But there were two driving conditions in which I noticed the car behaving a bit strangely. The first was when I drove the car at a steady state speed for a while, and then poked the throttle to accelerate. Immediately upon depressing the pedal, a knocking noise made itself heard from the rear of the car. I noticed this only when tapping the throttle from a slow cruise, but there’s a chance the same thing happened at higher speeds, and I just didn’t notice it.

The second instance that brought out the knocking noise was when I let off the gas after a heavy acceleration. As the load on the engine dropped, I heard a light crackling noise from the tailpipe.

In both instances, the noise really sounded more like a slightly rattly exhaust than a severe engine knock, and I couldn’t perceive any hesitation in power delivery whatsoever.

Having only driven the two prototypes for about 45 minutes each (and indeed, only with warm engines), I can’t say I’ve explored the entire realm of operating points or ambient conditions. In fact, if you look at the plots of my two drives (shown above), you can see that most of my time was spent at low engine speeds (unsurprisingly); I really didn’t drive around at the top of the tach, nor did I really give the Mazda the beans from a standstill.

Regardless, during my brief drive, the car felt to me like it was 90 percent dialed in. Power delivery was smooth, NVH was decent under almost all operating conditions, and torque was plentiful.

And lest you think Mazda pulled this off by just running the engine in spark ignition mode the whole time, take a look at that column on the right side of the two graphics above: over 90 percent of my driving was done in SPCCI mode.

On top of that, my fuel economy average of 38 mpg and 33 mpg significantly surpassed what Mazda’s model predicted I’d see had I driven the current SKYACTIV-G engine in the same fashion (I’m not sure how accurate that model is, but Mazda does claim a 20-30 percent improvement in efficiency over SKYACTIV-G.)

So yes, it appears that Mazda is nearly there with its gasoline compression engine. But, as one of their engineers told me when I said the car seemed 90 percent dialed in, it’s that last 10 percent—the refinement bit—that may end up being the toughest.

But Mazda is confident it’ll have that all figured out by 2019.