Operation:
The exhaust gases exiting the cylinders are directed from the exhaust manifold to the turbo. The exhaust gas pressure causes the turbine wheel to spin (the red gases). The exhaust gases then exit the turbo via the same turbine wheel to the exhaust. The compressor wheel is driven by a shaft (the blue gases). The compressor wheel draws in air from the side (where the air filter is depicted) and forces it under pressure (via the blue arrow) through the turbo hose to the intercooler. The intercooler cools the compressed air (the engine performs better with cooler air). Subsequently, the air ends up in the intake manifold.
By using a turbo, more air enters the cylinders during the intake stroke compared to a naturally aspirated engine, where air is only drawn in by the downward movement of the piston. By providing more air to the cylinders in this manner and adding more fuel, more power will be available.
The turbo pressure is measured by the boost pressure sensor. Based on the signal this sensor sends to the ECU, the turbo pressure is adjusted.
The turbo is mounted as close as possible after the exhaust manifold. Sometimes, the manifold and turbo are designed as a single unit. The turbo must be mounted as close as possible to the cylinder head to minimize the decrease in exhaust gas speed and pressure loss.
Turbo Lag:
Older turbos often suffer from the infamous turbo lag. The turbo operates on the engine’s exhaust gases. When the gas pedal is suddenly floored, the engine needs a large amount of incoming air from low RPM, but at that moment the turbo is still getting up to speed from the released exhaust gases. The turbo does not provide enough pressure yet. Only when the engine reaches a higher speed does the turbo come into full effect, usually around 2000 RPM, which is felt by harder acceleration.
This turbo lag is seen as a major drawback. That is why many people prefer a mechanical supercharger. It works constantly as it is directly driven by the crankshaft, rotating at the same speed as the engine. A supercharger will deliver pressure from idle immediately when the throttle is applied. Today’s turbos built into cars suffer less from this problem, thanks in part to the variable turbo.
Twin-turbo:
The term ‘twin-turbo’ indicates the presence of two turbos. These 2 turbos can sit next to each other on one bank of cylinders, or one turbo per bank of cylinders. This provides the driver with the advantage of more torque at low speeds, better performance in the high-speed range, and a smoother engine character. At low speeds, the air is supplied by a small turbo to the engine, and at higher speeds, the larger turbo becomes functional. The larger turbo has more turbo lag because it needs more air to get going, but this is compensated by the small turbo.
The following four images describe the situations in which one or both turbos operate. The four circles represent the cylinders, the red and blue sections are the exhaust and intake air. The intercooler is indicated as “I.C.”
Low engine speed and low load:
At speeds below 1800rpm, the volume flow of exhaust gas is small. With this small volume, the use of the small turbo is possible. The valve between the exhaust manifold and the large turbo is closed. The exhaust gas is thus only transferred from the small to the large turbo, thereby already bringing the large turbo up to speed. There is a series connection because both turbos are used.
Medium engine speed and moderate load:
Between 1800 and 3000rpm the valve between the exhaust manifold and the large turbo opens. At this point, both turbos are directly driven by exhaust gases from the engine. Again, there is a series connection because both turbos are used.
High engine speed and high load:
Above 3000rpm, the volume flow of exhaust gases becomes too large for the small turbo. The turbo is turned off to prevent the so-called “choke line” from being crossed (see the compressor characteristic chapter later on the page). The wastegate of the small turbo is opened so that all exhaust gases fed to the turbo are bypassed around the turbo. The exhaust gases then do not reach the compressor wheel.
The large turbo, however, is fully supplied with exhaust gases. The valve remains open, allowing the large turbo to reach high speeds and, consequently, move a lot of intake air to the intake manifold.
Tri-turbo:
Nowadays, “tri-turbo” engines are also being made. These engines are equipped with three turbos to achieve maximum filling degree across all engine speeds. BMW applies the tri-turbo technology, including in the M550d. The two small turbos use variable geometry, making them suitable for both low and high speeds. Depending on the engine speed, the turbo is adjusted for better response. The large turbo uses a wastegate.
Below are two situations described indicating which turbo is operational at what time.
Low engine speed and low load:
Only one of the two small turbos is driven. Due to the size of the turbo, it spools up quickly. The small turbo transfers the exhaust gases to the large turbo, thus already setting the large turbo in motion.
Medium and high engine speed and load:
Both small turbos are driven. The two small turbos drive the large turbo. Thus, maximum boost is achieved at all medium and high speeds.
Twin-scroll turbo:
When multiple exhaust gases converge in the exhaust manifold, interference problems can occur; the pressure waves hinder each other. In a twin-scroll turbo, the exhaust gases are separated in two channels leading into the turbo. The exhaust gases from cylinders 1 and 2 do not combine in the intake manifold but impact the turbine wheel independently. Using a twin-scroll turbo results in faster throttle response and higher efficiency. The image below shows that exhaust gases from cylinders 1 and 4 merge, as do those from 2 and 3.
