Audi’s all-new 3.0-L turbocharged gasoline V6, rated at 354 hp and 369 lb·ft (264 kW and 500 N·m, respectively) debuted recently in the 2018 SQ5 crossover. The V6 is the first engine produced under a new powertrain strategy that sees Audi lead development on V6 engines, handing responsibility for gas V8s to Porsche. Audi retains control of the 3.0-L and 4.0-L TDI diesel engines.
Development of vee engines and fuel cells is concentrated at Audi’s Neckarsulm R&D site with 2,500 engineers led by the recently appointed Dr. Franz Bäumel. Speaking to Automotive Engineering at the SQ5 media launch, Bäumel predicted that by 2025 one-third of Audi’s fleet would be electrically powered, the balance by ICEs. Beyond that date, the ICE would continue to be developed as it is “the best compromise for power and efficiency,” in addition to hydrogen and gas-powered vehicles.
Designated EA839, the new long-stroke V6's bore and stroke measure 84.5 x 89 mm. The deep-skirted cylinder block uses steel liners. Unusually for a contemporary V6, its cylinder banks feature a 90° vee; a factor in this design is to package the turbocharger within the vee. To offset the second-order vibration inherent in this layout, a balancer shaft is located within the vee. Newly developed rings for the aluminum pistons help reduce friction. In fully-dressed form, the engine weighs 379 lb (172 kg).
The new TFSI (direct injected and turbocharged) engine family, which includes a high-performance 2.9-L variant rated at 450 hp (335 kW)) and 442 lb·ft (600 N·m) include centrally-located injectors which Audi engineers claim are vital in the operation of the B-cycle combustion process that is also used on VW’s EA888 gasoline 4-cylinder unit—also a further development of the Miller cycle.
In this process the intake valves close well before the piston reaches BDC. This unusually short opening time keeps the fresh gas flow comparatively small, artificially creating a smaller displacement. When the piston moves back up after reaching BDC, the compression phase starts later than in a conventional engine. This allows a high geometric compression ratio of 11.2:1—with combustion occurring in a relatively small combustion chamber.
Compared with the compression phase, the expansion phase has been extended, and the longer expansion of the gas increases efficiency.
Timing of each of the engine's four camshafts can be phased up to a crankshaft angle of 50°. At higher loads the two-stage Audi valvelift system (AVS) closes the intake valves later. The opening time increases from 130 to 180° crankshaft angle, while at the same time the intake valves lift increases by 0.2 to 0.4 in (6.0 to 10.0 mm).
The twin-scroll turbocharger lies within the vee with the exhaust branches from both banks running separately until merging directly ahead of the turbine wheel. This configuration contributes to improved throttle response and the wide peak torque band from 1370 to 4500 rpm, according to Audi engineers. Peak power is developed from 5400 to 6400 rpm.
Engineering attention also was paid to the engine’s thermal management. The crankcase and cylinders have separate "strategic" cooling circuits: Under cold start conditions the water pump switches off coolant flow through the engine until operating temperature has been achieved. At that point, coolant is pumped through the entire engine, including the separate cylinder crankcase circuit.
The exhaust manifolds are integrated in the cylinder heads and into the cooling circuit, improving engine warm-up time. When the engine is hot, exhaust temperatures are kept lower, significantly reducing fuel consumption, according to the engineers.
Hybrids across the range
On the standard 20-in rims fitted to the SQ5, Audi claims a combined 28.8 US mpg and 304.2 g CO2 /mi. Powertrain engineer Andreas Fröhlich was keen to emphasize that the V6 starts Audi’s mild-hybrid program when it is fitted to the 2018 A8. It uses an integrated belt-driven starter to augment the standard starter, allowing the engine to be switched off and disconnected from the transmission for up to 45 s ‘coasting’ at speeds between 18 mph (30 kph) and 100 mph (160 kph).
Audi engineers claim 0.7 L/100km consumption savings on the current NEDC cycle. A lithium-ion battery in the 48-V loop restarts the engine within 300 ms. The fuel saving for the mild-hybrid version is 0.2 US gal/62.1 mi on the NEDC cycle.
The automatic coasting function is controlled via the forward facing camera and the radar system. However, if the camera is obscured by rain, for example, the system won’t operate.
Fröhlich explained that Audi is the "center of excellence" for the technology within VAG and that there will be a rapid roll-out of this mild hybridization across the model range. All Audis will feature the technology before the end of this decade and it will be used on both gasoline and diesel engines mated to either direct shift gearboxes (DSG) or torque converter automatics.Continue reading »