This page helps to explain how a turbocharger and it's related components work. There is no real difference between the way a turbo operates on a rotary or piston engine so before we start, here is a diagram of a turbocharged piston engine, it easily describes how a turbo charger works and will help to explain the rest of this information.
A turbocharger is used to force air/fuel mixture into an engine at a pressure greater then the natural atmospheric pressure of around 14.5 PSI. When a turbo produces 7 PSI of boost, this means how much extra pressure it applies on top of the natural atmospheric pressure. The way a turbo works is the exhaust coming out of the engine is pushed through a turbine. This turbine is mounted on a shaft, which in turn spins an air compressor. The compressor draws air in and blows it into the inlet manifold, and this produces BOOST. The whole point of forcing the air/fuel mixture into an engine is to allow it to burn more fuel and make more power with the same engine capacity. This can get complicated, as there are several factors that get in the way of efficency gain. For one thing when you compress air (with a turbo) it gets hotter. The problem with hotter air is that it contains less oxygen than cooler air, so there is less oxygen to help burn extra fuel that’s going into the engine. This is why many turbocharged cars use “intercooling” of various types, to cool the pressurised air back down into the engine.
It’s important to consider problem of too much heat inside the engine. Once the hot compressed air goes inside the engine it is then compressed again by the piston in the engine. Therefore by the time the mixture of fuel/air mixture is ignited it is really hot. When it is too hot it could ignite itself before the spark plugs fires (known as pinging). When the engine pings the smooth, well-timed normal ignition mishaps, costing you power and damaging ports to the engine. Damage is caused to the engine, and can therefore lead to blowing the head gasket, this then chain-reacts to more severe cases. Another problem when the engine has an air/fuel mixture which is too lean, that is not enough fuel for the air coming in, which creates too much heat, or having ignition which is not suited to the engine. With private turbo installations these problems are often ignored. The ignition timing required for optimum power and smoothness from a turbocharged engine is in fact totally different to that required for a non-turbo engine. This is because the more efficient the engine is, the less advanced the ignition timing needs to be to get power combustion of the fuel/air mixture. When the engine gets more and more boost, the ignition should happen later and later in each cylinders or rotors combustion cycle. To get best out of the turbocharged engine it needs a balance of boost, mixture richness, charge air temp and ignition timing that allows it to run smoothly and efficiently with the type of fuel you're using. If you use low-octane fuel, it is more susceptible to pinging as it ignites more suddenly and erratically. The higher the octane of your fuel, the more smoothly and progressively it will burn, which helps prevent pinging. This means that with better fuel you can run more boost or more advanced ignition without any engine difficulties, which indeed means more POWER.
Here is another diagram that explains how a turbocharged engine works.
The exhaust wheel is the unpainted steel housing where all the exhaust gas is flowed through on its way through the exhaust system. The wheel of the turbine is spun by the pressure of the exhaust gases which are of extreme speeds, these help the blades in the housing turn a shaft, which is connected to the compressor wheel.
The compressor housing looks like a reverse of the exhaust version. It contains bare cats aluminum on the outside with a centrifugal compressor wheel on the inside. This wheel is shaped differently in turn from the exhaust turbine because whereas the turbine is designed to generate spinning power, the compressor has to pressurise air.
Between the turbine and compressor is a compact housing which the connecting shaft, supported by plain bearings and sealed at both ends. Oil is pumped from the engine into the top pf the turbo. Then a much thicker pipe then allows the oil to drain back into the sump. The pipe needs to be thicker to provide space for the oil coming out of the turbo, as it is super heated and becomes very frothy. With technology now increasing ball-bearing turbos have come on the scene, using a proper ball bearing instead of a plain brass bearing. This free running nature allows the turbo to spin up to boosting RPM up to 40% faster than a conventional turbo.
Here you can see the exhaust housing (dark) and the compressor housing (light).
The purpose of a blow off valve is to release excess pressure from the turbo when driving on boost and changing gears. As the throttle is closed suddenly, the turbo is blowing air against a closed throttle body, this causes back pressure and slows down the turbo, so when you accelerate again the turbo needs to wind up. So the function of the blow-off valve is to vent this excess pressure and allowing the turbo to keep spinning fast. This helps to maintain reasonably full boost in the next gear. Some blow off valves release the excess air into the atmosphere, creating the 'psshhtt' sound while others recirculate the air back into the intake manifolding. There are split feelings on blow off valves, with some people saying they are only needed for high boost situations, and when you release the accelerator, there is less exhaust flow which will slow the turbo down anyway, but it is a matter of personal opinion. They do not add any performance to the engine though.
