Saturday, March 29, 2008
Design Considerations:
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 decrease 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. One sure way to reduce the inertia 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.
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. Luckily, there are some tricks used to overcome these challenges.
Most automotive turbochargers have a waste gate, which allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too quickly at high engine speeds. The waste gate is a valve that allows the exhaust to bypass the turbine blades. The waste gate senses the boost pressure. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the waste gate 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 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 boost.
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 a boost of 7 psi, the intercooled system will put in 7 psi of cooler air, which is denser and contains more air molecules than warmer air.
BLOW OFF VALVES:
Turbo charged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and the inlet of the engine. When the throttle is closed compressed air will flow to the throttle valve without an exit (i.e. the air has nowhere to go).
This causes a surge which can raise the pressure of the air to a level which can be destructive to the engine e.g. damage may occur to the throttle plate, induction pipes may burst. The surge will also decompress back across the turbo as this is the only path that the air can take. This sudden flow of air will often cause turbulence and a subsequent whistling noise as the air passes past the compressor wheel.
The reverse flow back across the turbo acts on the compressor wheel and causes the turbine shaft to reduce in speed quicker than it would naturally. When the throttle is opened again, the turbo will have to make up for lost momentum and will take longer to achieve the required speed, as turbo speed is proportional to boost/volume flow (This is known as Turbo Lag). In order to prevent this from happening, a valve is fitted between the turbo and inlet which vents off the excess air pressure. These are known as an anti-surge, bypass, blow-off or dump valve. They are normally operated by engine vacuum.
The primary use of this valve is to prevent damage to the engine by a surge of compressed air and to maintain the turbo spinning at a high speed. The air is usually recycled back into the turbo inlet but can also be vented to the atmosphere. Recycling back into the turbo causes the venting sound to be reduced and can actually help keep the turbo spooled while changing gears. The benefits of venting to the atmosphere are simply the ease of installation (because there is no need to run an extra hose to plumb the charge back into the system). There are no/little performance benefits for venting to the atmosphere, but because a Dump Valve is present the Turbo will slow down naturally rather than forcefully and will shorten the time needed to "spool-up" to counteract any turbo lag.
One way to decrease 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. One sure way to reduce the inertia 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.
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. Luckily, there are some tricks used to overcome these challenges.
Most automotive turbochargers have a waste gate, which allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too quickly at high engine speeds. The waste gate is a valve that allows the exhaust to bypass the turbine blades. The waste gate senses the boost pressure. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the waste gate 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 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 boost.
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 a boost of 7 psi, the intercooled system will put in 7 psi of cooler air, which is denser and contains more air molecules than warmer air.
BLOW OFF VALVES:
Turbo charged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and the inlet of the engine. When the throttle is closed compressed air will flow to the throttle valve without an exit (i.e. the air has nowhere to go).
This causes a surge which can raise the pressure of the air to a level which can be destructive to the engine e.g. damage may occur to the throttle plate, induction pipes may burst. The surge will also decompress back across the turbo as this is the only path that the air can take. This sudden flow of air will often cause turbulence and a subsequent whistling noise as the air passes past the compressor wheel.
The reverse flow back across the turbo acts on the compressor wheel and causes the turbine shaft to reduce in speed quicker than it would naturally. When the throttle is opened again, the turbo will have to make up for lost momentum and will take longer to achieve the required speed, as turbo speed is proportional to boost/volume flow (This is known as Turbo Lag). In order to prevent this from happening, a valve is fitted between the turbo and inlet which vents off the excess air pressure. These are known as an anti-surge, bypass, blow-off or dump valve. They are normally operated by engine vacuum.
The primary use of this valve is to prevent damage to the engine by a surge of compressed air and to maintain the turbo spinning at a high speed. The air is usually recycled back into the turbo inlet but can also be vented to the atmosphere. Recycling back into the turbo causes the venting sound to be reduced and can actually help keep the turbo spooled while changing gears. The benefits of venting to the atmosphere are simply the ease of installation (because there is no need to run an extra hose to plumb the charge back into the system). There are no/little performance benefits for venting to the atmosphere, but because a Dump Valve is present the Turbo will slow down naturally rather than forcefully and will shorten the time needed to "spool-up" to counteract any turbo lag.
INSIDE A TURBOCHARGER
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.
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 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.
There are many tradeoffs involved in designing a turbocharger for an engine. In the next section, we'll look at some of these compromises and see how they affect performance.
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