Is Mazda the Savior of the Internal Combustion Engine?

Automaker cracks the HCCI code and we sample an early interpretation

With the recent increase in popularity of hybrid powertrains and electric cars, you would be forgiven for thinking that we are beginning to see the demise of the internal combustion engine (ICE). The electric revolution is almost at full charge, but Mazda may have the technology that could save the ICE.

OEMs such as Tesla are revolutionizing the use of electrons to propel us down the road, and conventional manufacturers such as Volvo have even stated dates for when they’ll start producing only electric or hybrid cars, starting as soon as 2019. The electric revolution is exciting, but what about the humble ICE?

Emissions and efficiency regulations are forcing the conventional ICE to become downsized and turbocharged, or hybridized with electric motors to meet stringent targets. With this in mind, we were very surprised when Mazda invited us to see their new, efficient, low-emissions, four-cylinder ICE with not a turbocharger or electric motor in sight.

The Science

Let’s start with some context: the conventional gasoline engine we are so used to has a thermal efficiency of around 30%. This means that only 30% of the energy contained within the fuel is used to push the piston down, propelling you down the road. The rest is converted into heat that escapes through the exhaust or into the cooling system.

This efficiency is, to a certain extent, fixed, when a sparkplug is used to set off an explosion in a cylinder. There is an alternative method of exploding liquefied dinosaurs which has a theoretical thermal efficiency in the region of 30-40% better than the conventional ICE, and its name is homogenous charge compression ignition (HCCI).

The homogenous charge part means that highly atomized fuel particles are mixed evenly with incoming air, forming a homogenous (evenly distributed) air/fuel mixture. The compression ignition part means that instead of using spark plugs, the action of squeezing the mixture during the piston up-stroke is enough to auto-ignite the mixture—just like a diesel engine.

The whole process is significantly more efficient than the conventional spark induced explosion of a gasoline engine. The reason why we haven’t seen this in a commercial application yet is due to the difficulties in controlling this explosion.

With HCCI, ignition timing is very hard to control. A mixture too lean or rich will mean an ignition too early or late respectively. Some researchers have tried to control this by using something called ‘effective compression ratio control.’

This involves playing with the exhaust valve lift timings to change the effective volume of the cylinder, and therefore, the compression ratio. However, modulating exhaust valve timings on a cycle-to-cycle frequency is a hard task, and thus, HCCI technology has stalled at the lights. Until now.

The brainboxes at Mazda seem to have solved the issue of compression ignition timing, using a little device called a spark plug. “But that defeats the point of compression ignition!” I hear you scoff. Bear with me, it’s actually quite clever.

Given that pressure is the determining factor with auto-ignition, Mazda engineers had to think of a way to affect this variable with high frequency. The solution they subsequently developed is remarkable. They use an isolated area of the cylinder immediately adjacent to the spark plug to concentrate a tiny flame ball induced by a spark from the plug.

This flame ball sends a shockwave down towards the upcoming piston, producing an effective secondary piston, squeezing the air/fuel mixture. The increase in pressure pushes the homogenous mix over its auto-ignition threshold, setting off the efficient burn of the perfectly mixed air/fuel mixture.

This new engine family is code-named SKYACTIV-X and the proprietary technology is dubbed Spark Controlled Compression Ignition (SPCCI).

You may be asking yourself if Mazda is holding on to a dying technology and that electrification is inevitable due to its environmental benefits. Not according to Mazda’s senior technical fellow in charge of R&D, Mitsuo Hitomi, who was introduced to us as ‘Mr Engine.’ A one-to-one chat with Mr Engine revealed how close SKYACTIV-X emissions are to an electric car when we consider well-to-wheel emissions.

Currently, the vast majority of electricity used to charge cars comes from coal-fired power stations, which inherently involves masses of carbon emissions. Hitomi-san showed me how their SKYACTIV-X engine emits around 142g of CO2 per kilometer. Compare that to an electric vehicle’s emissions of 128g/km and one can appreciate how far Mazda have come with their new engine family.

