How a Knocking Engine Is the Sign of Automotive Innovation

Knock Knock. Who's there? A supercharged, mild hybrid, diesel-inspired gas engine.

Mazda

When you usually hear a knocking noise under the hood, it means a major headache and an expensive trip to the mechanic. But for Mazda’s latest engine—a mash-up of diesel and gasoline technologies—it's the sign of innovation.

“SKYACTIV-X thrives on knocking,” says Jay Chen, one of Mazda's powertrain engineers.

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Don't Knock the Knocking

Mazda

Usually knocking is what happens when the air-fuel mixture in an engine combusts when it’s not supposed to, usually because a spark plug is malfunctioning and little pockets of air are exploding because of pressure rather than spark. It’s called knock because it sounds like somebody rapping their knuckles on metal, and it’s a bad sign for most gas engines.

Mazda eliminated the audible side effect of knocking, but redesigned its purpose to harness that pressure-induced ignition without damaging the engine. Called combustion ignition (when it’s done on purpose), that sparkless combustion is central to Mazda’s newest engine.

Compression ignition is how diesel engines run. A diesel sprays fuel and highly compressed hot air into the combustion chamber to induce an explosion, rather than igniting the fuel-air mixture with a spark plug, as a gasoline engine does. Diesel's compression ignition is highly efficient because the fuel-air mix explodes at the furthest point the cylinder is able to travel in a combustion chamber. Unlike most spark-plug engines, it takes advantage of the cylinder's full range of motion.

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"Knock is the result of an ultra-lean fuel ratio.”

But where diesel engines are more efficient, they struggle in the high RPM department because diesel burns slower than gasoline. But the SKYACTIV-X merges the diesel engine's compression ignition—and its efficiency and low-RPM grunt—with the gas engine's high-RPM responsiveness, creating an engine that pulls strongly throughout the whole rev range.

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“It welcomes knock because knock is the result of an ultra-lean fuel ratio,” says Chen, noting that lean fuel ratio helps with fuel economy and emission numbers. “The goal with Spark Controlled Compression Ignition (SPCCI) is to eliminate what you hear and experience as knock.”

A Diesel-Gas Hybrid

This ignition is Mazda's sub-type of Homogeneous Charge Compression Ignition, but distinct in that it also has spark plugs. Engineers have tried to capture the diesel engine's efficiency and put it into a gas motor, but it never succeeded in displacing spark plugs in gas engines.

The SPCCI uses the spark plug as a sometimes-on, sometimes-off control mechanism to work a greater range of loads, temperatures, and operating conditions, says Chen.

It's Mazda’s smartest ignition yet, taking advantage of improved processor speeds to more finely manage fuel injection and spark.

It's why the SKYACTIV-X has spark plugs, and can switch back and forth between SPCCI mode and conventional spark-ignition mode. It runs on this new mode for the bulk of everyday driving, says Chen, and switches to spark-ignition for “less-than-ideal” conditions, such as cold starts and very high RPM.

The Next (But Not Last) Step

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But the new and improved ignition isn’t the only thing that separates Mazda’s new engine from the pack—it also a traditional supercharger in a very non-traditional way.

Usually, a supercharger packs extra air into combustion chambers so the engine can mix it with extra fuel, for the sake of more power. In the SKYACTIV-X, it's there not to add power but to keep ramming in enough air at high RPM to maintain an ultra-lean fuel-air ratio.

Mazda's i-ELOOP braking regeneration system helps lighten the engine load by powering accessories with power drawn from the brakes. A variable-voltage alternator captures braking energy and stores it in a capacitor to power things like the headlights, air conditioning, and radio.

“The beauty of SKYACTIV-X is that most components in the engine are proven in other applications, such as the supercharger,” Chen says. “They're just being used in different ways to optimize efficiency.”

Headed for a 2019 production date, the SKYACTIV-X is the second step of three in a long-term road map for Mazda's future, the third step being what Chen calls an ideal engine that’ll have even less internal friction and therefore greater efficiency. Current Mazda engines, like SKYACTIV-D and G, entered production in 2011 and quickly found a place in the Mazda 2, Mazda 3, Mazda 6, MX-5, CX-3, and CX-5.

Mazda won't say which models will get SKYACTIV-X, but it's not an exotic engine destined for small-time production. If Mazda is calling the SKYACTIV-X the next step in automotive power, we'll likely see it transform into Mazda’s engine of choice.

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