The wastegate is the most common form of boost control used in turbo charged cars. The function of the waste gate is to limit the speed of the turbine, and in turn limiting the amount of boost the turbo can produce. The way it works is based on the pressure produced by the compressor once you get to the desired boost level, a valve bypasses exhaust gas around the turbine and stops it from spinning any faster. As soon as the boost drops below the chosen level again, the valve will close. The result is that boost rises until you reach full boost and from then on the waste gate should make boost level off for the rest of the rev range.
Intercooling is a means of cooling down the pressurised “charge” air from the turbo, before it goes into the inlet. This is done both to reduce the combustion chamber temperatures, and to improve the oxygen-density of the charge air. The two main types you’ll see in streetcars are “air-to-air” and “water-to-water”. Air-to-air system is more simple and efficient. It simply involves running the charge air through a large heat exchanger (similar to a radiator), so that the heat is transferred to the air flowing across the outside of the heat exchangers core. The best place to mount the intercooler is at the front of the car so there is a better chance of cool air flowing through them. The main disadvantage is that all the piping involved results in a long inlet travel, which can slow throttle response. The water-to-water type solves the long inlet tract problem, as the charge air simply goes through a chamber, which contains a liquid-filled heat resistor. Because the inlet chamber is small it can usually be mounted directly between the turbo and throttle body. The disadvantages of the system include slightly poorer efficiency and the added complexity of needing an electric fluid pump that operates constantly.
There are two types of choices in a carburetor turbo setup: “Suck-through” or “Blow through”. The Suck-through (or draw through) setup involves mounting the carburetor before the turbo inlet (usually in front of the impeller mouth). This means that both fuel and air are drawn into the turbo already mixed and then blown into the inlet manifold. This is by far the simplest way to set up a turbo, as the carburetor doesn’t need to be especially modified and tuning is quite easy. The main disadvantages are that you can’t use any intercooling with such a setup, as it is dangerous to run air/fuel mixture through an intercooler core. The reason for this is that fuel can condense inside the intercooler core and stay there – if you then have an engine backfire the intercooler can explode. As a result water injection is about the only option for cooling the charge air with this setup. This also corresponds to a blow-off valve because instead of just venting pressurised air, it would be releasing a fuel/air mixture which is very dangerous. The Blow-through arrangement, logically enough, means the carburetor is mounted after the turbo compressor, so the turbo only draws in air and then blows it through the carburettor, which adds the fuel. To use a carburetor this way it has to be specially modified so that the jets will still add the right amount of fuel. This means specially sealing the carburetor and pressurizing the fuel bowls to match the turbo boost. The good thing is than an intercooler and also a blow-off valve can be used with such a setup.
Fuel Injection is the best setup for a turbo engine. As the injectors are controlled by a computer you have full control over the fuel delivery and can tune the engine’s fuel/air ratios much more accurately rather than with jets. A turbo charged EFI engine would have an air filter before the turbo, which then blows through a pipe to the throttle body. The throttle body controls how much air goes into the inlet manifold, which is where the injectors add the fuel. The intercooler (if fitted) is mounted after the turbo, but before the throttle body. When adding a turbo to a naturally aspirated EFI engine, more often then not the injectors themselves will have to be upsized to cope with the fuel demands of a turbo charged engine. The smaller injectors will fail to keep up with the engine and cause it to lean out. Likewise the fuel pumps will often need to be upgraded, because its no good having big injectors if the fuel pump can't keep the fuel pressure up to them at high RPM. Rather than designing a new fuel map and changing the factory injectors, an independent injector driver can be used to drive one or more extra injectors. This is cheaper and easier to do, so it is quite popular for cheaper turbo kits and upgrades. However, it does not offer the same degree of fine-tuning as complete fuel system upgrade. For the best drivability and reliability a full after market programmable computer system is definitely the way to go. Using a programmable computer also gives you much more flexibility for making future modifications.
