Twin-scroll turbochargers

Turbocharging is the most commonly used supercharging technology by internal combustion engines (ICE) for forced intake air induction. With turbocharging, the exhaust-gas energy is used to increase the inlet air destiny. Turbocharging is seen as the oldest heat recovery technology used in internal combustion engines.

The name “turbo” is given due to use of a turbine, which is using the thermal and kinetic energy of the exhaust gases to spin the intake air compressor. The exhaust gases will expand into the turbine, which will drive the compressor, which will compress the intake air, increasing its density.

For more details regarding turbocharging technology and turbocharger construction, read the articles:

• How turbocharging works
• How turbochargers work

Exhaust process

Most of the internal combustion engines used in road vehicles need 4 piston strokes for a complete engine cycle. During the exhaust process, the burnt gases are evacuated from the cylinder in order to make room for fresh intake air and fuel, for a new combustion cycle.

For a complete overview of the engine cycles, read the article How an internal combustion engine works.

The complete exhaust process of the burnt gases can be divided into 3 major phases:

• blowdown
• displacement (stroke)
• overlap (scavenging)

Blowdown phase

The exhaust valve is opened before the piston reaches the bottom dead center (BDC). Technically, the exhaust valve is going to be opened at the end of the power stroke, when the piston is pushed by the expanding gases (combustion).

When the exhaust valve opens (around 50° crankshaft angle, before BDC) the cylinder pressure is still high, around 4 bar and the temperature around 700 °C. Due to the massive pressure difference between the cylinder and exhaust manifold, when the exhaust valve opens, the burnt gases will start to flow rapidly into the manifold.

S – piston stroke
V_c – clearance volume
V_d – displaced (swept) volume
p₀ – atmospheric pressure
W – work
TDC – top dead center
BDC – bottom dead center
IV – inlet valve
EV – exhaust valve
IVO – inlet valve opening
IVC – inlet valve closing
EVO – exhaust valve opening
EVC – exhaust valve closing
IGN (INJ) – ignition (injection)

Thus, the main advantage of opening the exhaust valve before BDC is that the exhaust gases will leave the cylinder due to the pressure difference and the piston will have to use less energy to push the remaining gases out of the cylinder (during the exhaust piston stroke).

The blowdown phase will cause a rapid increase of the gas pressure into the exhaust manifold, followed by a rapid decrease due to the equalizing pressures between cylinder and manifold.

Displacement (stroke)

The exhaust stroke takes place when the piston moves from BDC to top dead center (TDC). During this phase, the gas flow is controlled (displaced) by the movement of the piston. In this phase, the gas pressure in the manifold is slightly above atmospheric pressure (for natural aspirated engines) or turbine inlet pressure (for turbocharged engines).

The complete closing of the exhaust valve takes place at around 40° after TDC.

Overlap (scavenging)

Since the intake valve is opened before TDC and the exhaust valve is closed after TDC, there is a small period in which both valves are open (overlap). This phase can last between 20 to 50° of crankshaft rotation, depending on engine.

There is an optimal valve overlap period for which the volumetric efficiency and the mean effective pressure of the engine can be increased. Further, for turbocharged engines with direct injection, an extended overlap period contributes to scavenging. which means that the fresh intake air flows through the cylinder and into the exhaust manifold, expelling all remaining burnt gases from the cylinder. The scavenging effect has several advantages on the engine, the main ones being improved volumetric efficiency and cooling of the cylinder (which allows higher compression ratio, thus higher mean effective pressure).

The exhaust gas pressure (p_eg) peaks during the blowdown phase. The gas pressure travels as a wave across the exhaust manifold. As the waves goes through, it’s causing a pressure drop after the peak, which can be lower than the pressure at the turbine inlet (p_T) (assuming it’s constant).

For example, for a 4 cylinders engine, with the firing order 1-3-4-2, the exhaust-gas pulse for cylinder 1 and 3 overlap. The same happens for every exhaust-gas pulse of two consecutive fired cylinders.

When all the exhaust ports of the cylinders are connected to a common exhaust manifold, there will be pressure interference between cylinders, which will causes overall exhaust gas pressure drop and kinetic energy loss. By separating the overlapping cylinders into separated scrolls (pipes, ducts) the pressure interference can be avoided.

Ideally, in order to maximize the usage of the exhaust-gas pressure and thermal energy in the turbine, there should be no pressure interference in the exhaust manifold.

Impact of turbochargers on engines

The gas energy which is lost in the exhaust (without turbocharging) account for approximately 30 – 40 % of the total energy released through combustion. With turbocharging a part of this energy is recovered and used to compress the intake air.

During the power stroke, when the exhaust valve opens (before BDC), the combustion process will continue to take place also in the exhaust manifold. The burned gases will expand further in the turbine, making it spin and drive the compressor wheel through the turbocharger shaft.

Turbocharging makes use of two types of exhaust gas energy (which would have been wasted in a natural aspirated engine):

• kinetic energy (given by the pressure waves)
• thermal energy (given by the expansion of the gas in the turbine)

The introduction of a turbocharger will also act as a restriction for the flow of the exhaust gas, which will cause the generation of a backpressure in the exhaust manifold. The backpressure will force the piston to consume more energy to displace the burnt gases out of the cylinder.

