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reflexx · 10-12-2006, 12:54 PM

The Aerodynamics of Turbocharged Engines

This article assumes you already understand a little bit about how turbochargers work. I will discuss a few parts of the system, and then how they relate to turbo selection, performance, etc.

Section 1: Components and their roles

Air

The most important part of the system. Clean, dry air can be described using a few measurements, and some common units for them:
- pressure (psi)
- volume (CFM)
- temperature (degrees F)
- density - dependent on the prior 3 variables.

The turbocharger

This piece of equipment consists of a compressor and a turbine connected with a shaft. The turbine is driven by the exhaust gasses from the engine, the compressor is driven by the turbine and increases the pressure of incoming air. The turbocharger, unlike a supercharger which is coupled to the crankshaft via a pulley, is a 'free floating' system and spins at an independent rate, which varies depending on gas conditions at both sides of the apparatus. Since the engine has a finite displacement, the turbocharger's job is to increase pressure while decreasing volume (higher density), so a fixed-displacement engine can burn more air (and fuel). Ultimately, the turbocharger blades are pushing against a column of air that goes all the way to the intake valves, and how hard the turbo has to work (how much pressure is needed) depends partially on how well the intake manifold, head and valves flow.

At air measurement point 1, the air properties are:
- ambient pressure (14.5 psi)
- high volume (~460CFM in our imaginary car)
- ambient temperature

At air measurement point 2, the air properties are:
- increased pressure, depending on turbo speed. Given a speed with a pressure ratio of 2.0, we get 30 psi
- reduced volume (~230CFM in our imaginary car)
- increased temperature, depending on compressor efficiency (more on this later).

The intercooler

The intercooler attempts to cool the air the turbo output back to a manageable temperature. Higher temperature air is less dense, detonates more easily and contains less oxygen molecules per unit of volume. Our goal is maximum density, minimum volume, so the intercooler will reduce the temperature and increase density by a certain amount, ideally while keeping pressure close to the same. Well-made intercoolers only create a pressure drop of 1 or 2 psi at "full flow", i.e. the maximum RPM and boost the engine is designed to run. With the absence of a cooling medium to cool the intercooler, the intercooler functions as a heat sink, and will become less efficient as more heat is absorbed. This is the case when using an air-air intercooler without fans at a standstill, such as doing a burnout at a drag track.

At air measurement point 3, the air properties are:
- hopefully only slightly less pressure than point 2 (29 psi)
- the same volume as point 2 (230 CFM)
- reduced temperature, cooler than point 2, but higher than point 1 (unless using icewater water-air intercooling)

The intake manifold

Here, the air exits the intercooler and collects in the 'surge tank'. The surge tank exists to even out pressure even with a turbocharger which is spinning faster and slower continuously. Here, is where the "boost pressure" is often measured with a MAP. The 'pressure' here is at point 3 on the diagram and represents the difference between the turbocharged air moving at high speed and pressure from the intercooler/turbo, and the backpressure from the engine's head and valves, unable to swallow all the air it is being fed. Ideally, the manifold is as large as possible with minimum corners and bends. Each bend reduces the air velocity and makes it harder to funnel the air into the cylinders. Velocity stacks may help here, just before each runner. Sometimes, the intake manifold may increase the pressure of the air slightly by allowing it to hit a back "wall" of the manifold before turning into the cylinders. This is known as "manifold stuffing", and can create more pressure at the end of the manifold (cyls 3,4). Often, these cylinders will run lean and fail first if a richer mixture is not sprayed into those cylinders.

The cylinder head

This complex component consisting of poppet valves, ducts and cams controls airflow into the head. Usually, this is where the most airflow restriction is found. Making the head flow more freely (by using larger valves, porting, bigger lift cams) will allow more air into the cylinders. This translates to increased CFM and decreased backpressure (less 'boost' psi measured at the intake manifold). This factor is also why an engine and turbo can make more power at "less psi", and why psi PLUS cfm is an accurate measurement whereas just psi is not.

Combustion chamber

Here, as much air as will fill the cylinder (the displacement or volume) enters, at whatever pressure was achieved in the intake manifold and head. When the valves close, the piston moves up towars the head, creating "dynamic pressure". The final combustion pressure is the incoming air pressure (boost pressure) multiplied by the compression ratio. This dynamic compression ratio is what's considered when resistance to knock is calculated. Higher boost with lower static compression and lower boost with higher static compression will knock at a similar octane. This is why turbocharged engines typically run a lower static compression ratio (9:1 or 8.5:1) so that more air/fuel molecules may be burned (more physical air present at the bottom of the stroke) with the same dynamic compression ratio.