Not content with only reinventing the engine, Hitmoi-san let slip about a new synthetic fuel that he is co-developing with university academics in Japan. Using genome editing techniques, the collaborative team is developing a microalgae–nannochloropsis oceania–that uses photosynthesis to produce synthetic fuels. Not only will this reduce the use of fossil fuels, but the higher purity fuels made in this way can be used in even higher compression ratio engines, increasing efficiency even more.

The Drive

After a couple of hours of technical presentations from the executive engineers, I was finally given the keys to a one-of-four R&D car. From the outside, it looks just like a current generation Mazda3, sporting a mean matte black paint job.

Under closer inspection, it was clear that the outer shell was the only aspect that’s shared with the current line-up. The new SPCCI engine was nestled into Mazda’s next generation chassis and shrouded in an existing Mazda3 body. For all intents and purposes, this was the next generation 3.

Pressing the engine start button is almost anti-climactic, as what happens next is distinctively normal. The sound and feel of an idling SPCCI engine is 90% ICE, 10% diesel. After pleasantries were exchanged with the German Mazda engineer—there to make sure we didn’t wreck a multi-million-dollar prototype—I pulled out onto German streets.

The engine pulls from low rpm with more torque than one would expect from a—technically–naturally aspirated engine, with a feel that lies between that of an NA and a turbo-diesel one. Under higher loads and at the higher band of rpm, spark ignition seamlessly takes over from SPCCI, resulting in the feeling of a turbocharged engine that is falling out of its power band at the top end. Overall, power delivery is comparable to that of a mildly supercharged ICE. However, there are slight differences that allude towards you driving something out of the ordinary.

After cruising around small German streets at a frustrating 20 mph, I saw the sign for the de-restricted autobahn. I stirred down through the gears until I hit 3rd and matted the gas pedal. The SPCCI engine responded well and pulled all the way up to its 6,000-rpm redline before I snatched another gear. The speed dial slowly crept up, but my attention was elsewhere in an effort to keep a priceless prototype in one piece. It’s only as I approach an Audi R8 blocking the outside lane that I look down and see 130 mph. What is extraordinary about this tale is not that an R8 was in the way of a hatchback with an eco-engine, but rather that the engine felt pokey all the way up to this speed, and it could have given more.

Analyze the fuel consumption curve across the entire rev range of the new SKYACTIV-X engine and we can see why:

The yellow area of the graph is the most economical part of an engine’s torque/rev range, and manufacturers design their transmissions to stay close to this area. With the current SKYACTIV-G engine, 6th gear intercepts the yellow range and tails off into the green areas. In the real world, this means that the gear ratio is rather high. Move over to the SKYACTIV-X plot and we see that the engine is so economical throughout the range, that the gear ratios can be lowered.

What does that mean to us? Well, this means that we can spend more time stirring gears and enjoy more of the rev range while still driving economically. This is a point stressed by Ichiro Hirose, managing executive officer in charge of powertrain development. His goal was to design an engine that could offer “driving pleasure that comes from the combination of performance & efficiency,” and you can’t say fairer than that.

Throughout the test drive, I noticed that there was a discernible rattle at high-loads and low rpm. After a chat with Hitomi-san after the drive, it became clear that this was engine knock (detonation) and was merely a tuning issue that would not be present in the production engine.

After all, we must bear in mind that the final product is at least two years away. Other than the intermittent knock, the prototype engine was remarkably polished. The transition from SPCCI combustion to only SI at higher revs was not even noticeable, and refinement was equal to the SKYACTIV-G engine that powered the car I drove to the test center.

The verdict

The electric revolution is almost at full charge, but Mazda may have the technology that could save the ICE. Although the SKYACTIV-X engine family is still a couple of years away from production, our early drive showed great promise. With almost as much performance as the current SKYACTIV-G engine in the Miata, but with comparable emissions to a 1.5 diesel motor, the SKYACTIV-X engine offers longevity to the gasoline burning engines that us petrol heads cherish.

What does the future hold? Hitomi-san admitted that there was a long way to go, but alluded towards many exploitable avenues to improve the engine in terms of performance. Long live the internal combustion engine.

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