Turbocharger Basics

A turbocharged internal combustion engine is quite literally a hybrid engine. A turbocharger is essentially a centrifugal jet engine, minus a combustor. An internal combustion engine acts as the combustion source, supplying the turbine with exhaust gases to drive the compressor, as well as allowing for the extraction of power through the crank.

While a turbo charger system can be explained in very simple terms, a lot of people tend to confuse a lot of the basics. There are many similar terms, and even more little nuances that are easy to get mixed up. I’d like to take a moment to help simplify and expand your knowledge.

Kind of like a tank of water with an adjustable valve on top feeding water into the tank and making it fuller, and a valve at the bottom that can be only fully open or fully closed. Someone, (the ECU) is turning the bottom valve open/closed and you don’t know how long it is open or closed, but you can adjust the water level by increasing or decreasing the water flow from the adjustable valve, (pill hole diameter).

See also:

• the intercooler, which cools the air flowing out of the turbocharger.
• lag and spool

How a Turbocharger Works

The process starts in your engine’s combustion chamber. After combustion has completed, the exhaust flows through the head, through the exhaust manifold and at some point, to your turbo’s turbine. The turbine is a simple fan wheel located in the exhaust housing of your turbo. When there is enough exhaust gas flowing through the turbine, it will spin. The turbine wheel is then connected by a shaft to the compressor wheel?. The compressor is a part of the intake system, and the spinning of its wheel? blows air in to your intake manifold.

In a normally aspirated engine, there is a vacuum in the intake manifold, as the movement of the pistons inside the combustion chamber tries to draw air in. As the throttle is opened, allowing more air to be drawn in, the vacuum approaches atmospheric pressure. But we want more. As the compressor blows more air in to the intake manifold, rather than reaching atmospheric pressure and stopping, the pressure will continue to rise. This pressure is called boost. With a pressurized intake manifold, your engine is now able to get more air to the combustion chamber, which means more power.

The Intercooler

From science class in 7th grade, we remember that when a gas is compressed (or pressurized), its temperature increases. 7th grade also told us that higher temperatures mean gases will be less dense, or contain less mass per unit of volume. We want as much mass of oxygen in the combustion chamber as possible, so we want the intake charge to be cool. The intercooler does this for us. It works like a radiator, by first transferring the intake’s thermal energy to itself, and then transferring it to the ambient air (or water in some cases).

There are some caveats to this, though. Many people tend to think the bigger intercooler the better, and in some ways this is true. By having more surface area, a larger intercooler is able to exchange more heat with ambient air. However, an intercooler that is too large will have more volume, which means the compressor will have to pressurize a larger volume of air (imagine inflating a small party balloon, and then inflating a hot air balloon to the same pressure).

A compressor bypass valve (CBV? or BPV) also known as a compressor relief valve is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged or centrifugally supercharged car when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.

This unique sound sometimes comes at a price. On a car with a mass airflow sensor (MAF), doing this confuses the engine control unit (ECU) of the car. The ECU is told it has a specific amount of air in the intake system, and injects fuel accordingly. The amount of air released by the blowoff valve is not taken into consideration and the engine runs rich for a period of time.*

Typically this isn’t a major issue, but sometimes it can lead to hesitation or stalling of the engine when the throttle is closed. This situation worsens with higher boost pressures. Eventually this can foul spark plugs and destroy the catalytic converter (when running rich, not all the fuel is burned which can heat up on and melt the converter).

    * Note that engines using a MAP? (manifold absolute pressure) system are not affected.

    * BOV’s are illegal in most countries now, your car will get defected for it

Blowoff valves are used to prevent compressor surge?. Compressor surge? is a phenomenon that occurs when lifting off the throttle of a turbocharged car (with a non-existent or faulty bypass valve). When the throttle plate on a turbocharged engine running boost closes, high pressure in the intake system has nowhere to go. It is forced to travel back to the turbocharger in the form of a pressure wave. This results in the wheel? rapidly decreasing speed and stalling. The driver will notice a fluttering air sound. In extreme cases the compressor wheel? will stop completely or even go backwards. Compressor surge is very hard on the bearings in the turbocharger and can significantly decrease its lifespan. In addition, the now slower moving compressor wheel takes longer to spool (speed up) when throttle is applied. This is known as turbo lag.

