Mazda Skyactiv-X kills the petrol spark

MAZDA has announced it will make a quantum leap in its development of the petrol engine within three years, with a new engine that runs on petrol but borrows the ignition principle of a diesel.

If the Japanese car maker’s goal is achieved, the so-called Skyactiv-X engines are set to smash combustion efficiency records with unprecedented fuel economy that, according to Mazda, will not compromise driving enjoyment.

The announcement is part of the company’s broader ‘Sustainable Zoom-Zoom 2030’ strategy which predicts a future where electrification goes hand in hand with high-efficiency combustion engines.

Until now, diesel engines have maintained a small efficiency lead ahead of petrol equivalents with an inherent ability to burn a ‘lean mixture’ where the amount of fuel injected into the cylinders is many times less than the quantity of air.

Petrols, on the other hand, are more bound to a widely accepted optimum ratio of air to fuel known as stoichiometric to avoid damaging combustion conditions - a fixed figure of 14.7 parts air to one part fuel - but Mazda says it has found a way to free the petrol engine from this binding principle.

Instead of using a spark to start combustion, the Skyactiv-X engine squeezes and heats the air in the cylinder to a point where the temperature is high enough to spontaneously trigger ignition of the fuel – exactly as a diesel does.

With conventional spark ignition, lean petrol and air mixtures cause very high combustion temperatures that can damage internal components and, although various manufacturers have successfully developed leaner running engines with technology such as exhaust gas recirculation, Mazda says compression ignition has allowed a mixture twice as lean as its rival’s best efforts.

With such low ratios of fuel to air a spark simply cannot ignite the mixture but compression ignition can and has the potential to cut fuel use under certain conditions by as much as half, without sacrificing power or torque.

The car maker isn’t quite ready to wheel its spark plug stock into the dumpster just yet though. The process is only possible with a preheated engine. During warm up, normal spark ignition is employed.

Another breakthrough was also required.

While diesels inject fuel into the hot compressed air at the exact moment ignition is required, the Mazda Skyactiv-X engine compresses already mixed air and petrol in a process called homogenous charge compression ignition (HCCI).

Typically, timing this type of ignition is almost impossible due to numerous variables that cause the mixture to spontaneously combust at an unpredictable point, but Mazda says it has cracked this challenge too with its ‘controlled compression ignition’ technology - although the company is not completely disclosing how the trick works beyond pressure charging (supercharged or turbocharged induction) and a principle described as spark controlled compression ignition (SPCCI).

Other details and specification will also have to wait including the engine’s compression ratio, cylinder layouts and power outputs – although the Skyactiv-X torque is said to be 10 to 30 per cent more than the current Skyactiv-G engines.

Quite how the revolutionary engine will sound is also an intriguing prospect. All eyes and ears will be on the Mazda launch diary for 2019.