Car designers (since the birth of the profession) is constantly concerned about increasing the power of engines. The laws of physics say that the power of the engine depends on the amount of fuel burned per cycle. The more fuel we burn, the more power. And let’s say we want to increase the number of “horses” under the hood – how to do it? This is where we have a few solutions.
The fact that the fuel for combustion requires oxygen. So in the cylinders not just fuel is burning but fuel-air mixture. We can’t stir fuel with air by rule of thumb, we need a certain ratio. For example, a gasoline engine fuel to air ratio is 1:15, so one part of fuel requires 14-15 parts of air – depending on the operating mode, fuel composition and so forth.
As we can see, quite a lot of air is required. If we increase the supply of fuel (it is not a problem), we also have to increase considerably the air supply. Conventional engines suck air automatically just because of a difference of pressure in the cylinder and in the atmosphere. The dependence is straight – the bigger volume of the cylinder, the more oxygen gets into it at each cycle. That is the classic recipe used back in 70s in USA , releasing huge engines with breathtaking fuel consumption. And is there a way to drive more air in the same volume?
There is a way. The first person came up to this brilliant idea was Mr. Gottlieb Wilhelm Daimler. A familiar name? Still, it is used in the name of former DaimlerChrysler corporation, famous for Mercedes-Benz brand. In 1885 Daimler figured out how to drive more air into the engine. He guessed inject air into the cylinders using a blower, which is a fan (compressor) which was obtained directly from the rotation of the motor shaft and drive the compressed air to the cylinders.
Swiss engineer and inventor Alfred J. Büchi went even further. He was in charge of the development of diesel engines in the Sulzer Brothers company, and he absolutely did not like that the diesel engines were big and heavy, and underpowered. He also doesn’t like the idea to takes energy from the engine to drive the compressor rotation. Therefore, in 1905, Mr. Büchi patented the world’s first compressor device that is used engine exhaust energy. Simply put, he came up with turbocharging.
How turbocharger works
This idea was clever, simple and even ingenious. As the wind rotates the wings of the wind mill, also the exhaust gases spin the wheel with blades. The only difference is that the wheel is very small, and has a lot of blades. Wheel with blades called turbine rotor and placed on one shaft with the compressor wheel. So turbocharger can be symbolically divided into two parts – the rotor and the compressor. The rotor is rotated by receiving exhaust gas and a compressor connected to it working as a fan pumps additional air into the cylinders. All this tricky structure called a turbocompressor (from the Latin words turbo – a whirlwind and compressio – compression) or more familiarly a turbocharger.
The air that enters the cylinders of the turbocharged engine, often have to be cooled additionally – then the air pressure can be made higher, driving more oxygen into the cylinder. It is much easier to compress cold air (in the cylinder of ICE) than hot one.
The air passing through the turbine is heated by the compression, as well as details of turbochargers, heated by exhaust gases. Incoming air is cooled by means of so-called intercooler (intermediate cooler). It is a radiator mounted in the path of air from the compressor to the cylinders of the engine. Passing through it, it gives its heat to the atmosphere. Cold air is denser – so even more of it can be driven into the cylinder.
The bigger amount of exhaust gas enters the turbine, the faster it rotates and the more additional air enters the cylinders, so the power is bigger. The effectiveness of this solution in comparison with, for example, a supercharger that only a small amount of engine power used to self-function of turbocharging – only 1.5%. The fact that the rotor of the turbine is powered not by the exhaust gases slowing down, but due to their cooling – after the turbine the exhaust gases are still going rapid but colder. Also the free energy spent on compressing the air increases the efficiency of the engine. Also, the ability to make more power from a smaller working volume of engine means less friction losses, a lower weight of the engine (and the whole car). All this makes the turbocharged cars more fuel efficient compared to their naturally-aspirated counterparts of equal power. It would seem, here it is, the ideal solution to all problems. But it is not so simple. The problems have just begun.
Pros and cons
Firstly, the turbine speed can reach 200 thousand revolutions per minute, and second, the temperature of hot gases reaches around 1832 °F (1000 °C)! What does all this mean? To make a proper turbocharger, which can withstand such load for a long time, is very expensive and difficult.
For these reasons, turbocharging became widespread only during the World War II, and even then only in aviation. In the 50s the US company Caterpillar was able to adapt it to their tractors, and engineers from Cummins designed the first turbo-diesel for their trucks. On mass-production passenger cars turbo appeared even later. It happened in 1962, when almost simultaneously Oldsmobile Jetfire and Chevrolet Corvair Monza become available on sale.
But the complexity and high cost of construction – are not the only drawbacks. The fact that the efficiency of the turbine is strongly dependent on engine speed. So at low engine speeds the amount of exhaust gases are relatively small so the compressor hardly blowing additional air into the cylinders. It happens that up to three thousand revolutions per minute the engine is not reaching additional power, and only then, after 4-5 thousand rpm, the system is working. This fly in the ointment is called a turbolag. And the larger the turbine, the longer it will spin up. Therefore, motors with very high power density and high pressure turbines are usually suffer turbolag the first place. Low-pressure turbochargers doesn’t suffer from turbolag, however it doesn’t increase the engine power much.
There are at least a few solutions of the turbolag effect. The first one is consecutive turbocharging when at low engine speeds the small low-inertia turbocharger works, increasing torque on the low rpm range; and the second one, which is bigger, turn on at high rpm with increasing pressure on the discharge. In the last century it was used to boost the serial supercar Porsche 959, and today turbo diesel engines of BMW and Jaguar-Land Rover use this scheme. In gasoline engines of VW group, the role of the low-rpm turbocharger belongs to a supercharger.
In-line engines often use a single turbo twin-scroll (a pair of “snail”) with dual working mechanisms. Each of the “snail” is filled with exhaust gases from different groups of cylinders. But in this case both cylinder groups give gases to one turbine efficiently unwinding it and at low and at high rpm.
However the most common solution of the turbolag is to install a couple of identical turbochargers serving a single group of parallel cylinders. Typical for the V-shaped turbo engine, where each block has its own turbocharger.
Making the turbocharger to operate efficiently throughout the rev range, you can still changing the geometry of the working part. Depending on the RPM inside the “snail” special blades are turning and shape of the nozzle is varying. The result is a “superturbo” that works well throughout the rev range. These ideas were in the air more than a dozen years, but only relatively recently it has been implemented. And the first turbine with variable geometry appeared on diesel engines because the temperature of the gases there is much less. The first gasoline car get this kind of turbocharger was the Porsche 911 Turbo.
The turbo engine design is polished for a long time, and recently their popularity has increased dramatically. And turbochargers proved promising not only in terms of speeding up the engines, but also in terms of increased efficiency and exhaust cleanliness. This is especially significant for diesel engines. Rare diesel today does not carry the prefix “turbo”. But the installation of the turbine on petrol engine allows you to turn an ordinary-looking car to a high-performance one.