kerosene
11-08-2004, 02:14 AM
I though we should write a general FAQ type of document about charging.
This would be a document which we should develope (and fix errors) and once satisfactory stick it on top of the forum to kill the repetitive over simple questions. I am no pro so feel free to correct any mistakes. Also I am not native english speaker so if some parts make no sense let me know.
Thanks,
Heikki
EDIT:
Was reading other posts and found pretty good link which seems to have most of the info with pics.
http://www.cse.uconn.edu/~yelevich/turbo/turbo.html
Forced induction general info.
Why to turbocharge or supercharge
With turbocharging or supercharging the intake air is pressurized - thus more air and fuel can go through same displacement engine on same RPM. Basically the amount of fuel you can burn equals to the amount of horsepower you can get out of an engine. To burn fuel efficiently the air to fuel ratio (AFR from now on) has to be right.
Supercharging refers to mechanically connected chargers (gear or belt) also known as blowers. There are several different kind of chargers used. The advantage of supercharging is that the pressure rise in manifold is linear to the rpm and pretty much instant - you do not get the famous turbo lag. On the other hand the blower takes power from crankshaft and that power gets "wasted".
Turbocharger is exhaust driven cahrger. It consists of turbine and compressior side. The hot exhaust leaving the engine is made to pass through the turbine side making it spin. On the same axis the compressior wheel sucks in air and compresses it for the intake. Turbos have better efficiency as most of the energy used to spin is "waste" energy. There are couple of minuses for turbos though. First the turbo doesn't compress equal amount of air in the whole RPM range (of the turbo, not the engine) also there is a lag in accelerating the turbo. This and the time it takes to pressurize the system account for the turbo lag. Turbo maps are used to define their optimal working range.
Limitations to charging and engine requirements
The power of an engine is defined not only by the amount of fuel air mixture going through an engine but also the compression there is in the cylinder when ignition takes place. The higher the compression the more you get out of your engine. The compression is limited by the octane of fuel. On normally aspirated engine the compression is defined by how freely flowing the intake system is and what is the compression ratio. Compression ratio defines the volume in the chamber when piston is in the top dead center (TDC from now on) in relation to the volume of the whole cylinder and chamber when the piston is in the lowest position.
On normally aspirated engine and pump gas you can't really go much higher than 10:1 or your fuel starts to ignite itself (due to the pressure) on the way up before TDC. There are a couple of things that affect the knocking limit - usually aluminum heads can take little bit more compression, richer fueal doesn't knock as easily.
In charged engine the mixture reaches similar pressures even with lower compression ratios as the intake air is already compressed and thus knocking can occur. So charged engines need to have lower compression ratio compared to similar normally aspirated engine. How low should you go is a discussion of its own. It depends how much boost you are planning to use and how much other tricks you are willing to do (some of which start to take away the general gaing of HP) like spark retard or. If you go around 8.5:1 you can have pretty high boosts with no problems and still use pretty optimal timing.
Another thing that turbo engines (well any engine really) don't like is lean mixture. In charged engine the fuel burns with higher temperature and if the mixture goes lean (too much air in relation to the fuel) the temperatures climb fast. I believe the most common way to break your charged engine is run it lean and melt your pistons. Because of the higher temps the charged engines often use forged pistons instead of cast ones. Forged pistons have tighter structure and distribute the heat from the piston top much faster. Genrally charged engines are kept little bit rich all the time to keep the burning temps down.
In general engine is easy to brake with over boost as you can create a lot of hp pretty easily. The part where it gets tricky is to make the engine last. Weather turbocharged or normally aspirated a performance engine has to be built so that it can take all the extra strain. Stronger cranks, better main caps etc. what ever it takes.
Intercooling - what is that
Engines like cool air. The cooler the air is the more given volume of air has molecules (and thus oxygen) . More air means more fuel means more power. Allways try to feed cool air to your engine.
When any gas is squeezed its temperature rises - on the other hand when gas is depressurized it cools down - you might have seen a barbeque gas canister that gets frosted even on a warm day if it looses lots of pressure). Some people think that the intake air in turbo is heated because the turbo is hot - no the real reason is the physics fact the compressing gas releases heat.
