and reduced exhaust emissions.

Basics of a turbo operation

A turbocharger has two "wheels" each of which look very similar to the plastic pinwheels that kids play with. The turbo uses the flow of exhaust gasses to make the turbine wheel of the turbo spin (same principle as a windmill, only on a very small scale). The turbine wheel is mounted to the same shaft as the compressor wheel. When the turbine wheel makes the compressor wheel spin fast enough, the compressor wheel starts to suck in air and spit it out toward the engine faster than the engine would normally suck in air on its own.

When this happens, the turbo has "spooled up" and begins to create turbo boost and the engine gets more air than it would normally, so the potential for more power increases along with the boost.

The way this system works has a drawback called ‘turbo lag’. A turbocharger doesn't help produce power until it can spin fast enough to create more air flow than the engine would on its own. So, when you step on the gas you have to wait for the engine to create enough exhaust flow to spin the turbine wheel, and the attached compressor wheel, of course.

It then creates more air flow than the engine would normally. The time between stepping on the gas and when the wheels start to spin fast enough to work, is the time referred to as ‘turbo lag’

Turbo lag can be reduced by correctly sizing the turbo to the car. A turbo which is too big for the engine won't be able to create usable boost until it's revving very high. This isn't too bad if you're using the car only to race since you can keep the revs up, but on the street, you'll have to constantly downshift even for moderate acceleration.

On the other hand, if the turbo is too small, then it will spool up quickly, but it won't be able to keep up with the engine... at high RPM's the engine will be sucking up more air on it's own than the turbo can provide and you car will fall on its face.

Turbo lag is often misunderstood. Just because you don’t have boost at the time you floor it doesn’t mean that the turbo is the wrong size. Turbos have a certain operating range of efficiency. If figured correctly, the right turbo head unit will allow a generous amount of power without causing the cylinder pressures to get too high resulting in a blown head gasket.

This typically happens when the exhaust housing is too small. You couldn’t put a stock 4 cylinder turbo on a car with 8 cylinders without running a very high risk. So proper turbo sizing per the application is the key factor in an efficient turbo system.

The lag of a turbo motor, (if the turbo is the proper size), is an advantage over naturally aspirated and supercharged motors because it gives you more control over traction; off the line or in the lower RPM ranges (typically Idle-2000rpm, any lower and you will run out at the top end or if you modify your motor it will require you to purchase a larger turbo).

The Wastegate

When enough exhaust gas flows past the turbine wheel of the turbocharger, boost is created. The faster it spins, the more boost you get... the more boost you get the more air the engine consumes... and the more air it consumes, the more exhaust gasses flow... and the more exhaust gasses flow, the faster the turbine wheel spins... and the faster the turbine wheel spins, the more boost you get... the more boost you get... well, you get the picture. Potentially, the engine will just put itself into an unending loop of increased power.

But at some point, the turbocharger or motor will eventually blow up (unlimited power just by having a turbocharger is just too good to be true!). This is why the wastegate is essential in any turbocharged engine. How it works is with a vacuum line that connects from the compressor outlet to the wastegate actuator. The vacuum line provides the wastegate with a constant input of what level of boost the turbo is making.

When a predefined boost level is reached, the wastegate opens and redirects some of the exhaust gasses to exit the engine through it instead of through the turbine wheel. When exhaust is vented through the wastegate, not all of the exhaust can make the turbine wheel spin as it goes directly out of the wastegate to the exhaust. This bypassing of exhaust keeps the boost under control and keeps the turbocharger from damaging the engine or itself.

Turbo Intercooler

An intercooler works on the same principle as the engine's cooling system's radiator (they even look very similar in design); all it does is exchange heat from what's in it to the air rushing by around it. The difference is most intercoolers only have air in them... the compressed air from the turbo.

A by product of compressing air to create boost is heat... in fact, anything that gets compressed gets hotter. The heat created in the air that the turbo compresses is not desired and in extreme cases can be dangerous to your engine. To address this, many turbocharged cars have an intercooler. The compressed air leaves the turbo but before making its way into your engine, it flows through the intercooler. The air transfers its heat to the intercooler's fins where it is dissipated into the surrounding air rushing by.
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Member Rating :   9.0 - 2 votes    
FAQ Posted by Dave-Retired
Info Created: 07 October 2009
Last Updated: 18 December 2015
 

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