Lag and Spool

Turbo lag and spool are words that very often get confused. They are similar problems both specific to turbocharged cars, and are both affected by many of the same variables, but differentiating between them is important.

The majority of turbochargers feature a wastegate—a valve which allows some of the exhaust gas to be directed around the turbine. This allows the turbo’s shaft to spin at a reduced speed, promoting increased turbo life (among other things). Think of it as a ‘stand by’ mode. Since the turbo isn’t needed during relaxed driving anyway, this effect is harmless…

This method is used for controlling the Wastegate and the Turbo Pre-Control. Both of these actuators are required to operate between simple fully open or fully closed. The actuators need to be moved/controlled to adjust for various operating conditions, i.e. Wastegate only needs to be opened 10% at lower RPMs and 75% at higher RPMs to regulate the boost pressure.

Driver factors are another matter. You basically need to understand how a turbo works and modify your driving style accordingly. To sum it up, don’t get caught with your pants down! If you feel that there may soon be a sudden need for serious thrust, downshift until your engine speed is at least 3000 RPM. This way there will be noticable boost almost as soon as you hit WOT. If you are going up a hill at WOT around, say 1800 RPM and your speed is dropping, you’ll need to downshift just like any other car in the same situation. Remember: turbos need exhaust gas in order to spin. Let them have some when they need it.

What is Duty Cycle Control?

This method is used for controlling the Wastegate and the Turbo Pre-Control. Both of these actuators are required to operate between simple fully open or fully closed. The actuators need to be moved/controlled to adjust for various operating conditions, i.e. Wastegate only needs to be opened 10% at lower RPMs and 75% at higher RPMs to regulate the boost pressure.

So what is this duty-cycle control?

The Wastegate and Turbo Pre-Control actuators are supplied air pressure directly from the Primary Turbo Compressor. Between the Primary Turbo and the actuators there are air restrictors or pills, these restrict the amount of air that can flow into the actuator. With a small opening it will take longer to pressurize the actuator than with a larger opening. Now add onto this a solenoid that can vent some of the pressurized air from the actuator. The ECU can only turn the solenoid On or Off, (no in-between analog stuff here, just digital). However, if the solenoid is turned On/Off quick enough that only some air is vented for intervals of time that are shorter than the time it take the pressurized air to leak in through the pill you can adjust the pressure in the actuator. Thus, a change in the On time versus the Off time, (duty cycle control) will give a different pressure and move the Wastegate or Pre-Control valve some amount.

Kind of like a tank of water with an adjustable valve on top feeding water into the tank and making it fuller, and a valve at the bottom that can be only fully open or fully closed. Someone, (the ECU) is turning the bottom valve open/closed and you don’t know how long it is open or closed, but you can adjust the water level by increasing or decreasing the water flow from the adjustable valve, (pill hole diameter).

Many things will affect the rate of spool in a system. The biggest is probably the size of the turbine. More later…

What is turbo lag (and how do I avoid it)?

The majority of turbochargers feature a wastegate—a valve which allows some of the exhaust gas to be directed around the turbine. This allows the turbo’s shaft to spin at a reduced speed, promoting increased turbo life (among other things). Think of it as a ‘stand by’ mode. Since the turbo isn’t needed during relaxed driving anyway, this effect is harmless…

…until you suddenly want to accelerate. Let’s say that you are loafing along, engine spinning 1500 rpm or so. You instantly floor the throttle. The exhaust gas flows through the turbo and cause it to spool (spin up to speed and create boost). However, at this engine speed there isn’t very much exhaust gas coming out. Worse still, the turbo needs to really get spinning to create a lot of boost. (Some turbos will spin at 150,000 rpm and beyond!) So you, the driver, need to wait for engine revs to raise and create enough exhaust gas flow to spool the turbo. This wait time—the period between hitting the throttle at low engine speed and the creation of appreciable boost—is properly called boost response. Many people incorrectly call it lag, which is really something different. Lag actually refers to how long it takes to spool the turbo when you’re already at a sufficient engine speed to create boost. For example, let’s say your engine can make 12 psi at 4000 RPM. You’re cruising along at a steady road speed, engine spinning 4000 RPM, and now you floor it. How long it takes to achieve your usual 12 psi is your turbo’s lag time. Between the two, slow boost response usually causes the most complaints.

