Mazda’s new SkyActiv-X engine will go against every convention you thought you understood about gasoline engines. It’s supercharged, but for efficiency purposes, not to increase power. It has a compression ratio of 16:1 (in current prototype form) yet runs on 87 octane fuel. It relies on the Miller cycle (or modern Atkinson cycle) when it wants to make power, yet runs a traditional cycle, albeit with compression ignition, when it wants to be efficient. It’s a gasoline engine that encourages knock, but in a controlled manner… What?
It sounds confusing at first, and unfortunately the complications accumulate as you dive deeper. Mazda is doing something nearly every auto-manufacturer has thought about, researched, and certainly desired, but they did it all with the help of a spark plug. Spark Controlled Compression Ignition (SPCCI) is a new method of combustion that allows gasoline engines to run on compression based ignition. In other words, it takes the combustion methodology of a diesel engine, while using the timing methodology of a spark-ignited gasoline engine.
The basic idea stems from attempting to accomplish the ideal engine: instantaneous combustion, no heat losses, and zero friction. It’s not possible to achieve in the real world, but we can certainly get closer than we are now. That’s Mazda’s mission. Ideally, gasoline engines would use Homogenous Charge Compression Ignition (HCCI) for maximum efficiency. During the compression stroke, the piston moves upward towards the cylinder head, increasing the pressure, and consequently the temperature, of the air-fuel mixture contained within. The pressure raises so high, and the temperature gets so hot, that even without a spark plug the vaporized gasoline combusts as the piston nears the top of its stroke. Full combustion occurs simultaneously, extremely quickly, and all of the pressure produced from combustion turns into useful work, forcing the piston downward. Well, that’s best case. The real world isn’t quite as lovely.
The challenge with HCCI is that there’s a very narrow range of RPM and load (throttle position) at which it’s possible. HCCI works best under low engine load at moderately low RPM. Outside of this range, the combustion timing is chaotic and difficult to control. And if detonation (knock) occurs when you don’t want it to, you at best lose efficiency, and at worst destroy your engine. So at this point we know two things: 1. There’s a narrow range at which HCCI is possible. 2. Outside of this range, spark-ignition is required. Thus the challenge is figuring out a way to switch between spark-ignition and compression ignition in a seamless manner.
But what if you didn’t need to fully switch over? What if you always used the spark plug to time ignition? Well that’s exactly what Mazda has done. In the ranges where compression-ignition isn’t possible, the SkyActiv-X engine runs like any other gasoline engine. The piston moves up, compressing an air/fuel mixture until top dead center, where a spark plug ignites the mixture and a flame front travels outward, pushing the piston down for the power stroke. Normal combustion, yawn…
When possible, the engine operates in a manner similar to diesel engines, albeit with premixed air and directly injected fuel. Air comes in, swirling about, creating a nice homogenous air-fuel blend as the piston reaches the top of its travel. Ideally, the air-fuel ratio is about 37:1 at this point. You’ve read that correctly, about 2.5 times as lean as a traditional gasoline engines! In the center of that swirling air-fuel mixture, the eye of the storm remains calm. Here, the direct fuel injector spritzes a light addition of gasoline, dropping the air-fuel ratio nearest the spark plug to approximately 29:1.
These two distinct regions of air-fuel ratios play important individual roles. As the piston moves upward, the fuel is getting dangerously close to igniting all on its own from the heat and added pressure from compression (remember, this is regular octane fuel and a compression ratio of 16:1). Just before knock starts to occur, the spark plug fires away. The less lean area of air near the plug combusts, creating a tiny expanding flame front.
With the added pressure from the expanding flame front, the surrounding ultra-lean air can’t hold on any longer. Nearly instantaneously, the entire mixture bursts into flames. And suddenly Mazda’s strategy makes so much sense. Use a spark plug for perfectly timed combustion, and use compression ignition for its incredible efficiency benefits. Wait, but what about that supercharger, the low octane fuel, and whatever the Miller cycle is? Why all the quirks and complexities? Everything that at first seems counterintuitive has a rational explanation:
1. Mazda's Lean Supercharger
Mazda doesn’t want you to call the roots blower attached to the side of the SkyActiv-X engine a supercharger; they’re currently calling it the “high response air supply” but don’t be fooled, it’s a roots-style supercharger. The reason they don’t want you calling it what it is, is because superchargers are generally associated with horsepower rather than efficiency. In this case, the supercharger is being used to lean out the air-fuel mixture, a lean supercharger, if you will, to improve fuel economy. This reduces combustion temperatures, lowers NOx emissions, improves the amount of useful work produced with the same amount of fuel, and reduces heat loss to the cylinder bore. Wonderful!
2. Regular Octane Meets A 16:1 Compression Ratio
If Mazda had it their way, they’d be running 80 octane fuel in their SPCCI engines rather than the elitist 87 you find on the affordable nozzle at the gas station. That’s because these engines essentially crave knock. Not exactly the knock you’re thinking of, but very similar.
Remember, compression ignition means you want the fuel to ignite all on its own. Using a higher octane fuel would in this case make combustion more difficult. The more sensitive the fuel is to pressure changes, the more easily Mazda can control exactly when it ignites by using a spark plug.
3. The Powerful Miller Cycle
The Miller cycle (nearly synonymous with the modern Atkinson cycle) is a trick used on modern engines for improved efficiency. The idea is to leave the intake valve open for a portion of the compression stroke, pushing some of the air and fuel out of the cylinder back where it came from.
This reduces the effective compression ratio, and together with a now (relatively) higher expansion ratio, reduces fuel consumption. But Mazda uses the Miller cycle for exactly the opposite reason: torque! When the engine wants peak power over great fuel economy, the supercharger only provides a modest amount of boost. The intake valve allows air to escape during compression to ensure knock doesn’t occur with the wildly high compression ratios used. Plenty of air, the equivalent of a commoner’s engine’s glutinous fuel supply, and we’re taking our eyes off the mpg-O-meter and enjoying the road ahead. Now we’ve seen it all: an efficient supercharger and a powerful miller-cycle. Just don’t expect anyone to believe you when you show up to the car meet.