Slow Down There Turbo!

If, like me, your introduction to the word “turbo” came from old school video games and you never learned otherwise then you still think that when actors hit the NOS buttons on the steering wheel of their cars in The Fast and The Furious (2001) that the buttons could alternately called “turbo” buttons.  Well, unfortunately for you and me and all of those lead astray by these video games and episodes of Knight Rider and the infamous KITT with his “Turbo Boost” feature, that understanding of turbos is incorrect.  Turbos are power boosters but they have nothing to do with the little red buttons on the steering wheel.

In this video you’re watching a Chrysler K-car which has been turbocharged. Just before the car takes off down the track you can hear a loud whine which starts low and goes to a high-pitched whine or almost whistle sound right at about 32 or 33 seconds.  That sound is the turbo charger spooling up.
 
Basics
 
Looking back at the previous article about engines needing air, fuel, compression and ignition, we can see that if we add more of these items we should get more power right?  Enter the turbocharger.  The turbocharger works by compressing air into the intake of the engine, increasing the amount of air (aka boost) and therefore the amount of fuel (remember the stoichiometric fuel ratios) and pre-compressing the air before the engine goes through the final compression for ignition.  Now with the turbocharger we have three of the important things for “the big bang” in greater quantity.
 
The turbo compresses the air with a spinning compressor wheel that is connected by a small shaft to an exhaust turbine in the exhaust stream after the engine as shown in the diagram.  Heat and exhaust velocity spin the turbine and compressor wheels to speeds well over 100,000 rpm
 
Turbo speed limit
 
The turbo speed I mentioned above is actually low compared to what many turbos are capable of.  The limit of turbo speed is determined by the strength of the material that is used for the compressor wheel and how resistant it is to flying apart and the diameter of the compressor wheel.  The velocity of the compressor wheel tips are a factor of rotational speed and the distance from the turbo shaft.  When over-speeding, the velocity of the tip of the compressor is so high that little pieces of material will actually separate from the wheel and fly into the outside wall, unbalancing the wheel.  When a compressor wheel becomes unbalanced at 230,000 rpm you can imagine the bad stuff that happens. 
 
Turbo speed is controlled by two main methods, one of the most common and widely used is a wastegate, which is essentially a hole in the exhaust manifold with a valve in it that will allow exhaust to bypass the turbine.  By bypassing the turbine the energy that was in that exhaust is not applied to the turbine to increase or maintain turbo speed.  A newer method for controlling turbo speed is with variable geometry turbos (VGT) and variable nozzle turbos (VNT).  The speed of air hitting the turbine vanes is a large factor in how efficiently the turbine collects energy from the exhaust and converts it to speed.  VGTs and VNTs work by lowering the exhaust velocity through the turbine and changing where the high-speed gas hits the turbine.  By doing this the efficiency of the turbine is reduced and less of the energy is converted into speed. 
 
Intercooling
 
When air or any type of material is compressed its temperature rises drastically.  After air is compressed by a turbo it can often be heated to temperatures over 350°F (180°C).  If you recall from my last blog about combustion I mentioned a type of ignition called auto-ignition; the increased temperature of air combined with the compression of the engine can cause this auto-ignition to happen way before the engine is ready for ignition to happen.  This is really bad.  Intercooling, or more correctly charge air cooling cools off the combustion air before it goes into the engine by passing the compressed air from the turbo through an air to air radiator in the front of the car.   Some production cars don’t cool off the air after turbo compression; these cars will usually have less benefit from the turbo than the intercooled counterparts.

Bigger is not Better

The next part about turbochargers that is important is that bigger is not always better.  In the case of the K-car the driver is using a very large turbo charger which takes a long time to spool up and be useful.  If the driver was not on a race track and didn’t have time to floor the engine and get the turbo spinning he’d never have any boost during normal driving because the large turbo has so much momentum to overcome before spinning fast enough to be useful.  Small turbos will give a car much better throttle response at low engine speeds and normal driving since there is very little momentum in smaller wheels and the speed can be increased quickly, but larger turbos will give an engine much better high engine speed power because of higher airflows through the engine.

There is also a delicate balance between the compressor size and the turbine wheel size.  If these aren’t balanced correctly then something called turbo surge can happen where the air actually stops going through the turbo and goes backwards out the compressor inlet.  It sounds like a great big bark or “chunt” noise when it happens.  The only time I’ve heard this in person is on a big-rig semi engine, so if you hear something wierd like that when a semi truck changes gears next to you on the highway you’ll know what is happening.  In the video below the wierd pulsing noise you hear at the end is the air surging and going backwards through the compressor.

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One Response to “Slow Down There Turbo!”

  1. suz kaemingk Says:

    i can feel your emotion ha ha

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