First off, the Mazda3 i isn’t the fastest Mazda3, powered by the smaller 2.0-liter four, rated at 155 horsepower rather than the 184 horsepower and 185 lb-ft of torque of 2.5-liter of the Mazda3 s models. But the smaller engine is in some ways more technologically advanced than its big brother, fully engaged with what Mazda has improbably named Skyactiv technology.
That said and before we go further, Skyactiv isn’t a special engine, as in just a special engine, but a whole suite of optimizations that improve efficiencies without adding weight or the complexity of added equipment—such as a turbocharger—which increases an engine’s efficiency by increasing the power of an engine of a certain size, then reducing the size of the engine to obtain improved economy from the smaller engine while maintaining performance with the horsepower added by turbocharging to improve the engine’s volumetric efficiency.
That’s the philosophy of Ford’s EcoBoost system, as the name suggests, but it’s arguable that it’s a backdoor approach to fuel saving. The Mazda2.0-liter four as used in the Mazda3 i improves its volumetric efficiency, however, without the pumping loss of conventional piston engine by the use of a very high compression ratio of 13:1, and valve timing designed to prevent knock, the premature violent combustion of the fuel/air mixture.
Here’s how the Skyactiv valve timing the 2.0-liter engine works. The flow of intake air to the engine isn’t restrained by a throttle plate, which normally is kept nearly closed to reduce engine output by limiting the air flowing into the engine. But when the intake valve opens on the intake stroke of the engine, the vacuum that’s formed behind the throttle plate pulls back on the piston. It’s this resistance that’s called pumping loss, reducing low speed fuel economy.
The Mazda Skyactiv 2.0-liter engine, on the other hand, completely opens the throttle plate and intake valve, letting air flow into the engine unrestrained. To keep the engine from running away at full throttle, however, the intake valve is held open longer, into the compression stroke. As the piston moves up, some of the air that was sucked in past the intake valve to blown back out. That reduces the effective air taken into the engine without the pumping loss caused by the throttle plate.
In an engine with port fuel injection (or, God forbid, a carburetor) this air would have had a fuel mixture, and pushing it back into the intake port would result in an even richer mixture as more fuel is injected with the next intake stroke. But the Mazda Skyactiv 2.0-liter engine has direct injection, so the air flowing back and forth has no fuel mix in it.
Mazda explains, however, this would normally cause “destabilized combustion,” and with a name like that, it can’t be a good thing. The 2.0-liter Skyactiv engine, however, has a remarkably high 13.0:1 compression ratio, which Mazda says has a stabilizing effect on the combustion process.
That still doesn’t eliminate all tendency for an engine with ultra-high compression to knock, but the Mazda 2.0-liter Skyactiv engine also comes with a long four-into-two-into-one exhaust manifold that helps scavenge the combustion chamber quickly and with less backpressure than an exhaust manifold with shorter runners, reducing backflow of hot exhaust gases into the cylinder, thereby inhibiting engine knock.
The length of time combustion takes also affects knocking, so the 2.0-liter Skyactiv a special piston is used with an unusual pocket in the center of the piston face which, according to Mazda, allows “initial combustion flames to propagate without interference.”
All in all, it’s a remarkable refinement of the Otto cycle engine.
It’s not all airflow trickery, however. Mazda decreased the weight of engine internals, with 20 percent lighter pistons and connecting rods that weigh 15 percent less. That’s weight that in itself must be rotated before the car can be moved by the engine. And as another boon to performance and economy, internal friction was reduced by 30 percent.
Overall, the changes to the engine alone increase fuel economy by 15 percent.
Mazda also increased fuel economy via two new transmissions, a six-speed automatic and six-speed manual. The automatic gets new technology, with full-range lockup, with an up to an 88 percent lockup ratio.
Mazda knows our preferences here at CarBuzzard, however, and provided us a manual transmission-equipped Mazda3 i for testing. The new lightweight transmission was benchmarked against the Mazda MX-5 Miata for feel and operation. One of the ways Mazda got that Miatafeel was to have initial resistance which decreases through the 1.8-inch throw of the shift lever. Mazda says it gives the feel of sliding into the next gear.
The reduced weight of the transmission means less for the engine to haul about, which aids not only acceleration but fuel economy as well (plus helps with the zoom-zoom braking and cornering). Beyond the gearbox, the new Mazda3 body is eight percent lighter than last year’s and the chassis’ weight has been reduced by 15 percent. At the same time, stiffness has been increased by 30 percent. It’s all through new design—which for example includes strengthening elements that are “continuous” rather than “noncontinuous—and with additional spot welds and adhesives. At the same time, significant changes were made to the suspension, both for increased nimbleness along with a smoother ride.