Engine Notebook: Cat gets ACERT-ive

A key goal in Caterpillar’s development of ACERT technology was to provide commonality – one basic strategy to meet in-place and upcoming emissions standards for both on- and off-highway applications. It’s working. The same basic design that satisfies current Tier 3 nonroad standards will be the platform on which Tier 4 nonroad engines will be built. (Tier 4 gets phased in by engine horsepower size during the years 2011 through 2015.) And ACERT on-highway engines that meet the current (2004) standards will provide the basis for engines that will meet new standards in 2007 and 2010.

Corresponding on- and off-highway ACERT engines share 90 percent of their core iron, including blocks, crankshafts, cylinder heads, valve trains, oil and water pumps, rods and bearings.

Given the dissimilarity in duty cycles between on- and off-highway applications, achieving this high level of commonality was no easy task. The secret was to incorporate some flexibility into the fundamental design.

A variety of variations
In some instances, the variations are slight. Camshafts, for example, have the same blanks but the finished profiles are different. Injector bodies are the same, although the tips differ. (Injector design also varies between hydraulic and mechanical actuation, but this is dependent on displacement, not application.)

Some variations are significant. While both on- and off-highway engines use turbochargers, the turbo setups are different. On-highway engines use series turbos, while off-road engines use waste-gated units. The effect is the same – turbo response is tailored to the need of the engine, providing sufficient boost at a wide range of engine speeds without excessive boost under extreme conditions.

Electronics are more comprehensive on off-highway engines. While both on- and off- highway engine electronics must interface with transmission controls, off-highway engines must also communicate with hydraulic systems, PTO-driven attachments and other auxiliary systems.

Do the right thing
Maintaining combustion efficiency requires that the right amount of air and the right amount of fuel be delivered at the right time. But what’s “right” changes with engine speed and load.

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On-highway engines help control the combustion event with variable-valve actuation, where the timing of the intake valves controls when air enters the cylinder. As engine load increases, for example, intake valves stay open longer so the incoming charge air has sufficient time to completely fill the combustion chamber.

Both on- and off-highway engines use multiple-injection fuel delivery. At idle, the injector may deliver a single pulse of fuel. Under more demanding conditions, there may be two, three or more separate pulses, each precisely controlled. The result is the same in either case: Emissions are reduced.

Both engines also use cross-flow cylinder heads. Clean air comes in one side and dirty exhaust goes out the other. Combustion efficiency is improved, so power goes up and emissions go down. But cross-flow heads can be a manufacturing challenge, and the placement of hardware on both sides of the block can lead to a tight fit in the engine bay, so until emissions standards got tighter there was no compelling reason to use the design.

The cylinder heads have been improved in other ways, too, according to John Dutton, Cat’s off-road ACERT introduction manager. “As we worked on the heads we improved their aerodynamic design at the ports, valves and stems to get better flow. It used to be that you’d look into the intake manifold and see a maze of baffles. Now you cut one up and look inside and the aerodynamic design is obvious.

Aftertreatment not an afterthought
On-highway emissions regulation started earlier and has been more stringent, so on-highway engines have been using exhaust aftertreatment for some time. From the several available aftertreatment strategies, Cat chose to use a diesel oxidation catalyst. The catalyst unit is placed in series in the exhaust system between the engine and the muffler.

Aftertreatment will be required for Tier 4 off-road engines, and Cat engineers are mulling their options. Randy Huber, technical manager in the Caterpillar machine engine group, said, “We’re well positioned with the ACERT technology as our basis in meeting the Tier 4 regs. Beyond that most of the potential technologies would require some sort of aftertreatment device, whether that will be filters, NOx absorbers, SCR [selective catalytic reduction] or something else. At the end of January Caterpillar announced the formation of Cat Environmental Technologies, a new group that will bring the development and manufacturing of all aftertreatment technology and products in house. The group will also manufacturer the diesel particulate filter for Cat on-highway ACERT engines for 2007.

One of the issues of grafting on-highway aftertreatment technology to off-highway engines has been finding materials robust enough to endure the pounding they would receive.

Substrates that form the core of many aftertreatment devices have been especially vulnerable to failure.

Huber said those problems have been resolved, or soon will be. “I don’t want to belittle those matters,” he says. “There are significant challenges in getting some of these devices to live in a ripping application on a D11. But I don’t know that we’ll be limited by that technology. In fact, I’m highly confident that we’ll be able to solve those issues.

“The bigger challenge our industry will face regarding aftertreatment is just the sheer size of the components. How do you physically fit them underneath the hood or beneath the line-of-sight structures of off-road products? People tend to think that off-road machines have a significant amount of room [under the hood] and that’s just not the case.”

OEMs will have to find a way to make the parts fit. Meeting the next level of emissions standards will require aftertreatment, especially as regulatory agencies seek a convergence of standards. Soon the requirements for on- and off-highway engines will be similar. California, the rest of North America and Europe will also come closer to uniformity.

Developing solutions to the emissions regulations has been neither easy nor cheap. “[Cat] has spent more than a billion dollars on ACERT technology and other clean diesel programs,” said Dutton. “Because it touches nearly all of our businesses, this has been the company’s largest product development program.”

But ACERT satisfies that original key goal of commonality, said Dutton. “ACERT will take us to the 2010 truck regs, and it’s going to take us to Tier 4.”