Cylinder Deactivation Coming for GM’s 3.0-liter Duramax Diesel?

Cylinder deactivation once meant periodically shutting off a set number of cylinders—typically half the total (or one on a 1.0-liter EcoBoost three-cylinder)—and it was always the same cylinders that got shut down. Then in 2014,I covered a startup called Tula Technologies, which proposed a Dynamic Skip Fire technology that assessed the instantaneous torque demand being made of the engine every 90 degrees of crankshaft rotation and then decided which cylinders to fire and which to idle. This cylinder-deactivation system came to market in 2019 on the Chevrolet Silverado and GMC Sierra 5.3- and 6.2-liter V-8 engines, and will soon arrive onthe 2021 Chevrolet Tahoe/Suburban, GMC Yukon/XL, and Cadillac Escalade/ESV full-size SUVs. Now Tula is readying this promising technology for use in diesel engines under the name diesel Dynamic Skip Fire (dDSF).

How Does Cylinder Deactivation Save Fuel?

Towing a big trailer up Davis Dam may require a big, burly V-8 engine, while cruising along a level freeway when that same truck is empty barely requires a tiny two-or three-cylinder engine. When eight big cylinders only need to burn the air and fuel of two or three little ones, the throttle plate feeding air to them stays mostly closed, making the engine work to suck what little air it needs through the small throttle opening. This work is referred to as "pumping losses." But if you shut down five or six cylinders, the two or three remaining cylinders get to work as hard as they were when all eight were lugging that trailer uphill, so the throttle opens wider and the engine no longer works hard to breathe. Of course, diesel engines run unthrottled all the time, so there's less in the way of pumping losses to eliminate, but there is still efficiency to be gained by running fewer cylinders at higher load.

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How Does Dynamic Skip Fire technology Work?

All the more recent cylinder deactivation systems work the same way: The intake and exhaust valves are deactivated in such a way as to capture a volume of air (and perhaps some recirculated exhaust gas) in the deactivated cylinder that acts like a spring, consuming a bit of energy when the piston goes up, giving it back again on the way down while fuel flow is cut off. In the overhead-valve GM V-8s this is accomplished by hydraulically collapsing the lifters that actuate the pushrods and valves in the deactivated cylinders. (Overhead-cam engines typically require a special collapsible hydraulic valve tappet on each valve.) With no air flowing through these idled cylinders the exhaust catalysts stay hot and fully functional.

How Much Fuel Can Dynamic Skip Fire/Dynamic Fuel Management Save?

Tula claimed the technology would boost the fuel economy of a full-time gas V-8 engine by 15 to 18 percent, and indeed comparing 2018 and 2019 Silverado/Sierra 5.3-liter eight-speed automatic models, EPA combined economy improves by 11.8 percent on RWD models and 5.9 percent on 4WD models, and the 2018s featured the more basic "Active Fuel Management" V-8-4 cylinder deactivation. Tula's fuel efficiency expectations for dDSF are more modest for engines like GM's Duramax diesel—1.5 to 4.0 percent, though CEO Scott Bailey points out that for heavy-duty diesels, 3.0 percent of 20,000 annual gallons of diesel fuel burned by a long-haul semi-tractor adds up to meaningful savings. He adds that an 80,000-pound rig traveling at a steady 65 mph on a flat highway requires 214 horsepower, and that most diesels will run all cylinders under this situation, but on the "line haul" cycle used in the commercial industry, representing a 151-mile stretch of road in Texas, speeds average 61 mph and 35 percent of this cycle will be driven at less than all cylinders. In such use there is significant opportunity for dDSF to generate meaningful benefits, even at higher speeds. Bailey also reiterates that DSF in gas or diesel engines is most beneficial under partial-load operation.

The Diesel Emissions Challenge

Running unthrottled is great for diesel fuel economy, but it's terrible for emissions. With engine load and speed being regulated strictly by the amount of fuel they're supplied, under light load and idle conditions, when there's a lot of extra (cool) air flowing through the cylinders, the exhaust temperature plunges. When this happens, the selective catalytic reduction (SCR) NOx catalyst—which ideally needs to operate at a temperature above about 480 degrees F—can stop working entirely, spiking emissions. The fix for this is to "increase conventional thermal management." That involves injecting a surplus fuel somewhat late so that it can burn in the exhaust system to heat the catalyst. This approach obviously torpedoes the fuel economy.

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How Well Does dDSF Work at Saving Fuel and Reducing NOx?

Tula has worked with Cummins and Jacobs Vehicle Systems (of "Jake Brake" fame, for the diesel valvetrain deactivation hardware) to equip a 15-liter 500-hp, 1,850-lb-ft Cummins X15 Efficiency Series inline six-cylinder variable-nozzle turbodiesel engine with dDSF cylinder deactivation. Its standard aftertreatment system included a diesel oxidation catalyst, diesel particulate filter, and a copper zeolite SCR catalyst for tailpipe NOx control. An engine "map" that plots turbocharger outlet temperatures at various different torque outputs and engine speeds without running "thermal management" illustrates the light-load challenge. The catalyst only functions in the brown, orange, or yellow regions. Note how much more of the map involves these colors when dDSF in engaged. Basically, dDSF elevates exhaust temperatures between 50 and 200 degrees depending on the operating conditions. Test results indicate that on the government's Low Load Cycle, NOx is reduced 74 percent while the engine consumes 5 percent less fuel (and hence produces 5 percent less CO2) compared to a baseline engine running conventional thermal management. Note that results are most impressive for city use and port drayage applications; less so for long-haul trucks carrying heavy loads at constant highway speeds.

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Will dDSF Work on Smaller Engines Like GM's 3.0-Liter Duramax?

"The technology itself scales to virtually any diesel. So, ultimately, we should find our way into light-duty trucks and passenger-car diesels," says Tula CEO Scott Bailey. He is, of course, not authorized to speak about any such plans for future products. And while the sexy new 3.0-liter I-6 diesel shares its basic configuration with the Cummins test engine, the Duramax's use of DOHC valvetrain complicates the task and cost of cylinder deactivation, while the engine's lack of an exhaust retarder like the Jake Brake presents no potential net cost savings as it does on the Cummins X15. The overhead-valve 6.6-liter Duramax turbodiesel V-8 (with exhaust retarder) might be a more likely candidate for dDSF, given the working arrangement GM and Tula already have. But these engines aren't facing the same emissions hurdles yet, so don't hold your breath for DSF (or DFM) coming to a diesel pickup or SUV any time too soon. We do expect to see it enter production on a heavy-duty semi-tractor engine soon, but no production plans have been announced.