Compressed air can reach temperatures over 200F/100C. This is not good. Thats why high performance engines use some kind of cooler to cool down the pressurized intake air. Most common type is air to air inter cooler (IC from now on). It is basically a radiator type of structure in which the compressed air tarvels and cool air cools the fins and thus the hot air.
Good Intercooler has low flow restriction which results in low pressure drop.
Also IC should have as big as possible surface area to emit the heat and it should be given a good flow of cooling air - let that be scoops or fans.
Any higher performance application should use IC.
Typically the factory intercoolers (in factory turbo cars) are really small and restrictive.
Components of Turbo system.
Turbo charger
Selecting turbo charger is usually trade off between low/mid range power and high rpm power. With relatively big turbos (obviously in relation to the engine size) one can usually get the highest horsepower. On the other hand on the lower RPM the charger doesn't create full boost and accelerating the charger is slower - more lag. If you are driving land speed record vehichle big is the way to go. On street or road racing you need more lower rpm power and throttle response.
Smaller turbos are usually set to reach the maximum boost on relatively low rpm and after that the extra boost potential of the exhaust is leaked by the charger through a waste gate.
Choosing right size and kind of turbos is critical - use professional help for this.
Exhaust manifold
Exhaust manifold of turbocahrged engine hast to cope with much higher strains than normally aspirated. The temperature is much higher as the exhaust gasses are under pressure. Also the actual charger is often attached staright to the manifold so it has to be strong enough to support the weight of the charger even in rough bumps on the road.
If custom building a manifold use something like 3/8" or 8-10 mm flanges and thick tubing. 3-4 mm wall.
For highest performance the tubes of the manifold for each cylinder should be of equal length and as short as possible to keep the turbo lag low. As any part of engine with flowing gasses the shape should be as smooth and as freely flowing as possible.
Also if external waste gate (further more) is used it should be connected to the manifold in a way that flow either to the turbo or waste gate is smoot. Think of Y connection instead of T connection.
Pictures of pretty sweet exhaust manifold by finnsih company proturbo (http://www.proturbo.fi)
BMW 6 cyl:
http://www.proturbo.fi/pakosarjat/bmw%20e30%206%20turbomanifold.gif
Nissan 200SX 4 cyl:
http://www.proturbo.fi/pakosarjat/nissan-s13-rstturbomanifold.gif
Usually the practical space under the hood limits a lot what you can actually do. Still stock turbo manifold are often very restrictive.
Waste gate
For boost control (avoiding too high boost) waste gate is most common solution. Waste gate is attached to the exhaust manifold before turbo and it has a line to intake manifold. It is a spring loaded valve that opens when pressure from intake manifold is high enought to push it open. When opened some of the exhaust gasses are allowed to bypass the turbo and this way limiting the boost.
In street legal car the bypassing exhaust has to be redirected back to the exhaust system and in emission controlled vehicle before the cat. Generally the reconnection should be smooth and as far from the turbo as practically possible so that it wouldn't interrupt the flow of the gasses exiting the turbo.
Many chargers have internal waste gate so that it is part of the charger. Usually they compromise the flow characterstics of the charger a little bit.
Blow off Valve (BOV)
When throttle is opened, flow through the engine rises, boost and turbo rpm get higher. Now if the throttle is released suddenly turbo still has very high speed but there is no place for the air to go in the intake manifold. This can cause damage to the compressior as it is forced to slow down quickly. Weather it hurts the turbo or not at least it slows it down which means it has to be spooled up again for full boost. For this reason a blow off valve is used. If there is a sudden pressure rise in the intake system the BOV lets the pressure out. This creates the hissing Tssccchhhhhu sound of turbo engine if it is revved. One important point has to be made. If you have a system where fuel gets mixed to air before BOV the mixture sparying out is extremely flammable and it has to be directed back to the system in a safe way.
Oil system
Turbochargers spin often over 100 000 rpm so cooling and lubricating it is essential. Turbo gets pressurized oil from oil pump and after doing its job it flows back to the system with gravity. The return lines have to be without kinks and curves and have as little restriction as possible. If the oil flow in turbo is too slow it starts to over heat and it burns and sticks to the surfaces. You do not what to burn your bearings.
Later more updates.
Please comment in replys and I'll make corrections/addition where needed.