At air measurement point 4, the air properties are:
- pressure point 3 multiplied by the static compression ratio (270psi in our hypothetical engine)
- a volume that is cyl displacement/(static compression ratio) or 55 cubic centimeters
- a vastly increased temperature from compressing the air 9 times

Exhaust valves, manifold, wastegate

This component allows the exhaust gas to exit the cylinder, and ducts it to the turbocharger's turbine (hot) side. Of paramount importance here is maintaining exhaust velocity, pressure and temperature. Flow is improved by making the bends as gradual as possible. Pressure is maintained by using properly sized runners (big but not too big). Temperature is improved by making the manifold not too long, and maybe ceramic coated or heat wrapped (provided the manifold is of a strong structural construction to deal with the increased temperatures this brings). The exhaust gas then passes by the wastegate just before entering the turbine. At this point, the wastegate can vent gas to the atmosphere to bypass the blades, keeping the turbocharger speed (and boost pressure) at the desired level. The wastegate must be sized appropriately to bypass enough gas if needbe, or the turbocharger will spin too fast (overboost), damaging the engine.

At air measurement point 4, the air properties are:
- high pressure, from the energy of combustion
- back to a similar volume as before entering the cylinder head (230CFM?)
- a temperature 200-400 degrees cooler than at point 4 after combustion

Turbine exhaust housing and downpipe

The idea here is to channel the exhaust gas onto the turbine blades. If the exhaust housing is small, the exhaust flow has a higher velocity (finger over gardenhose effect) and will spool the turbo faster. However, at the same time, the exhaust housing will flow less overall gas, so at high gas volumes (high rpm), the exhaust housing will limit the power production of the turbocharged system by reducing the turbine blade efficiency as well as create backpressure all the way back to the cylinder valves and push against the piston itself. Some builders will pick a larger overall size turbo with a smaller exhaust housing to flow more air but keep spool manageable. The downpipe and exhaust system post-turbo also contributes to exhaust backpressure. In this case, not only does this backpressure push back on the piston, but also on the turbine blades, slowing down spool. Unlike a normally aspirated car, the less the better is the rule here, even to the point of running just a straight downpipe without an muffler or exhaust (yes, this is probably illegal to do on the street).

Section 2: Technical and Operating Principles

Compressor Efficiency

Compressors of different sizes and designs behave differently given the same gas volume and required pressure ratio. A compressor's efficiency is defined as the ratio of the input temperature to the output temperature when asked to perform at a given pressure ratio (boost). An ideal compressor wouldn't heat the air at all, but all turbo compressors will heat the air to some degree, in the best cases only about 10-15%.

If picking a compressor that is much too large:

- the compressor enters the surge line, the blades stall, potentially causing damage, efficiency plummets, exit temperatures skyrocket.

If picking a compressor that is too small:

- the compressor reaches reduced efficiency at the right side of the graph, outputs very hot air, runs out of the ability to move more air (CFM), potentially causes detonation/knock from the hot exit temperature air if pushed harder, reduced lifespan from spinning faster than designed to meet the desired pressure ratio.

Pressure ratio is measured as (boost + atmospheric)/atmospheric. 10 psi boost is 14.5+10 = 24.5/14.5 = 1.68. Calculate your engine CFM (max RPM * displacement in liters / 56). Multiply by your volumetric efficiency (85-93%, depending on headwork). Determine desired pressure ratio (depending on aftermarket pistons, type of gas you will run [1.75 max for 93 octane], desired power level). Plot the airflow and pressure ratio point. Use this plotting to find a compressor where this point is in the highest or close to highest efficiency island on the map. This compressor will output the coldest/densest air at the desired operating conditions. Plot a few other RPM points other than max RPM. Make sure the compressor is reasonably efficient at 5000, 6000 rpm.

