How Mazda Plans To Save The Petrol Engine
evo India|July 2018

Car makers may be falling over themselves in the rush to electrification, but for its advanced Skyactiv-X engine series, due to debut in the next 3 model in 2019, Mazda has other ideas

Antony Ingram

MAZDA MUST SPEND ITS MONEY wisely. With a global output of around 1.6million vehicles per year, it’s a long way down the automotive pecking order, and its independence means it lacks the economies of scale present in companies such as the Volkswagen Group, Toyota, Hyundai and Kia, or General Motors.

Yet this independence also gives Mazda relatively free rein to forge its own development path. No other company persisted with Wankel rotary engines for as long as Hiroshima’s engineers (not necessarily a wise investment, but one indicative of Mazda’s focus on finding engineering solutions to problems), and as the rest of the industry turned to turbocharging and downsizing to reduce its environmental impact, Mazda implemented what it calls ‘rightsizing’: refining conventional combustion engines with improved technology and making them appropriately sized for their application.

The company’s upcoming combustion engine series, dubbed Skyactiv-X, takes the concept of constant technological refinement to its next logical stage and introduces a technology several companies have tried, and failed, to implement in the past with petrol engines: compression ignition.

The idea in itself is nothing new. Diesel engines already operate on the principle of compression ignition and several companies have experimented with it in petrol cars at the prototype stage. Infiniti’s recently introduced variable-compression engine, while not running on compression ignition as yet, features technology that could make it possible with petrol vehicles in the future. But Mazda has found a way to do it today, and with much less complication than previous experiments with the technology.

To explain how it works, one must first understand the basic principles of homogenous charge compression ignition (HCCI). HCCI engines operate by the fuel and air mixture spontaneously igniting at the end of the compression stroke in a four-cycle combustion engine. The ignition is caused by the increase in temperature of the mixture as a result of its compression – to a higher ratio than would be used by a spark-ignition engine. The process is more efficient than spark ignition, partly because the charge ignites throughout the entire combustion chamber at once, rather than propagating from around a spark plug, and partly because the engine uses a leaner mixture. As Mazda puts it, faster combustion gets more work out of the same energy.

However, HCCI is also difficult to control and works effectively in only a limited window of operation: at relatively low engine speeds and at relatively low loads. Introducing a spark to the process and switching between spark ignition and HCCI is a solution to that problem, but a complicated and expensive one, requiring different conditions in the cylinder depending on the mode of operation.

Mazda’s solution is called SPCCI, or spark controlled compression ignition. Yes, it uses a spark, and the engine can operate as a conventional spark-ignition engine when required, but the spark is also used to initiate compression ignition without the need to mechanically increase the compression ratio of the engine, and therefore complexity and expense are reduced. So how does SPCCI work?

In its regular state, the Skyactiv-X engine runs a high compression ratio of 16:1 – but that’s not high enough to cause compression ignition on its own. During the induction stroke, air is pushed into the cylinder with a belt-driven supercharger and fuel from a high-pressure injector. The shapes of the combustion chamber and piston crown promote swirl and even distribution of the lean mixture.

During the compression stroke, the same injector then fires again, delivering a slightly richer mixture to the area around the spark plug. When the spark plug fires at the start of the expansion stroke, it ignites the richer mixture and creates what Mazda describes as an ‘air piston’: a rapidly expanding flame front, raising compression in the chamber, which in turn spontaneously ignites the remaining, leaner charge.

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