Can you explain the term turbocharger


A turbocharger, also known as exhaust gas turbocharger (ATL) or, colloquially, a turbo, is used to increase the performance of piston engines by increasing the mixture throughput per work cycle, which is achieved by a compressor in the intake tract. The compressor is driven by an exhaust gas turbine that uses the energy from the exhaust gases. Turbochargers can use either the pressure (accumulation charging) or the kinetic energy of the exhaust gases (shock charging) as an energy source.

In the case of non-supercharged piston engines (naturally aspirated engines), when air is drawn in by the piston, a negative pressure is generated in the intake tract. The negative pressure increases with increasing speed and limits the achievable power of the engine. One of the ways to counteract this is to charge the cylinders using a turbocharger. A turbocharger consists of a turbine and a compressor, which can be constructed very similarly. The turbine and the compressor are connected by a shaft. The turbine is set in rotation by the exhaust gas flow and drives the compressor via the shaft, which sucks in air and compresses it. The compressed air is fed into the engine. Due to the higher pressure, a larger amount of air enters the cylinders during the intake stroke than with a naturally aspirated engine. This means that more oxygen is available for the combustion of a correspondingly larger amount of fuel. This leads to an increase in the engine mean pressure and torque, which increases the power output. Because of the larger amount of gas, gasoline turbo engines usually require a reduction in the compression ratio compared to comparable naturally aspirated engines, as otherwise excessive pressures and the resulting high temperature can lead to uncontrolled ignition of the fuel-air mixture (knocking).

In contrast to the naturally aspirated engine, in which the air cools adiabatically during intake due to the negative pressure, the compression in supercharged engines significantly warms the charge air. Depending on the degree of charging, the compressed air in series engines can be heated to around 120 to 150 ° C [3]. In addition to the additional temperature load on the engine, this also reduces the achievable power, as the degree of filling of the engine deteriorates. The reason for this is the lower density of the hot air, which means that less oxygen is supplied to the engine. To avoid this, the charge air in practically all modern supercharged engines is cooled by charge air coolers. Since the charge air cooler opposes a certain resistance to the flow of charge air and thus slightly reduces the pressure generated by the compressor, the temperature difference of the charge air cooling should be greater than 50 K [3] in order to achieve an effective increase in performance compared to an engine without charge air cooling. In order to reduce the thermal load at full load, the air-fuel mixture can be enriched with additional fuel. In engines where the highest possible power output has priority over the service life, the charge air can also be cooled by an additional water injection or injection of a water-alcohol mixture directly into the intake tract, which enables a further increase in performance.

For turbines or compressors, impellers or paddle wheels are used, which convert the flow energy into a rotary motion or, in the case of the compressor, conversely, the rotary motion into flow work. Modern turbochargers can reach speeds of up to 290,000 revolutions per minute (e.g. smart three-cylinder turbo diesel). These speeds can only be achieved because the turbocharger shaft is mounted in a hydrodynamic slide bearing. In addition to the oil supply connections, some turbochargers also have connections to the water circuit for cooling purposes. The development of ceramic ball bearings makes the turbochargers more robust and durable. There are one or two ceramic bearings in addition to the plain bearing. Ball-bearing turbochargers have a smaller sliding friction surface, which allows them to respond more quickly. This results in a faster increase in the speed of the supercharger and a boost pressure that starts earlier.

  • Use in gasoline engines

In petrol engines with external mixture formation, the boost pressure is limited by the heat of compression produced by the fuel-air mixture in the 2nd stroke. Exceeding this means uncontrolled spontaneous ignition and thus engine knocking or engine ringing. The start of knocking can be increased by means of high-octane fuel, an effective charge air cooler or water-methanol injection. In most cases, however, the valve timing is changed and the compression is reduced to prevent this effect.

In diesel engines for passenger cars as well as for trucks, the exhaust gas turbocharger is meanwhile "state of the art", since the diesel engine can only achieve liter performance approximating that of the (petrol) gasoline engine by means of turbocharging. Without turbocharging, a comparably powerful engine would have to have almost twice the cubic capacity and thus significantly higher weight. In addition, the specific torque characteristics of a turbo diesel compared to a naturally aspirated diesel shift the range of maximum power development to lower speed ranges. As a result, such motors offer a high degree of "elasticity", so that you have to shift into lower gears less often to accelerate.

  • Use in diesel engines

In principle, diesel engines do not need a throttle valve. Therefore, a gas flow is present at the turbocharger even during overrun. This means that the speed of the turbine does not drop as much as in a gasoline engine, which improves the response behavior when there are load changes. Diesel technology is therefore very well suited for the effective use of a turbocharger. In addition, diesel engines have a higher degree of efficiency, lower speeds and a lower exhaust gas temperature, so the material of the diesel turbocharger is less exposed to high loads.

Large diesel engines were equipped with turbochargers or external compressors at an early stage (e.g. marine diesel engines for the first time in 1925). Exhaust gas turbochargers were also used in the first diesel locomotives at the end of the 1930s. In 1951 MAN equipped an engine for trucks with a self-developed turbocharger, with the 8.72-liter engine being increased in output from 130 to 175 hp. The truck manufacturer Volvo built a turbocharger into its engines from 1954, which brought about the breakthrough in truck engine construction due to its reliability. Turbochargers have been used in a very high proportion of the large commercial vehicles delivered since the 1960s. Diesel engines have been equipped with turbochargers in passenger cars since 1979. Since 1988, passenger cars with diesel turbocharged engines with intercoolers and direct injection have become very important in Europe.