If the backpressure is too high, there is a risk of a backflow, which meas that the exhaust gases will flow back into the cylinder and intake manifold, decreasing the volumetric efficiency and the overall performance of the engine.

The turbocharger has also a significant impact on the transient response of the engine (acceleration). The power output of an engine is directly dependent on the intake air mass. For a turbocharged engine, to quickly increase the air mass in the cylinders, the turbine needs to accelerate and drive the compressor. The bigger the mass moment of inertia of the turbine+shaft+compressor, the longer the time required for acceleration (turbo-lag).

On the other hand, using a small turbine, which can accelerate faster, will cause problems at higher engine speeds and loads, due to the fact that if will choke the exhaust, unable to absorb high exhaust gas flow. Therefore, the process of matching a turbocharger with an engine is very complex and needs to take into account a lot of factors.

Types of turbochargers

The architecture of the exhaust manifold has a very important role in the performance of the turbocharger, in terms of efficiency and response time (the time taken to spin faster). The exhaust manifold must be designed taking into account the following requirements:

• the interference between the exhaust process of the cylinders needs to be kept at a minimum, ideally without having any pressure interference between the connected cylinders (during the exhaust process)
• the energy of the exhaust gas should reach the turbine with minimum losses
• the deployment of the exhaust gas into the turbine must be done consistently over time, to insure maximum efficiency

From the exhaust gas energy point of view, there are two types of turbocharging systems:

• constant-pressure turbocharging
• pulse turbocharging

Constant-pressure turbochargers are mainly used in diesel engines for passenger vehicles. Having the exhaust ducts for all the cylinders integrated in the same component has the advantage of a compact design which can be easily integrated in any engine application.

Constant-pressure turbochargers are also called single-scroll, because all the exhaust gas flow goes into the turbine through a common (single) duct (scroll).

A constant-pressure turbocharging system has a common pipe/exhaust manifold for all the cylinders. The exhaust ports of each cylinder are connected to a common volume, called a collector. Thus, before reaching the turbine, the exhaust-gas pressure waves from each cylinder interfere with each other and dampens out the pressure peaks. The exhaust gas pressure before the turbine will only have small fluctuations around a constant value.

Because of the integrated design, in a constant-pressure turbocharging system, the number of cylinders of the engine does not play a significant role. For example, from the turbocharging point of view, the behavior of a 4 cylinder turbocharged engine will be the same with the one of a 6 cylinder engine.

Constant-pressure turbochargers are also called single-scroll turbochargers because the use a single common pipe (scroll) to transport the exhaust gas from the cylinders to the turbine.

The advantages of single-scroll (constant-pressure) turbocharging systems are:

• high turbine efficiency, given by the steady flow of exhaust gas
• good performance at high load (high exhaust gas flow)
• simple, easy to manufacture and cost effective exhaust manifold and turbine casing

The disadvantages of single-scroll (constant-pressure) turbocharging systems are:

• lower exhaust gas energy at the turbine inlet
• poor performance at low – medium engine speed and load
• poor performance during transient engine operation (acceleration)

1. compressor
2. compressor housing
3. bearing (central) housing
4. turbine housing (single-scroll)
5. turbine
6. wastegate

How twin-scroll turbochargers work

In a pulse-turbocharged system, depending on the number and firing order of the cylinders, different routing pipes connect the exhaust ports of the cylinders with the turbine. In this case, the pressure interference between cylinders is eliminated and the pressure waves (high peak pulse) travel up to the turbine inlet.

For a 4 cylinders engine, with the firing order 1-3-4-2, the cylinders 1 and 4 have a common exhaust pipe and cylinders 2 and 3 have a second exhaust pipe. Both pipes transport the exhaust gas up to the turbine inlet. Since it uses two pipes for the exhaust gas, the system is called twin-scroll turbocharging.

Twin-scroll turbocharging takes full advantage of pulse energy, which means that the exhaust gas energy available for conversion to useful work in the turbine is bigger.

Compared to a single-scroll (constant-pressure) turbocharger, a twin-scroll (pulse) turbocharger has the following advantages:

• higher turbine inlet energy due to exploitation of pressure waves (pulse energy)
• good performance at low – medium engine speed and load
• good performance during transient engine operation (acceleration)

The disadvantages of twin-scroll (pulse) turbocharging systems are:

• poor efficiency at high engine load and speed
• complex and expensive exhaust manifold and turbine casing

The exhaust streams from the two pairs of cylinders are routed to the turbine via separated spiral-shaped channels (scrolls) of different diameter.

The larger channel (A), which connects the exhaust of the cylinders 2 and 3, directs one exhaust stream to the outer edge of the turbine blades, helping the turbocharger to spin faster.

The smaller channel (B), which connects the exhaust of the cylinders 1 and 4, directs the other exhaust stream to the inner surfaces of the turbine blades, improving the response of the turbocharger during transient operations (engine acceleration).

Twin-scroll technology combines optimal low-end response with excellent top-end power increase.

Credit: BMW

Single-scroll turbochargers are only using the thermal energy of the exhaust gas in order to compress the intake air through the compressor.

Twin-scroll turbochargers are using both thermal and pulse (pressure wave) energy of the exhaust gas in order to obtain mechanical work to drive the intake air compressor.

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