|A comparison of the benefits of two different cylinder deactivation operating strategies for downsized 3-cylinder engines such as the 1.0L EcoBoost. The largest difference proved to be in cost. Schamel et al. Click to enlarge.|
Ford’s 1.0-liter EcoBoost engine recently received its eighth award in four years at the 2015 International Engine of the Year Awards. The judges selected the 999cc EcoBoost as the winner (for the fourth consecutive year) in the “Sub 1-Litre” category, awarding it 444 points. The second place challenger, GM’s 999cc EcoTec 3-cylinder turbo, scored 176 points. (Although the EcoBoost received 61 fewer points than last year, the gap between first and second place widened.) Last year, the 1.0‑liter EcoBoost became the first engine to be named overall International Engine of the Year three times in a row, and was in 2012 named “Best New Engine.”
One of the judges, Hormazd Sorabjee of Autocar India said, “That so much can be achieved from a sub 1-liter, three-cylinder motor is simply astounding.” That may be, but Ford is exploring ways to further improve the fuel efficiency of the three-cylinder design, including the possible use of cylinder deactivation. (Earlier post.)
Even for an aggressively downsized engine such as the 1.0-liter EcoBoost, a significant improvement in vehicle fuel economy could be found by exploiting cylinder deactivation. The highest priority in the development of new combustion engines for automotive applications is the ongoing reduction of fuel consumption.—Carsten Weber, advanced powertrain manager, Ford of Europe
In a paper presented at the International Vienna Motor Symposium in May, Dr. Andreas Schamel, director, Ford Global Powertrain, Research & Advanced Engineering, described Ford’s and its partners’ analytical and experimental investigation into potential cylinder deactivation strategies for use on a downsized three-cylinder engine. Co-authors of the paper, in addition to Carsten Weber, were Dr. Martin Scheidt of Schaeffler AG and Dr. Hartmut Faust of LUK GmbH.
Their focus was resolving—and optimizing—two conflicting attributes of cylinder deactivation in such an engine: fuel economy vs. vibration behavior.
In a 2013 interview with Automotive World, Dr. Schamel suggested that deactivation on a three-cylinder engine would be was “somewhat tricky.” Asked about that comment now, Schamel observed that just because it was tricky, “it doesn’t mean we give up on it. We were pleasantly surprised that we overcame some of the difficult elements.”
Cylinder deactivation is a well-known and increasingly applied approach to achieving a downsizing benefit on larger displacement engines. The de-throttling effect and reduced friction under part load conditions resulting from deactivation improve the overall efficiency of engines which would otherwise suffer from the excess displacement.
Based on drive torque demand, even three-cylinder engines could benefit from cylinder deactivation (depending upon load; a high load cycle such as US 06 hardly benefits from deactivation, the team noted). However, while in larger engines an even firing sequence is maintained, the three-cylinder design requires additional technology. The Ford team investigated two options:
A fixed cylinder deactivation—i.e., applying an appropriate valve deactivation mechanism to one cylinder. On a 3-cylinder engine this leads to the capability to run a 2⁄3-engine (666cc active displacement for the 1.0L EcoBoost). This, however, results in an uneven firing order leading to a challenging low engine order excitation. Successfully implementing this approach commercially requires a way to avoid compromising customer expectations on NVH.
A rolling cylinder deactivation—i.e. varying the number and sequence of deactivated cylinders. This allows the engine to be run in d could be used to run the engine in 1⁄2-engine mode—corresponding to a 500 cc active displacement for the EcoBoost—but with the advantage of an even firing order. This approach requires a significant effort in controls and complexity.
To gather real-world fuel consumption results, the Ford team took to the roads in and around Cologne with a Ford Focus equipped with the 1.0L EcoBoost engine mated to a six-sped manual transmission. The route combined Autobahn, city driving and rural roads over approximately 55 km (34 miles), including altitude differences with different gradients.
