First turbocharger with variable compressor geometry

Photo: © Geber86 / Getty Images

Trimmed for
the future

Is it possible to significantly improve boost pressure build-up at low engine speeds without having to accept any compromises in terms of rated power output? Although it may sound almost impossible, this is precisely what BorgWarner is currently doing with completely new turbocharging technology. The innovative solution, referred to as switchable trim compressor (STC), provides greater torque at low revs and faster torque generation, while also delivering better fuel consumption figures. BorgWarner is keen to use the new STC technology to prepare both gasoline and diesel engines for stricter emissions regulations.

Turbocharger with STC switchable trim compressor. An actuator activates and deactivates the STC.

Turbocharger with STC

For decades, internal combustion engine turbocharging has made a decisive contribution to increasing performance and efficiency, and reducing pollutant emissions. Turbocharging technologies have become more and more complex as a result of ever stricter requirements regarding emission limit values, the combustion process, and performance figures – from single-stage boost, through multi-stage boost, all the way up to systems with electrical support. On the turbine side, significant efficiency gains have been achieved through the use of designs with variable turbine geometry.

BorgWarner evaluates variable compressor geometry

Until now, evolution on the compressor side has mainly concentrated on improving the flow geometry. The stable operating range of the compressor is restricted by various physical limits, which makes it difficult to match a turbocharger to a specific engine application. To push the boundaries of the compressor characteristics and increase efficiency close to the conventional surge limit, the development process at BorgWarner involved testing to determine what kind of improvements can be achieved using variable compressor geometry.

There are various ways to adjust the characteristic using variable geometry in the compressor stage. To ensure that the best solution for passenger vehicle applications was found, the engineers at BorgWarner systematically examined a range of concepts to determine their performance and technical feasibility. Several options were assessed, including solutions with adjustable inlet guide vanes, with a switchable trim compressor upstream of the compressor wheel, with axial retractable diffusor guide vanes, as well as with rotatable diffusor guide vanes. Of all the concepts tested in simulations, on the test bench, and on actual engines, the switchable trim compressor proved to be the best solution.

Five percent greater efficiency

The STC is positioned directly upstream of the compressor wheel. Narrowing the inlet area alters the effective trim of the compressor wheel. Using the STC technology, it is then possible to combine the benefits of a compressor wheel with small trim at low flow rates with those of a wheel with larger trim at higher flow rates. When the STC is activated, a more homogeneous air flow meets a reduced area of the compressor wheel, which significantly improves the inflow of the wheel. At lower flow rates, this results in increased efficiency compared to a standard compressor stage and a shift of the surge line toward lower throughputs.

Smart integration of the STC also allows the negative efficiency impact of the recirculation flow to be lowered, as mixing losses and the driving pressure difference for the recirculation flow are reduced. Once the operating point in the compressor map is outside the beneficial range, the STC is deactivated so that the standard compressor inlet can be used.

STC efficiency gains

Chart 1

Inactive STC (red) and active STC (blue) in comparison. The STC shifts the surge limit significantly to the left, leading to efficiency gains of up to 5 percent.

Chart 2

Maximum torque increased by 50 percent at 1,500 rpm, rated torque improved by 18 percent, time-to-torque shortened by 13 percent.

performance evaluation

Testing with prototypes on the combustion chamber test bench and on an actual engine showed that the STC technology, when deactivated, does not suffer any disadvantages whatsoever over a turbocharger without STC. The influence on the efficiency of the compressor is less than one percent, while the limits and characteristics remain unchanged. Activating the STC, on the other hand, moves the characteristic map of the compressor markedly to the left (see Chart 1), whereby the efficiency gains are around five percentage points. The developers at BorgWarner investigated the benefits of the technology in two example scenarios.

Greater power output for highperformance engines

Due to the high pulsation in mass flow, three-cylinder gasoline engines with high specific power represent highly challenging applications in terms of compressor design. A high-performance 91.5 cubic inch (1.5-liter) inline three-cylinder engine, calibrated to deliver 147 hp per liter (110 kW/l), was used for evaluation purposes. The turbocharging system was equipped with variable turbine geometry (VTG). The simulation focused on delivering maximum torque at 1,500 rpm and the minimum engine speed at which the rated torque is achieved.

As shown in Chart 2, the STC concept significantly improves the performance of the gasoline engine. Thanks to movement of the surge limit, a significant increase in efficiency, and a moderate pressure increase at low mass flows, the BorgWarner solution delivers significantly higher torque. The steady state torque at 1,500 rpm can be increased by almost 50 percent to achieve the LET targets, which also results in a reduction of the minimum engine speed at rated torque by 18 percent. The time to torque at 1500 rpm is also improved by 13 percent, while maintaining the T/C speed margin. This results in significantly faster engine pick up and greater pulling power at low revs.

Greater agility for Miller engines

With regard to the second version examined by BorgWarner, the developers decided to go with an application designed specifically to deliver efficiency. To this end, a highly efficient four-cylinder GDTI Miller engine with displacement of 122 cubic inches (2 liters) and a specific output of 107 hp per liter (80 kW/l) was simulated. In keeping with the performance of the turbocharger, the Miller rate was optimized to deliver the greatest possible fuel efficiency. The STC system was matched so that it would remain active up to 1,900 rpm under full load and then be deactivated above this engine speed. During testing, the BorgWarner solution demonstrated an improvement in response of up to 30 percent (Chart 3). The greater efficiency of the compressor at low engine speeds and high loads also leads to a reduction in specific fuel consumption of two to four percent, as shown in Chart 4.

However, the STC technology can be used for more than simply improving the response of the turbocharging system. In fact, it can also be used to maintain the dynamics of the engine while low-pressure exhaust gas recirculation is active for efficiency reasons. When using the STC system in combination with low-pressure EGR, savings of up to two percent are achievable at low partial load. Using the engine in a passenger car weighing 1.6 tons, this results in consumption benefits that are directly dependent on vehicle agility and the load spectrum. The engineers at BorgWarner determined this in three different driving cycles (see Chart 5). In the WLTC and in a moderate RDE representative driving cycle, small CO2 reductions of around 0.3 to 0.5 percent are possible. With highly dynamic driving in an aggressive RDE scenario, savings of around 0.8 percent can be achieved. In combination with low-pressure EGR, moderate dynamic cycles permit savings of 1 to 1.25 percent, while savings of 1.7 percent can even be achieved if the car is driven hard. The switchable trim compressor (STC) from BorgWarner therefore makes it possible to lend Miller engines trimmed specifically for efficiency significantly more agility – while also making additional fuel savings.

Initial projects already started

With its innovative STC switchable trim compressor, BorgWarner is supporting the automotive industry in producing highly efficient and high performance gasoline and diesel engines that meet the latest emission regulations. The STC technology can be combined with any type of turbine, although it is particularly useful in helping high-performance turbine stages unleash their full potential – for example VTG, Dual Volute turbines, Twin Scroll units or eTurbos. Advanced development of the STC was concluded in April 2018. Since this time, the innovative solution has been in basic development. Engine developers have shown a great deal of interest, and the STC technology is already being tested by various vehicle manufacturers in projects.


Chart 3
Thirty percent reduction in time-to-torque at low revs.

Reduction in specific fuel consumption

Chart 4
Reduction in specific fuel consumption of 2 to 4 percent thanks to greater efficiency of the compressor.

STC technology

Chart 5
Significant fuel savings can be achieved in all driving cycles thanks to the STC technology.


Photos: © BorgWarner

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