New Ways of Turbocharging the Heavies!



Peter Els
01/10/2017

For the layperson whose knowledge of turbocharged engines comes primarily from their experience behind the wheel of a car it may come as quite a shock to learn that the car engine is a bit of a latecomer when it comes to enjoying the benefits of turbocharging: Unlike mainstream passenger car engines that have only recently turned to forced induction, heavy commercial and off-highway vehicles have relied on turbocharging for several decades, with turbochargers having been around for more than 110 years.

Turbocharged bus

© gumpapa - fotolia

These large displacement commercial-vehicle engines are commonly fitted with state of the art turbochargers to increase their power output, reduce fuel consumption and, most recently, to dramatically lower emissions of both harmful particulates and greenhouse gasses.

Motivated by customers’ demands to reduce fuel costs, and environmental pressures from lobby groups and governments to improve exhaust gas emissions more and more, OEMs and first tier suppliers have managed to refine these systems even further.

Developed for better performance and reduced emissions

Already a leading industry provider of fuel saving serial two-stage turbocharger technologies, Honeywell recently launched its newest generation, which moves away from matching two single-stage turbochargers in favor of a real two-stage turbocharger design with a dedicated and optimized aerodynamic package.

The new two-stage aerodynamic design allowed Honeywell engineers to use a smaller diameter turbine wheel in the high-pressure turbo to improve efficiency. Similarly, the diameters of both compressor wheels in the high-pressure and low-pressure turbos have also been reduced and redesigned to increase peak efficiency and improve transient response.

With a pressure ratio of up to 5:1  and a dual wastegate port, Honeywell’s newest serial two-stage application with will first be seen fitted to the MAN 12.4L engine.

This ultra-high efficiency, serial two-stage system is further enhanced with an axial turbine wheel in the low-pressure turbo. Coupled to the sector-divided, fixed vane nozzle, high-pressure turbine, the system delivers optimum performance for heavy-duty long-haul trucks that call for fuel efficiency and low CO2 emissions.

Honeywell developed this system in part due to a European Union request for technology targeting reductions in CO2 emissions which could be implemented by 2020. The new system has a number of features that contribute to an overall turbo efficiency of nearly 64 percent with a 6 % reduction in fuel consumption on a typical road test duty cycle. 

The new design also supports turbo-compounding powertrains using a waste heat recovery system that can further reduce fuel consumption by up to 5 %.

Turbo-compounding technologies can either be mechanical or electrical: 

  • In mechanical turbo-compounding, exhaust flow is used to spin a turbine that connects through a mechanical transmission directly to the crankshaft. This results in higher torque and power outputs whilst reducing exhaust energy losses for a given fuel input. 
  • In electrical variants, the turbine is connected to an electrical generator. The electricity thus generated can be stored and then used to power electrical-accessories or to provide torque assist to the engine through an electric motor (in hybrid powertrains).

Although turbo-compounding has been around for some time now, due to ongoing refinement and the appearance of hybrid heavy commercial vehicles manufacturers are showing a renewed interest in the technology, with Volvo and Mack trucks both offering turbo-compounding technologies in their 2017 lineup:

  • Volvo Trucks’ D13TC will be available as an optional engine with claimed fuel efficiency improvements of up to 6.5 %, as well as power ratings that top 335 kW and 2,500 Nm of torque. A one-point increase in the compression ratio undoubtedly contributes to the improvement, but according to Volvo, turbo-compounding adds as much as 37 kW. 
  • For customers with long-haul applications, Mack will offer the 2017 MP8 engine with a turbo-compounding system engineered to give customers a no-compromise increase in power and efficiency. Similar to Volvo the system also adds up to 37 kW, enabling a substantial increase in fuel efficiency approaching 8.8 %. Furthermore, Mack claims the additional power generated by the turbo-compounding system allows the engine to maintain full torque as low down as 900 rpm. Thus not only enabling a broader operating range in top gear, but also allowing the vehicle to hold top gear longer when overtaking on a gradient; even with falling engine speed.

There’s no doubt that forced induction using a turbocharger is the best solution for heavy-duty on- and off-highway applications, but the Achilles heel of the turbo has always been driveability, and principally turbo-lag while the turbo spools up.

The eTurbo reduces lag while increasing performance

The electrified turbocharger is a critical technology in engine hybridization. It combines a variable geometry turbocharger (VGT) and an electric machine (EM) within a single housing. The EM is capable of bi-directional power transfer, so excess energy can be recuperated by the EM to supply electrical accessories or to be stored in a battery for later usage.