They found that a significant portion of the commuter drive was performed in cylinder deactivation-mode providing a fuel economy benefit under real driving conditions. The biggest contribution of cylinder deactivation was reached in the mid-speed area—up to 30% of the overall distance.
For reasons of cost, flexibility and packaging, Ford chose to use switchable roller finger followers (SRFF) as the deactivation mechanism in its EcoBoost prototype engine. SRFF can implement either deactivation strategy. Based on the chosen strategy, the deactivation mechanism would be used for one or for all cylinders.
There are four boundaries to the operating map of cylinder deactivation: two related to engine speed and two depending on the engine load.
At higher engine speeds the deactivation mechanism and its controls are not fast enough to enable the mode switch, so that an operation in deactivated mode at higher speeds is not possible.
The lower speed limit of the operation range is dictated by powertrain vibration. At low engine speeds, eigenfrequencies of the powertrain will be excited which will lead to an unacceptable NVH behaviour.
The maximum possible load of cylinder deactivation is given by the fuel economy benefit. At higher engine loads, the engine is more efficient running with all cylinders. At lower engine speeds, there is a load- and speed-dependent NVH limit. Increasing the engine speed and hence the excitation frequency of the main engine order, decreases the vibration angles which enables operation at higher engine loads.
|Boundaries to the operating map of cylinder deactivation. Click to enlarge.|
To mitigate the NVH effects, the Ford team developed a system used in the prototype that combined a dual mass flywheel (DMF) with pendulum absorber with a tuned clutch disc. The DMF with the tuned clutch disc counteracts the engine excitation, and the circumferential Pendulum Damper reduces the engine order vibration by more than 90%. he introduction of this decoupling system allowed a wider operating range of cylinder deactivation.
In fact, they found that with the optimized DMF and the circumferential Pendulum Absorber, the vibration behavior was similar for all three operation modes: i.e. fixed cylinder deactivation, rolling cylinder deactivation and the base operation with all cylinders.
Depending on the drive cycle and the cylinder deactivation strategy, the researchers observed a fuel economy reduction of 4 to 6% for a C-car such as a Ford Focus. The greatest difference between the various modes was cost.
For the cylinder deactivation strategy with one fixed cylinder, one single cylinder has to be activated so that the actuators are required only for this cylinder. In the case of the rolling deactivation, actuators are required for all cylinders. The controls effort is higher for an engine with cylinder deactivation than for a conventional operating engine with all cylinders. The rolling cylinder deactivation is the most complex system with actuators for all three cylinders including their drivers. Because of the required oil pressure for the actuators, the parasitic losses are higher for an engine with rolling cylinder deactivation, too. The acoustic noise radiation in all three operation modes is similar due to improved hardware. The rolling deactivation mode has a small advantage compared to the fixed deactivation mode because of the more even combustion excitation. Comparing the fuel economy benefit, there is a significant benefit for the fixed cylinder deactivation compared to the base operation mode with all cylinders. The rolling deactivation mode shows a further small benefit and has the best fuel economy.
… The ratio of total functional benefit and cost seems to be advantageous for the single cylinder deactivation strategy versus the rolling approach. A high level contemplation shows a 90% fuel economy benefit for 40% of the cost.—Schamel et al.
Although the study was “a really nice joint piece of research,” Schamel said, “the specifics on the rolling versus the fixed strategies is really unique to the three-cylinder problem statement.” Ford has not announced if it will take the next step and commercialize the cylinder deactivation technology in the 3-cylinder unit.
I am not excluding it, but nor am I announcing it.—Andreas Schamel
Dr. A. Schamel, Ford Forschungszentrum, Aachen; Dr. M. Scheidt, Schaeffler AG, Herzogenaurach; Dipl.-Ing. C. Weber, Ford Werke GmbH, Cologne; Dr. H. Faust, LuK GmbH & Co. KG, Bühl. “Is Cylinder Deactivation a Viable Option for a Downsized 3-Cylinder Engine?” 36th International Vienna Motor Symposium (2015)