Skyactiv engine in Mazda CX-9 in
Skyactiv engine in Mazda CX-9 in

Japanese carmaker Mazda has released details of its “sparkless” ignition engine SkyActiv-X, which it says improves the efficiency of petrol engines by 30%.

The engine uses a combustion method called “Spark Controlled Compression Ignition” to enable a petrol engine to run in the same way a diesel engine and thereby combine the best characteristics of the two types, said Mazda. “Our engine will be the first commercial HCCI engine in the world” and is “a dream for engineers” said the company at a launch even earlier this month.

According to Mazda, the efficiency limits of conventional, spark-plug petrol engines have been reached, while it’s “next generation” engine cleanly and rapidly combusts a very lean air-fuel mix, twice as thin as any other spark ignition engine can. This improves thermal efficiency, halving the amount fuel used and reducing Nox emissions.

Diesel engines use a combustion method called compression ignition, where the fuel and air are mixed and ignited by compression temperature and pressure, not a sparkplug. The version of compression ignition Mazda and other companies have been trying to develop for a number of years is called homogenous charge control ignition (HCCI).

HCCI engines are capable of running with a ratio of fuel to air reduced to a level that would not ignite in a conventional petrol engine. In a HCCI petrol engine, after the piston moves from the top dead centre, combustion takes place spontaneously and rapidly everywhere in the combustion chamber, enhancing the force and pushing the piston for a longer time.

Controlling the compression ignition is key in the HCCI engine and Mazda’s engineers have solved this problem by…using a sparkplug. A sparkplug has to be used in a HCCI engine because it is the only way to solve the problem that HCCI only works at a limited range of RPM and load.

The problem then becomes how to switch between HCCI and spark-plug ignition in a smooth and stable way. The proprietary control technology developed by Mazda uses the sparkplug to overcome this problem. Engineers have also raised the compression ratio in the engine for mid to high loads, by putting a large amount of air is put into the engine using superchargers.

The SkyActiv-X engine works in all conditions apart from “super low temperatures”, said Mazda, when spark ignition is used. Fuel economy is improved by up to 30% compared to Mazda’s latest Syactiv-G engine. At low speeds fuel economy is improved by a maximum of 45% compared to the company’s 2008 petrol engine.

The engine performs as well or better than the latest Skyactiv diesel engines in terms of fuel efficiency.

More information on the Skyactiv-X engine will be revealed at the Tokyo Motor Show later this year.

Mazda is well-known for developing alternative engine technologies, after spending many years developing the Wankel rotary engine, which today is only used in racecars.

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