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Mazda blows off the turbo rule book

Petrol grunt: With 170kW and 420Nm produced by a 2.5-litre powerplant, Mazda’s new SkyActiv four-cylinder petrol engine has characteristics more commonly associated with a diesel donk.

‘Real world’ performance drives turbo development for Mazda, not emissions tests

23 Jun 2016


AFTER years of resisting turbocharged petrol engines for its mainstream models, Mazda is now adopting a more widespread introduction of forced induction, starting with the imminent CX-9 large SUV, but the Japanese car-maker says it has a twist in its turbo tech.

Many of its rivals have already turned down the turbo trail in the pursuit of greater fuel efficiency and lower emissions without compromising on performance, but Mazda says the figures do not paint the full picture.

Speaking at the demonstration of Mazda’s new G-Vectoring technology in California, Mazda North America vehicle development engineer Dave Coleman said that in many cases the fuel economy figures quoted in vehicle specifications were not representative of day to day use.

“Fundamentally, they are trying to get the same peak performance but out of a smaller engine,” he said. “That’s something that generally pays off really well in fuel economy tests and not that well in the real world.

“On a fuel economy test you drive like a grandma and you never get on boost and you get just the benefits. In the real world you get on boost a lot more of the time and, depending on the way you drive, you’re going to get close to the figure or you’re going to be way off target.

“It helps in the catalogue but seldom pays off in the real world.”

Mr Coleman explained that downsizing to smaller capacity turbocharged engines is not always the silver bullet for offering both low emissions and high performance and that, depending on the driving style, a turbocharged engine can offer efficiency or performance.

Smaller turbo engines can offer greater thermal efficiency than the equivalent larger capacity naturally aspirated engines thanks to smaller internal surface areas and higher charge density, but Mr Coleman explained other turbo engine design parameters can be detrimental.

“You give up a lot with a turbo,” he said. “You give up a bunch of compression, which takes a lot of efficiency. When you are off boost you have the benefit of a lot less friction from a smaller engine.

“That little engine is working fairly hard (on boost) but it’s still less efficient than a naturally aspirated engine which, through higher compression, you get more efficiency.

“When you go on boost you have to do so much more enrichment to control combustion temperatures… and that’s wasting fuel.”

Mr Coleman also cited lag as an ongoing disadvantage to turbocharging where the a delay in turbo response to throttle opening delays power production and acceleration, but while he highlighted the shortcomings of other brand’s progress the development engineer said Mazda has approached the challenge from a different angle to its rivals.

Rather than simply reducing engine displacement and adding turbocharging ancillaries, Mazda’s approach aims to combine a reduction of cylinders in conjunction with a different application of turbos.

“We’re taking a very different approach to doing it,” he said. “We are only doing it where we can lop cylinders off. The advantage in reduced friction is a lot bigger when cutting cylinders rather than just going to a smaller engine.

“We’ve developed a system that doesn’t have turbo lag and is tuned for massive low-end torque where you can have that really direct throttle response.”

For the new-generation CX-9, the 3.7-litre naturally aspirated V6 of the outgoing car has been swapped out for a turbocharged 2.5-litre with four-cylinders, producing 170kW and 420Nm, which are figures more akin to a diesel engine of similar capacity.

“You have to tune an engine for massive amounts of torque and give up on horsepower. That’s why we have kind of weird specs. It seems underpowered but it has got huge amounts of torque. That ends up giving you much better drivability.”

Unlike a sportier segment, Mr Coleman said the CX-9 was less likely to be shunned for its topsy-turvy power and torque figure.

“We are hoping that that segment is not as horsepower sensitive as others. The torque figure is impressive so we’ve got at least one number that’s bigger than the other guys.”

To arrive at the high torque output, Mr Coleman’s team applied forced induction differently to many other manufacturers who have adopted turbocharging earlier.

Exhaust gas recirculation (EGR) is also used by other brands to reduce NOX emissions (the pollutant at the centre of the Volkswagen emissions scandal), but reintroducing hot gasses to the combustion chamber causes problems of its own. Mazda’s system reduces the gas temperature through a cooler before reintroduction.

“Everybody understands how cooled EGR works in their engines — they are common in diesels but very few gas engines use it because on a fuel economy test, you don’t drive hard enough to get into that fuel enrichment so the EGR doesn’t pay off,” Coleman said.

As a happy bi-product, the cooler combustion chamber creates a better environment for high boost pressure without pre-ignition.

“When you throw the exhaust back in you’ve burned up all the oxygen and that slows down combustion instead of throwing in a bunch of extra fuel that it can’t burn,” said Coleman.

“It turns out that, that raises your knock limit so much that you can turn the boost up even higher and make up for the lost oxygen.”

With a higher knock or pre-ignition point, Mazda was also able to raise the compression ratio to even greater than the naturally aspirated V6 — a ratio that typically has to be lowered in high-boost applications. Where the ratio of the atmo V6 was 10.3:1, the new four-pot turbo is 10.5:1.

The unorthodox features continue with the new engine’s cylinder head, where the two centre exhaust ports (two and three) have been siamesed so that the inertial forces of consecutive exhaust pulses can help clear the waste gasses from its neighbouring cylinder.

“We arrange the ports next to each other so they have a scavenging effect on each other. You get a venturi effect where one exhaust pulse will pull out the other,” Coleman said.

Complementing the various technological innovations is a dual port turbine housing, which directs exhaust gasses through three smaller finger-tip sized holes at low speed to accelerate the turbo past its boost threshold earlier reducing lag, before opening the second larger port at higher engine speeds.

Mr Coleman likened the effect to a variable geometry turbine nozzle turbo but less expensive and less prone to high temperature damage.

“We focused on real world fuel economy so we did stuff very differently than everybody else does,” he said.

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