What is SKYACTIV?

Car manufacturers find themselves confronted with tough targets to reduce greenhouse-gas emissions and fossil fuel use over the next few years. As a consequence many manufacturers have latched onto futuristic ideas like hybrid-drive, battery-electric vehicles and fuel cells.

Mazda is different however. The Japanese manufacturer believes there's plenty of development potential left in internal combustion drivetrains. As we've reported previously, Mazda is going its own way with a raft of 'known quantity' solutions that have come to the fore over years of internal combustion engine development. The engineering program resulting from Mazda's 'sustainable zoom-zoom' philosophy is named 'SKYACTIV' and it embraces affordable fixes that can collectively reduce the fuel consumption of Mazda's product range by 30 per cent, between now and 2015.

As a taste of what SKYACTIV holds in store, Mazda shipped out a TPV (Technical Prove-out Vehicle) for a brief drive program around Melbourne's Sandown Racing Circuit earlier this week (pictured). Practically every facet of SKYACTIV development short of the SKYACTIV-G (petrol) engine was incorporated in the TPV.

The car (pictured) is a mule -- a current Mazda6 'top hat' on a new platform that is 50mm longer and about 120kg lighter than an equivalent variant from the current range. It's powered by the SKYACTIV-D diesel engine, which runs a remarkably low compression ratio of 14:1 -- roughly the same as the SKYACTIV-G petrol engine. With the lower compression ratio, the engine can make do with lighter internals (pistons, conrods, crankshaft) since these don't need to withstand the same pressures as turbodiesel engines running 18:1 or higher. The lighter mass of the reciprocating and rotating parts allows a higher rev range for the new engine. During the drive around Sandown, the diesel was consistently reaching 5500rpm, although Kiyoshi Fujiwara, the company's powertrain development guru, said that the recommended maximum speed for the engine was 5200rpm.

SKYACTIV is a four-fold approach, mostly centred around finding efficiency gains from: existing internal-combustion technology (petrol, diesel), transmission (epicyclic automatic, manual), body and chassis. By honing existing technology, it's a cheaper alternative to hybrids and battery-electric vehicles that require additional hardware, some of which impose a substantial cost on the production of the car -- lithium-ion batteries being one example.

According to Mazda, SKYACTIV is a 'building block' strategy that can combine with 'electric devices' -- hybrids and EVs -- to reduce global fossil fuel consumption and greenhouse gas emissions. The company's idle-stop is one link between the internal combustion technology and hybrid-drive systems. A guiding element in the SKYACTIV program is that the engines developed will be compatible with both idle-stop and hybrid-drive systems. Hybrids are the link to EVs and fuel-cell vehicles. Mazda believes, as do other manufacturers, that the majority of road transport will remain powered by internal-combustion engines for the next decade. That was why the SKYACTIV program focuses principally on improving internal combustion technology.

Engines
The typical internal-combustion-engine (ICE) car is only around 30 per cent efficient. According to Mazda, future technological developments might yield ICE engines up to 60 per cent efficient. This will result from HCCI technology (Homogeneous Charge Compression Ignition) -- a next-generation technology that employs lean-burn combustion in an engine with a super-high compression ratio to reduce NOx and CO2 emissions beyond the levels possible with today's technology. HCCI, which is possibly better known by the Mercedes-Benz name 'DiesOtto', is still some way off from real-world application, but it's just over the horizon.

Four types of energy loss contribute to the ICE's lack of efficiency: Exhaust, Cooling, Mechanical and pumping. In the early stages of the SKYACTIV program, Mazda engineers settled on compression ratio as the best means of controlling the level of efficiency loss in both petrol (SKYACTIV-G) and diesel (SKYACTIV-D) engines, but while the tactic was to raise compression as high as possible in the petrol engine, the diesel required a lower compression ratio. The ideal solution was a ratio of 14:1 for both.

SKYACTIV-G is aimed at increasing thermal efficiency through raising the compression ratio and reducing residual exhaust gases in each cylinder during the intake stroke. Fuel economy is improved to around the same level as a current-generation diesel of the same capacity. The petrol engine also gains from low-friction components for the crankshaft, pistons, conrods, the valvetrain, oil pump and water pump.

SKYACTIV-D (the diesel engine with two-stage turbocharger), relies on its lower compression ratio to allow engineers the opportunity to adjust ignition timing so that the engine meets current and coming emissions standards, while delivering a 20 per cent improvement in fuel consumption. Power delivery is smoother too, says Mazda, and the engine isn't hobbled with turbo lag as current generation engines can be. Importantly, the new diesel meets Euro 6, US Tier2/Bin 5 and Japan's Post New Long-Term Emission Regulations -- without aftertreatment systems.

