Turbochargers aren’t universally popular but the truth is they work well at developing more power, and as BMW is now using them in all its engines it’s worth knowing the basics. Words: Simon Holmes. Photography: BMW.
Turbochargers have been around for many years and BMW has been using them in road cars since the early 1970s with the BMW 2002 Turbo E20 . Since then, the company has slowly introduced them into its range, first through diesel engines and more lately throughout the petrol engine line-up.
They have become particularly popular in recent times because they are able to massively increase the power output of an engine but without the same kind of emissions that would be associated with a naturally aspirated engine producing the same power. So rather than using a big, highly-tuned, 5.0-litre V10, a twinturbocharged V8 will do the same job but be more efficient. This is exactly why BMW followed the turbocharger route with the M5.
The main problem traditionally associated with turbocharging, though, has always been that of lag. Particularly in the early days, turbos had a bad reputation for their slow response. However, over the years new technology has improved the way turbos perform to counter this side effect. The use of better bearings and twin-scroll designs mean lag is far less of a problem now than it was.
What is a turbocharger?
A turbocharger is effectively a turbine that is powered by the exhaust gases that are exiting the engine. Usually, these gases would escape through the exhaust manifold where they are then collected into a single exhaust pipe to exit at the back of the car. But rather than exit straightaway, a turbocharger would typically be mounted on the exhaust manifold itself, so all the gases are collected together as usual but they are then sent into the turbo. Inside the turbo is a turbine wheel and as the gases pass by on the way out into the exhaust pipe the wheel spins. Its rpm is directly related to the engine’s rpm so its speed will increase as more exhaust gases are produced.
This spinning turbine wheel is located on a shaft, the shaft sits on bearings that are lubricated by oil so it can spin freely and on the other end of the shaft in a separate enclosed housing sits a compressor wheel. So as the turbine wheel spins, the compressor wheel spins and by doing that it draws in vast amounts of fresh air. This air is then compressed into positive boost pressure as it is sent into the compressor housing and forced out towards the engine’s inlet. The compressed air is then mixed with more fuel than usual before being ignited to produce a more powerful explosion, therefore creating more power. The added benefit of this process is that even more exhaust gases are then sent back out into the turbocharger’s exhaust wheel so it can all start again.
What does an intercooler do?
The temperature of the compressed air as it exists the turbocharger on its way to the engine is relatively high, so if it can be cooled it will be become denser still, which means it can be mixed with more fuel for a bigger explosion. It’s the job of the intercooler to do this, and by passing the boost pressure through a sealed core it works in just the same way as a normal car radiator does by using passing airflow to cool the charge.
Alternatively, some cars – including BMWs – use a water cooler, or chargecooler as it is known. This works in the same way but instead of using airflow to cool the sealed core it uses water or coolant, which is kept circulating around the core using a separate pump and circuit. It also has its own radiator to keep the fluid cool. This is crucial, as the downside to water-cooling is that if the fluid gets too hot it becomes ineffective and that ultimately decreases power as the boost isn’t sufficiently cooled.
What is lag?
Turbo lag is the one major downside to turbos. As the turbine wheel works on the speed and quantity of exhaust gases exiting the engine it will only make positive boost pressure once it is up to speed. It is the dead time when the engine is still trying to produce enough gases to spin the turbine wheel fast enough that creates what is known as boost threshold, or lag, as you wait for it to respond. The same thing will happen during gear changes as the throttle is momentarily closed shut. It isn’t for long but it’s enough to have an impact and make the turbo fall off its optimum rpm level, meaning engine revs have to build up again before the turbo will produce boost pressure.
How is the turbo regulated?
Eventually, the engine will reach a point when it can’t, or won’t, consume anymore of the boost produced by the turbo, so it has to be regulated. It is the job of the wastegate to do this by creating another exit for the exhaust gases to escape before they reach the turbine wheel. This separate escape path is a small trap door that opens at a set pressure to stop the turbo over-spinning and producing too much boost.
It makes sense that bigger engines are capable of producing and consuming larger amounts of gas and air, and twin-turbocharging takes advantage of this, but there is more than one way of using two turbos. Firstly, there are basic setups that use two identical turbochargers to work in unison together, each doing an equal share of the work. The boost they produce is then mixed together and sent towards the inlet manifold. However, because bigger turbochargers are less responsive lowdown and smaller turbochargers run out of puff higher up the rev range, a sequential turbo setup is designed to improve both lag and response. It does this by using valves to specifically direct the engine’s exhaust gases. In the lower rev range they are diverted towards one small turbo that produces boost quickly to give the engine low-down power. As the revs rise an electronically controlled valve is then triggered at either a specified boost pressure or a certain rpm point. Then, the larger quantities of exhaust gas are seamlessly diverted towards a second, larger turbocharger that is able to take over from the smaller unit. The result is the best of both worlds as the two turbos work with each other to create a single wave of power.
What is a twin-scroll turbocharger?
Twin-scroll turbochargers have been developed in the last few years and are the latest evolution designed to increase efficiency and response and BMW now use them in most of its cars. A twin-scroll turbocharger works in the same way as any other turbocharger in principle, however, it makes much better use of the engine’s exhaust gases, or more specifically, how and when they exit the engine. Conventional engines never fire all the cylinders at the same time, therefore exhaust gases exiting the engine come in pulses. These pulses all meet together in the exhaust manifold just before entering the turbocharger. The trouble with this is that when these varied and mixed pulses of gas meet it disrupts the flow and therefore decreases the speed of the gases. This decrease of speed in turn means the exhaust wheel turns slower than it could do, reducing the efficiency of the turbo and increasing the response and lag.
However, by dividing the exhaust manifold up into separate sections and keeping the gases apart all the way until they pass the turbine wheel, it is possible to feed the turbo in alternating pulses that don’t interfere with each other. This way, exhaust gases from certain cylinders will only meet together for the first time once they are in the exhaust pipe. This creates a more fluent gas speed and the results are a turbo that spins quicker, therefore reducing lag.
The twin-turbo setup from the E60 535d can be seen here with each turbo consuming exhaust gases from three cylinders.