In a conventional turbo, the exhaust gases encounter each other in the exhaust manifold. This is referred to as “interference.” The image below shows the pressure pulses that originate from one cylinder within the exhaust manifold.
Due to valve overlap (the intake and exhaust valves are both open during the transition from exhaust to intake stroke), negative pressures (lower than atmospheric pressure) also occur. With valve overlap, the exhaust gases help suck fresh air into the combustion chamber and expel residual exhaust gases. Thus, the combustion chamber is supplied with more oxygen, increasing the volumetric efficiency.
When examining the pressures in the exhaust manifold of a four-cylinder engine, significant interference is observed. Each positive pulse is diminished by the negative pressure resulting from valve overlap. This is detrimental to the turbo lag (response time to spool up).
The application of the twin-scroll turbo improves the response time because the exhaust gases from cylinders 1+4 and 2+3 are separated. The pulses, not influenced by negative pulses at that moment, are much stronger. The designer can therefore increase the duration of valve overlap to achieve even higher volumetric efficiency.
Variable Geometry Turbo:
A turbo with a wastegate suffers from turbo lag; only when the engine reaches a certain number of RPMs does the turbo receive enough exhaust gases to operate. A variable geometry turbo lacks a wastegate but features adjustable vanes in the exhaust channel. These vanes can be adjusted by rotating an actuator ring, which is turned by vacuum. The required vacuum is provided based on engine load and speed by a solenoid valve (magnetic valve), controlled by the ECU.
By adjusting the vanes, the airflow can be directed. With a change in airflow, the turbo can still spin faster at low speeds and lower exhaust gas pressures. The position of the vanes restricts the volume of exhaust gas that can flow in. To achieve higher speeds, the vanes are adjusted inward at higher engine speeds. Both at low and high engine speeds, high boost can be obtained. Therefore, the turbo’s operation is optimal over a wide RPM range, as the engine will already receive the same boost at low speeds as at high speeds.
Dump Valve:
The dump valve is also known as a “blow-off valve.” It is mounted on a turbo hose, where the air is directed from the turbo to the engine intake side. When the gas pedal is pressed, the turbo of a passenger car can reach 200,000 rotations per minute. At that speed, maximum boost pressure is reached. If the gas pedal is suddenly released, there is an excess of air pressure on the intake side of the engine, but the throttle is closed.
Without a dump valve, back pressure is created towards the turbo, causing the supplied boost air to rapidly decrease the turbo’s speed. When accelerating again, it takes a long time for the turbo to return to speed. The dump valve prevents this. When the gas is released, it will blow off a certain amount of incoming air. The excess air is then removed from the intake system. The turbo blades are not slowed down and will, therefore, spool up faster when the gas is reapplied. The dump valve closes immediately after the excess air is vented. Contrary to popular belief, a dump valve does not increase power.
The dump valve produces the characteristic blow-off sound when the gas is released during acceleration in a turbocharged vehicle.
Wastegate:
Every turbo without variable vanes is equipped with a wastegate. The wastegate ensures that the pressure in the turbine housing (at the exhaust side) does not become too high. When the turbo is operating and building up pressure, the wastegate is closed. All the air leaving the cylinders during the exhaust stroke is used to drive the turbine wheel, reaching maximum boost.
However, at idle, no turbo pressure is needed. At this point, the wastegate opens. Some of the exhaust gases are diverted to the exhaust; they can flow directly to the exhaust. The wastegate is essentially a valve between the exhaust manifold and the engine’s exhaust; all air flowing through the wastegate does not pass through the turbo. Thus, potentially available energy is not utilized. This explains the term wastegate; “Waste” translates to “loss.”
Also, upon reaching a certain speed, the wastegate opens; during acceleration, the turbo must spool quickly, but as the speed of the turbine—also the compressor wheel—reaches a certain level, it must be held constant. By opening the wastegate at this speed, the excess exhaust gases can be directed to the exhaust. The turbo’s speed can be controlled by adjusting the wastegate’s opening angle. The ECU regulates how much the wastegate is activated based on data from the boost pressure sensor.
Intercooler:
The temperature of the compressed air can become extremely hot (more than 60 degrees Celsius). For better combustion, it is necessary for the air to cool. This is what the intercooler takes care of. The intercooler is a separate component and is therefore described extensively on another page; see the intercooler page.