Soot and NOx emissions occur when heterogeneous combustion takes place in localised parts of the combustion chamber. This is due to the very high temperatures when the piston reaches Top Dead Centre. To overcome this and meet diesel emission standards, manufacturers have opted for combustion to be timed with the descent of the piston on the power stroke, when internal temperatures are slightly cooler, but the downside is that the engine uses more fuel than desireable.

SKYACTIV-D's lower compression ratio ensures cooler temperatures in the combustion chambers, even at Top Dead Centre. Fuel can be injected at that point in the cycle and, because combustion is slower, there's an opportunity for the fuel and oxygen to burn in a leaner ratio, right across the combustion chamber.

The lower compression ratio places a lower level of stress on the engine's internal components, as mentioned already. During Mazda's Technical Forum, the press were able to lift pistons and crankshafts of current engines and compare them for weight against the SKYACTIV-D equivalents. There was no doubt that the newer components were lighter and that weight reduction was clearly evident just from manhandling them.

These lighter parts lead to lower reciprocating and rotating mass, which provides the means for the engine to reach speeds not normally associated with diesels -- speeds above 5000rpm. The engine block too, has been lightened by 25kg, to reduce the weight the car is carrying around -- and consequently reduce fuel use. In addition, the crankshaft journal diameter is reduced from 60mm to 52mm, reducing friction and increasing efficiency.

One factor working against low-compression diesels in the past was the degree of difficulty associated with cold starting. Mazda claims to have overcome this through specifying multi-hole piezo injectors and a variable valve lift mechanism. The piezo injectors deliver the fuel very precisely and the variable valve lift mechanism allows exhaust gas to be drawn back into the cylinder during the cold-starting intake stroke. This warms the combustion chamber in anticipation of the next fuel/air charge to be drawn into the chamber.

Transmissions
Mazda is working on more efficient transmissions in the two most popular forms: manual and epicyclic auto. SKYACTIV-DRIVE, as the auto is known, will be offered in two forms, one capable of handling up to 270Nm of torque and the other rated up to 460Nm. Mazda estimates the fuel reduction using this type of transmission will be in the region of four to seven per cent. Being a six-speeder, it offers an immediate benefit over the auto boxes Mazda currently offers in its product range. Added to that, the SKYACTIV-DRIVE units incorporate a greater range of damper-clutch operation, locking up the torque converter over a range 64 per cent greater than the company's current five-speed auto.

Among the upgraded features that set the SKYACTIV-DRIVE transmission apart from current-generation automatics are: compact torus, more durable clutch, a direct linear solenoid in the actuator and a mechatronic module within the transmission to carry out the functions of separate hydraulic control device and external ECU. In combination, these features reduce fuel consumption naturally, but also help the transmission to meet Mazda's various design criteria. The company claims that the transmission offers the launch feel of a CVT or conventional epicyclic box and the direct in-gear feel of a dual-clutch transmission.

SKYACTIV-MT is the manual transmission developed through the SKYACTIV program. As for the automatic equivalent, the manual is rated for two different torque figures: 270 and 460Nm. Both gearboxes are designed to confer the same "crisp" shift quality of the MX-5, without being any heavier to operate. Among the various ways of achieving this, Mazda opted for a system with the first gear placed at the top of the cluster, exploiting the gear's weight through the shifter to ease quick shifting into higher gears. Other elements enhancing shift quality included: a lock-ball-type synchroniser, shift-load canceller and slide ball bearing.

Mazda has devised ways of reducing the overall size of the transmissions, as well as cutting out weight and efficiency losses due to internal friction. The smaller unit (up to 270Nm) features two shafts, whereas the larger transmission is based around three shafts. To keep dimensions compact, the larger transmission features a common gear for second and third (as is the case too with the smaller transmission), plus a common first/reverse gear that allowed the engineers to dispense with a separate idle shaft for reverse. All up, the larger transmission's geartrain is 3kg lighter.

Where box sections and subframes curved over suspension mounting locations in the current cars, the SKYACTIV team has straightened out those members, which are often constituent parts of the multi-load crash structure.

Mazda has also determined that a dual brace system (joining framework directly to the body) and creating a circular structure for reinforcement adds to the car's total structural integrity. The roof rail section is to be bonded (by welding) to the rear frame and use of high-tensile strength steel will rise from 40 per cent in current models to 60 per cent for the next-generation SKYACTIV models.

The chassis and underpinnings of SKYACTIV models will further reduce fuel consumption, but not at the expense of driver satisfaction, Mazda claims. Electric power steering reduces power take-off from the engine and the front and rear suspension subframes are lighter, but offering enhanced rigidity. Finally, Mazda will do away with welded flanges in its cross member joins, instead having the narrower, lower section welded in place, sitting within the upper section.

The engineering involved in the cross-member joining calls for different manufacturing work-arounds, but that's symptomatic of the whole SKYACTIV program. It's holistic, but also realistic, as far as technological advances go.

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