A turbocharger does the same operation as a regular supercharger, with a slight caveat. A turbocharger is powered by the engine’s exhaust gases, not directly by the engine via a belt or chain. Consequently, we can utilize those fumes’ energy to develop more power, and not just expunge them into the atmosphere. Similar to the superchargers, turbochargers also come in a variety of nuances.
6 Types of Turbochargers
Without further ado, here are the six types of turbochargers available on the market.
1. Singular turbocharger
The working principle of a turbocharger is simple and effective. When exhaust gases are expelled out of the engine, they pass by a component known as a turbine. This turbine is a complex-looking fan built specifically to spin as fast as it can when pushed by fluid, in this case, exhaust gases. After these gases hit the turbine, they slow down and are free to leave out of the tailpipe.
This turbine is built into a shaft that has an impeller at the opposite end. The impeller does the opposite operation to the turbine. This impeller is linked to the engine’s intake. Consequently, when air reaches this impeller, it quickly shoots it out and inside the engine at greater speed, hence increasing airflow and pressure. By doing so, right at the front of the impeller, a low-pressure area will form that the outside air will want to fill right up, allowing this compressor to gather as much air as it can until it reaches an equilibrium. The result is that more air enters the engine!
Such a simple mechanism, however, has issues. The main fault is how this turbocharger’s output peaks at a certain RPM, and anything over isn’t as effective. Moreover, balancing such a turbocharger is difficult, because designing it for low RPMs avoids turbo-lag but makes the engine feel gutless at higher RPMs. On the flip side, developing the turbocharger to maintain adequate performance at redline translates into copious amounts of turbo-lag, along with making the car unresponsive and borderline undrivable.
Despite this, a singular turbo is a preferred alternative for power, efficiency, all while keeping the costs down. Therefore, this is why naturally aspirated engines are rare nowadays, especially knowing that an improvement in efficiency also translates into a more environmentally friendly engine (1).
2. Twin Turbocharger
As the name indicates, instead of having one simple turbo, you mount two. However, it isn’t as simple as mounting two of them. You can mount these turbos in specific ways according to the engine layout and the manufacturer’s desired range of usage. Apart from this, any other name apart from “twin-turbo” is marketing.
You can mount these turbochargers one after another, one for low RPMs and another for high RPMs. How each manufacturer sets up this system differs widely. In one case, a low RPM turbocharger could work until a certain threshold is met and start sending exhaust gases to the second turbo. At higher RPMs, a valve could close completely and allow the engine to work with only the bigger turbocharger. This is called a two-stage serial sequential system, utilized mainly in twin-turbo diesel engines.
You could have a similar system, but always have both turbochargers running when at high RPMs. This one is called a serial-parallel system, employed again in diesel engines. You could have a turbocharger on each cylinder bank in a V-type engine and have each one work independently of one another.
Variants are endless, and each manufacturer can fine-tune them to get the desired effect out of the turbochargers. Generally, such a system reduces turbo-lag by either employing a low RPM turbo or by mounting two smaller turbochargers instead of one that’s substantially bigger. Moreover, two turbochargers allow more air to get into the engine, translating into more power, regardless of how the system is set up. The renowned Toyota Supra Mk. IV with the famous 2 JZ-GTE employed a sequential twin-turbocharged system (2).
3. Twin-scroll turbocharger
A twin-scroll turbocharger is similar to a singular turbocharger. There’s still a singular turbo mounted; however, there’s a substantial difference regarding the turbine’s case, called a volute. The chamber is split into two seemingly two distinct pipes, each for a pair of cylinders chosen specifically.
To keep things simple, let’s assume we are mounting such a turbo on a regular in-line four with a firing order of 1-3-4-2. The twin-scroll turbocharger has two different pipes, one for cylinder one and cylinder four (pipe A), another for cylinder two and cylinder three (pipe B).
If we’re following the firing order, we will observe that each firing cycle will alternate the pipe, sending an exhaust pulse first on pipe A, then pipe B, then A again, and so forth. Thanks to this design, we can better harness the exhaust gases speed, therefore allowing the compressor to suck in more air, translating into more power and better engine response.
The biggest issue with this design is the complexity of the exhaust manifold and the turbine’s volute. Splitting an exhaust manifold into two all while using the same header, and splitting the volute into two, a case that is already quite complex to manufacture is not an easy task by any means. Moreover, this advantage starts to dwindle at high engine speeds, resulting in plenty of torque in the low and mid-range, but lackluster performance near redline. The Mitsubishi Lancer Evolution X used such a turbocharger design (3).
