Delphi’s Dynamic Skip Fire (DSF) technology is making its production debut in some versions of the fully redesigned 2019 Chevrolet Silverado with 5.3- and 6.2-liter V-8 engines, in which it will be called Dynamic Fuel Management.
For GM, it marks the most significant upgrade the technology has received since it was introduced in its current form in 2005. Chevrolet hasn’t talked Silverado-specific improvements, but Delphi said that, in general, the new technology could boost fuel economy by 5 percent compared with existing deactivation systems that shut off specific cylinders or entire cylinder banks during cruising or coasting.
Cylinder-deactivation systems have been under the hood of a wide range of U.S.-market vehicles for decades—even the V-12 Lamborghini Aventador and the new Mercedes-AMG S63—but the common criticism is that engines so equipped are less refined in their cylinder-cutoff modes and that their transitions on and off are quite noticeable. This new dynamic system could do away with the harsh transitions and vibrations that can accompany such systems.
DSF doesn’t prioritize specific cylinders. While the firing pattern it follows might appear totally random, it’s dictated by an algorithm created by Tula, a California company founded in 2008 by Adya Tripathi, with General Motors one of its original strategic investors. Still, although there’s an algorithm, there are no preordained templates or patterns. The decision on whether a cylinder fires or not is made on an event-by-event basis. And while the algorithm aims to smooth vibration and keep heat distributed well, the priority is cylinder loading. Based on a broader snapshot, at a certain sustained engine load, the system targets a specific firing density—how many cylinders fire over a specific time to achieve the right loading.
Running Wide Open
The technology makes a bigger difference on engines that are less loaded during steady-state cruising and light acceleration; it is claimed by Delphi to deliver a U.S. driving-cycle carbon-dioxide reduction (and mileage gain) of more than 15 percent on a V-8 or about 7 percent on a four-cylinder. One key to Dynamic Skip Fire’s efficiency gains is that it reduces pumping losses by putting far more load on the individual cylinders that are fired—so the efficiency of the entire engine increases.
That max-loading concept means that the system adjusts output with the firing frequency, not the throttle, and cylinders fire less often than with existing cylinder-deactivation systems. In light- or medium-load highway driving, DSF can fire a four-cylinder engine at the equivalent of a 1.5-cylinder engine; in very light-load conditions and under 31 mph, it can run the engine at what amounts to the level of a 0.8-cylinder engine. GM hasn’t yet revealed specifics of how the system works in the trucks, but the company said it is using its own calibration and noted that it can operate on as small a scale as one cylinder.
But with fewer individual pulses of combustion, those pulses are stronger. The algorithm aims to counter vibrations, but adding a belt-based mild-hybrid system and the flat torque output from the electric motor helps smooth out the engine, and it can widen the range—both in revs and engine load—at which cylinder deactivation can take place, including coasting with all the cylinders off. (GM’s version does not incorporate the electrified components.)
And there’s one really neat other thing that the system does in conjunction with the motor system: Tula’s software controls help further smooth the system by pulsing the hybrid motor in sync with Skip Fire’s engine-torque pulses.
To prove this concept, last year Delphi installed the technology—called eDSF when it’s joined with a 48-volt mild hybrid system—in a current-generation Volkswagen Passat with a turbocharged 1.8-liter four-cylinder engine and a six-speed automatic transmission. At this year’s CES, Delphi took the car a step further by adding an electric turbocharger, which it calls an e-charger. The e-charger doesn’t replace the existing turbocharger but instead supplements it—enabling a supercharger-like kick of torque off the line, as well as quickened throttle response to help when the engine is restarted and to make up for the slight lag that it and the other components can introduce.
DSF requires a Tula co-processor that operates with the powertrain controller; it also adds a few more sensors, plus output drivers and deactivation components for the valvetrain. A cylinder is deactivated via a simple mechanism that places a pin at one end of a roller finger follower; engine oil pressure moves an anchor pin out of the way, allowing the follower to move while keeping the valve closed.
Heard, Not Felt
A few weeks ago, we previewed the updated system on public roads, and it works exactly as described. Keeping light on the accelerator enabled DSF, but pressing past maybe a quarter of the accelerator’s travel bypassed it. And because of the e-charger in the prototype Passat we rode in, off-the-line acceleration felt especially perky. Delphi’s display in the car showed us which cylinders were firing and when, but we couldn’t perceive the transitions as it went from firing all four cylinders as in a non-DSF-equipped car to its various partial-firing modes and back. The only sensory indication when the system was operating was auditory, not physical, sounding as if the engine were being lugged at much lower rpm than it was actually spinning.
In this four-cylinder demonstration, the company only enabled DSF from about 1500 rpm but said it could lower that threshold if combined with the full suite of technologies, adding that it would be quite a bit lower for a torque-rich V-8. The test VW, according to Delphi, achieves more than a 15 percent boost in fuel economy, increased low-end torque, and a zero-to-19-mph time that is improved by 20 percent.
On its own, Dynamic Skip Fire produces mileage gains in the range of 5 to 10 percent on a V-8 engine. And while it’s actually bringing up the NOx emissions level a little bit from each individual cylinder fire, those cylinders aren’t firing nearly as often, so overall NOx levels are down, too.
The reality is that over the next decade, the vast majority—about 95 percent of the vehicles sold in 2025—will still have an internal-combustion engine under the hood. Delphi said it sees hybrids growing to 31 million vehicles annually by then, or which it believes 65 percent will be 48-volt mild hybrids. The market for nonhybrid gasoline-powered vehicles is expected to fall by 20 percent during that time, but with tech like smarter cylinder deactivation—in these upcoming GM V-8 trucks, for instance—the workhorses aren’t going to get left behind.
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