Intake Throttle and Pre-swirl Device for Low-pressure EGR Systems Knowledge Library Knowledge Library Intake Throttle and Pre-swirl Device for Low-pressure EGR Systems Low-pressure EGR systems to reduce emissions are state of the art for diesel engines. They offer efficiency benefits compared to high-pressure EGR systems and will gain further importance. BorgWarner shows the potential of a so-called Inlet Swirl Throttle to make use of the losses and turn them into a pre-swirl motion of the intake air entering the turbocharger to improve the aerodynamics of the compressor. By Urs Hanig, Program Manager for PassCar Systems at BorgWarner and a member of BorgWarner’s Corporate Advanced R&D Organisation Technology to meet future Emission the compressor. Obviously, pre-swirl will have a Standards positive impact on the compressor also in are- Low-pressure EGR systems (LP EGR sys- as where no throttling is required. So the IST tems), see Figure 1 , for gasoline engines yield can be used to improve engine efficiency and significant fuel consumption benefits, they are performance also in regions where no throttling also an important technology to meet future or EGR is required. emission standards (e.g. Real Driving Emissi- ons) [1 ]. To achieve the targeted EGR rates in Approach and Modes of Operation particular on diesel engines throttling the LP With IST the throttling effect is achieved by ad- EGR path is necessary in some areas of the justable inlet guide vanes in the fresh air duct. engine operating map. This can be done either In other words, IST is an intake throttle desi- on the exhaust or the intake side but to throttle gned as a compressor pre-swirl device. This the intake side is the more robust and by far approach is expected to have positive impact the more cost efficient solution. In state of the on the combustion engine, like: art EGR systems, which means EGR systems - higher low-end torque with low pressure drop, the impact of intake - reduced emissons and exhaust throttling on engine efficiency is - lower fuel consumption. comparable. To gain most out of the IST it needs to be ope- rated in different modes depending on the en- The IST (Inlet Swirl Throttle) replaces the con- gine operating point, see Figure 2. The angle of ventional LP EGR throttle in the intake side be- the inlet guide vanes is adjusted continuously fore the compressor which is usually a simple with changing engine load and speed and the butterfly valve. Throttling always means indu- set point of the vanes is determined by a con- cing losses. The approach of the IST is to trols algorithm, also taking the position of the make use of the losses and turn them into a VTG (variable turbine geometry) and the EGR pre-swirl motion of the intake air entering the valves into account. turbocharger to improve the aerodynamics of BorgWarner Knowledge Library 201 5 1 Figure 1 . Engine with low-pressure and high-pressure EGR system In mode 1 some areas of the engine operating engine speeds (esp. relevant for downspee- map the target EGR rate can not be met even ding concepts) the operating point in the com- with the EGR valve fully open. In these areas pressor map tends towards the surge line as it IST is used to throttle the intake air to drive moves to higher pressure ratios and lower EGR so that the desired EGR rate can be rea- mass flows. Close to the surge line compressor ched. As a secondary effect of the throttling a efficiency drops steeply. Here positive pre-swirl pre-swirl with a rotational component with the improves compressor efficiency and thus redu- same sense of direction as the compressor ces engine pumping losses and the IST angle wheel (positive pre-swirl) is induced. Because is set to make full use of this effect. At maxi- the operating point in the compressor map is mum load and low engine speed (low-end tor- still away from the surge line, swirl has only que area) the IST induced pre-swirl extends minor effect on compressor aerodynamics. The the compressor map towards lower mass swirl helps, however, to improve mixing of EGR flows. This provides extra air allowing for gas and fresh air. This increases compressor higher low-end torque. efficiency significantly. During mode 3, IST reduces turbo speed. Ac- In mode 2, IST improves compressor efficien- cording to the Euler equation negative pre-swirl cy. At higher engine loads EGR is faded out by reduces the speed of the turbocharger for a closing the EGR valve. For high loads and low given operating point. This effect can be benefi- cial because a turbocharger matching is always a compromise between achievable low- end torque and allowed maximum turbo speed at rated