Compressor Characteristic (Surge & Choke Line)
In designing an engine, the size of the turbo must be considered. Matching the size of the turbo to the engine is known as “matching.” A too-large turbo will exhibit significant ‘turbo lag.’ The turbo will take longer to spool up since the turbine housing is too large for the small amount of exhaust gases. Only at higher engine speeds will the turbo be fast enough to provide high pressure. A too-small turbo will almost eliminate turbo lag. The turbine wheel will spool up quickly with a small amount of exhaust gas. High turbo pressure is obtained even at low speeds. However, at higher speeds, the quantity of exhaust gas is too much for this small turbo. More exhaust gas is present than can fit inside the turbo; in such cases, the wastegate must open earlier to divert much exhaust gas. Waste translates to “loss,” which applies here; the exhaust gases flowing through the wastegate do not contribute to driving the turbo.
The size of the turbo is therefore extremely important in the design of the engine. Every turbo, during design, receives a compressor characteristic. Based on the compressor characteristic, it can be determined if it is suitable for a particular engine. The image below shows an example of a compressor characteristic.
The pressure ratio P2/P1 (on the Y-axis) is the ratio between the turbo’s inlet (P1) and outlet (P2). The pressure behind the turbine wheel is always lower than before it. A dimensionless pressure ratio of 2.0 means the pressure before the turbine wheel is twice as high as after it. The volume flow factor (on the X-axis) is the amount of air flowing through the turbo. The curved, horizontal lines indicate the speed of the turbo shaft.
The image shows the red line as the surge line and the blue line as the choke line. The surge line, also known as the pump limit, is where the speed of the compressor wheel is too low. The surge line marks the limitation of airflow through the too-small compressor wheel. The pressure ratio is too high and the volume flow too low. The air ceases to be drawn in by the compressor, thereby stopping and later resuming speed. This unstable airflow causes pressure fluctuations and pulsations in the intake path. This pulsating is also known as the “surging” of the compressor. Hence the name “surge line.” The back-and-forth moving air causes large forces that can overload the turbo. The compressor wheel blades can break, and the bearings can become overloaded.
The choke line is another limit that the compressor must not exceed. Here, the maximum volumetric flow occurs at a low-pressure ratio. The compressor housing diameter determines the maximum volumetric flow. When surpassing the choke line, the compressor wheel is too small to handle the (larger) volumetric flow. Consequently, much engine power is lost. The choke line is also known as the “overspin choke.”
The image shows the compressor characteristic when the engine is in partial load. During partial load, the engine should have the lowest fuel consumption. The lowest specific fuel consumption is achieved at the smallest island. The wastegate regulates the pressure so that it crosses the middle island. Initially, the wastegate is closed, causing the turbo pressure to rise. The engine management system opens the wastegate, as shown by the green line in the image. The turbo shaft speed is then between 8000 and 9000 revolutions per minute.
When driving in the mountains, a greater geographical altitude is involved, causing the air to be thinner. This impacts the turbo’s operation because thinner air contains less oxygen, reducing the pressure before the compressor. Consequently, the pressure ratio and the compressor speed must increase to reach the final boost pressure. This situation is depicted in the image.
The green line indicates the partial load situation at sea level and the orange line in the mountains. Due to the thinner air, the compressor speed will increase to 100,000 rotations per minute.
Due to the higher compressor speed, the temperature of the intake air fed to the engine will rise. Therefore, the intercooler will have to dissipate more heat. Now the difference in fuel consumption can also be seen; fuel consumption increases in the mountains due to the higher pressure ratio P2/P1 and the higher turbo speed.
Combination of Turbo and Compressor:
Nowadays, car manufacturers increasingly choose to equip engines with both a turbo and a supercharger. The turbo is usually larger and equipped with a wastegate. The supercharger is intended to prevent turbo lag; at low engine speeds, the supercharger provides boost pressure and sets the turbo in motion. At higher speeds, the turbo takes over.
The compressed air travels through the supercharger or bypass valve to the turbo, and from the turbo through the intercooler to the intake manifold.
Click here for more information about the Roots supercharger.
Electronic Turbo:
A conventional turbo suffers from turbo lag at low speeds because exhaust gases are needed to drive the turbine wheel. A supercharger does not have this issue and provides boost pressure from idle. A combination of these seems ideal. However, a mechanical Roots supercharger must be driven by the crankshaft, resulting in energy loss. Automakers are therefore experimenting with multiple exhaust gas turbos or electric turbos to eliminate the turbo lag of the exhaust gas turbo.
The electronic turbo is controlled by the engine control unit. In just 250 milliseconds, the compressor wheel reaches up to 70,000 revolutions per minute. The electric motor in the turbo drives the compressor wheel, rapidly moving the intake air to the exhaust gas turbo’s compressor wheel. The compressor wheel spins very quickly when the electric motor is activated.
With the help of the electronic turbo, the engine has a faster response. At higher speeds, where the exhaust gas turbo can provide full boost, the electronic turbo is disabled.