The latest Mazda 3 is soon to be launched with the revolutionary Skyactiv-X engine, which uses both spark and compression ignition.
If that sounds like gobbledygook, we’ll explain it as best as our limited technical knowledge will allow.
In a petrol engine, the fuel and air mixture is compressed on the piston’s upward stroke and ignited by a spark plug. In a diesel engine, the fuel/air mixture is compressed until it self-ignites, with a little encouragement from the heat of a glow plug.
The combustion process – the burning of the fuel/air mix – takes longer and is less complete in a petrol than a diesel engine. That’s why petrol engines emit more carbon dioxide than diesels.
In its efforts to make petrol engines as clean as possible, Mazda has invented what it calls Spark Controlled Compression Ignition for the Skyactiv-X engine.
The unit runs a very high compression of 16 to 1 – i.e. at the bottom of the piston’s stroke, the combustion chamber is 16 times bigger than at the top of the piston’s stroke. That’s very nearly a high enough ratio to achieve compression ignition of petrol. A spark lights the mix and, as with diesel, the highly compressed mix burns more rapidly and completely than a conventional petrol engine.
If that’s as clear as mud, the statistics lay out the benefits.
The 2.0-litre, four-cylinder Skyactiv-X unit produces 178hp and 165lb/ft of torque, which is quite a lot of power and torque for a naturally-aspirated engine this size.
But by far the most remarkable stats are those for economy and emissions. Mazda claims a combined 52.3mpg (WLTP) and Co2 emissions of 96g/km (NEDC converted) – with a manual gearbox and 16in wheels.
That combination of power and torque, with such low fuel economy and emissions is nothing short of miraculous.
The 3 is assisted in achieving those numbers by a 24-volt mild-hybrid system that recovers energy under braking, then deploys it via an electric motor that assists the engine when needed.
The Skyactiv-X engine will be available in both the hatchback and saloon versions of the 3, with a choice of manual or automatic gearbox. The hatchback will be available with four-wheel-drive, too. They go on sale later this year.
By Graham King