What’s so special about Mazda SkyActiv engines anyway?

The latest Mazda 3 is a very good-looking hatchback. There is a sedan version, but the hatchback just looks so much better. According to my colleague, the allure of the Mazda 3 isn’t just skin-deep. I reckon he quite liked it. Mazdas have been known as fun to drive, but I'm more curious about what lies underneath .

Now, Mazda loves to spout its “Jinba Ittai” driving philosophy and “Kodo” design philosophy, but if we’re talking about the core of a car, it has to be the powertrain. And in this sense, Mazda has the SkyActiv technology.

Q: What exactly is the deal with SkyActiv engines anyway?

A: In short, Mazda just uses different engine cycles than what is usually found in other cars.

Before we go on, it is important that you know the following engine cycles:

• Otto Cycle
• Atkinson Cycle
• Miller Cycle

The Otto cycle is the most conventional engine cycle. In an Atkinson cycle, the expansion stroke is longer than the compression stroke. The Miller cycle on the other hand, is an advancement of the Atkinson cycle with the help of forced induction (supercharger).

Variations of the SkyActiv engine include the SkyActiv-G, SkyActiv-D, and SkyActiv-X. The”G” denotes gasoline while the “D” denotes diesel. The latest SkyActiv-X is a gasoline engine which employs Spark Controlled Compression Ignition (SCCI). We'll get to that later.

The first SkyActiv engine was the SkyActiv-G. If you bought a recent Mazda, it would most likely have a SkyActiv-G engine (the SkyActiv-G comes in several engine sizes).

Q: What’s so special about the SkyActiv- G?

A: The high compression ratio and the Otto-Atkinson cycle.

High compression is a characteristic of diesel engines and not gasoline engines. Gasoline engines risk “knocking” if compression is too high.

With the higher than usual compression ratio, SkyActiv-G engines are capable of extracting more power out of the fuel.

Q: How does the SkyActiv-G solve the knocking problem?

A: By using a long 4-2-1 exhaust and piston cavities.

The long exhaust reduces exhaust pulses returning to the engine (reduce heat hence reduced knocking) and the piston cavity allows smooth initial combustion-flame growth (more controlled spread of flame hence reduced knocking).

The SkyActiv-G runs on both Otto cycle and Atkinson cycle. The former at high load and the latter at low load. Combining the two, we get the Otto-Atkinson cycle.

Q: What is the difference between the Otto cycle and the Atkinson cycle?

A: In the Atkinson cycle, the expansion stroke is longer than the compression stroke. In the Otto cycle, both compression stroke and expansion stroke are of equal lengths. The original design of the Atkinson cycle was achieved via an over-center-arm on the crankshaft.

Modern day Atkinson cycles are achieved via adjusted valve timing. Besides Mazda, Toyota has also meddled with the Otto-Atkinson cycle. However, Otto-Atkinson Toyota engines like the 5.0 L V8 in the Lexus RC F did not achieve compression ratios as high as the Mazda SkyActiv engines.

Q: What’s the benefit of the Otto-Atkinson cycle?

A: Better fuel efficiency at low load and maximum power at high load. A gross oversimplification would be “It’s something like VTEC”, but that would upset Mazda engineers.

Q: Is the SkyActiv-G more powerful than standard Otto cycle engines?
A: No. It’s more efficient.

The SkyActiv-D is a Mazda’s diesel engine and we’ll talk about that in another entry. There’s some interesting stuff about that too.

Mazda has also introduced the SkyActiv-X in 2017 which finally made it into Mazda cars last year. Unfortunately no Mazda cars in Malaysia come with the SkyActiv-X engine yet.

What's brilliant about the SkyActiv-X is that it runs on the Miller Cycle. In the Miller Cycle, the opening time of both intake and exhaust valves overlap. The intake valve opens and closes later than it would in the Otto Cycle.

The SkyActiv-X uses a Spark Controlled Compression Ignition (SPCCI) which means the engine operates like a diesel engine at lower engine speeds, making it more efficient.

The SkyActiv-X is also supercharged, although the function of this supercharger is to ram as much air possible to maintain a very lean air-fuel ratio (more air, less fuel).  The lean air-fuel ratio helps to prevent knocking in the high compression design engine.

So there you have it, that is what’s special about the Mazda SkyActiv gasolin engines. They run on different engine cycles to achieve maximum efficiency.

Of course, the SkyActiv of today also extends to the entire car, but that is a story for another entry.

In summary:

1. There are three types of cycles for the ICE reciprocating engine: Otto Cycle, Atkinson Cycle, Miller Cycle

2. The SkyActiv-G runs on the Otto-Atkinson Cycle while the SkyActiv-X runs on the Miller Cycle.

3. Mazda achieves the Atkinson cycle by modifying the valve timing. In the case of the Miller cycle, a supercharger is used to create a lean air-fuel mixture (more air less fuel) for high compression.

4. The objective of SkyActiv is to achieve maximum efficiency.

5. Although not available in Malaysia, the SkyActiv-X is available in Europe in cars like the Mazda 3 and Mazda CX-30.