How does a 4-cylinder twin-scroll single turbocharger work?

The 4-Cylinder Twin-Scroll Single Turbocharger is a turbocharging system that combines twin-scroll technology with a single-turbine design to improve 

the engine's responsiveness and efficiency. Here's a detailed step-by-step explanation of how it works: 1. Basic Structure: Turbocharger Components

• Turbine: Driven by the engine exhaust.

• Compressor: Driven by the turbine, compressed air is sent to the cylinder.

• Twin-Scroll: The turbine inlet is divided into two independent scrolls, which are connected to different cylinder exhaust manifolds.

2. Exhaust Characteristics of Four-Cylinder Engines

• Firing Order: Four-cylinder engines are usually fired in a 1-3-4-2 order, with the exhaust of adjacent cylinders spaced 180° apart.

• Exhaust Pulse Interference: If all cylinder exhaust enters the same pipe, the exhaust pressure waves of adjacent cylinders will interfere with each other, 

resulting in reduced turbine efficiency (called "exhaust pulse interference").

3. Flow splitting design of twin-scroll

• Grouped cylinder connection: two scrolls connect two groups of cylinders respectively (for example: cylinder 1+4 is one group, cylinder 2+3 is another group).

• Isolate exhaust pulses: the exhaust of each group of cylinders enters an independent scroll to avoid exhaust pressure waves canceling each other out, 

ensuring that exhaust energy is efficiently transferred to the turbine.

4. Efficient use of exhaust energy • Pulse Turbocharging effect:

◦ The twin-scroll separates the exhaust pulses with large intervals (such as the exhaust interval of a four-cylinder engine is 180°), 

allowing the turbine to continuously receive high-kinetic energy exhaust gas.

◦ Compared with a single scroll, a twin scroll can also maintain a higher exhaust speed at low speeds, reducing turbo lag.

5. Advantages of a single turbine

• Simplified structure: a single turbine is lighter and less expensive than a twin-turbo system.

• Wide speed coverage: through twin-scroll optimization, a single turbine can achieve fast response at low speeds while maintaining large flow rate boost at high speeds.

6. Workflow example (taking a four-cylinder engine as an example)

1. Exhaust stage:

◦ Cylinder 1 completes work, and the high-temperature exhaust gas rushes to the turbine through the first scroll.

◦ After 180° crankshaft rotation, cylinder 4 exhausts and drives the turbine through the same scroll.

◦ The exhaust of cylinders 2 and 3 alternately drives the other side of the turbine through the second scroll.

2. Turbine drive:

◦ Two sets of alternating exhaust pulses make the turbine speed increase faster, especially at low speeds, significantly reducing hysteresis.

3. Intake boost:

◦ The turbine drives the compressor to send compressed air into the intercooler, and then inputs it into the cylinder after cooling, improving combustion efficiency.

7. Performance advantages

• Low-speed torque improvement: The dual-scroll design allows the turbine to work efficiently at 1500-3000 RPM of the engine, enhancing low torque.

• Reduced turbo lag: Exhaust pulse isolation makes the turbo response speed close to that of a supercharger.

• Fuel economy: improve combustion efficiency while meeting emission regulations (such as reducing rich combustion before turbo intervention).

8. Typical application scenarios

• Household performance cars: such as BMW N20 engine, Volkswagen EA888 Gen3, etc., balance power and cost.

• Balance economy and sportiness: suitable for models that require linear power output and value fuel efficiency.

Summary The four-cylinder twin-scroll single turbocharger achieves a response speed close to that of a twin turbo in a single turbo structure 

by diverting exhaust pulses and optimizing exhaust gas energy utilization, while taking into account lightweight and low cost. 

This design is a classic solution for balancing performance and practicality in modern turbocharging technology.