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Mazda plots new course with SkyActiv diesel

Mazda is again charting its own path toward fuel efficiency.

Published March 15, 2013

Mazda is again charting its own path toward fuel efficiency.

This time, the signposts proclaim: “diesel.”

The technology, long popular in Europe, seems primed for North America, including a new Cruze from Chevrolet.

But Mazda, whose green strategy is to make internal combustion as efficient as possible, says it’s leading diesel in a new direction — the world’s lowest-compression version.

Its “SkyActiv” approach designs vehicles from scratch to cut weight, and improve strength and aerodynamics, with a propulsion system that squeezes maximum work from the fuel.

Gasoline powered the initial SkyActiv engine. It went into an otherwise conventional Mazda3, then, with the complete package, into the CX-5 compact SUV. The first diesel is a Mazda6 sedan, available in Japan and Europe and coming here this year.

With gasoline, Mazda developed an ultra-high compression ratio. For diesel, it’s gone the other direction, dropping the ratio from the typical 16:1 to 14:1.

This seems nuts since diesel works without spark plugs, and to do so the rising piston must compress the air-fuel mixture so tight it combusts on its own.

So, I returned to product engineer Dave Coleman. Last year, he demystified the gasoline technology. Now, it’s diesel’s turn.

“There’s a trend toward lower and lower compression” in diesel, Coleman says. “We’re way out on the leading edge.”

Weight reduction is crucial for fuel efficiency, and it’s a challenge with diesel: The high compression and combustion heat demand a heavy, cast-iron engine block and the increased torque requires a robust transmission.

As well, diesels need equipment to eliminate emissions of soot-like particles and smog-producing nitrogen oxide, or NOx. Particle filters add some weight but can’t be avoided. NOx is another story: Current controls spray liquid urea into the exhaust stream. So, the vehicle must carry a tankful — nearly 20 litres for the Cruze — and devices to deliver it.

SkyActiv cuts weight two ways. The lower compression allows a lighter, cast-aluminum engine with less structure and smaller components. And it meets NOx standards without scrubbing.

“Instead of trying to clean NOx from the exhaust, we’re creating combustion conditions that don’t produce it,” Coleman says.

But how can diesel work without the high compression at its heart?

In most diesels, Coleman says, fuel injection and combustion occur slightly after the piston begins its downstroke after full compression. This cools things so NOx emissions can be handled by the urea system. But it also reduces power because some expansion has occurred before combustion. You get less work than you’d expect from the stated compression ratio, but you’ve had to build an engine that can handle the high compression.

Mazda puts injection and combustion right at the top of the piston’s compression stroke. The lower compression produces a slower, more thorough burn, which generates fewer pollutants. And since combustion begins with the piston at its highest position, more fuel energy is put to work.

Nothing is perfect. The reduced compression impedes cold-weather starts. Diesels use glow plugs to initially warm the combustion chamber. Mazda’s technology needs more. The solution: A small ceramic plug that instantly heats to 800 C.

Traditional “dumb” plugs simply heat up to some natural steady-state, Coleman says. Mazda’s “intelligent” plugs quickly lower the voltage once the target temperature is reached. Since there’s no danger they’ll overheat, they can be designed to warm up faster.

Once the engine is producing hot exhaust, special valve timing allows a bit into the combustion chamber to help achieve operating temperature.

It works at -35 C.

Mazda is “very bullish” on diesel, which combines “environmental performance” and “a rewarding driving experience,” Coleman says.

That remains to be seen. While I’m fascinated by SkyActiv’s workings, I haven’t driven the diesel; fuel-consumption numbers aren’t yet available; and price might be an issue. Diesel will be $2,500 to $3,000 above gasoline, which isn’t bad. But it will be offered only on the top two of three trim levels, putting it $6,500 over the base model.

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