How Cylinder Deactivation Can Help Meet 2027 Emissions Targets

Engine makers will have their hands full meeting the ultra-stringent emissions reduction mandates coming our way in 2027. Jacobs Vehicle Systems may have a solution.

The U.S. Environmental Protection Agency and the California Air Resources Board are targeting NOx (nitrogen oxides) emissions and carbon dioxide emissions simultaneously, which poses a double challenge for engineers. Treating one has traditionally meant an increase in the other. But coming to the rescue is Jacobs Vehicle Systems, makers of the not-so-humble “Jake Brake.” Leaning heavily on its experience with variable valve actuation (VVA), Jacobs has developed an in-cylinder solution that promises drastic cuts in NOx along with more modest improvements in fuel efficiency.

So confident is Jacobs in this two-pronged strategy, the company is taking a custom-built International LT625 tractor with an A26 diesel engine right to fleets and other stakeholders to demonstrate the effectiveness of cylinder deactivation.

“We’re excited to begin this important tour,” said Steve Ernest, vice president of engineering and business development at Jacobs. “We will be working with a wide variety of important commercial vehicle shareholders in the coming months to schedule test drives and showcase our latest technologies.”

To kick off the 2022/2023 North American Technology Tour, Jacobs invited trade-press editors to a demonstration event near its corporate headquarters in Bloomfield, Connecticut. Journalists saw cylinder deactivation (CDA) and active decompression technology (ADT) in action on an in-cab display while driving on a test track.  

Cylinder Deactivation and NOx Emissions Reduction

CDA is a modular valve actuation technology that essentially shuts off some or all engine cylinders under certain conditions. The results are two-fold. When operating under less than full load, three cylinders may fire while the remaining three (or any combination up to 5 and 1) work harder. The resulting higher exhaust temperature keeps the catalyst in the selective catalytic reduction system at or closer to its optimum operating temperature of about 482 degrees (250 degrees C), maximizing its NOx reduction efficiency.

“In order to achieve the peak conversion rate, the mid-bed temperature of the SCR needs to be in the 250-degrees C range,” said Robb Janak, director of new technology for Jacobs. “When you're at the 95%-plus conversion rate, any engine-out NOx is going to get scrubbed by the SCR system. Whenever temperatures fall below 250 degrees [C], you start getting tailpipe NOx.”

This becomes critical in 2027 with the advent of a low-load emissions test cycle. Currently there are exemptions for low-load, start-up and idle conditions where NOx isn’t counted. But the next round of regulations will have a low-load test cycle, with the goal of capturing all engine operating conditions.

“There's no more ‘get out of jail free’ card,” said Janek. “This is all about real-world operating conditions.”

When the truck is loaded, the engine works hard enough most of the time to keep exhaust temperatures high enough to maintain adequate temperatures at the heart of the SCR catalyst. But in high transient conditions (starting and stopping) or when coasting, the SCR temp drops. Enabling CDA prevents cold air (ambient temperatures) from being pumped through the engine and on into the aftertreatment system.  

This was shown clearly on the demo truck’s in-cab display. With CDA turned off (what we would call normal operation today), the exhaust temperature began dropping almost immediately, losing about 150 degrees C within a few seconds of cutting off fuel to the engine. With CDA switched on (all the valves in the cylinders stay closed so there’s no air flow through the system at all), the exhaust temp stayed high, dropping only about 50 degrees C.

There was less fluctuation in SCR intake temperature compared to the exhaust temperature, but the SCR temp dropped only a few degrees from its “fueled” state after the fuel was cut off. That may not sound like much, but higher temperatures have a profound effect on the catalyst’s NOx conversion rate. CDA helps maintain the temperature inside the catalyst without dosing it with fuel to raise the internal temperature.

“As an ‘in-engine’ technology, CDA can add a small cost to engine production, but that cost can be mitigated by reducing the size and complexity of the aftertreatment system or eliminating the need for other technology that reduces emissions,” explained Ernest. “Because CDA ultimately reduces fuel consumption and requires less maintenance than other external solutions designed to raise aftertreatment temperatures, it will result in a lower total cost of ownership.”

Active Decompression Technology

The Jacobs “Drive the Future” technology demonstration truck also offers fleets a chance to experience the benefits of easier engine start-stop cycles without cab shake.

An older, lower-tech way to do this came in the form of a spring-controlled lever in the cab that opened a port in the cylinder head to release the charge of compressed air. Today, Jacobs' active decompression technology uses variable valve actuation to hold the intake or exhaust valves open momentarily when starting and stopping the engine.

The primary benefit is for drivers using an engine start/stop system as an alternative to idling to maintain cab temperatures and keep batteries sufficiently charged.

With current engines, when shutting down, once the fuel is cut off, the engine spins down under its own momentum until there’s insufficient momentum to overcome a compression cycle in one cylinder. When that piston cannot push past top-dead-center, the compressed air within the cylinder acts as a spring and pushes the piston back down. That produces the cab shake drivers are familiar with.

“The initial goal for ADT was to prevent drivers getting woken up each time the engine shut down,” said Janek. “When the truck auto-starts in the middle of the night to recharge the hotel batteries, drivers might be able to sleep through the startup, but they get shaken out of bed during shut-down. With ADT, our hope is drivers will be less reluctant to use engine start/stop as an idle reduction strategy.”

With an engine valve held open, the engine simply spins down until it runs out of momentum. Jacobs claims there’s a 90% reduction in vibration at shutdown with ADT.