4. Variable Geometry Turbocharger
A variable geometry turbocharger is a complex piece of engineering, and very sensitive to external factors. This turbocharger works by using a system that allows it to change the turbine’s volute completely by inserting movable vanes. Consequently, therefore it’s called “variable geometry.”
The turbine changes its shape thanks to its assembly, having each vane mounted on the hub via braces and rivets. These braces can be rotated slightly thanks to a small electronically controlled arm mounted on that hub that moves up and down. When the braces are rotated, their angle changes, and as a result exhaust gases hit the blades at different angles.
As an analogy, you can think of it as a ship’s mast when navigating in headwinds, and how sailors rotate the sails based on the wind’s direction. A similar process happens inside the turbine, but the direction doesn’t change, speed does. By employing such a system, a variable geometry turbocharger is more efficient across a broader range of RPMs, by working both as a small turbo and a big turbo.
Diesel engines utilize this kind of turbochargers extensively, thanks to their low operating temperature. Lower temperatures indicate lower mechanical stress, something that cannot easily be achieved at 7000 RPM. However, we also saw their adoption in gasoline engines, like the old-school 1988 Honda Legend on its 2.0L V6, or on the newer Koenigsegg One:1 in a twin-turbo layout (4).
5. Dual Volute Turbocharger
A dual volute turbocharger is a better optimized twin-scroll turbo, using the same principle as the latter. BorgWarner developed this one, a bit after another prototype of a twin-scroll turbocharger in 2014. Its concept is quite similar to the twin-scroll one but optimized better.
Simply put, a twin-scroll turbocharger’s turbine has its volute split by a wall, so to say, to allow the exhaust gases to come at different times and move the turbine by pulses. BorgWarner optimized this design by removing that volute wall altogether and opting for, you guessed it, a dual volute design.
Instead of having the volute split by a wall, you have one small volute inside a bigger volute, each one for a pair of cylinders. Thus, this translates into two turbochargers inside one another that share the same turbine and impeller.
The result is a superior-performing twin-scroll turbocharger, with improved optimization across the board. It was introduced recently, on the in-line four found on the Chevrolet Silverado and the Cadillac CT4-V (5).
6. Electrical Turbocharger
This is where the future of turbocharging is at. Electrical Turbochargers are simple pieces of machinery, but we were not able to develop them until recently thanks to technological limitations. They work by linking an electric motor to the turbine-impeller shaft and assisting the turbine when the engine runs at low RPMs.
This resolves all the issues regarding turbo-lag neatly and simply. Instead of optimizing exhaust flow massively to reduce turbo-lag or creating intricate volute designs, we are just slapping an electric motor on that turbo and calling it a day. Whenever the engine is running too slow, an electric motor will kick in and spin the compressor alone while assisting exhaust flow.
As soon as the turbine reaches its parameters, the motor stops and the turbo works like any other turbo, resulting in a neat way of operation. This was not possible until recently because this motor requires significant power and we could not link it only to a regular car battery. As a result, and thanks to our familiarization with hybrid powertrains, we have higher voltage alternatives at our disposal.
This system was extensively employed on Formula 1 cars, with major companies wanting to adopt such a system, like Audi and Mercedes-Benz. This is considered the future of turbocharging, thanks to the striking new European pollution standards that require maintaining a constant stoichiometric air-fuel ratio (6).
Widespread Turbocharger Usage
Turbochargers revolutionized automotive design since their development. Nowadays most cars employ such a system thanks to the emergence of downsizing. Notwithstanding this, turbochargers are popular for enthusiasts alike, being an accessible mod for their beloved cars.
1. Hannu Jääskeläinen, Magdi K. Khair. Turbocharger Fundamentals. [Online] November 03, 2017. [Cited: January 05, 2022.].
2. Dorofte, Adrian. How does BMW TwinPower Turbo work: The technology explained. [Online] September 02, 2020. [Cited: January 05, 2022.].
3. BorgWarner. BorgWarner’s Twin-Scroll Turbocharger Delivers Power and Response for Premium Manufacturers. [Online] [Cited: January 05, 2022.].
4. Garrett Motion. variable Geometry Turbos for Diesel Engines. [Online] [Cited: January 05, 2022.].
5. BorgWarner. BorgWarner Introduces Dual Volute Turbocharger for Gasoline Engines. [Online] [Cited: January 05, 2022.].
6. Garret Motion. Award-Winning E-Turbo Innovation. [Online] [Cited: January 05, 2022.].
4. Charles01, Public domain, via Wikimedia Commons
5. Photo copyright Stumpwater Media Group. All rights reserved.
6. Borg-Warner E-Turbo.