There has always been some debate over the relative fuel efficiency of engines of varying displacement. No matter the displacement, a certain amount of fuel is required to deliver a certain amount of horsepower. When considering factors like mechanical drag, pumping and compressing efforts, and fuel-to-air ratios, there would not be huge differences between a 13-liter engine and a 15-liter engine.

However, the dynamics change quite a bit when considering the relative efficiency of a 15-liter engine compared to a 7.5-liter engine. That’s where cylinder deactivation (CDA) comes in. By “turning off” three of the six cylinders you effectively reduce the engine’s working displacement. And the inherent mechanical efficiencies can be surprisingly high.   

CDA also promises to reduce NOx in a big way. It will play a significant role in meeting the 2024 and 2027 emissions targets and the new NOx-reducing and low-load-cycle emissions regulations currently being developed by the Environmental Protection Agency and the California Air Resources Board.

How does CDA work?

Diesel engines work across a broad range of power requirements — from basically zero at idle, up to a full rated output. At full power, all cylinders contribute to the effort and the user needs every cubic inch of displacement. Under low-load conditions, the required power can be easily produced by three or fewer cylinders.   

If you were using all six cylinders, you might be at 10% or 20% throttle, but if you run on three cylinders at a 25% load, those three cylinders are more thermodynamically efficient. They will run hotter, the turbo is going to be more efficient, and the engine’s air-mass-flow will drop.

Reducing the mass-flow through the engine helps a couple of ways. It reduces the pumping effort required to move the air, and the lower air volume allows for a lower air-to-fuel ratio. Depending on the precise duty cycle that can reduce fuel consumption by about 40%, said Dr. Greg Shaver, professor of engineering at Purdue University.

“CDA allows us to put less fuel into the engine for the same load,” he shared during a webinar on the topic. “If the air-to-fuel ratio was 32 before, it might be 27 under CDA. It’s not starving the engine of air, therefore we don’t run into a soot-formation problem, either.”

CDA is accomplished by hydraulically managing the valvetrain to keep the exhaust and intake valves closed on certain cylinders, while cutting off fueling to those cylinders, says Robb Janak, director of new technology for Jacobs Vehicle Systems.

“We basically just turn off the intake and exhaust valve main events with a bridge mechanism. Even though the rocker arms are still moving up and down, they’re no longer opening the intake and exhaust valves. That, combined with turning off fuel injection, shuts down entire cylinder and forces the remaining cylinder to do the work — but more efficiently,” Janak says.

When you add electronics to control the intervals when CDA is applied, and then manage the number of affected cylinders and the duration of the CDA events, you can precisely control the CDA performance. That’s what Jacobs and Cummins have be doing with Silicon-valley-based Tula.

Tula’s Dynamic Skip Fire is an advanced cylinder deactivation control strategy that makes decisions for an engine’s cylinders on an individual basis, to best meet torque demands while saving fuel and maintaining performance. The company’s original Dynamic Skip Fire (DSF) software has been shown to significantly reduce CO2 emissions in gasoline engines.

Testing of the Tula algorithms technology on a Cummins engine with Jacobs CDA technology has already reduced NOx emissions by 74%.

It has been in production since 2018, with more than 1 million passenger cars and light trucks already on the road.

Emissions benefits

Taking some cylinders out of service during low-load operation does reduce fuel consumption, but the real benefit involves lower NOx emissions. Under low-load conditions, the exhaust temperature (from six lightly loaded cylinders) is too low to maintain NOx conversion in the SCR catalyst. While in CDA mode, the working cylinders work harder and produce hotter exhaust.

Running loaded at highway speed, the SCR inlet temperature of the aftertreatment system would be around 250 C or higher. But while idling or driving in stop-and-go traffic, that temperature can drop to around 150 C, which is well below the catalyst’s optimum operating temperature.

“You’re producing a lot of NOx in traffic,” says Gabe Roberts, director of product development at Jacobs. “NOx conversion goes from 100% basically down to 10% or 20% in what we call the cruise-creep cycle.”

With CDA, the temperature will still drop, but it will plateau at about 180 C.

“That means the catalyst stays hotter for longer,” he says. “Under this cycle, it still eventually cools off, but it stays hotter longer, so NOx conversion takes a lot longer to drop off. You produce a lot less NOX in CDA mode than in full six-cylinder mode.”

CDA is also helping reduce the frequency and duration of active regen events. High exhaust temps help to efficiently oxidize ash in the diesel particulate filter. In the absence of high exhaust temperatures, hydrocarbon dosing raises the temperature inside the diesel oxidation catalyst and the DPF. Using CDA to maintain high exhaust temps when coasting, for example, or while under any light-load condition, helps reduce or even eliminate the need for hydrocarbon dosing. Fuel efficiency improves as a result.

Detroit Diesel is doing something similar to maintain exhaust temperatures for more efficient regeneration. Rather than valve-controlled cylinder deactivation, it uses asymmetric injection, where fuel is cut off to three cylinders while fueling to the remaining three cylinders is increased. Detroit claims this is especially useful at startup to heat the aftertreatment system faster, and at idle to keep exhaust temperatures high, thus producing less soot. It can also be used along with the engine brake, while coasting, to boost exhaust temperatures.

“With the DD15 Gen 5, we can activate the engine brakes during driving regens to reduce the temperature drops while the truck is coasting and the engine is at very low load,” says Len Copeland, Detroit Diesel’s heavy-duty product marketing manager. “We can apply engine braking to three cylinders and add fuel to the remaining cylinders. In effect, the cylinders are fighting each other just for the purpose of generating heat.”

Copeland says process is invisible to the driver.

There are no CDA-equipped heavy-duty diesel engines in production, but Jacobs is researching CDA on engines from eight engine manufacturers around the world. Eaton is also developing several CDA and variable valve actuation technologies it believes will be needed to reduce carbon dioxide by up to 20% by 2024, and up to 27% by 2027.

Mandated emissions reductions are coming in Europe, Japan, China, India and other markets in the coming years, but they are expected to be significantly different from the U.S. EPA rules. CDA is seen as a way to approach those targets without making substantial modifications to existing base engine designs.