Power envelope

Unlike a normally aspirated car, a turbocharged car's torque curve is not very linear. Because of this, the turbocharged car strives to stay within the highest torque band for as long as possible to take advantage of mechanical gearing. A turbocharged car which begins to produce near-full-torque at 4500RPM with a redline of 6500RPM will be considerably slower and harder to drive than a car which produces near-full-torque at 4700RPM with a redline of 9000RPM, since the second car can stay "in boost" longer. Each gear change despins the turbocharger by a small amount as well as creates time the car is not applying torque to the tires. Therefore, simply having more gears is not the solution so much as optimizing spool, redline and airflow.

Boost lag

Lag is the time difference between going Wide Open Throttle (WOT) at a given RPM, and the turbocharged engine reaching "operating pressure", whatever that setting may be.

What causes boost lag?
The turbocharger, since it is a 'free floating' system, spins at variable rate. Pressure developed by the turbocharger is a function of its rotating speed. The wheel has weight, and the incoming air to pressurize creates a resistance against the blades, which means the turbocharger needs time to spin up to the speed at which it creates the required boost pressure. Furthermore, that pressurized air needs to make it through the intercooler and charge piping into the engine before the necessary horsepower can be produced.

Why is boost lag difficult to measure and calculate?
A free-floating turbocharger may be at a variety of speeds at the time of going WOT. The turbo may be fully spun up just after prior boosting, or may be spinning very slowly with the throttle almost completely closed, or any number of positions in between. Furthermore, after WOT, the engine accelerates, producing more gasses as time goes on, changing the amount of accelerating gas the turbocharger sees. Additionally, the compressor sees an increasing amount of resistance as boost pressure grows, increasing the resistance to acceleration the turbocharger experiences. Because of these factors, simple statements such as "full boost by 4500 rpm" are meaningless and unquantifiable without more information about the state of the system before and after WOT.

How do I reduce boost lag?
1) choose a smaller turbocharger. A smaller turbocharger has less rotating mass and also compresses less air, which means the rotating assembly spins up faster.
2) reduce exhaust restriction. Exhaust restriction decreases the exhaust gas speed, limiting spool speed.
3) be in the right gear at the right time. Don't try to 'make power' or pass on the highway with the engine at 1500 rpm.
4) increase your redline, through valvework, electronics, or otherwise. With more available time to stay "in boost", you can pick a lower gear than otherwise, increasing the exhaust gas volume at that given roadspeed.