There are two aspects to consider when dealing with boost response: engine factors and driver factors. As far as engine factors go, there are many things which affect turbo lag… although most are directly related to the design of the turbo itself. Turbos can be designed to minimize lag but this usually comes at the expense of top-end flow. In other words, you can barter for instant boost response by giving up gobs of horsepower in the upper third of your RPM range. (Behold the catch-22 in designing one turbo for all uses.)

Driver factors are another matter. You basically need to understand how a turbo works and modify your driving style accordingly. To sum it up, don’t get caught with your pants down! If you feel that there may soon be a sudden need for serious thrust, downshift until your engine speed is at least 3000 RPM. This way there will be noticable boost almost as soon as you hit WOT. If you are going up a hill at WOT around, say 1800 RPM and your speed is dropping, you’ll need to downshift just like any other car in the same situation. Remember: turbos need exhaust gas in order to spin. Let them have some when they need it.

Definitions

A compressor bypass valve (CBV) also known as a compressor relief valve is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged or centrifugally supercharged car when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.

A blowoff valve, (BOV, sometimes hooter valve, dump valve) does basically the same thing, but releases the air to the atmosphere. This creates a very distinctive sound desired by many who own turbocharged sports cars. Some blowoff valves are sold with trumpet shaped exits that amplify the “Psshhhh” sound, these designs are normally marketed towards the keen boy racer. For some owners this is the only reason to fit a BOV. Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere. In the United States, Australia and Europe cars featuring unmuffled blowoff valves are illegal for street use.

Downsides of releasing air to atmosphere

This unique sound sometimes comes at a price. On a car with a mass airflow sensor, doing this confuses the engine control unit (ECU) of the car. The ECU is told it has a specific amount of air in the intake system, and injects fuel accordingly. The amount of air released by the blowoff valve is not taken into consideration and the engine runs rich for a period of time.*

Typically this isn’t a major issue, but sometimes it can lead to hesitation or stalling of the engine when the throttle is closed. This situation worsens with higher boost pressures. Eventually this can foul spark plugs and destroy the catalytic converter (when running rich, not all the fuel is burned which can heat up on and melt the converter).

    * Note that engines using a MAP (manifold absolute pressure) system are not affected.

    * BOV’s are illegal in most countries now, your car will get defected for it

Purpose of Relief and Blow Off Valves

Blowoff valves are used to prevent compressor surge. Compressor surge is a phenomenon that occurs when lifting off the throttle of a turbocharged car (with a non-existent or faulty bypass valve). When the throttle plate on a turbocharged engine running boost closes, high pressure in the intake system has nowhere to go. It is forced to travel back to the turbocharger in the form of a pressure wave. This results in the wheel rapidly decreasing speed and stalling. The driver will notice a fluttering air sound. In extreme cases the compressor wheel will stop completely or even go backwards. Compressor surge is very hard on the bearings in the turbocharger and can significantly decrease its lifespan. In addition, the now slower moving compressor wheel takes longer to spool (speed up) when throttle is applied. This is known as turbo lag.

With the implementation of either a bypass valve or a blowoff valve the pressurized air escapes, allowing the turbo to continue spinning. This allows the turbocharger to have less turbo lag when power is demanded next.

A blow-off-valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, underpressure develops in the intake manifold after the throttle plate and “sucks” the blowoff valve open. The excess pressure from the turbocharger is vented into the atmosphere or recirculated into the intake upstream of the compressor inlet.

Tuning adjustable valves

Most aftermarket valves are adjustable leaving customers curious on how to set them properly for their vehicle. Typically the adjustment lies in the spring preload. Here is how to set it.

You want the spring as soft as possible without leaking boost at peak pressure. If the spring is set too soft then the valve will not close fully resulting in a boost leak and idle problems. If you set it too hard then the valve will not fully open, close too early, and have compressor surge.

Trial and error with an accurate boost gauge is the perfect way to find the right setting for your vehicle….

Allard, Alan. Turbocharging and Supercharging. Cambridge, England: Patrick Stevens Limited, 1982.

Gorla, Rama, and Khan, Aijaz. Turbomachinery Design and Theory. New York, New York: Marcel Dekker, 2003.

