Tighter emissions limits drive new heavy-duty diesel control strategies

  • 20-Jan-2009 08:54 EST
Hanover show 2008 - 17.jpg

The requirement for more injection pulses will see Delphi adopt common rail in place of EUI for heavy-duty diesels for U.S. EPA 2010/Euro VI. (John Kendall)

Euro VI, with a likely implementation date of 2013/14, is not yet finalized, and the introduction of World Harmonized Heavy Duty Certification (WHDC) could replace test procedures in Europe, North America, and Japan.

Switching to WHDC test cycles could have several effects on fuel injection system design, because the proposed cycles include a World Harmonized Transient Cycle (WHTC) and a hot start steady-state cycle (WHSC). WHTC includes both hot and cold start cycles with a 10% weighting for the cold start cycle. The WHSC test also includes full-load test modes, which means that exhaust emissions control systems must be effective at full load and high exhaust temperatures. The WHTC test includes a lower average load factor, and hence exhaust temperatures would be lower.

“We have to think about managing the exhaust temperature for the cold start,” said Chris Such, Chief Engineer, Heavy Duty Diesel Engines at Ricardo. “There are various things you can do. For example, you can have a seventh injector with a six-cylinder engine. Then you can have that injector injecting fuel ahead of the catalyst.” The technology can be very effective in reducing catalyst light-up times.

The U.S. EPA 2010 and Euro VI limits are bringing a change of strategy for Delphi’s heavy fuel injection systems. The company has been one of the leading developers of electronic unit injector (EUI) technology for heavy-duty diesel engines, culminating in the E3 injector, which introduced two valves to control injection and extended the range of injection possibilities to include multiple injections.

The need for more injection pulses will see Delphi introduce heavy-duty common-rail systems, previewed at the IAA Show in Hannover last September. The systems on display included a conventional pump-driven common-rail system, as well as “distributed pump” and “remote pump” systems, utilizing Delphi’s unit pump and unit injector pumping units to charge the rail. Delphi claims that these systems can deliver injection pressures up to 3000 bar (43.5 ksi).

“The drive for reduced emissions and the resulting use of aftertreatment has lead to the use of multiple fuel injection pulses—from the pre, the pilot, the main, and then to a post for the aftertreatment,” Gary Forward, Engineering Manager, Electronics (heavy-duty diesel) at Delphi in London, told Automotive Engineering International. “The requirement for multiple injections is forcing a larger injection window. The advantage with common-rail-based systems is that the injection window is not limited to the duration of the pumping lobe on the cam as is seen with EUP/EUI-based systems.

“One challenge involved in implementing multiple injection pulses in the heavy-duty environment is in controlling very small quantities of fuel for doing pre and pilot pulses and how you actually maintain that quantity over the life of the system,” Forward continued. “Variation in quantity can lead to changes to the desired combustion profile, like insufficient or excessive pilot injection, which then obviously has effects—increased noise, advancing combustion, or perhaps excessive speed variation. The use of multiple injections gives rise to problems when switching between injection modes. The move to torque-based control helps to maintain torque continuity during these mode switching scenarios.

“Using post injection for an exhaust heating phase, we need to demand fuel quantity to enable control of exhaust temperature. We’ve been looking at modeling interactions between the main injection and the post injection to understand how we can use software strategies to maintain pulse-to-pulse independence. So, if we demand a post injection of x milligrams, we still deliver that post injection of x amount of milligrams independent of pulse to pulse separation,” said Forward.

To meet Euro VI and EPA 2010, both exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) combined will probably be needed to meet the very low NOx emissions limits. “There is the possibility to use large amounts of EGR without an SCR catalyst,” said Such. “That’s ‘Plan B’, if you like. ‘Plan A’ is EGR plus SCR with a DPF [diesel particulate filter]. 'Plan B' would be much higher levels of EGR with very high injection pressures.”

Such believes that other issues that need to be considered to meet future emissions limits include reduced variability: “The Euro VI and U.S. EPA '10 limits are so low that any variability from one engine to another, or even one cylinder to another, has the potential to take it over the limit. It’s really down to the combination of the engine manufacturer and fuel-system manufacturer to reduce that variability as far as possible.

“One way of achieving that, which we are very interested in, is a cylinder pressure-based management system," Such explained. “There is a cylinder pressure sensor in each cylinder, which enables you to track the cylinder pressure curve. Normally, the combustion control is based on the start of the injector needle lift at the start of injection. Mapping the complete cylinder pressure characteristic enables combustion to be managed on whatever parameter you like. For example, you could have it based on a 50% burn angle—when 50% of the fuel is burnt.”

Delphi has developed an in-house prototype system for use with its distributed pump and remote pump common-rail systems. The company calls this “angle-based mapping.”

“It forms a system that replaces the need to map between either torque or fuel to your output angle or time demand,” explained Anthony Potter, Systems Manager (heavy-duty diesel) at Delphi in London. “So with a very brief sweep of a number of speed, load, and rail pressures, you can build a rough calibration to be able to get some initial data and get the system up and running. This type of system is mainly used for prototype development, customer demonstration, and subsequent concept evaluation.

“When we supply prototype fuel systems to customers for evaluation, we want to send out an electronics package which is not too difficult to calibrate and will enable them to get a feel for system performance to evaluate emissions capabilities. In addition to this, we’re looking at integrating that with an auto calibration scenario, so in the future we’re hoping to improve system accuracy for demonstration without the overhead of an extensive calibration activity, yet maintaining control flexibility.”

Delphi’s productionized control systems handle the vehicle, engine, fuel injection equipment (FIE), and air management control strategies. “If you look at it in a modular architecture in that form,” continued Potter, “the FIE layer with the rail pressure control system could essentially be plug-and-play with any other customer application, which would comprise air management, engine, and vehicle control features. So in that manner, our architectural system design, which has been developed over a number of years, is at a point where the productionized application code for an EUI-type system could essentially be used as a base application for a common-rail-based system.”

Delphi is developing technologies that utilize accelerometers to look at combustion vibration or interrogating engine speed signals for cylinder-to-cylinder speed fluctuations, detecting changes in system performance for diagnostic purposes.

“In terms of helping to ensure emissions over life for the system,” said Potter, “we’re looking at monitoring delay times in injector valve operation by looking at injection drive waveform as well as looking at minimum drive pulse detection strategies, which detect when minimum combustion happens.”

 

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