Cummins Progressing In Development of More Efficient Light-Duty Diesel Engines; Targeting 10.5% Fuel Economy Improvement Over 2007 Baseline With Tier 2 Bin 5 Emissions

21 May 2009

Cummins’ Light Duty Efficient Combustion technology is designed to scale down across smaller light-duty engines. Source: Cummins. Click to enlarge.

Cummins is progressing in the development of Light Duty Efficient Clean Combustion (LDECC) technology for light-duty diesel engines. The objective of the DOE co-funded R&D program, which began in October 2007 and runs through November 2010, is to develop and design an advanced diesel combustion system that meets Tier 2 Bin 5 standards while delivering improvements in fuel efficiency. Project partners include Chrysler and BP.

Cummins is developing LDECC on a V8, and is targeted at improving the fuel efficiency of a 5,000 lb test weight LDV over the FTP city drive cycle by 10.5%. Cummins is designing LDECC with the intention of scaling it down across its family of smaller I-4 light duty engines, with displacements down to 2.8L.

In a presentation at the Department of Energy (DOE) Merit Review this week in Washington, D.C., Don Stanton, Corporate Director, Advanced Engine R&D at Cummins, said that company started out by focusing on expanding early PCCI (Premixed Charge Compression Ignition) and smokeless rich combustion to provide for a simultaneous reduction in NO_x and PM while maintaining improvements in fuel economy.

Lifted flame combustion at high loads. Click to enlarge.

Cummins is now transitioning to lifted flame combustion with air entrainment at intermediate loads to avoid the high noise and cylinder pressures of early PCCI while still maintaining the improvements in fuel economy. The lifted flame combustion at high load improves NO_x vs. PM, has a higher EGR tolerance, and provides a more favorable combustion centroid. Implementing lifted flame combustion in this application entails work with the nozzle tip geometry and injection pressure, and controlling the charge temperature, Stanton said.

Cummins is investigating four distinct areas of enabling technologies to drive fuel economy improvements:

• Fuel injection systems. Cummins is looking at two systems, one piezo, one solenoid. In this phase of the development, the engineers are working with an injection pressure of more than 2,200 bar, with up to 7 injection events.

• Air handling systems. Cummins is going after a high capacity combined low-pressure and high-pressure EGR system. There is currently an EGR cooler bypass on the high pressure loop. Cummins is also designing SOHC and DOHC variable valve actuation systems, implementing three different concepts; the Miller cycle is used for part of the regime. Other elements include variable swirl and a two-stage turbocharger.

• Controls and sensing system. Cummins is implementing a closed loop combustion control system, which Stanton called the key to the entire LDECC effort. It uses in-cylinder sensors and piezo actuator feedback for fuel delivery and combustion phasing. Other elements in the control system include fuel quality sensing, charge management embedded models, and drive cycle optimization.

• Aftertreatment. In addition to the particulate filter, Cummins has decided to explore the inclusion of an SCR system. Although the original goal of the project was to do away with NO_x aftertreatment systems via lower engine-out NO_x, Cummins is concerned about the potential impact of the SFTP2 supplemental test procedures being considered by the California Air Resources Board (ARB). Moreover, the need for high power densities in the light-duty engines makes it even more challenging to achieve the requisite levels of NO_x reduction in cylinder, Stanton said.

The technology currently can deliver up to 7.2% lower fuel consumption, but would require an SCR system for NOx control. Click to enlarge.

With a basic implementation of an optimized combustion system design (including piston bowl, injector nozzle and intake swirl combination); fixed geometry, sequential turbomachinery; a high-pressure loop EGR system; and DOC and DPF aftertreatment, Cummins is close to achieving engine-out Tier 2 Bin 5 emissions, along with an improvement in fuel consumption of 3.1%.

Using the same set of enabling technology, but allowing more engine-out NO_x—which would need to be addressed by the SCR aftertreatment system—Cummins currently shows a 7.2% improvement in fuel consumption compared to a 2007 baseline engine. There is additional room for system optimization on top of that, and the 10.5% target could be conservative.

Cummins has a number of milestones to hit this year, including:

1. A Closed Loop Combustion Control Strategy that will adjust fuel injection parameters based on processing of the cylinder pressure signal to achieve optimal combustion phasing.
2. Engine testing with the first prototype VVA system, exploring Miller cycle, trapped residual, and aftertreatment thermal management.
3. Analysis and single cylinder engine testing to determine the benefits of a combustion recipe with lower compression ratio. The goal is to determine if a lower compression ratio piston can help extend the early PCCI operating range without efficiency degradation.
4. Selecting an engineer architecture for the upcoming third funding phase (budget period 3, BP3).
5. Development of a VVA controls strategy. Integration of the VVA system controls will be done in two steps. First, the VVA controller will be incorporated into the engine air handling controls framework. Second, closed loop combustion control feedback will be added to the controls strategy.