Bumpy road to Mazda compression-ignition petrol unit

MAZDA has revealed that it suffered many setbacks on the road to developing a reliable and efficient new combustion system for its upcoming SkyActiv-X range of petrol and diesel engines.

Known as Spark Controlled Compression Ignition (SCCI) – or SkyActiv-X in Mazda speak – and said to be the world’s first commercial application of a petrol engine using compression ignition (in which the fuel-air mixture ignites when compressed by the piston), the system has been in development in its final guise for two years, but the idea originated almost 10 years ago.

Among the many solutions was the implementation of spark-plug ignition for low rev activity or during heavy engine loads, which led to difficult to modulate switching between spark and compression ignition.

When a solution was finally worked out, Mazda realised that the resulting fireball that occurred when the spark plug sparked also helped the piston to compress down further and harder, aiding efficiency.

However, there were pre-ignition issues to work out, so a super high-pressure fuel spray system was devised. Additionally, a supercharger from Rootes is employed to help the air-fuel mix essentially an off-the-shelf item, it has been modified to Mazda’s specs.

The benefits include improved launch response, more expansive high-rev performance and at least a 20 per cent decrease in fuel consumption – or 30 per cent in real-world conditions.

“Since we thought of this, it has been two years,” according to Mazda Motor Corporation managing executive officer in charge of powertrain and vehicle development and product planning, Ichiro Hirose, speaking to GoAuto through a translator at the Mazda Global Tech Forum 2017 in Frankfurt late last month.

“But I want to add that because the basis of this is compression ignition, and that’s what’s used in diesels as well, we’ve been developing it for a long time.”

Mr Hirose detailed the issues the engineering team faced during development.

“I talked about some barriers we had to face – every single time we faced a barrier, we had to suffer and moan and groan and start again, and then when we got over that we’d hit the next time and it was a repeat.

“The biggest breakthrough and effort was coming up with and making that spark plug work with the compression ignition engine. Because although you might at a glance have a look at this technology and say ‘yes, it’s the same as a diesel engine technology’, but what is different is the nature of the fuels themselves. Because petrol is different to diesel in that if you mix it into air it becomes homogenous really quickly. So if you take the petrol, the ideal combustion state only exists in an instant, and trying to get that pinpoint precision was difficult, especially the controls required for us to get that state.

“It may seem like something completely convoluted and intricate and complex, but it’s not really so, because the base engine is basically a regular direct-injection engine, and what we’ve done in terms of the structure and hardware is that we’ve added the ‘Air Supply’ supercharger, and then added a super-high pressure injector so the injector pressure is higher. So it’s an extension of what we have now.”

Mr Hirose said that inspiration for the SCCI system came from a number of disparate sources.

“There wasn’t a particular engine out there that we took our interest from,” he said. “But it was basically from academic forums and associations like the SAE Society of Automotive Engineers because they have published papers of research, and perhaps also some technical papers from other OEMs (Original Equipment Manufacturers) that was the source of our inspiration that led us to this.

“With the original SkyActiv-G we achieved a super-high compression ratio, and so we pulled up the compression ratio just enough so we can suppress knocking, which means we knew where the boundaries where you cannot go past this line which means with this new engine we took it the other way around so that these are the conditions that we needed to pass to give us the effective (knock-free) combustion we knew how to supress the phenomena which means we know all the factors how to pause the things that cause the knock.

“And, as a matter of fact actually, 91 RON gives you better performance than 95 RON.”