A closer look at Mazdas Sky Activ Technology

Alex Jamieson reports

Mazda have been hard at work over the past few years pushing the boundaries of the compression ratios of their internal combustion engines. The development of their SKYACTIV-G engine represented the first mass production petrol engine to achieve a compression ratio of 14.0:1 (most mass production vehicles run at around 10:1 to 12:1). For those of us without degrees in engineering, the compression ratio of an internal combustion engine refers to the change in capacity of the combustion chamber as the piston travels from the bottom of its stroke (during intake)  to the top of its stroke (during in compression). The higher the pressure of the fuel-air mixture, the higher the thermal efficiency. This means the same combustion temperature can be reached using less fuel while generating a longer expansion cycle. This produces more mechanical power while also lowering the exhaust gas temperature (which is simply wasted energy). In layman’s terms, a higher compression ratio gives you more bang for your buck.

Diesel engines have long been considered the ‘Holy Grail’ of combustion engine efficiency (let’s pretend we don’t know about all the new-found harmful emissions). Part of the reason for this is that diesel engines don’t use spark plugs to detonate the air-fuel mixture in the combustion chamber, but instead rely on the compression of air in the chamber to generate the heat required to ignite the diesel fuel. This is known as homogeneous charge compression ignition. Consequently,  diesel engines have to run at a very high compression ratio (around 15:1 to 23:1). Diesel engines typically have a higher peak combustion temperature, however, the longer expansion cycle means less energy lost, lower exhaust temperatures and therefore, you guessed it, better efficiency.

Right, now that we have all of that out the way, what have Mazda been up to? Well, they’ve managed to develop a ‘best of both worlds’ engine- a petrol powered homogeneous charge compression ignition (HCCI) engine. Named SKYACTIV-X, the new engine represents a ‘’world’s first’’ and just like a diesel engine, uses sparkless ignition- well at least most of the time. Under certain circumstances, the spark plugs will take over to ensure the optimal combustion of the fuel-air mixture. ‘’Worlds first’’ should, however, be used loosely here. In fact, the technology to allow for sparkless ignition petrol engines has been around for about 100 years, but the tendency for petrol to detonate prematurely in the chamber and effectively destroy the engine has been its major downfall. High-octane and leaded petrols were developed with the sole intention of combatting premature combustion but, until now, the ability to create a reliable mass production HCCI petrol engine has been seemingly out of touch.

Allegedly, the SKYACTIV-X engine will produce 10-30 percent more torque than the current SKYACTIV-G engine. That being said, the former will employ a supercharger, so that torque jump isn’t quite as impressive as you may be lead to believe. What is impressive, though, is the incredible 35-45% increase in fuel efficiency.  In fact, Mazda claims that the efficiency ‘’equals or exceeds’’ that of the SKYACTIV-D turbodiesel engine. No mean feat considering the 1.5L SKYACTIV-D can achieve as little as 3.5L/100km… Could this possibly be the saviour of the internal combustion engine? We can only hope so. The SKYACTIV-X engine is set to debut in 2019 and in our opinion, it can’t come soon enough.

Read our review of the Mazda CX-3 later this week.

Industry NewsUncategorized

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