10-12-2006, 12:54 PM	#1 (permalink)
reflexx Banned Join Date: Aug 2006 Posts: 539 Thanks: 5 Thanked 0 Times in 0 Posts iTrader Rating: (1/100% )	The Aerodynamics of Turbocharged Engines The Aerodynamics of Turbocharged Engines This article assumes you already understand a little bit about how turbochargers work. I will discuss a few parts of the system, and then how they relate to turbo selection, performance, etc. Section 1: Components and their roles Air The most important part of the system. Clean, dry air can be described using a few measurements, and some common units for them: - pressure (psi) - volume (CFM) - temperature (degrees F) - density - dependent on the prior 3 variables. The turbocharger This piece of equipment consists of a compressor and a turbine connected with a shaft. The turbine is driven by the exhaust gasses from the engine, the compressor is driven by the turbine and increases the pressure of incoming air. The turbocharger, unlike a supercharger which is coupled to the crankshaft via a pulley, is a 'free floating' system and spins at an independent rate, which varies depending on gas conditions at both sides of the apparatus. Since the engine has a finite displacement, the turbocharger's job is to increase pressure while decreasing volume (higher density), so a fixed-displacement engine can burn more air (and fuel). Ultimately, the turbocharger blades are pushing against a column of air that goes all the way to the intake valves, and how hard the turbo has to work (how much pressure is needed) depends partially on how well the intake manifold, head and valves flow. At air measurement point 1, the air properties are: - ambient pressure (14.5 psi) - high volume (~460CFM in our imaginary car) - ambient temperature At air measurement point 2, the air properties are: - increased pressure, depending on turbo speed. Given a speed with a pressure ratio of 2.0, we get 30 psi - reduced volume (~230CFM in our imaginary car) - increased temperature, depending on compressor efficiency (more on this later). The intercooler The intercooler attempts to cool the air the turbo output back to a manageable temperature. Higher temperature air is less dense, detonates more easily and contains less oxygen molecules per unit of volume. Our goal is maximum density, minimum volume, so the intercooler will reduce the temperature and increase density by a certain amount, ideally while keeping pressure close to the same. Well-made intercoolers only create a pressure drop of 1 or 2 psi at "full flow", i.e. the maximum RPM and boost the engine is designed to run. With the absence of a cooling medium to cool the intercooler, the intercooler functions as a heat sink, and will become less efficient as more heat is absorbed. This is the case when using an air-air intercooler without fans at a standstill, such as doing a burnout at a drag track. At air measurement point 3, the air properties are: - hopefully only slightly less pressure than point 2 (29 psi) - the same volume as point 2 (230 CFM) - reduced temperature, cooler than point 2, but higher than point 1 (unless using icewater water-air intercooling) The intake manifold Here, the air exits the intercooler and collects in the 'surge tank'. The surge tank exists to even out pressure even with a turbocharger which is spinning faster and slower continuously. Here, is where the "boost pressure" is often measured with a MAP. The 'pressure' here is at point 3 on the diagram and represents the difference between the turbocharged air moving at high speed and pressure from the intercooler/turbo, and the backpressure from the engine's head and valves, unable to swallow all the air it is being fed. Ideally, the manifold is as large as possible with minimum corners and bends. Each bend reduces the air velocity and makes it harder to funnel the air into the cylinders. Velocity stacks may help here, just before each runner. Sometimes, the intake manifold may increase the pressure of the air slightly by allowing it to hit a back "wall" of the manifold before turning into the cylinders. This is known as "manifold stuffing", and can create more pressure at the end of the manifold (cyls 3,4). Often, these cylinders will run lean and fail first if a richer mixture is not sprayed into those cylinders. The cylinder head This complex component consisting of poppet valves, ducts and cams controls airflow into the head. Usually, this is where the most airflow restriction is found. Making the head flow more freely (by using larger valves, porting, bigger lift cams) will allow more air into the cylinders. This translates to increased CFM and decreased backpressure (less 'boost' psi measured at the intake manifold). This factor is also why an engine and turbo can make more power at "less psi", and why psi PLUS cfm is an accurate measurement whereas just psi is not. Combustion chamber Here, as much air as will fill the cylinder (the displacement or volume) enters, at whatever pressure was achieved in the intake manifold and head. When the valves close, the piston moves up towars the head, creating "dynamic pressure". The final combustion pressure is the incoming air pressure (boost pressure) multiplied by the compression ratio. This dynamic compression ratio is what's considered when resistance to knock is calculated. Higher boost with lower static compression and lower boost with higher static compression will knock at a similar octane. This is why turbocharged engines typically run a lower static compression ratio (9:1 or 8.5:1) so that more air/fuel molecules may be burned (more physical air present at the bottom of the stroke) with the same dynamic compression ratio. At air measurement point 4, the air properties are: - pressure point 3 multiplied by the static compression ratio (270psi in our hypothetical engine) - a volume that is cyl displacement/(static compression ratio) or 55 cubic centimeters - a vastly increased temperature from compressing the air 9 times Exhaust valves, manifold, wastegate This component allows the exhaust gas to exit the cylinder, and ducts it to the turbocharger's turbine (hot) side. Of paramount importance here is maintaining exhaust velocity, pressure and temperature. Flow is improved by making the bends as gradual as possible. Pressure is maintained by using properly sized runners (big but not too big). Temperature is improved by making the manifold not too long, and maybe ceramic coated or heat wrapped (provided the manifold is of a strong structural construction