TECHNICAL FEATURE:

Mazda SKYACTIV Technology

by David Finlay (19 May 2011)

On the face of it, Mazda might not seem to be paying much attention to environmental affairs, at least compared with other manufacturers. There is, for example, no hybrid Mazda in production, no electric prototypes have been announced, and the company doesn't even go to the trouble of creating low-CO2 specials with names like BlueMotion, BlueEFFICIENCY, ECOnetic or what have you.

It turns out, however, that Mazda is playing a long game. There will certainly be a hybrid Mazda in the future, but for the moment there are other matters to be attended to. And they are being attended to in a wide-ranging programme called SKYACTIV, whose first fruit - the CX-5 compact crossover SUV pictured above in its guise as the Minagi concept car - will go on sale early in 2012.

SKYACTIV sounds almost magical, as if its work is carried about by little elves hovering above the car and firing Cupid-like arrows at it, and no doubt that's the kind of effect Mazda is aiming at. In reality, it's a bit more prosaic. The sub-activity known as SKYACTIV-Chassis, for example, for all the airy talk about "oneness between car and driver", seems to be normal ongoing suspension development with a fancy name attached.

In fact, there is nothing about any of SKYACTIV which represents a complete unending of How Things Are Done. This is more about taking existing technology and making it slightly better in a number of ways. But the overall effect of that, as Charles Darwin would have told you, can be dramatic, and in the case of the SKYACTIV programme that's most obvious when it comes to engines.

The intended result of the programme is that future Mazda petrol engines will be more economical and emit less CO2 (according, at the very least, to official test procedures) than current diesel ones, while future diesels will be better still. Both will also produce more torque, with consequent benefits for performance.

This is being achieved in many ways, but in each case the starting point is a rethink of what the engine's compression ratio should be. Broadly speaking, the compression ratio is the extent to which the piston compresses the fuel/air mixture as it whizzes up the cylinder, and there's no reason for an average motorists to consider it remotely important. But it is.

In a petrol-fuelled engine, higher compression ratios are better because the fuel is burned more efficiently, so you can get the same power while using less of it or use the same amount to get more power. If the compression ratio is too high, though, the temperature and the pressure of the mixture go beyond acceptable levels, the combustion process goes haywire and you get what is called knocking (or pinking), about which there is nothing good to say.

With SKYACTIV, Mazda is raising the compression ratio to 14:1 (which is huge - much more the kind of figure you'd find in a race car than a road-going one) to get the extra performance and reduced fuel usage, while also going to great lengths to eliminate knocking.

One method employed is to change the design of the exhaust manifold to reduce back pressure (very briefly, instead of the pipes from each cylinder being combined into one as soon as possible, they are paired further downstream, and combined into one still further away from the action). Another method is to redesign the piston so that it has a domed top with a small central indentation, giving a very specific location for combustion to take place.

Diesel engines have an opposite problem with regard to compression ratios. Diesel fuel is made to combust by being compressed, not ignited as petrol is, and you normally need a very high ratio to make this happen. But this leads to compromises regarding power, fuel economy and emissions (especially of oxides of nitrogen and particulates), so Mazda is reducing the ratio to 14:1 - yes, the same figure which is extraordinarily high for a petrol engine yet also very low for a diesel.

This reduction helps remove the compromises during combustion, but it causes other problems. SKYACTIV aims to fix these by employing very precise fuel injection (up to nine squirts of diesel at a time, in a pre-determined pattern according to what the engine is being asked to do), variable lift for the exhaust valves and two-stage turbocharging.

In both types of engine, Mazda is working on reducing internal friction by various methods. With the diesel, it is taking advantage of the reduced compression to make the engine lighter. Diesel units are normally much heavier than petrol ones because their very high compression ratios would otherwise shake them to bits, but at 14:1 Mazda is confident enough to be able to use, among other things, an aluminium cylinder block which on its own saves 25kg.

In yet another part of SKYACTIV, a similar philosophy is being used to reduce the weight and increase the strength of the car's bodyshell. This includes high-tech processes such as the increased use of light but high-tensile steel and innovative ways of bonding one structural component with another. It also involves much simpler examples of rethought design, such as abandoning what would normally be curved components in favour of straight ones.

A lighter structure is a Good Thing in every possible way, as it enhances not only every improvement mentioned above but also advances in the design of both manual and automatic gearboxes. The combined effect, Mazda hopes, is a 30% improvement in fuel economy (and, by extension, CO2 emissions) over the 2008 average by 2015, without resorting to hybrid technology. And whenever a hybrid Mazda does arrive - "in a few years" was the best answer we could get to that question - its benefits, when added to those of SKYACTIV, should result in a car which combines formidable green credentials with impressive power and a high-quality driving experience.

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