Factory twin turbo systems are invariably designed to make the engine more tractable, rather than more powerful. To understand why this is so, you have to realise that big turbos and small turbos behave quite differently. A small turbo has minimal inertia, so it takes more gas flow and boost. On the other hand a small turbo may not be able to flow boost up to a maximum level when a large engine is revving to its greatest. A larger turbo will have greater further capacity, so although it takes longer to speed up, it wont run out of puff at the top end of an engines rev range. To give an example say we have a 2.0 litre engine with a small turbo like a Garret T2. Such an engine would come “on boost “ at very low engine speeds possibly before below 2000RPM, but due to the limited flow capacity of the turbo, by 4000RPM boost would start to trail off and by 6000RPM you might have less then half your normal boost level. If you change this turbo for something very large like a Garret T4, the engine wouldn’t come on boost until much later – probably around 4000RPM, which may make the car a bit difficult to drive, but the good thing is that by 6000RPM the turbo would still happily provide full boost. Normally you would look for a compromise somewhere in between, like a T28, so you would get boost just before 3000RPM and only trail of very slightly at the top end. Ready to get the best of both turbos, however, some manufacturers have used a set up called “sequential turbo charging”. Put simply, this system uses a small turbo to give boost at low RPM, with an additional larger turbo kicking in at higher RPM. These systems take a lot of development to get working smoothly and are also expensive to do in an aftermarket engine, but factory sequential turbo engines like the 13B-REW rotary in the series 6 RX7 are certainly very impressive. A slightly simpler approach is to use two small turbos instead of one big one. This way both turbos come on boost at relatively low RPM, but because you have twice the flow of one small turbo, boost doesn’t tend to drop off at higher RPM quite as much. For ultimate power potential, however, one big turbo still has more potential due to superior to its flow efficiency. This is why drag cars generally use one huge turbo even if the engine came with a twin turbo setup originally.
Turbo sizes need to be properly calculated to get optimum performance and drivability for a given engine. Generally a hi-flow turbo will use the existing exhaust turbine from a factory turbo, with either a larger compressor wheel in the existing housing, or a whole new compressor assembly grafted on. It is generally the case that the existing turbines on factory turbochargers can support a larger compressor, so this is convenient way to improve the flow capability of your turbocharger while still keeping the original manifolding, oil feeds and etc. This process is relatively cheaper than going for a new turbo and associated parts required for that turbo.
One of the most popular upgrades for a turbocharged engine is using “forged pistons”. Forged Pistons are made using a different process to cast pistons, which strain especially under high temperature and pressure loads. As such they are more suitable for engines running high boost (14-15PSI) and also running a large hit of nitrous oxide. Forged pistons often cost twice the amount of normal performance pistons. Generally when builders change pistons in a turbo engine, the compression ratio will also be reduced. The less compression you have, the more boost you can run without pinging. But the less compression you have the worse your car will perform “off boost”. For a street engine, a compression ratio of 8.0:1 compression will be sluggish off boost.
The O-Ring is a metal ring slightly larger than the bore size, which helps the head gasket get a more positive seal around the edge of the bore (where the head gasket is most likely to fail). A groove is actually cut into the head or block to locate the thin ring, which bites into the head gasket when the head is torqued down.
When fitting a turbo system to an engine that wasn’t turbocharged before, it is critically important to have the ignition advance curve altered to suit a turbocharged engine. Boosted engines need to have their ignition retarded at higher revs, rather than advanced like a naturally aspirated engine – otherwise they will ping extremely.
Turbocharged rotaries are among the best engine setups for a small car, where space can be a problem with engine conversions.. Turbocharging a rotary is the same as turbo charging a piston engine, but there are other factors to consider with this type of engine design. Firstly, rotaries are even more sensitive to pinging than piston engines. When a rotary suffers pinging, the Apex seals often chip, causing scoring of the rotor housings, which cannot be repaired. If severe pinging causes the apex seals to break completely, not only will the engine be destroyed, but the seals will be further pushed out to the exhaust and can destroy the turbo’s exhaust turbine. Apex seals in turbo rotaries are usually steel, because steel is more resistant to pinging than ceramic seals, which shatter easily when the engine starts pinging. But other than that there are not many restrictions, as most rotaries have relatively low compression already so all you need is suitable fuel delivery and ignition. Another modification that is normally done to rotary engines running higher boost is called 'dowelling', this involves strengthening the block using dowels, in an attempt to prevent the block from 'twisting' during high boost.