Turbo Chargers

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This section is very technical. Make sure that you have a basic understanding of how the engine works before you try to get into the heavy realms of turbo and super charging. Turbo charges and super charges are add on's that car manufacturers use to get more power and performance out of their cars. One way to get more power out of an engine is to increase the amount of air and fuel that it can burn. You could add more cylinders or make the current cylinders bigger, but these changes may not be possible, because the engine can only be so big. Bigger cylinders equals far more engine expense and less fuel economy. Having said that, fuel economy isn't such an important consideration in performance car design . . . people who buy high performance sports cars don't usually worry about the extra fuel expense. Another way of increasing engine power is a turbo charger. A turbo can be a simple, more compact way to add power, especially as an after market accessory. The results can be dramatic.

Turbochargers allow the engine to burn more fuel and air mixture by forcing more mixture into the cylinders. In other words, the fuel and air mixture in the compression chamber is at a far higher pressure than in a normal engine. The typical boost provided by a turbocharger is 6 to 8 pounds per square inch (psi). Since normal atmospheric pressure is 14.7 psi at sea level, about 50 percent more air fuel mixture is pumped into the engine. Therefore, you would expect to get 50 percent more power. But because the efficiency is not perfect, a loss of about 10% or more can be expected. Still, that's a pretty good increase in power. In order to achieve this boost, the turbocharger uses the exhaust flow from the engine to spin a turbine which then spins an air pump. The turbine in the turbocharger spins at speeds of up to 150,000 rotations per minute (rpm). That's about 30 times faster than most car engines can go! And since it is hooked up to the exhaust, the temperatures in the turbine are also very high.

As I said, the efficiency is not perfect. One cause of the inefficiency comes from the fact that the power to spin the turbine must come from somewhere, and having a turbine in the exhaust flow increases the restriction in the exhaust. This means that on the exhaust stroke, the engine has to push against a higher back-pressure thus subtracting a little bit of power from the cylinders that are firing at the same time.

Cars with carburettors automatically increase the fuel rate to match the increased airflow going into the cylinders, while modern cars with fuel injection system relies on oxygen sensors in the exhaust to determine if the air-to-fuel ratio is correct. So, carburettor systems will automatically increase the fuel flow if a turbo is added. If a turbocharger with too much boost is added to a fuel-injected car, the system may not provide enough fuel. Either the software programmed into the controller will not allow it, or the pump and injectors are not capable of supplying it. In either case, other modifications will have to be made to get the maximum benefit from the turbocharger.
 

The exhaust from the cylinders passes through the turbine blades, causing the turbine to spin. The more exhaust that goes through the blades, the faster they spin.

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 centre 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 metal 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 too much friction.

Problems with turbochargers?

With air being pumped into the cylinders under pressure by the turbocharger, and then being further compressed by the piston, there is more danger of knocking, meaning that the gas ignites by compression rather than by combustion. Knocking happens because as you compress air, the temperature of the air increases, thus increasing the likelihood of the air and fuel mixture being ignited. The temperature may increase enough to ignite the fuel before the spark plugs fire. Cars with turbochargers often need to run on higher octane fuel to avoid knock problems. If the boost pressure is really high, the compression ratio of the engine may have to be reduced to avoid knocking.

Turbo Lag

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 or so 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 surges ahead once the turbo gets moving. One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight, allowing the turbine and compressor to accelerate quicker, and start providing boost earlier.

Small or large Turbocharger?

One sure way to reduce the inertia effects of the turbine and compressor is to make the turbocharger 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 quickly at higher engine speeds, when lots of exhaust is passing through the turbine, thus creating the problem of overheating and mechanical problems. (see the section on ‘Dump valves' below for the solution to this problem)

A large turbocharger can provide lots of boost at high engine speeds, but may have lots of turbo lag because of the longer time it takes to accelerate its heavier turbine and compressor. However, it is able to provide better boost at higher speeds.

Dump valve or ‘wastegate'

Most automotive turbochargers have a dump valve, which allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too quickly at high engine speeds. The dump valve allows the exhaust to bypass the turbine blades of the turbo charger. The dump valve senses the boost pressure in the system. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the dump valve  ‘dumps' some of the exhaust gasses, bypassing the turbine blades, allowing the blades to slow down. You can hear the distinctive sound of the dump valve releasing the excess pressure from the system on many modern production and modified cars. There's a loud ‘hiss' as the pressure is released, just before the driver squeezes the gas on again. One of the best dump valve sounds around is produced by the Subaru impreza STi, pictured below.

Ball Bearings

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 to accelerate more quickly, further reducing turbo lag, but this addition is more likely be used in a larger turbocharger than a smaller one.

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.

Sequential Turbochargers

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 boost. Great, but far more expensive to implement.

Inter coolers

When air is compressed, it heats up, becoming expanded as a result. Thus, 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 to be burnt, not necessarily more air pressure. An inter cooler or charge air cooler is an additional component that looks something like radiator, except air passes through the inside as well as the outside of the inter cooler. 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 inter cooler further increases the power of the engine by cooling the pressurised air coming out of the compressor before it goes into the engine. This means that if the turbocharger is operating at a boost of 7 psi, the inter cooled system will put in 7 psi of cooler air, which is denser and contains more air molecules than warmer air. More air molecules equals a bigger explosion in the cylinders which equals more engine power.

What's the difference between a Turbocharger and a Supercharger?

Well, let's start with the similarities. Both turbochargers and superchargers are called ‘forced induction systems'. They compress the air flowing into the engine. The advantage of compressing the air is that it lets the engine pump more air into a cylinder. More air means that more fuel can be forced in, so you get more power from each explosion in the cylinder. A turbo or supercharged engine produces more power overall than the same engine without the charging.

The typical boost provided by either a turbocharger or a supercharger is 6 to 8 pounds per square inch (psi). Since normal atmospheric pressure is 14.7 psi at sea level, you can see that you will get about 50-percent more air into the compression chamber. Therefore, you would expect to get 50-percent more power. It's not perfectly efficient, though, as we have discussed earlier so you might get a 30-percent to 40-percent improvement instead.

The key difference between a turbocharger and a supercharger is its power supply. Something has to supply the power to run the air compressor. In a supercharger, there is a belt that connects directly to the engine. It gets its power the same way that the water pump or alternator does - from the engine. A turbocharger, on the other hand, gets its power from the exhaust stream - the gasses that are forced out of the cylinders by the rising pistons on the exhaust stroke. The exhaust runs through a turbine, which in turn spins the compressor. In a gas turbine, a pressurised gas spins the turbine. In all modern gas turbine engines, the engine produces its own pressurised gas, and it does this by burning something like propane, natural gas, kerosene or jet fuel. The heat that comes from burning the fuel expands air, and the high-speed rush of this hot air spins the turbine.

There are trade offs in both systems. In theory, a turbocharger is more efficient because it uses the "waste" energy in the exhaust stream for its power source, thus recycling the waste energy. On the other hand, a turbocharger causes some amount of back pressure in the exhaust system and tends to provide less boost until the engine is running at higher RPM's. Superchargers are easier to install but tend to be more expensive.

Remember, whatever car you drive, every road in the UK has a speed limit!

I hope you found this discussion of turbo and super charging interesting and informative. Enjoy your driving lessons