Two engine options in new Mazda6; updated 2.5L with cylinder deactivation, turbo 2.5L with Dynamic Pressure Turbo
The updated Mazda6 (earlier post) introduced at the Los Angeles Auto Show this past week offers two powertrain options in the US: an upgraded SKYACTIV-G 2.5 naturally aspirated engine and the turbocharged SKYACTIV-G 2.5T engine, first unveiled at the 2015 LA Auto Show for the 2016 CX-9.
Mazda6’s 2.5-liter, four-cylinder engine comes with a number of upgrades to reduce internal friction and improve efficiency across all rpm, and also adds a greater level of refinement. A new technology that is unique to Mazda in North America is cylinder deactivation in a four-cylinder engine.
The outside two cylinders can shut down when the vehicle is operated at steady speeds between 25 and 50 mph, but all four cylinders work instantaneously when needed for maximum performance. A centrifugal pendulum has been adopted in the torque converter of the six-speed SKYACTIV-DRIVE automatic transmission, counterbalancing any vibration that might otherwise be felt when running on two cylinders. The result is an imperceptible switchover between two- and four-cylinder modes.
Higher-grade models will also be available with Mazda’s turbocharged SKYACTIV-G 2.5T engine. First available in the current-generation Mazda CX-9 midsize crossover SUV, the SKYACTIV-G 2.5T produces 310 lb-ft of torque from just 2,000 RPM and 250 hp on 93-octane fuel (227 hp with 87-octane gasoline). The SKYACTIV-G 2.5T engine received a 2017 Wards 10 Best Engines award.
Mazda’s Dynamic Pressure Turbo varies the degree of exhaust pulsation depending on engine speed. At low rpms (below 1620 rpm), the volume of the exhaust ports is reduced by closing a valve located just before the turbine that drives the turbocharger. This reduces interference between exhaust pulses and maximizes the energy of each pulse to obtain a high turbine driving force. This allows the turbocharger to spool up quickly, creating near-instant boost.
At higher rpms there is sufficient energy in the exhaust flow and the valve opens, allowing the turbine to be driven by a steady flow of exhaust gases as in a traditional turbocharger. Whereas existing variable turbochargers adjust the speed or direction of exhaust gas flowing into the turbine, Dynamic Pressure Turbo varies the degree of exhaust pulsation.
A pulse-scavenging 4-3-1 manifold prevents exhaust backpressure and helps the engine breathe freely.
Cooled exhaust gas recirculation (EGR) is used to reduce combustion temperatures, preventing knocking and therefore reducing the need for fuel-enrichment (when extra fuel is dumped into the cylinders as a means to cool the engine). Many times when turbocharged engines fail to achieve EPA-estimated fuel-efficiency numbers in the real world, it is because laboratory tests don’t replicate the higher-load driving styles that necessitate fuel-enrichment.
In too many instances, turbocharged engines are mated to continuously variable automatic transmissions (CVT), operating in a narrow band of efficiency, sacrificing a connectedness between driver and car. Mazda’s six-speed SKYACTIV-DRIVE automatic transmission, paired as standard to the SKYACTIV-G 2.5T engine, keeps its torque converter locked through most of its operation, giving a more connected sense of controllability.
Finally, as a champion of the manual transmission, Mazda6 will continue to offer the SKYACTIV-MT, paired with the SKYACTIV-G 2.5 engine.