No end in sight for Mazda diesels

Diesel engine technology doesn't face imminent death in the Mazda product range, according to Ichiro Hirose, the company's Managing Executive Officer in charge of powertrain development.

In fact, while road transport remains heavily reliant on oil refineries for fuel, there's also a sound environmental reason for persevering with diesel combustion – contrary to what many automotive brands and EU legislators would have you believe.

In Geneva last week for the unveiling of the Mazda CX-30, Hirose spoke with carsales through an interpreter, explaining why the SKYACTIV-D (diesel) engine will remain a cornerstone of Mazda's powertrain offerings.

"If you go to a textbook for engineers, about engine efficiency, there are only really two factors that we have to consider in improving thermal efficiency," he explained.

"One is improving the compression ratio, second is increasing the specific heat ratio, meaning trying to combust in a lean situation as much as possible."

These two objectives are key to what Mazda refers to as 'Generation Three' powertrain development. A 'Gen Three' petrol engine could ramp up thermal efficiency by as much as 30 per cent more than the 'Gen Two' SKYACTIV-X engine, which is currently state-of-the-art powertrain technology for petrol engines.

But these efficiency gains aren't limited to petrol engines. Hirose insists that the efficiency of diesel engines can be improved along exactly the same lines. The Mazda powertrain expert explained why, having originally reduced the compression ratio of the SKYACTIV-D engine, the company was now preparing to bump it up again for the 'Gen Three' diesel powerplant.

"What happened with the SKYACTIV-D is that we lowered the compression ratio in order to reduce the NOx [Nitrogen Oxide] substances that will be produced in the combustion mechanism," he revealed.

"We wanted to have the most amount of time possible to let the air/fuel [charge] mix as much as possible for that to happen. As you know with diesel fuel, the characteristics [are] that if you put the fuel under high pressure and high temperature, it is going to combust by itself.

"But the lower compression ratio allows time before it combusts, so that's why we've lowered the compression ratio for Gen Two."

Recent advances in diesel fuel delivery systems underpin the development of 'Gen Three' diesel engines, according to Hirose.

"Injection technology is evolving at a very quick rate, year on year. Also, mixing technologies – there are so many different elementary factors that are contributing to improvement of how to mix, which means that we can get a similar well-mixed air/fuel mixture in a shorter amount of time.

"If the [charge] can be mixed thoroughly, in a short amount of time... we can increase the compression ratio for diesel as well.

"Increased compression ratio means improved thermal efficiency. So what we were trying to get at is... almost like a cross-pollination.

"All the things that we find through petrol-engine development we are sometimes able to implement for diesel. And sometimes we find things in diesel that we can put back into the petrol-engine development.

"So this is always going to be evolving... this is also in scope for the Generation Three diesel engines to improve... efficiencies there. And eventually, what we are trying to aim for is that the mechanism and the combustion itself is the same, but the only difference is fuel.

"That's the ultimate goal."

The reason why Mazda is committed to developing an engine that could conceivably run equally well on diesel or petrol can be traced back to an unexpected outcome of the world's fixation with reducing CO2 emissions.

According to Ichiro Hirose, more stringent emissions legislation out of Europe has led to increased demand for petrol-engined vehicles in lieu of diesels. But the production rate of diesel fuel to petrol hasn't kept up with the changing demand.

"When you refine oil, you'll always get a constant ratio of gasoline and diesel. What's happening now is that there is an increased demand for gasoline, and they are now reprocessing the diesel that's already produced back into petrol to meet... those demands, meaning that in this process you're producing a lot more CO2 just to get the extra petrol.

"And if we are really thinking about trying to protect the environment, meaning how to lower CO2 levels at a global scale, we need to be thinking about well-to-wheel. How really do we lower CO2 emissions [for] the whole world?

"As long as there are refineries, there's always going to be gasoline and diesel produced, so let's keep this ratio. It's always going to be produced, and if we can provide the customers with the choice of using both fuels, as long as oil refineries are there then that's always going to be contributing to keeping the CO2 emissions as low as possible..."

Hirose says that Mazda's powertrain development timeline is actually a rolling process. There are no distinct steps per se, with Gen One engines developed, built and sold alongside Gen Two (SKYACTIV-X).

Gen Two engines are expected to remain in production after Gen Three engines reach the market. The powertrain expert believes Mazda will still be building internal-combustion engines in 2030.

"Because of the way it's been explained – we talked about Gen One, Gen Two, Gen Three – it sounds like a step-wise development, but in actual fact, how the development has been made was rather more continuous," he said.

"Because the development is continuing, by installing Generation Two we've found a lot of clues and hints that will allow us to unlock Generation Three."

Hirose said that the Gen Three engines (which we're told by another Mazda exec could commence production in three to five years) will include an upgrade for the current SKYACTIV-X engine.

"Actually... they are all a continuation of the same development... That's including the original SKYACTIV engines, 'X' is also on the continuum as it were. There will be a next iteration of that that will be Gen Three."

Despite the rolling nature of Mazda's engine development program, the generational step changes do mark specific differences in engine combustion technology.

"Generation Two is really based on increasing the specific heat ratio. And looking back on Generation Two, have we met our limits of increasing these two factors? No, we don't think so, because Generation Three is now based on trying to limit and suppress the heat loss that you can get, which means that if you can suppress heat loss, you can increase the compression ratio even further. This is what we are taking apart now in our development.

"If we take the current [engine] architecture that's available to us and try to raise the compression ratio even more, you can see that the air/fuel mixture has gone under severe compression, but that then leads to heat generation – and this heat is being lost through the combustion chamber walls, with the current architecture.

"If we can limit heat loss through the chamber walls... we can increase the compression ratio even more. Increasing the compression ratio even more means... increased efficiency.

"So what we're trying to do is lessen the heat loss. Lessening the heat loss means that we allow for the compression ratio; increasing the compression ratio means that there is more useful energy that we can pull out of the system, meaning that there is increased efficiency in the engine."

In summary:

Generation One – original SKYACTIV technology
Generation Two – SKYACTIV-X (HCCI) technology to increase specific heat ratio
Generation Three – Diesel, petrol and HCCI technology to suppress heat loss