power (including some margin for high altitude operation). IST can help to enhance the quality of the turbo matching by inducing a negative pre-swirl at rated power to reduce tur- bo speed. This gives an additional degree of freedom for turbo matching and can ultimately yield efficiency or performance benefits. IST is set to fully open (or neutral) mode in the Figure 2. Operating modes of the IST other areas of the engine operating map. In BorgWarner Knowledge Library 201 5 2 this mode the optimised aerodynamics of the inlet guide vanes reduces the pressure loss to approximately 6 mbar at maximum air flow, which was proven to have negligible impact on engine performance. Design and Realisation Most important development targets of the IST were cost efficiency, robustness and low weight, see Figure 3. This was achieved by a modular design, reduced number of parts, low number of seals, especially to the outside, and a careful choice of materials. In addition, the Figure 3. IST mounted upstream of the LP EGR IST had to be compact and easy to integrate junction and the compressor housing with only little necessary modifications to the interfacing components. caused by excitations through the intake flow is avoided. In its final design stage the IST has a size comparable to a conventional butterfly valve. It Test Results can either be a stand-alone solution or partially To confirm the expected performance of IST be integrated with the LP EGR system and the experimental investigations with prototypes compressor housing. And importantly, the were conducted on an engine dyno and a hot aerodynamics of IST can cope with a bend in gas test stand. Some exemplary results are the fresh air path upstream of the IST, like of- shown in Figure 4. ten found in real applications. From literature it is known that pre-swirl does For further cost reduction the guide vanes are improve the efficiency of a turbo compressor actuated by an off-the-shelve actuator from an operating close to the surge line [2, 3]. The EGR valve and a simple mechanism similar to cause for this improvement are reduced losses a VTG actuation. The actuator is integrated in in the intake area of the compressor wheel: a the IST housing. The vanes can completely positive pre-swirl changes the direction of the close the fresh air duct; they can also be set velocity vector of the intake flow relative to the into every intermediate position between com- compressor wheel making the interaction be- pletely open and closed. In order to induce ne- tween flow and wheel more efficient. How IST gative pre-swirl the vanes can also be set to works in that respect was investigated on the negative angles. A fail-safe mechanism opens hot gas test stand to confirm the optimisation the vanes completely in case of an actuator work done by CFD on both the IST guide va- failure. nes and the housing geometry. The measure- ments show that the pre-swirl induced by IST The aerodynamics of guide vanes and flow improved compressor efficiency by up to 2 % channel was optimised by comprehensive CFD for constant flow and pressure ratio, see Figure simulations. Goal was to minimise the losses in 4 (top). Moreover, the shift of the surge line to the fully open, but also in the intermediate po- a mass-flow reduced by 50 % was also con- sitions of the vanes. Of great importance was firmed (mode 2). also to generate a stable swirl profile at the inlet plane of the compressor wheel with a de- Throttling behaviour, linearity and controllability dicated radial velocity gradient. Finally, the va- of the IST are key for a successful application nes had to be designed in a way that on a real engine (mode 1 ). To investigate this a high-cycle fatigue of the compressor wheel comprehensive controls approach was needed BorgWarner Knowledge Library 201 4 3 Figure 4. Results from engine dyno and hot gas test stand (modes in the image on the left as shown in Figure 2) to determine optimal VTG and EGR valve po- prevented. And most important, the desired sitions taking the impact of IST position and EGR rates were also met in steady state and IST operating mode on the air system into ac- transient operation. count. BorgWarner has a fully functional air system controller [4] available as a develop- This first set of investigations was conducted ment tool to which the control of the IST was using the engine’s series turbocharger. added. However, this turbo was not able to deliver the higher turbine power at low engine speeds The test engine was a state of the art 2.0-l die- which the compressor needed to provide more sel engine with a rated power of 1 00 kW mee- air with the shifted surge line.