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Reducing pumping losses. (Graphics: Delphi).

Delphi and Tula show NVH benefits from Dynamic Skip Fire

Delphi and partner Tula Technology have revealed test data showing NVH reduction benefits with use of their jointly-developed Dynamic Skip Fire (DSF) cylinder deactivation system that was designed to reduce engine fuel consumption and emissions.

Speaking at the 2017 Vienna Motor Symposium, Martin Verschoor, Vice President Engineering, Delphi Powertrain Systems, said the DSF system delivers its efficiency benefits “with unchanged perception of NVH even in turbocharged, downsized 4-cylinder engines.” Delphi is claiming its DSF system can deliver greater than 10% reduction in CO2 emissions, depending on the engine application.

The Tula-designed system has been the subject of various SAE Technical Papers since its 2013 debut as well as presentations at WCX17 (the 2017 SAE World Congress) and the 2016 and 2017 Vienna symposia. Now approaching production configuration, DSF is claimed to be the industry’s first fully-variable engine cylinder deactivation technology.

Systems development currently is focused on 4- and 3-cylinder engines, including a Volkswagen 1.8-L TSi demonstrator that was discussed at the WCX17. Tula's early DSF development focused successfully on a GM small-block V8 installed in a full-size SUV.

During DSF operation, the decision to fire or skip a cylinder is made immediately prior to each ignition event, with each event considered independently and in sequence. DSF also allows all-cylinder cutoff during deceleration. Many factors are considered when deciding which cylinders to fire; these include torque demand, NVH characteristics of the engine and the frequencies and amplitudes that must be avoided in order to eliminate any impact on occupant comfort.

Avoiding resonant frequencies

Although occupants of vehicles fitted with production cylinder deactivation systems may not be aware of NVH effects as the system triggers, Delphi and Tula believe there is a need to smooth even subtle changes. The partners have thus focused on gaining deep knowledge of the effects of frequencies and amplitudes of vibration. Active management of NVH will allow more aggressive calibrations of the cylinder deactivation system, leading to further CO2 and fuel savings without compromising refinement, state the companies.

The companies have carried out significant research into human perception of, and sensitivity to, vibration. Testing their findings on a range of vehicle platforms, they have tailored their control algorithms to avoid sequences that excite those vibrations that vehicle operators and passengers can feel, typically 0.5 to 20 Hz.

The knowledge of frequencies and sequences that will be imperceptible to vehicle occupants dramatically reduces the calibration effort required by Tula, Delphi and their OEM customers.

Another commonly experienced source of NVH-related discomfort is due to the natural resonant frequencies of the vehicle. These can be excited by the engine, leading to noise and vibration. Because the DSF firing frequency is no longer directly coupled to the crankshaft frequency, calibration of the Tula algorithms allows these resonant frequencies to be completely avoided.

In a production vehicle that Tula tested, conventional operation yielded what was termed an “objectionable” audible boom at 47 Hz with production calibration. However, under DSF operation all firing patterns resulting in frequencies between 44 Hz and 48 Hz were avoided.

Tula CEO Scott Bailey revealed to Automotive Engineering the new focus on 4-cylinder downsized/boosted applications for DSF over two years ago, explaining that these represent global vehicles. The Jetta mule uses Tula-created algorithms and a Delphi GDi fuel system and engine controller. Significant mechanical aspects also include Delphi’s Deactivation Roller Finger Follower (DRFF) valve control system. Described by the company as an “advanced variable valve actuation technology” it uses a rocker arm mechanism working with a 3-lobe cam that switches between full and zero-lift profiles.

Full cylinder deactivation can be achieved in a single cam revolution at speeds up to 3000 rpm, states Delphi. High valve spring loads are unnecessary.

DSF + 48V coming next

The two companies report that studies using this engine have shown what is described as “excellent correlation” of NVH characteristics between operation without DSF and with the system activated. Results for different levels of firing density are stated to have shown that a more aggressive calibration (i.e. more cylinder deactivation, placing higher loads on each firing cylinder to increase efficiency) can be implemented with no reduction in refinement discernible to vehicle occupants.

The Delphi team also is developing a turbo-GDi demonstrator vehicle that integrates DSF with Delphi’s 48V mild hybrid system. Integrated control of the two systems has been designed to achieve smooth torque delivery even at low engine speeds and firing densities. When the demonstrator is fully optimized, the combined system is expected to deliver a total CO2 reduction of up to 20% compared to the 4-cylinder turbo-GDi baseline engine.

By the time the first OEM customer for DSF is unveiled, Delphi's Powertrain operation may be either a new standalone division or a new company, possibly with a new owner. In May 2017 Delphi announced that it is spinning off its $4.5-billion powertrain operation into a separate publicly traded company, as it continues to transform its portfolio with a strong focus on self-driving vehicle technologies. 

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