The internal combustion engine, now more than 200 years old, will be with us for a long time to come yet.
Powering more than 1 billion vehicles on the planet and almost 100 million new ones each year, it is the default way to move vehicles along the world's roads.
With its carbon emissions now conclusively implicated in climate change, electric cars may seem like a panacea—but they will take many decades to become even half of the planet's vehicle fleet.
With that in mind, small Japanese carmaker Mazda has put a huge amount of research into improving the thermal efficiency of the standard combustion engine to lessen its ill effects on the environment.
The average gasoline engine wastes 70 to 75 percent of the fuel's energy content in heat and noise. Suppose that could be improved on, without expensive add-on ancillaries or hybrid systems with battery packs?
Two weeks ago, we drove running prototypes of the company's latest evolution of its SkyActiv engines, which will be the first mass-produced engines to operate on the principle of homogenous charge compression ignition, or HCCI.
What Mazda calls its SkyActiv-X series represents the next evolution from the SkyActiv-G gasoline engines it launched in 2012.
With the highest compression ratio of any gasoline engine in production—14 to 1—those 2012 powerplants burned a leaner air-fuel mixture and consumed roughly 20 percent less fuel than engines of similar output from other makers.
Now, the 2.0-liter SkyActiv-X inline-4 offers another 20-percent improvement, meaning real-world gas mileage of almost 40 mpg in a prototype 2020 Mazda 3 compact hatchback during a mixed drive cycle that included some short 100-mph blasts on the German autobahn.
That new engine will arrive for 2020, and it represents another leap in improvement. Mazda's achievement in putting an HCCI engine into production means it will have reached what engine designers for decades have considered a Holy Grail of efficiency.
The idea is to create an engine in which an exceptionally lean air-fuel mixture spontaneously combusts under compression alone. That’s how a diesel engine works, but the SkyActiv-X engine burns gasoline and its compression ratio of 15 or 16 to 1 is lower than that of many diesels.
While HCCI requires less fuel for the same specific output, any such engine must also be capable of operating with much richer air-fuel mixtures, ignited by a spark plug, to develop maximum power—which requires a lower compression ratio.
Mazda SkyActiv-X engine: additional compression from sparked fuel-air mixture ignites lean air-fuelEnlarge Photo
Mazda SkyActiv-X engine: spark-controlled compression ignition (SPCCI)Enlarge Photo
Mazda SkyActiv-X engine: using the spark plug as a control factorEnlarge Photo
Mazda 2.0-liter SkyActiv-X engine with spark-controlled compression ignition (SPCCI)Enlarge Photo
Previous attempts at HCCI engines from Daimler, General Motors, Nissan, and other makers varied the compression ratio within the combustion chamber mechanically.
From hinged connecting rods to crankshafts whose throws could be varied, rather like valve timing, they were fiendishly complex and added numerous additional parts within already complex internal combustion engines.
Mazda has taken a different path: the compression ratio applied to the lean air-fuel mixture is increased inside the combustion chamber by combusting a very small amount of rich air-fuel mixture close to the spark plug during the piston’s compression stroke.
That spreads a pressure wave that further compresses the lean air-fuel mixture, which then ignites on its own, not as a result of being sparked.
Mazda has dubbed the technology Spark-Controlled Compression Ignition, abbreviated as SPCCI.
Its simplicity is indicated by the use of a slightly strengthened conventional SkyActiv-G block, with three alterations to ancillary components that make the lean combustion possible.