When the former Chrysler Corp. introduced the Pentastar 3.6-L gasoline V6 in the 2011 model year, it was engineered to replace seven incumbent V6 engines, and along with 3.0-L (for China) and 3.2-L variants. It has done exactly that. However, despite all the flexibility built into it, the Pentastar remained relatively uncomplicated—port fuel injection, no exhaust gas recirculation (EGR), and it was naturally-aspirated at a time when turbochargers were proliferating elsewhere.
This approach kept costs down, and for 2016 Fiat Chrysler Automobiles is phasing in a Pentastar update in the same cost-conscious vein—important because the V6 serves as a value offering in virtually all FCA's North American nameplates, including the Ram pickup. The revamped V6 delivers a 6% increase in fuel economy on the combined U.S. Federal Test Cycle, along with a 5.0 hp/3.7 kW bump (to 295 hp/220 kW) and 15% more torque under 3000 rpm, according to Bob Lee, FCA North America Vice President of engine, powertrain and electrified propulsion.
Still naturally aspirated and port injected, the upgraded V6 also is now capable of transitioning to direct fuel injection and turbocharging—both technologies are "on the shelf" in Auburn Hills—as well as particulate traps, should future regulations require them.
Variable valve lift
A two-step variable-lift intake valve system (https://youtu.be/YZtqCq9TXZg) was added, activated by oil pressure under control by solenoid valves—two for each cylinder head. One solenoid is in front, for the first cylinder intake valve; the other is at the rear, for the second and third intakes. Each roller cam follower incorporates a pivotable high-lift section held in place by a spring-loaded lockpin; the high-lift mode (10.3 mm/0.41 in) is the default.
On acceleration, a solenoid valve opens and oil pressure pushes the lockpin, releasing the high-lift follower section. It pivots down on a bushing, and the roller follower runs on low-valve-lift (5.75 mm/0.23 in) cam lobes, in which the engine stays through to the 2800-rpm switchover point. Lee explained that this switchover reduces pumping power draw and contributes to improved combustion, which delivers both the modest increase in horsepower and boosts fuel economy by 2.7%.
Thanks to steps taken to reduce combustion knock, the Pentastar compression ratio was raised from 10.2 to 11.3:1. Combined with new eight-hole injectors (vs. the previous four-hole), tumble intake ports and high-energy (100 mJ) ignition coils, the result is a claimed 1% fuel economy improvement.
The engine's variable intake and exhaust valve timing (VVT) authority has been increased to a range of 70°, vs. 50° previously. VVT on the old engine was part of the control system that eliminated the need for EGR. On the 2016 Pentastar, VVT incorporates an engine-torque-based cam phasing strategy, which reduces cylinder pumping losses. The enhanced cam phasing also is one contributor to the compression ratio boost by minimizing engine knock during hot starts.
In addition, the VVT was specifically calibrated to combine with longer intake runners in a redesigned intake manifold, contributing to the improved low-range torque. And the system expands the operating range of the idle stop/restart system, a carryover feature that not only improves fuel economy in real-world operation but also gives FCA a substantial CAFE credit.
After the long period without EGR, engineers added a liquid-cooled system that has normally been a design feature only on many heavy-duty trucks. It reduces temperature of the exhaust gases from 650°C to 130°C.(1202°F/266°F), Lee told Automotive Engineering. The reduced gas temperature helps enable the higher compression ratio by suppressing knock at higher loads. And the EGR in itself delivers a 0.8% improvement in fuel economy and low NOx emissions. High-tumble intake ports and shrouded valves further enhance fuel, air and exhaust mixing.
The Pentastar's high production volume (over 5 million units to date) puts an exclamation point on the value factor. Direct fuel injection (DI) was engineered into the engine design but it remains "on the shelf," said Lee.
DI is a positive addition only where loads are higher, particularly in applications with supercharged and turbocharged engines and when it also can be justified by the benefit of a higher compression ratio, noted Steve Gorgas, the Pentastar Chief Engineer. He added that a DI hardware package includes an engine-mounted high-pressure fuel pump and rail, which adds both weight and cost. Further, DI also uses more expensive fuel injectors that deliver 5-7 pulses per power stroke and requires very precise distribution of the fuel sprays to control formation of particulates.
Lee explained that as particulate standards tighten by 2018, the potential need to add particulate traps also has to be considered. Gasoline port-injection systems have lower efficiency, he admitted, but they also provide some additional time for the fuel to atomize on the back of the intake valves, which minimizes particulates.
Turbocharging, Gorgas said, isn't all free power. Although the overall result is a plus, there is greater parasitic loss at under 2000 rpm. So for the Pentastar, a turbo doesn't fit yet. However, like DI, a turbo was engineered in and is "on the shelf."
Weight, friction reductions
The various upgrades added 13 lb (6kg) to the engine's overall mass, with the EGR and variable lift system accounting for over 7.5 lb/3.4 kg. Gorgas' development team used a "look everywhere" approach to weight reduction, and employed thin-wall castings for the heads, windage tray and front cover. Across the range of installations, the upgraded V6 weighs about 326-332 lb/148-151 kg fully dressed, an average of 4.0 lb/1.8 kg less than the previous generation Pentastar.
Except for trail-rated Jeep applications, the oil pan is smaller, and oil capacity drops from six to five quarts (5.7 to 4.8 L), saving about 2.0 lb/0.9 kg. The block was modified to maintain structural strength while thinning out some of reinforcement ribs. The structural provision for an air pump, which has never been used, was eliminated. Overall, block weight was pared by almost 2.0 lb (0.9 kg). The main bearing caps are nodular cast iron, vs. the previous powdered cast iron. Nodular iron costs more, Gorgas noted, but it permits a level of contouring that saved 0.8 lb (0.4 kg) overall.
The intake manifold, as part of the redesign that lengthened the runners, is now a two-piece design, vs. the previous three section, for a 0.7 lb/0.3 kg reduction. There's a pair of visually obvious frontal modifications: the crankshaft damper retaining bolt has a recess in the head, saving 43 g/1.5 oz and the water pump pulley has drilled holes in the front face, saving almost 110 g/4.0 oz.
Many improvements reduced friction to improve fuel economy 1%, led by reductions in crankshaft journal width (from 72 to 68 mm) and crankpins (from 59 to 54 mm). Fillet rolling was increased to maintain strength and the overall redesign was verified to maintain the previous NVH characteristics. The timing chain guide is faced with HDR-1, an ultra-low- friction plastic.
Finally, the valve train was given a super-polish treatment, valve spring loading was reduced, and the two-stage oil pump was refined to reduce parasitic loss.Continue reading »