Modern car engines strive for the ideal balance between power, efficiency and environmental friendliness. One of the key decisions in this race was the system twin turbocharged, known as biturbo. Unlike a conventional turbine, here engineers use two compressors, which can significantly improve the performance of the power unit.
The operating principle is based on more efficient use of exhaust gas energy. If in a standard turbo engine one large turbocharger can create a delay in response, then a combination of two devices allows you to flexibly control the boost pressure. This is especially true for engines with large cylinder volumes, where a single supercharger is physically insufficient.
In this article we will look in detail at how does a biturbo work, what turbine connection schemes exist and why automakers choose this technology. Understanding these processes will help you better understand the technical characteristics of your car and operate it correctly.
Basic operating principle of the dual charging system
The system is based on using the energy of the exhaust gas flow to rotate the turbine. A compressor is attached to the turbine shaft, which sucks in atmospheric air, compresses it and supplies it to the intake manifold. In the system biturbo this process is optimized through thread splitting or staged compression.
When the engine is running at low speeds, the volume of exhaust gases is small. An ordinary large turbine “sleeps” under such conditions, not creating the required pressure. The twin-turbo system solves the problem of inertia, providing powerful thrust almost from idle. This phenomenon is called absence of turbo lag, which is the main trump card of the technology.
It is important to note that gas flows are controlled by a complex system of valves and dampers. The electronic control unit (ECU) constantly reads data from sensors and regulates opening wastegate (bypass valve). It is the accuracy of the settings that determines how smoothly and efficiently the engine will operate under load.
When warming up the engine on a biturbo system, try to avoid sudden pressure on the gas pedal in the first 3-5 minutes of operation to ensure uniform heating of the turbocharger bearings.
Parallel turbine connection diagram
The parallel circuit (Parallel Turbocharging) is the most common in V-twin engines. In this configuration, each bank of cylinders is served by its own turbine. This allows the size of each compressor to be reduced, which reduces their inertia and improves overall motor response.
The design implies symmetrical operation of the nodes. The exhaust manifolds are separated, and gases from the left and right "head" of the block enter their turbines. The compressed air is then combined into a single intercooler or supplied through two separate heat exchangers. This scheme is often found on powerful V6 and V8 engines from BMW, Audi and Mercedes.
Key features of the parallel circuit:
- 🚀 High performance at high speeds due to the large total volume of pumped air.
- ⚖️ Symmetrical load on the exhaust system, which reduces back pressure in the manifolds.
- 🔧 Compactness separate components, which simplifies the arrangement of wide engines in the engine compartment.
However, such a system has its own nuances. Synchronizing the operation of two independent turbines requires very precise tuning of the ECU. If one of the turbines operates with a delay, this may cause a thrust imbalance and uneven wear of the cylinder-piston group elements.
Sequential circuit: intelligent power distribution
The sequential circuit (Sequential Turbocharging) is more complex and is often used on diesel engines or gasoline engines, where elasticity throughout the entire speed range is important. It uses two turbines of different sizes: one small and one large.
At low speeds, only exhaust gases are driven to a small turbine. It spools up quickly, delivering excellent boost pressure without lag. When the engine speed increases and the volume of gases increases, a second, larger turbine comes into operation. Special valves redirect the flow of gases or connect a second circuit.
Advantages of the sequential system:
- 📉 No traction failures throughout the entire engine speed range.
- 🛡️ Reduced thermal load on exhaust system parts under partial loads.
- 📈 Maximum power at high speeds thanks to the connection of a large compressor.
Technical difficulties of implementation
The sequential circuit requires a highly complex system of pneumatic valves and actuators. Any malfunction in the damper control system can cause the engine to go into emergency mode or lose a significant portion of power.
It is worth noting that the transient moment when the second turbine turns on must be perfectly calibrated. In older implementations of this technology (for example, on some models Mazda RX-7 or BMW M57) drivers felt a slight “kick” when the second stage was connected. Modern systems have learned to hide this moment, making acceleration absolutely linear.
Performance Comparison: Bi-Turbo vs Mono-Turbo
To understand the feasibility of using two turbines, it is necessary to compare their characteristics with the classic single supercharger. The difference lies not only in the maximum power, but also in the nature of its output, as well as in reliability and maintenance costs.
A single large turbo (Big Turbo) is capable of producing huge power at the top, but at low speeds it is inert. The Small Turbo responds quickly, but chokes at high speeds. Biturbo tries to combine the advantages of both options, although it complicates the design.
| Parameter | Mono-turbo (Large) | Biturbo (Parallel) | Biturbo (Sequential) |
|---|---|---|---|
| Low RPM response | Low (Turbojam) | High | Maximum |
| Maximum power | High | Very high | High |
| Design complexity | Low | Average | Very high |
| Maintenance cost | Low | High | Maximum |
When choosing a car, it is worth considering that resource biturbo systems directly depend on the quality of service. Double the number of turbines means double the number of bearings, seals and pipes that can fail. However, with proper operation, such engines can run hundreds of thousands of kilometers.