Additional benefits were later realized for decompressing cylinders during startup, too. Opening valves to relieve compression takes a big load off the starter and the ring gear. Janek said it lowers cranking torque by about 40%, allowing the engine to crank, while getting the engine up to twice its normal starting speed before compression is restored and fueling begins.     

“Not only does ADT make it easier to start the engine, but it also reduces wear on the starter and the ring gear,” he said. “Because the engine stops at random positions, it’s no longer engaging the ring gear at the same point every time.”

ADT further improves start-ups in cold temperatures by enabling the engine to reach its critical ignition speeds. When combined with supplemental air inlet heaters, ADT enables the engine cylinders to be pre-warmed without the engine load from compression, which is especially useful when freezing temperatures reduce battery charge levels.

“We have 30% faster cold-starts and we can lower the cranking torque by about 40%,” Janek said.

It could also open the door for easier starting in a mild hybrid system when the engine is transitioning from electric to ICE operation. That function is not yet officially in the option books, but Jacobs says it’s under consideration.

Tula Adds Precision Combustion Control

In 2019, Jacobs began collaborating with internal combustion controls company Tula Technology, combining Tula’s Dynamic Skip Fire (DSF) controls technology with Jacobs’ CDA capabilities.

Tula DSF makes dynamic firing decisions based on how much torque is requested, then it selectively chooses which cylinders are active or deactivated to meet performance demand. When more torque is required, the firing density increases, and when there is less demand for torque, firing density decreases. The control algorithm effectively creates an engine with optimal displacement for the torque required.

All of which begs the question, will drivers notice the difference? If it weren’t for the display mounted in the demo truck, I would hardly have known CDA was active or not.

This whole process is seamless to the driver, and the driver has no control over its function. The demo truck has switches to turn the system on or off for comparison purposes only.

The six green dots represent the six cylinders in the engine; when they are green, they are active, when they are white, they are deactivated. In fixed mode, a predetermined number of cylinders are active, for example 4-on/2-off or 3-on/3-off. When DSF is active, the active/deactivated state seems less coherent, and the green dots flicker on and off randomly like the lights on an internet router.

When the driver demands some torque from the engine, the system weighs the request and activates the appropriate number of cylinders. With a full power request, all cylinders become instantly active and the exhaust temp shoots up. Conversely, going from coast mode to engine braking, the transition is instant and seamless. The driver would never know that the engine has just transitioned from all cylinders off to all cylinders in retarding mode.

By having the ability to select which cylinders are fired every engine cycle, and by considering the frequencies and amplitudes of vibration produced by cylinder combinations, the control algorithm determines the cylinder firing sequences that deliver smooth operation. Jacobs says Tula’s control system ensures production-level NVH (noise, vibration, and harshness).

Seat-of-the-Pants Impressions

As I noted previously, without the display, I wouldn’t have been aware of any of the trickery going on under the rocker cover. It was utterly seamless. When driving bobtail at low load, running on three or four cylinders, I could discern no difference. After all, demand on the engine was light and three cylinders could deliver more than enough power to satisfy that demand. When more power was requested, the other cylinders came on stream and away we went.

I could, however, tell the difference when idling on two or three cylinders. If I wasn’t aware of the CDA technology, I’d say it felt like the engine was “misfiring” — which, of course, it was, but by design, not some mechanical fault. The slight vibration was noticeable, but barely.

The ADT start and shut-down sequences were distinctly different from the usual driver experience and will not go unnoticed.

When starting, the driver turns the key and there’s a whining sound as the starter spins the engine free of any internal compression. Jacobs says the engine spins up to about 200 rpm or so. This lasts for a second or two before the valves are returned to normal operation and fuel is introduced. It starts normally from that point.

When shutting down, the driver turns off the key, and fuel is cut off to the engine as the valves are held open to expel the air from the cylinder without compressing it. The engine spins down until the momentum is exhausted and rotation ceases. There’s no cab shake, no shudder. Just silence. It’s pretty cool, and it won’t shake drivers out of bed.

This technology will require some explanation. Drivers, being drivers, will want to know why things aren’t behaving as they always have. A little bit of training and awareness of the system is a far better alternative than larger, more complex, and exasperating aftertreatment systems.

Extensive Independent Testing

There’s no magic here; CDA has been proven to increase fuel economy and reduce tailpipe emissions in heavy-duty diesel engines in the recognized Heavy Duty Federal Test Procedure (FTP) test cycles, including, importantly, the Low Load Cycle test. This will become increasingly important as EPA and CARB finalize their 2027 ultra-low NOx regs.

The benefits have been measured and quantified on Jacobs’ own dyno and through testing at the Southwest Research labs, among others.

In another endorsement of the technology, Cummins acquired Jacobs earlier this year, in part because of this technology. Cummins sees engine braking and cylinder deactivation technologies as key components to meeting current and future emissions regulations. Both Cummins and Jacobs have been working with Tula to test more sophisticated algorithms for cylinder deactivation technology.

Independent testing has shown in low-load cycles, CDA can reduce NOx emissions by up to 86%, while fuel consumption (and by extension, CO2), decrease by 12%. While driving a low-load (bobtail) test route around Hartford, CDA was active 49% of the time. On-highway, with a loaded trailer, CDA was active 22% of the time.

Jacobs has published several papers highlighting results from various tests using different configurations of CDA. The results are all very affirming.