SQUASHING an air/fuel mixture until it spontaneously combusts has, until now, been the preserve of diesel engines. In 2008, Mazda started a project to develop a petrol engine with Homogenous Charge Compression Ignition (HCCI) but, like Daimler before it, was defeated by the technical challenges. Mitsuo Hitomi, Mazda’s Managing Executive Officer in charge of the Technical Research Centre, explains.
“Initially we started with pure HCCI combustion but we realised it’s impossible to control. If the intake air temperature is too high the combustion rate is too high and the engine is very noisy. If it’s too low, the combustion process is unstable. You have to control the intake temperature within three degrees. Impossible. So we changed the concept,” he said.
Reintroducing spark plugs was the Hiroshima-based company’s masterstroke, in this case using the spark plugs as a secondary control mechanism in the combustion process. The SkyActiv-X powerplant employs a small fireball from spark ignition, effectively raising pressure in the rest of the chamber such that compression ignition can occur.
The SkyActiv-X engine has a hefty Roots-type supercharger bolted to its front end tasked with pumping huge quantities of air into the combustion chamber.
Mazda realised that in order for compression ignition to occur, the air/fuel mix had to be incredibly lean. Conventional spark-ignition petrols mix air and fuel in the stoichiometric ratio of 14.7 to 1. The SkyActiv-X engine, in its lean spark-ignition mode, burns in a 29.4:1 ratio and, when operating in ‘pure’ compressionignition mode, ups that to 36.8:1.
In other words, the SkyActiv-X architecture requires a lot of air to deliver its best.
The calibration required to control this Spark Controlled Compression Ignition (SCCI) process is dizzyingly complex, and developing a stable switching point between the two modes has proven a huge technical challenge.
The engine has also been designed for future electrification, and hybridisation would deliver enough low-end torque to subtly extend the band in which the engine can operate in SCCI mode.
“We’ll combine a hybrid system
with this engine, but it will be a mild hybrid,” confirmed Hitomi.
Another huge benefit of this engine architecture – especially in the Australian market – is that its economy improves as fuel octane is reduced. Many modern engines can’t operate on this fuel for fear of pre-detonation in the combustion chamber, something that compression ignition, in effect, strives for.
Mazda is confident of the scalability of this unit, and it’s likely that we’ll first see it on the market in the next-gen Mazda 3, due in 2019. Between now and then, the engine has to undergo a whole raft of durability tests but Mazda seems confident that the tech is 90-95 percent production-ready. Is this the next big thing in automotive technology? Experience with the production version will allow us to deliver the definitive verdict, but early signs suggest that Mazda is onto something big. oc en fo n ha du co pe th te pr bu M
First off, no, it doesn’t sound like a diesel. It’s marginally more sibilant than a conventional petrol engine. Rolling gently onto the throttle pedal provokes a half-second diesel-like flutter of pinging, but Mazda is confident that this can be calibrated out of existence by the time the engine is ready for production.
Otherwise it’s wholly transparent tech. The engines as tested made 140kW and 230Nm, and Mazda promised the performance of a 2.0-litre MX-5 with the economy of a 1.5-litre Mazda 2 diesel. Driven with a fairly light right foot, I managed 5.3L/100km on an 86km test route. That’s remarkable for an engine with that power output.