Turbotalk: what exactly does a turbo do?

When we think of turbos, we rightly think of increasing performance. Nevertheless, the turbo is an excellent attribute to increase the efficiency of an engine. Whether it concerns petrol, diesel, LPG or hydrogen, they all react in a fixed ratio with oxygen. That’s just high school chemistry, nothing more, nothing less. If you are going to burn more fuel for the sake of higher performance, you will therefore also need proportionally more air that contains that oxygen. That’s why the throttle has to open to let more air through. With the turbo-less, naturally aspirated engine, that amount of air – or the mixture already prepared in the intake duct – is limited by what the downward moving piston can suck into the cylinder through the open inlet valves. The piston can’t take in more than the swept volume, and due to all kinds of obstacles, including the intake valves, and flow losses, it’s not even possible to get close to the swept volume at all.

Compressor

If you want a better filling of the cylinder, you will have to increase the displacement (there’s no substitute for cubic inches was the adage in America for many years), or ensure that air is forced in (supercharged), so that more oxygen is present. to be able to react with more fuel as well. As for those cubic inches, a bigger engine means bigger losses. Think of a larger moving mass in the form of pistons, connecting rods, crankshaft, camshafts, valves, and so on. In addition, due to the larger surface of the combustion chamber (piston bottom, cylinder walls and head), more heat is also released to the environment: all losses. Increasing performance in this way is therefore not the most efficient solution, especially when only limited performance is required most of the time. With a small engine the losses are smaller and you can press in the required extra air with a compressor. Back in the early days of the automobile, we saw mechanical compressors driven by the crankshaft, known as superchargers. These Roots blowers and G-loaders all provided extra air. However, because they are driven by the crankshaft, they also require energy. It is more efficient to use the residual energy that is usually discharged via the exhaust valves and flows freely into the world in an atmospheric engine.

The hot exhaust gases (red) move the turbine and then flow towards the exit (right). The compressor, which is connected to the turbine with a shaft, draws in fresh air (blue) and forces it towards the engine.

By pass

When supercharged with a turbocharger, the waste stream transfers its residual energy to an exhaust gas turbine. That is a paddle wheel in the exhaust duct, which can reach speeds of more than 300,000 per minute. The turbine (from which the word turbo is derived) is connected via a shaft to a paddle wheel in the inlet duct: the compressor. It sucks in fresh air and then presses it towards the engine with overpressure. The turbo can only go full throttle when enough exhaust gases flow past the turbine. That is not immediately the case when you suddenly step on the accelerator; the pressure is built up with a slight delay. The bigger the turbo, the longer it takes to get going. A compact turbo, on the other hand, responds faster, but also has a more limited capacity to blow air towards the engine. In addition, such a compact turbo, when it is at its maximum capacity, forms a resistance in the exhaust duct. The latter can literally be circumvented by installing a by-pass, which is accessible via a valve. We know this as the waste gate. Another option is to adjust the geometry of the volute-shaped turbine housing with adjustable blades. With little exhaust gases (and therefore little pressure) you can turn the slats so that the space between them becomes smaller. As a result, the speed of the gas towards the blades increases and the impeller starts up faster. If the pressure is too great, the fins are turned in such a way that more space is created and the speed of the gas flowing past decreases.

Lower Density

We also come across engines that – to avoid delays – have several smaller turbos, connected both in parallel and in series. Regardless of the setup, a turbo that does its best literally gets red-hot. This also increases the temperature of the intake air. And because hot air has a lower density, it also contains less oxygen per unit volume. That’s unfortunate, but not insurmountable. By allowing the air to flow through a heat exchanger (a radiator), it cools down. The air shrinks, so that the oxygen content per unit volume rises again. We are talking about charge air cooling, also known as intercooling. Thanks to the cooling of the compressed air, the engine receives more oxygen and can burn more mixture. On the one hand, a turbo – with or without an intercooler – is able to make a smaller engine perform like a larger one. On the other hand, supercharging allows you to get the performance of a large engine from a more efficient smaller engine. Better performance and more efficiency go hand in hand in this way.