Cheers,
Heikki
This would be a document which we should develope (and fix errors) and once satisfactory stick it on top of the forum to kill the repetitive over simple questions. I am no pro so feel free to correct any mistakes. Also I am not native english speaker so if some parts make no sense let me know.
Thanks,
Heikki
EDIT:
Was reading other posts and found pretty good link which seems to have most of the info with pics.
http://www.cse.uconn.edu/~yelevich/turbo/turbo.html
Forced induction general info.
Why to turbocharge or supercharge
With turbocharging or supercharging the intake air is pressurized - thus more air and fuel can go through same displacement engine on same RPM. Basically the amount of fuel you can burn equals to the amount of horsepower you can get out of an engine. To burn fuel efficiently the air to fuel ratio (AFR from now on) has to be right.
Supercharging refers to mechanically connected chargers (gear or belt) also known as blowers. There are several different kind of chargers used. The advantage of supercharging is that the pressure rise in manifold is linear to the rpm and pretty much instant - you do not get the famous turbo lag. On the other hand the blower takes power from crankshaft and that power gets "wasted".
Turbocharger is exhaust driven cahrger. It consists of turbine and compressior side. The hot exhaust leaving the engine is made to pass through the turbine side making it spin. On the same axis the compressior wheel sucks in air and compresses it for the intake. Turbos have better efficiency as most of the energy used to spin is "waste" energy. There are couple of minuses for turbos though. First the turbo doesn't compress equal amount of air in the whole RPM range (of the turbo, not the engine) also there is a lag in accelerating the turbo. This and the time it takes to pressurize the system account for the turbo lag. Turbo maps are used to define their optimal working range.
Limitations to charging and engine requirements
The power of an engine is defined not only by the amount of fuel air mixture going through an engine but also the compression there is in the cylinder when ignition takes place. The higher the compression the more you get out of your engine. The compression is limited by the octane of fuel. On normally aspirated engine the compression is defined by how freely flowing the intake system is and what is the compression ratio. Compression ratio defines the volume in the chamber when piston is in the top dead center (TDC from now on) in relation to the volume of the whole cylinder and chamber when the piston is in the lowest position.
On normally aspirated engine and pump gas you can't really go much higher than 10:1 or your fuel starts to ignite itself (due to the pressure) on the way up before TDC. There are a couple of things that affect the knocking limit - usually aluminum heads can take little bit more compression, richer fueal doesn't knock as easily.
In charged engine the mixture reaches similar pressures even with lower compression ratios as the intake air is already compressed and thus knocking can occur. So charged engines need to have lower compression ratio compared to similar normally aspirated engine. How low should you go is a discussion of its own. It depends how much boost you are planning to use and how much other tricks you are willing to do (some of which start to take away the general gaing of HP) like spark retard or. If you go around 8.5:1 you can have pretty high boosts with no problems and still use pretty optimal timing.
Another thing that turbo engines (well any engine really) don't like is lean mixture. In charged engine the fuel burns with higher temperature and if the mixture goes lean (too much air in relation to the fuel) the temperatures climb fast. I believe the most common way to break your charged engine is run it lean and melt your pistons. Because of the higher temps the charged engines often use forged pistons instead of cast ones. Forged pistons have tighter structure and distribute the heat from the piston top much faster. Genrally charged engines are kept little bit rich all the time to keep the burning temps down.
In general engine is easy to brake with over boost as you can create a lot of hp pretty easily. The part where it gets tricky is to make the engine last. Weather turbocharged or normally aspirated a performance engine has to be built so that it can take all the extra strain. Stronger cranks, better main caps etc. what ever it takes.
Intercooling - what is that
Engines like cool air. The cooler the air is the more given volume of air has molecules (and thus oxygen) . More air means more fuel means more power. Allways try to feed cool air to your engine.
When any gas is squeezed its temperature rises - on the other hand when gas is depressurized it cools down - you might have seen a barbeque gas canister that gets frosted even on a warm day if it looses lots of pressure). Some people think that the intake air in turbo is heated because the turbo is hot - no the real reason is the physics fact the compressing gas releases heat.
