Mazda SkyActiv's the new limit

Mazda's SkyActiv has been named as having one of the best engines of the year. Photo / Supplied

Mazda hasn't exactly reinvented the reciprocating engine with its SkyActiv technology, but it has given it more breathing room, believing that it will be around for many years yet.

The carmaker's 2-litre SkyActiv engine in the updated Mazda3 range has been named one of the 10 best engines for 2011-12.

SkyActiv is a bunch of technologies around which Mazda aims to drastically improve fuel economy. It claims it delivers more power and torque but cuts fuel use by 25 per cent - a figure that Mazda will stand by but won't shout from the rooftops.

"If we do, someone is sure to ring up and say they can't replicate it," says Mazda NZ managing director Andrew Clearwater. "It depends on the driving style."

The powertrain - engine and gearbox - is at the core of the system, but SkyActiv also includes chassis design. For example, it uses more pieces of straight, lightweight steel sections to form the body shell's reinforcement channels.

The first model to carry the complete SkyActiv G (petrol) and D (diesel) bag of tricks will be the CX-5 sports utility vehicle, due in a few months. The updated Mazda3 that has just been launched here uses the powertrain only.

This is the heartbeat of SkyActiv: more efficient combustion, improved cooling, a largely conventional automatic gearbox with the precision of a Volkswagen-type DSG unit but considerably lighter in weight, a new manual gearbox for front-wheel-drive cars with a snickerty-click shift, and an improved exhaust manifold.

But the big news is compression ratios. Mazda has changed the rules of combustion by increasing the ratio in its spark-ignition petrol engines and decreasing it in compression-ignition diesels. For years, petrol engines with carburettors ran at a ratio between 8:1/9:1. Direct fuel injection moved the ratio on to 10:1 and as high as 12:1. That was as good as it got before the risk of "detonation", or when the fuel ignites too early in the combustion process and tries to muscle in on the piston stroke.

Mazda overcame that problem, using crowned pistons with a cavity in the centre (very much like a diesel engine), valve gears that can be adjusted to change the operating cycles and improved fuel injection to enable the 2-litre Skyactiv G engine to run at 14:1 on 91 unleaded, a ratio that means a better bang when the fuel/air mixture ignites. (The SkyActiv Mazda3 runs at 12:1, an agreed ratio for New Zealand and Australia.)

The result is that more of the power in the fuel is used to power the vehicle rather than being lost in the exhaust or cooling systems, or through mechanical resistance or friction.

The upcoming diesel engine in the CX-5 is another breakthrough. SkyActiv D lowers the traditional diesel compression ratio from 17:1 to 14:1, a change that will allow the carmaker's 2.2-litre oil-burner to rev to 5200rpm and beyond.

That's new territory for diesel. The current best run out of puff at around 4400rpm.