Turbocharger Knowledge
Technical Knowledge about the Turbocharger
A turbo is a rather small component. At least the chargers are mostly small. However, there is a lot of technology, know-how, and technical terms in such a small precision part. We would like to explain some of these terms related to the turbocharger to shed some light on the subject.
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A/R Ratio
The turbine size of the turbocharger largely influences the volume flow. The A/R ratio is a method of fine-tuning between the sizes, where A describes the turbine inlet area. A affects the speed at which the gases hit the turbine wheel, so a smaller cross-section results in higher gas velocities. R describes the angle of incidence of the gases on the turbine wheel. If it is small, higher turbine speeds are generated. A small A/R ratio stands for quick response/boost, a large one for more power and less back pressure at high speeds.
Efficiency
The exhaust gas turbocharger has an efficiency of about 75%, which is higher the less heat build-up of the air occurs, meaning the higher the efficiency, the lower the temperature that the turbo delivers to the intake area (on the compressor side) and the less back pressure (heat) on the exhaust side. This back pressure causes high temperatures between the turbine and the combustion chamber, which may need to be counteracted (sodium-cooled valves, larger exhaust system, oil and water coolers…).
Maps
The compressor and turbine maps are their performance diagrams. Which provide insights into their efficiency and behavior. The compressor map compares the pressure ratio to the volume flow (speed/throughput). The left boundary of the efficiency islands represents the surge limit, where no throughput takes place because the airflow separates at the compressor blades. This occurs, for example, when the throttle valve is closed, resulting in high pressure buildup but low volume flow. Backward-curved blade ends, as well as a recirculation valve, can prevent gas backflow and thus positively shift the limit. On the right side, the efficiency islands reach the choke limit, where the throughput is limited at high volume flows. At this limit, the compressor is at the limit of its flow rate, which occurs when the speed of sound is reached at the compressor wheel. By offsetting every second compressor wheel blade, manufacturers achieve an extension of the choke limit. The turbine map compares the turbine pressure ratio to the throughput. The behavior of the turbine is determined by the temperature and pressure difference before and after the blade wheel.
Intercooler (IC)
In addition to increasing density (boost pressure build-up), the turbocharger also causes an undesired temperature increase of the charge air, which represents higher engine loads (knock limit, combustion chamber temperature, pressure, ...). Furthermore, air has less density the warmer it is, i.e., less oxygen and less engine power. The goal is to cool the air, which is provided by the IC. Its advantages are: more power, torque, compression, durability, pre-ignition, as well as less boost pressure for the same power, consumption, and octane number requirement. The size of the IC depends on the amount of air to be enforced and the charge air temperature. See Air/Air IC and Water/Air IC.
Clearance
It describes the dimension between the housing and the turbine/compressor wheel. Some of the air escapes through these clearances, which is naturally bad for efficiency. The clearance should be as small as possible. But large enough to ensure sufficient play of the wheel in the radial and axial directions. These tolerances are needed by the wheel to compensate for the leverage and bending vibrations.
Turbocharger
It essentially consists of the central bearing housing and at the ends a turbine and compressor housing, each containing a impeller, which are rigidly connected to a shaft (in the bearing housing). The turbine housing is mounted directly on the exhaust manifold. And the turbine wheel is driven by the engine exhaust gases. Since both wheels are rigidly connected, the compressor wheel now rotates and sucks in fresh air. At sufficiently high speeds, a pressure build-up now occurs, allowing the engine to be supplied with significantly more oxygen than it could suck in itself, provided there is enough fuel, resulting in higher engine power. The bearing housing should be water-cooled, as this drastically reduces the risk of oil coking.
Turbine
The task of the turbine side is to supply the compressor wheel with energy so that it delivers the required air flow and pressure quickly enough. Under the same exhaust conditions, a small turbine responds faster than a large one, but it offers higher exhaust back pressure at high speeds, which presents the difficulty in selecting the turbine size.
Trim
Describes the ratio between the inlet and outlet diameters on the respective turbo sides. The wheel geometry is particularly important because even small differences cause large differences in diameters. On the turbine side, a large trim reduces back pressure after the turbine (high efficiency), while on the compressor side, a large trim means higher throughput even at low pressure.
Wastegate
Boost pressure control using a wastegate / pressure actuator is the most common and best method. Here, a portion of the exhaust gases is directed around the turbine wheel once the desired boost pressure is reached. Regulation via WG is the best possible, yet it wastes valuable exhaust gas energy. Because the valve / flap opens before the turbo reaches the desired pressure to be able to divert enough exhaust gas at maximum pressure, this energy could still be used to accelerate the turbine wheel before it reaches the target speed. The internal WG / pressure actuator is installed in / on the turbo itself. A disadvantage of this system is that the diverted exhaust gas usually meets the exhaust gas that drove the turbine wheel directly behind the turbine wheel before the exhaust pipe (downpipe), creating high turbulence there. With an external WG, these two exhaust gas streams should be combined much later at a freely selectable location (minimum distance from the turbine outlet should be about 50 cm), where, for example, a cross-sectional enlargement can also take place. However, this external WG has the disadvantage that if not arranged optimally, it can already create vortices before the turbine, which disturb the main mass flow. The WG outlet in the exhaust manifold should ideally be located in the flow of all cylinders, at a shallow angle departing from the main mass flow (not a right angle) and symmetrically with the turbine housing.
Compressor
A good efficiency of the compressor side is determined by the pressure ratio and the volume flow. With the optimal size, the efficiency optimum (about 75%) must be positioned in a frequently used speed range. The lower the efficiency, the higher the temperatures, i.e., the efficiency should be kept as high as possible over the entire speed range.
Many thanks for the support to Stefan Pieper (VW-Heideseen).
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