Mazda's SKY-High compression

Mazda faces a small but important matter to resolve this year. Will the company's first iteration of SKYACTIV-G technology run 13:1 compression, or 14:1 in the Aussie market?

The super-frugal petrol engine is scheduled to go on sale before the end of the year and will deliver a major leap in fuel efficiency. But the magnitude of that leap will depend in part on compression ratio. Through the use of an innovative '4-2-1' exhaust system with long manifold runners that channel cooled exhaust into cylinders on the intake stroke, Mazda has found a means of reducing uncontrolled detonation ('knock') to the point where the compression ratio can be bumped up to 14:1 -- a number practically inconceivable in production cars before now.

However, that number 14 is dependent on the combustion of premium (95 RON) unleaded petrol. In Australia, where most drivers continue to buy regular unleaded (91 RON), the high-efficiency engine's compression must be reduced to 13:1. That much was explained to the Carsales Network by Mazda's visiting Executive Officer in charge of Product Planning and Powertrain Development, Kiyoshi Fujiwara.

Fujiwara-san, out here for Mazda's Technology Forum showcasing the company's SKYACTIV development program, explained that the difference in fuel consumption between 14 and 13:1 wouldn't be that great -- about "three to four per cent".

All the same, that's not as great a difference as the percentage difference in price -- at the bowser -- between regular unleaded and the premium fuel. For owners, the 91 RON alternative would likely be more economical than the 95 RON fuel, despite higher consumption. And as a fellow journalist noted, petrol retailers often discount the lower-cost fuel, but rarely discount premium unleaded.

All of this is getting ahead of the field, suggests Steve Maciver, Mazda's Public Relations Manager. The company is yet to make a decision as to whether it will go with the lower- or higher compression for the first SKYACTIV-G model to be sold here -- anticipated by many to be the Mazda2.

"No final decision has been made on that essentially, but for that engine to run at its [optimum] -- at a 14:1 compression ratio -- [Fujiwara-san] is right in saying that you have to use premium unleaded.

Would Mazda stipulate its vehicle buyers use 95 RON unleaded in order to bring in the more frugal version? There's a precedent in the recent past, but the current Mazda6 did revert to 91 RON subsequently.

"One thing to note is that [when] the current generation Mazda6 came on board, that was originally running off premium, 95 RON," replied Maciver.

"What we did there is we managed to get some changes done so we could then run that car on 91. Obviously, as far as possible, we are keen to have customers running on 91 if possible -- but again, there has to be a decision made...

"Our line-up of passenger cars has consistently run on regular unleaded -- and that probably wasn't the norm with Mazda6 when we moved to premium. More than anything it's a decision to keep our product line-up as consistent as possible."

The compression ratio conundrum is in a whole different league from pumping 91 octane fuel into a Peugeot or a Volkswagen -- and leaving it to the engine management system to detune the engine on the fly in order to handle the lower-quality fuel. There are potential durability problems involved in running an inappropriate fuel in an engine with an extremely high compression ratio.

"We can only recommend to customers what they put in the car," Maciver responds to that.

"Obviously everything is engineered to specific tolerances -- and when we bring a product into the country, we provide a recommended schedule, in terms of what fuels and what lubricants you put into that car. If we recommend that you have to run a car on 95, we would be expecting customers to put 95 into the car..."

In other words, use the wrong fuel and your warranty may be void. Use the wrong fuel beyond the warranty period and your engine may be void. But decision made or not, the odds seem to be in favour of Mazda going with the lower-compression engine when the new technology arrives.

Mazda first named its marketing plans for the future 'Sustainable Zoom-Zoom' back in 2007. SKYACTIV is, in essence, the engineering side of that coin. It encapsulates the idea that "'Fun to drive' remains absolutely fundamental to Mazda," as local Mazda MD Doug Dickson explained it to the press earlier in the week.

SKYACTIV-G delivers a 15 per cent improvement in fuel economy against a current engine of the same displacement. In fact, according to Mazda, its fuel efficiency is on a par with a diesel engine of similar capacity. Keeping in mind the 'Zoom-zoom' element of the new engine technology, the SKYACTIV-G also produces around 15 per cent more torque at mid-and high-range revs.

A much higher compression ratio is the major contributor to the fuel efficiency gains. Mazda has achieved this principally through a 4-2-1 exhaust system. Just like tuned extractors from the good old days, the exhaust manifold comprises runners that merge from four to two to one, as the name suggests. This results in longer runners, which cool the exhaust gas faster and reduce the primary shock wave that builds up in conventional manifolds and pushes exhaust gas from individual cylinders back into the cylinder while the exhaust valve remains open. The air drawn into the cylinder for the next power stroke is displaced by the residual exhaust gas in a conventional engine and manufacturers set the compression ratio relatively low to reduce uncontrolled detonation (knock) to a minimum. Not only can knock reduce the engine's durability in the longer term, it also reduces torque when required.

By contrast, Mazda's innovative exhaust system employs runners 600mm long, but in a loop to fit within relatively compact packaging.

"The layout of the engine in the engine compartment is quite interesting..." said Steve Maciver on that point. "It's tilted slightly backwards. One of the ways of [controlling combustion temperatures and thermal efficiency] is to try and cool down the exhaust gases and the in-cylinder temperatures before we have that combustion; we get a cleaner combustion that way.

"[The] 4-2-1 manifold system is longer than an existing set-up. To allow that to fit into the new chassis, the new car, the engine is tilted slightly backwards... and that allows for the flow of exhaust gases to run more smoothly."

With the new system the primary shock wave from one cylinder takes longer to reach another on the intake stroke through the manifold -- and the exhaust gas is cooler, allowing for a higher compression ratio. The one downside of this is that 'light-off' time for the catalytic converter takes longer, because the gas is cooler -- the converter being further removed from the engine by the long runners of the manifold. Mazda has overcome this though, by specifying pistons with a cavity in the crown, and combining that with a particular formulation of stratified fuel/air charge around the spark plug, thanks in part to the direct-injection system employed.

SKYACTIV-G is just one pillar of the SKYACTIV program. Keep an eye out for our upcoming first drive review of the next-generation Mazda6 prototype, and a feature on the new, cleaner technology.

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