Compressed air can reach temperatures over 200F/100C. This is not good. Thats why high performance engines use some kind of cooler to cool down the pressurized intake air. Most common type is air to air inter cooler (IC from now on). It is basically a radiator type of structure in which the compressed air tarvels and cool air cools the fins and thus the hot air.
Good Intercooler has low flow restriction which results in low pressure drop.
Also IC should have as big as possible surface area to emit the heat and it should be given a good flow of cooling air - let that be scoops or fans.
Any higher performance application should use IC.
Typically the factory intercoolers (in factory turbo cars) are really small and restrictive.
Components of Turbo system.
Turbo charger
Selecting turbo charger is usually trade off between low/mid range power and high rpm power. With relatively big turbos (obviously in relation to the engine size) one can usually get the highest horsepower. On the other hand on the lower RPM the charger doesn't create full boost and accelerating the charger is slower - more lag. If you are driving land speed record vehichle big is the way to go. On street or road racing you need more lower rpm power and throttle response.
Smaller turbos are usually set to reach the maximum boost on relatively low rpm and after that the extra boost potential of the exhaust is leaked by the charger through a waste gate.
Choosing right size and kind of turbos is critical - use professional help for this.
Exhaust manifold
Exhaust manifold of turbocahrged engine hast to cope with much higher strains than normally aspirated. The temperature is much higher as the exhaust gasses are under pressure. Also the actual charger is often attached staright to the manifold so it has to be strong enough to support the weight of the charger even in rough bumps on the road.
If custom building a manifold use something like 3/8" or 8-10 mm flanges and thick tubing. 3-4 mm wall.
For highest performance the tubes of the manifold for each cylinder should be of equal length and as short as possible to keep the turbo lag low. As any part of engine with flowing gasses the shape should be as smooth and as freely flowing as possible.
Also if external waste gate (further more) is used it should be connected to the manifold in a way that flow either to the turbo or waste gate is smoot. Think of Y connection instead of T connection.
Pictures of pretty sweet exhaust manifold by finnsih company proturbo (http://www.proturbo.fi)
BMW 6 cyl:
http://www.proturbo.fi/pakosarjat/bmw%20e30%206%20turbomanifold.gif
Nissan 200SX 4 cyl:
http://www.proturbo.fi/pakosarjat/nissan-s13-rstturbomanifold.gif
Usually the practical space under the hood limits a lot what you can actually do. Still stock turbo manifold are often very restrictive.
Waste gate
For boost control (avoiding too high boost) waste gate is most common solution. Waste gate is attached to the exhaust manifold before turbo and it has a line to intake manifold. It is a spring loaded valve that opens when pressure from intake manifold is high enought to push it open. When opened some of the exhaust gasses are allowed to bypass the turbo and this way limiting the boost.
In street legal car the bypassing exhaust has to be redirected back to the exhaust system and in emission controlled vehicle before the cat. Generally the reconnection should be smooth and as far from the turbo as practically possible so that it wouldn't interrupt the flow of the gasses exiting the turbo.
Many chargers have internal waste gate so that it is part of the charger. Usually they compromise the flow characterstics of the charger a little bit.
Blow off Valve (BOV)
When throttle is opened, flow through the engine rises, boost and turbo rpm get higher. Now if the throttle is released suddenly turbo still has very high speed but there is no place for the air to go in the intake manifold. This can cause damage to the compressior as it is forced to slow down quickly. Weather it hurts the turbo or not at least it slows it down which means it has to be spooled up again for full boost. For this reason a blow off valve is used. If there is a sudden pressure rise in the intake system the BOV lets the pressure out. This creates the hissing Tssccchhhhhu sound of turbo engine if it is revved. One important point has to be made. If you have a system where fuel gets mixed to air before BOV the mixture sparying out is extremely flammable and it has to be directed back to the system in a safe way.
Oil system
Turbochargers spin often over 100 000 rpm so cooling and lubricating it is essential. Turbo gets pressurized oil from oil pump and after doing its job it flows back to the system with gravity. The return lines have to be without kinks and curves and have as little restriction as possible. If the oil flow in turbo is too slow it starts to over heat and it burns and sticks to the surfaces. You do not what to burn your bearings.
Later more updates.
Please comment in replys and I'll make corrections/addition where needed.
Cheers,
Heikki