What is a twin turbo engine? What are the various types of turbos? What are the differences between single-stage or dual-stage turbos? Turbo working principles, benefits and disadvantages
Since the beginning of time, turbochargers have been used in power recovery for vehicles. Turbochargers push the engine blocks further by providing additional horsepower through high heat output. Turbo technology is found in various vehicle brands today. It is composed of two parts: the turbine and the compressor. It works in a very simple way. The heat and the kinetic energy in the exhaust gases compress intake air, increasing its density. This then turns the turbine to drive it.
Turbocharging is not necessary as the gas energy lost in exhaust makes up approximately 30%-40% of total combustion energy. Turbocharging allows some of that energy to be recovered and used for compressing the intake air.
Turbocharging can also have a significant impact on the engine’s transient response. The air mass absorbed directly affects the engine’s power output. To increase the volume of air in the cylinders, the compressor must be run faster by the turbine. The turbo will take longer to accelerate if it is large enough (turbine + shaft + compressor mass moment inertia)
To avoid turbolag, a smaller turbine is possible. A small turbine can be used to avoid turbo lag, but it will not absorb the exhaust gases flow at high speeds. Matching the turbo to an engine is a complex process that requires many factors to consider.
The design of the exhaust manifold is crucial in determining the performance of the turbocharger. These are the requirements for exhaust manifold design:
– To keep the interaction between the exhaust and the cylinders as minimal as possible.
– The turbine must receive as much energy from the exhaust gases as possible.
To achieve maximum efficiency, exhaust gas must be synchronized with turbine.
Two types of turbocharger systems are available in terms of exhaust gas energy.
– Single Stage (Constant Pressure) Turbocharger / Single-Scroll Turbocharger
– Dual Stage Turbocharger / Twin-Scroll Turbocharger
Single Stage (Constant Pressure) Turbochargers
In diesel-powered passenger cars, constant pressure turbochargers are used. It is compact in design and can easily be integrated into any engine application. Single-stage turbochargers, also known as fixed pressure turbochargers, are also known. Because all exhaust gas flow flows through the same channel and reaches the turbine.
Because of the integrated design, the number and size of the cylinders does not matter in a constant-pressure turbocharger system. A turbocharged 4-cylinder engine will behave the same way as a 6-cylinder engine.
These systems have the following benefits:
– High turbine efficiency combined with regular exhaust gas flows
– Excellent performance at high revs (high exhaust gases flow).
– Easy to make and affordable exhaust manifolds and turbine housings
These are the disadvantages:
– Reduced exhaust gas energy at the turbine inlet
– Low and medium revs are not very efficient
– Performance at sudden accelerations is poor
Dual Stage Turbochargers (Twin-Scroll Turbochargers)
Different guide pipes connect the exhaust ports of cylinders to the turbocharger in a dual stage turbocharged system. This depends on how many cylinders are being fired and the firing order. This eliminates the pressure interaction between cylinders and allows pressure waves to travel up to turbine inlet.
A 4-cylinder engine with 1-3-4-2 fire order has cylinders 1 through 4. Cylinders 2 and 3 share a common exhaust pipe, while cylinders 3 and 4 have a separate exhaust pipe. Both pipes carry exhaust gas to the turbine intake. Dual-stage turbochargers are two pipes that carry exhaust gas to the turbine inlet.
Twin-stage turbos can convert more exhaust gas energy because they take full advantage vibration/impact energy. This means the turbine can turn more.
The advantages it offers over a single-stage turbo include:
– Pressure waves provide more exhaust gas energy
– High performance at low engine speeds and medium engines
– Performance at sudden accelerations
These are the disadvantages:
– Poor performance at high engine speed
Complex and expensive turbine housing and exhaust manifold
Single-stage turbos only use the thermal energy from exhaust gases to compress air from the compressor. Double-stage turbos combine pressure wave energy with the heat from the exhaust gas.
Parallel Twin Turbos (Parallel Twin Turbos)
Two identical-sized turbochargers are used to pump air into the cylinders using parallel twin turbos. The exhaust gases from the turbos are split equally, but they are recombined at the same inlet before being sent into the cylinders. Every operation on a single turbo is performed between two turbos. This system has the biggest advantage that it reduces lag and increases air intake, compared to a single turbo. This system is used in V-type engines. Instead of using sinuous pipe to deliver air to the cylinders each cylinder bank has its own turbocharger. Combining the two turbines produces enough power to prevent turbo lag.
Sequential Twin Turbos
Two sizes of turbochargers can be used to power these systems. It uses a small turbo with a blade to balance exhaust gas flow at low revs, and a secondary turbocharger for high revs.
A compression valve is located in front of the large turbo. This valve ensures that low-speed exhaust gases are sent to the small turbocharger. As the revs increase, the compression mechanism opens slightly, allowing the exhaust gas to accumulate in the larger turbo. At a certain airflow volume, the valve opens completely. The second turbo is activated, maximising efficiency.
Sequential turbochargers almost eliminate all the drawbacks of single turbochargers. It is also more efficient and superior to parallel connected turbos.
Stepped Twin Turbo
They are based on the same principles as the sequential type turbocharger. It uses a “stepped” process to produce high levels air compression and sends it the cylinders. It uses a small turbo to start. The compressed air is then transferred into a larger turbo. Cascade systems can have a higher air pressure than regular twin-turbos. The reaction times however are slower. It is therefore used in diesel engines with high compression ratios, low rpm ranges, and high compression.
