Turbo engines: Measuring the boost pressure is the key to success

In order to cope with ever stricter statutory emissions regulations worldwide, OEMs are increasingly turning to ever smaller gasoline engines. These ever smaller engines consume less fuel and emit far fewer emissions. However, they require engine turbocharging, a method of increasing the efficiency of internal combustion engines by supplying air at increased pressure, to give consumers the power they know from modern vehicles



The driving experience of these smaller turbocharged engines must be at least equal to that of their larger, free-aspirated counterparts. This requires full drive pressure at low engine speed. At the same time, loss of power at full speed should be avoided. This can only be achieved with a sophisticated boost pressure control system.

One of the main challenges here is to precisely control the air-fuel ratio close to the stoichiometric value at different charge pressures.

Pressure control with turbine side bypass

Turbine side bypass control is the simplest form of boost pressure monitoring.

Once a specific boost pressure is reached, part of the exhaust gas flow is bypassed around the turbine. A spring-loaded diaphragm usually controls the boost pressure control valve, which opens and closes the bypass depending on the boost pressure.

Pressure control with variable turbine geometry

To control boost pressure, manufacturers have recently resorted to variable turbine geometry. This approach allows the flow cross-section of the turbine to be adapted to the operating parameters of the engine.

At low speeds, the flow cross-section is reduced by closing the guide vanes. The boost pressure and thus also the torque of the engines are increased as a result of the greater pressure drop between turbine inlet and outlet. When accelerating from low speeds, the accesses open and adapt to the corresponding motor requirements.

By regulating the flow cross-section of the turbine for the respective operating point, the exhaust gas energy and thus also the efficiency of the turbocharger can be optimized. The efficiency of the motor is further increased thanks to this method compared to bypass control.

Electronic boost pressure control systems

Electronic boost pressure control systems are now mostly used in modern gasoline engines. Compared with purely pneumatic control, which can only act as a limit on full-load pressure, flexible boost pressure control allows the optimum boost pressure to be set at part load.

The operation of the flaps (or valves) is subject to a modulated control pressure instead of a full boost pressure and can be adjusted depending on various parameters such as charge air temperature, ignition point adjustment and fuel quality.

Simulation reduces production time and development costs

Given the abundance of complex variables, manufacturers are turning to simulation during the design and testing phases.

Another hurdle to overcome is the narrow range in which the centrifugal compressor must operate stably at high charging pressures.

Extensive testing under real-world conditions is the only way to develop an effective simulation model. The tests are mainly carried out on engine test benches in climatic chambers.

The following pressure information is recorded during the open and partially throttled test runs:
-Suction pipe pressure
-Charging pressure
-Air pressure

In order to obtain a clear picture of the engine performance over the complete engine speed range, the tests run taking into account the engine temperatures (coolant and oil).

During the test run, it is important that engineers record any performance deviation. Occurrences such as exhaust pulsations, which can cause standing waves at certain engine speeds and excite the impeller at critical frequencies, reduce the life of the turbo or even lead to catastrophic failures.

Therefore, measuring the compressor and turbine pressure performance is critical to developing an accurate extrapolation model for implementation during simulation.

A well-developed simulation model saves developers time and money in bench and road testing. However, this requires detailed records of the pressures that occur.