Society of Automotive Engineers. Turbochargers and Turbocharged Engines. Warrendale, PA, 1979.

Watson, N, and Janota, N. Turbocharging the Internal Combustion Engine. London, England: Macmillian Press Ltd, 1982.

-Wikipedia

The process starts in your engine’s combustion chamber. After combustion has completed, the exhaust flows through the head, through the exhaust manifold and at some point, to your turbo’s turbine. The turbine is a simple fan wheel located in the exhaust housing of your turbo. When there is enough exhaust gas flowing through the turbine, it will spin. The turbine wheel is then connected by a shaft to the compressor wheel. The compressor is a part of the intake system, and the spinning of its wheel blows air in to your intake manifold.

In a normally aspirated engine, there is a vacuum in the intake manifold, as the movement of the pistons inside the combustion chamber tries to draw air in. As the throttle is opened, allowing more air to be drawn in, the vacuum approaches atmospheric pressure. But we want more. As the compressor blows more air in to the intake manifold, rather than reaching atmospheric pressure and stopping, the pressure will continue to rise. This pressure is called boost. With a pressurized intake manifold, your engine is now able to get more air to the combustion chamber, which means more power.

From science class in 7th grade, we remember that when a gas is compressed (or pressurized), its temperature increases. 7th grade also told us that higher temperatures mean gases will be less dense, or contain less mass per unit of volume. We want as much mass of oxygen in the combustion chamber as possible, so we want the intake charge to be cool. The intercooler does this for us. It works like a radiator, by first transferring the intake’s thermal energy to itself, and then transferring it to the ambient air (or water in some cases).

There are some caveats to this, though. Many people tend to think the bigger intercooler the better, and in some ways this is true. By having more surface area, a larger intercooler is able to exchange more heat with ambient air. However, an intercooler that is too large will have more volume, which means the compressor will have to pressurize a larger volume of air (imagine inflating a small party balloon, and then inflating a hot air balloon to the same pressure).

The Blow-Off Valve

Later..

Lag and Spool

Turbo lag and spool are words that very often get confused. They are similar problems both specific to turbocharged cars, and are both affected by many of the same variables, but differentiating between them is important.

Spool

Most often, when people use the terms spool or lag, they are talking about spool. Spool is, simply, how much exhaust needs to be flowing through the turbine to reach the desired boost level, and is referenced by engine speed. For example, your engine will flow roughly twice as much exhaust at 7000rpm as it does at 3500rpm. A given car may need to be running at 3500rpm before it is flowing enough exhaust to spin the turbine fast enough to pressurize the intake to 12psi.

Many things will affect the rate of spool in a system. The biggest is probably the size of the turbine. More later…

While a turbo charger can be explained in very simple terms, a lot of people tend to confuse a lot of the basics. There are many similar terms, and even more little nuances that are easy to get mixed up. I’d like to take a moment to help simplify and expand your knowledge.

How a Turbocharger Works

As a baseline, we all need to know how a turbo works. I’ll refer to the diagram below (source: Turbo Technics)

In a normally aspirated engine, there is a vacuum in the intake manifold, as the movement of the pistons inside the combustion chamber tries to draw air in. As the throttle is opened, allowing more air to be drawn in, the vacuum approaches atmospheric pressure.

http://www.turbotechnics.com/docs/images/Diagram.GIF

The process starts in your engine’s combustion chamber. After combustion has completed, the exhaust flows through the head, through the exhaust manifold and at some point, to your turbo’s turbine. The turbine is a simple fan wheel located in the exhaust housing of your turbo. When there is enough exhaust gas flowing through the turbine, it will spin. The turbine wheel is then connected by a shaft to the compressor wheel. The compressor is a part of the intake system, and the spinning of its wheel blows air in to your intake manifold.

In a normally aspirated engine, there is a vacuum in the intake manifold, as the movement of the pistons inside the combustion chamber tries to draw air in. As the throttle is opened, allowing more air to be drawn in, the vacuum approaches atmospheric pressure. But we want more. As the compressor blows more air in to the intake manifold, rather than reaching atmospheric pressure and stopping, the pressure will continue to rise. This pressure is called boost. With a pressurized intake manifold, your engine is now able to get more air to the combustion chamber, which means more power.

The Intercooler