On the other hand, the EM can also accelerate the turbocharger to improve engine response, especially for transient torque demands. Due to its location in the air system, reasonably small electric systems can have a large effect on transient behavior and engine efficiency. In this sense, the technology is superior to conventional hybrid systems that act on the engine crankshaft.

Following similar developments in the light vehicle market, automotive parts giant BorgWarner demonstrated an electric turbocharging system that reduces turbo lag at a recent technology preview at its suburban Detroit headquarters. 

The eTurbo delivers a nearly immediate boost because it initially uses an electric motor to spool up or spin the turbine, Chris Thomas, BorgWarner’s chief technology officer, told a gathering of journalists at the company’s headquarters.

Furthermore, instead of using a wastegate to blow off turbo pressure, once exhaust gas pressure reaches an optimum level, the eTurbo motor will act as a generator, providing an electric current that reduces demand on a truck’s conventional alternator.

However, according to Thomas, the system is still a “couple of years” away from production.

While the electrified turbocharger may capture and store the excess energy in the air system for re-use when required, this new configuration needs a new control structure to manage the air path dynamics. Also, the selection of optimal setpoints for each operating point is crucial for achieving the full fuel economy benefits. 

There is general consensus amongst engineers that the development of a systematic strategy in both real-time energy management and multi-input-multi-output (MIMO) control is essential for exploring the benefits of the electrified turbocharger. 

In a paper titled “Real-Time Optimal Energy Management of Electrified Engines” researchers Dezong Zhao, Edward Winward,¬¬ Zhijia Yang, ¬Richard Stobart and ¬ Thomas Steffen constructed a real-time two-level energy management strategy for a structure where the electrified turbo is integrated into a hybrid system. 

The control inputs and outputs were identified based on a detailed control-oriented dynamics analysis of the electrified engine, and the boundaries of control variables were established. 

From this, a model-based MIMO decoupling controller was designed to ensure the performance under transient conditions and in steadystate operation was fast and smooth. 

Furthermore, the supervisory level controller optimizes the fuel economy in real-time, while battery life is optimized through a closed-loop control of the battery State Of Charge.

It’s not surprising that manufacturers of large-displacement, heavy duty engines are investing in a broad range of turbo based technologies: According to a 2016 report by ExxonMobil, titled ‘The Outlook for Energy: A View to 2040,’ heavy-duty vehicles will be the largest energy-consuming segment of the transportation sector by 2030. 

This is not unexpected, given the role of trucking in sustaining modern life and the projected growth in economic activity and trade. On the back of this growth, global energy demand for heavy-duty vehicles is expected to increase by about 45 percent in the period from 2014 to 2040.

So when viewed in this light, turbocharging these engines is no longer a nice-to-have, but rather it’s critical in safeguarding resources and the environment for decades to come.

Sources:

  • Brian Lohnes; Truck Trend; The new Volvo D13 With Turbo Compounding Pushes Technology and Efficiency in Trucking; May 2016; http://www.trucktrend.com/features/1605-the-new-volvo-d13-with-turbo-compounding-pushes-technology-and-efficiency-in-trucking/
  • Paul A. Eisenstein; Trucks.com; BorgWarner Puts New Spin on Turbochargers; November 2016; https://www.trucks.com/2016/11/22/borgwarner-turbochargers/
  • Mike Stoller; Honeywell Transportation Systems; Honeywell Turbocharger Technology And Software Applications Target Wide Range Of Needs For Commercial Vehicle Manufacturers; September 2016; http://www.honeywell.com/newsroom/pressreleases/2016/09/honeywell-turbocharger-technology-and-software-applications
  • ERTRAC Working Group: Energy and Environment; Future Light and Heavy Duty ICE Powertrain Technologies; June 2016; http://www.ertrac.org/uploads/documentsearch/id42/2016-06-09_Future%20ICE_Powertrain_Technologies_final.pdf
  • Cummins Turbo Technologies; The Future of Turbocharger Technologies in the Off-Highway Sector; https://www.cumminsturbotechnologies.com/The%20Future%20of%20Turbocharger%20Technologies%20in%20the%20Off-Highway%20Sector
  • ExxonMobil; The Outlook for Energy: A View to 2040; 2016; http://cdn.exxonmobil.com/~/media/global/files/outlook-for-energy/2016/2016-outlook-for-energy.pdf

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