Intercoolers and biturbo cooling system
Compressing air causes it to heat up, which reduces oxygen density and increases the risk of detonation. Therefore, in dual charging systems, special attention is paid to charge cooling. Intercoolers (air-to-air or liquid coolers) are a critical element.
Parallel systems often use two intercoolers located on the sides of the engine (front-mount or side-mount). This allows you to shorten the air path and reduce pressure losses. Series designs sometimes use two-stage cooling, where air passes through two heat exchangers in series to achieve a minimum temperature.
☑️ Checking the boost system
It is necessary to monitor the condition of pipes and connections. Increased pressure in the system (which can reach 1.5–2.0 bar and higher in tuned versions) creates a huge load on rubber products. The slightest leak will lead to loss of power and incorrect operation of the engine.
Maintenance and common problems
Operating a car with a biturbo system requires discipline from the owner. Turbochargers spin at speeds of up to 200,000 rpm and operate in extreme temperature conditions. Any disregard for the rules can lead to expensive repairs.
⚠️ Attention: Never turn off the engine immediately after vigorous driving at high speeds. Allow the turbines to cool at idle for 1-2 minutes to prevent the oil from coking in the bearings due to residual heat.
Typical problems owners face:
- 💨 Wear of oil seals: leads to oil getting into the intercooler and subsequent oil burn.
- 🔊 Whistle or howl: Indicates damage to the impeller or plain bearings.
- 📉 Insufficient boost: often caused by leaking pipes or a faulty wastegate actuator.
The quality of the engine oil plays a decisive role. Special tolerances have been created for turbocharged engines (for example, ACEA A3/B4 or manufacturers' specifications) that provide thermal stability. Oil changes should be done more often than required, especially if the car is operated in urban mode.
The main enemy of a turbine is dirty oil and overheating. Compliance with oil change intervals and the correct procedure for stopping the engine will extend the life of the biturbo system significantly.
It is also worth mentioning the problem of “oil starvation” during cold starts. In winter, the oil thickens, and until it warms up, the lubrication of the turbine bearings may not be sufficient. Therefore, in winter, the engine warm-up time before driving should be increased.
Development prospects and electrification
Dual supercharging technologies continue to evolve. Modern engineers are introducing variable geometry turbines (VGT) and electric compressors. The electric compressor (e-turbo) is able to instantly create pressure, completely eliminating inertia until the main turbine enters operating mode.
Such systems are already found on modern Mercedes-AMG and Audi. They allow you to take advantage of the advantages of a biturbo without the complex mechanical connection of two turbines. The future is likely to be a combination of traditional turbos and electric superchargers, which will provide ideal response in all conditions.
⚠️ Attention: When purchasing a used car with a twin-turbo, be sure to have the cylinders endoscoped and check the service history. Repairing such a system can cost up to 50% of the cost of the car itself.
In conclusion, the twin-turbo is a compromise between complexity and efficiency. For the driver, this means access to a huge reserve of power and torque, but in return, increased attention to the technical condition of the “iron horse” is required.
Myth about the resource
There is an opinion that biturbo does not last long. In fact, the resource does not depend on the number of turbines, but on the quality of the oil and driving style. Engines with one huge turbine often experience greater thermal loads than two small ones operating in gentle mode.
What is the main difference between parallel and sequential twin turbo?
A parallel twin-turbo uses two identical turbos to increase performance (usually on V-twin engines), while a sequential twin-turbo uses different sized turbos to expand the operating range: a small one works at low speeds, a large one is connected at high speeds.
Is it possible to install a biturbo on a regular naturally aspirated engine?
Theoretically, it is possible, but this requires a complete replacement of the piston group (reducing the compression ratio), installing intercoolers, reflashing the ECU and strengthening the gearbox. Simply installing turbines on a standard engine will lead to its rapid destruction due to detonation.
How often do you need to change the oil in a biturbo engine?
The recommended oil change interval for turbocharged engines is 7,000 - 8,000 km, even if the manufacturer allows 15,000 km. This is due to high temperature loads, which quickly age the oil.
Why does the turbine whistle?
A slight whistle when accelerating is the normal sound of a turbocharger (exhaust gases pass through the nozzle apparatus). However, a loud howl, grinding or whistling sound at idle indicates a bearing failure or system depressurization.