to deal with the increased temperatures this brings). The exhaust gas then passes by the wastegate just before entering the turbine. At this point, the wastegate can vent gas to the atmosphere to bypass the blades, keeping the turbocharger speed (and boost pressure) at the desired level. The wastegate must be sized appropriately to bypass enough gas if needbe, or the turbocharger will spin too fast (overboost), damaging the engine. At air measurement point 4, the air properties are: - high pressure, from the energy of combustion - back to a similar volume as before entering the cylinder head (230CFM?) - a temperature 200-400 degrees cooler than at point 4 after combustion Turbine exhaust housing and downpipe The idea here is to channel the exhaust gas onto the turbine blades. If the exhaust housing is small, the exhaust flow has a higher velocity (finger over gardenhose effect) and will spool the turbo faster. However, at the same time, the exhaust housing will flow less overall gas, so at high gas volumes (high rpm), the exhaust housing will limit the power production of the turbocharged system by reducing the turbine blade efficiency as well as create backpressure all the way back to the cylinder valves and push against the piston itself. Some builders will pick a larger overall size turbo with a smaller exhaust housing to flow more air but keep spool manageable. The downpipe and exhaust system post-turbo also contributes to exhaust backpressure. In this case, not only does this backpressure push back on the piston, but also on the turbine blades, slowing down spool. Unlike a normally aspirated car, the less the better is the rule here, even to the point of running just a straight downpipe without an muffler or exhaust (yes, this is probably illegal to do on the street). Section 2: Technical and Operating Principles Compressor Efficiency Compressors of different sizes and designs behave differently given the same gas volume and required pressure ratio. A compressor's efficiency is defined as the ratio of the input temperature to the output temperature when asked to perform at a given pressure ratio (boost). An ideal compressor wouldn't heat the air at all, but all turbo compressors will heat the air to some degree, in the best cases only about 10-15%. If picking a compressor that is much too large: - the compressor enters the surge line, the blades stall, potentially causing damage, efficiency plummets, exit temperatures skyrocket. If picking a compressor that is too small: - the compressor reaches reduced efficiency at the right side of the graph, outputs very hot air, runs out of the ability to move more air (CFM), potentially causes detonation/knock from the hot exit temperature air if pushed harder, reduced lifespan from spinning faster than designed to meet the desired pressure ratio. Pressure ratio is measured as (boost + atmospheric)/atmospheric. 10 psi boost is 14.5+10 = 24.5/14.5 = 1.68. Calculate your engine CFM (max RPM * displacement in liters / 56). Multiply by your volumetric efficiency (85-93%, depending on headwork). Determine desired pressure ratio (depending on aftermarket pistons, type of gas you will run [1.75 max for 93 octane], desired power level). Plot the airflow and pressure ratio point. Use this plotting to find a compressor where this point is in the highest or close to highest efficiency island on the map. This compressor will output the coldest/densest air at the desired operating conditions. Plot a few other RPM points other than max RPM. Make sure the compressor is reasonably efficient at 5000, 6000 rpm. Power envelope Unlike a normally aspirated car, a turbocharged car's torque curve is not very linear. Because of this, the turbocharged car strives to stay within the highest torque band for as long as possible to take advantage of mechanical gearing. A turbocharged car which begins to produce near-full-torque at 4500RPM with a redline of 6500RPM will be considerably slower and harder to drive than a car which produces near-full-torque at 4700RPM with a redline of 9000RPM, since the second car can stay "in boost" longer. Each gear change despins the turbocharger by a small amount as well as creates time the car is not applying torque to the tires. Therefore, simply having more gears is not the solution so much as optimizing spool, redline and airflow. Boost lag Lag is the time difference between going Wide Open Throttle (WOT) at a given RPM, and the turbocharged engine reaching "operating pressure", whatever that setting may be. What causes boost lag? The turbocharger, since it is a 'free floating' system, spins at variable rate. Pressure developed by the turbocharger is a function of its rotating speed. The wheel has weight, and the incoming air to pressurize creates a resistance against the blades, which means the turbocharger needs time to spin up to the speed at which it creates the required boost pressure. Furthermore, that pressurized air needs to make it through the intercooler and charge piping into the engine before the necessary horsepower can be produced. Why is boost lag difficult to measure and calculate? A free-floating turbocharger may be at a variety of speeds at the time of going WOT. The turbo may be fully spun up just after prior boosting, or may be spinning very slowly with the throttle almost completely closed, or any number of positions in between. Furthermore, after WOT, the engine accelerates, producing more gasses as time goes on, changing the amount of accelerating gas the turbocharger sees. Additionally, the compressor sees an increasing amount of resistance as boost pressure grows, increasing the resistance to acceleration the turbocharger experiences. Because of these factors, simple statements such as "full boost by 4500 rpm" are meaningless and unquantifiable without more information about the state of the system before and after WOT. How do I reduce boost lag? 1) choose a smaller turbocharger. A smaller turbocharger has less rotating mass and also compresses less air, which means the rotating assembly spins up faster. 2) reduce exhaust restriction. Exhaust restriction decreases the exhaust gas speed, limiting spool speed. 3) be in the right gear at the right time. Don't try to 'make power' or pass on the highway with the engine at 1500 rpm. 4) increase your redline, through valvework, electronics, or otherwise. With more available time to stay "in boost", you can pick a lower gear than otherwise, increasing the exhaust gas volume at that given roadspeed.