Like all carmakers, Mazda must face constricting emissions standards and the need to squeeze more miles out of each gallon of fuel. And though the company will be borrowing Toyota's hybrid system in 2013, in the meantime, it has brought forward some refinements to its gas and diesel engines that promise to increase efficiency while retaining Mazda's "zoom-zoom" character. We got a sneak peek at two new engines and a chance to experience them in two prototypes. Let's take a closer look at these new motors.
Sky-G Gasoline Engine
The thing that you'll hear from Mazda that you won't hear anywhere else is that there is no plan to downsize gasoline engines and add turbos to meet efficiency goals. "In the relentless pursuit of ideal combustion," said Seita Kanai, the company's Director and Senior Managing Executive Officer, R&D and Program Management, "we are realizing a naturally aspirated engine has better efficiency than downsized, turbocharged engines."
So, starting in 2012, Mazda will unveil a new 2.0-liter naturally aspirated four cylinder that the company claims will enable its Mazda 6 midsize sedan to achieve 40 mpg. As is often the case, the new motor does not have one new part that makes this possible but a combination of new technology and careful engineering. The most eye-popping detail is the lofty 14:1 compression ratio, which is closer to diesel range than typical gas engines. A high compression increases cylinder pressure and temperature, which can produce harmful detonation. So what has Mazda done to allow such a high number?
For starters, the engine runs on the Atkinson cycle. A brief primer on the Atkinson cycle: The intake valve remains open after the piston reaches bottom dead center on the intake stroke. This spits out some of the intake charge, reducing the effective compression ratio and engine power, but since the full expansion stroke is employed, the cycle increases efficiency. The Toyota Prius uses the Atkinson cycle, but it has an electric-boost motor to offset the power loss. The new engine only runs on the Atkinson cycle at light load, thanks to a camshaft adjuster that can rotate the intake cam up to 70 degrees, much further than anything else on the market. When full power is requested—the U.S. engine will have a 13:1 compression ratio to deal with our 87 octane fuel—the computer switches the intake timing to more closely resemble the Otto cycle. The result is 158 horsepower and 151 lb-feet of torque.
Sky-D Diesel Engine
A major enabler of the Atkinson cycle is a direct-fuel-injection system that quirts in the fuel just before the spark plug fires. So, during the compression stroke, when the intake valve is still opened, the air being pushed back into the intake manifold is not mixed with gas, as it would be with port injection.
Even with the wide-range adjustable-intake camshaft, under full load, the engine could still suffer from detonation and therefore require the engine computer to retard ignition timing, hurting efficiency. To reduce combustion-chamber temperatures, the Sky-G employs a equal-length exhaust header, which scavenges exhaust gases from all cylinders, allowing cool air to enter in its place through the intake valves. In a nutshell, the almost equal-length design allows the downstream pressure wave from a previously fired cylinder to "suck" the hot gases from a just-opened exhaust-valve set. The concept isn't entirely novel—you've seen it on one race engine after another—the innovation is simply that this isn't a race engine. And so getting back to the whole heat-as-the-main-enemy thing, engineers found that the new header, along with some tricky timing adjustments, resulted in a decrease in chamber temperature of some 322 degrees F, which, as we know, is enough to cook a chicken.
Combine all that with a 20 percent reduction in friction and you have an innovative update on a gas engine that's still simple enough to be suitable for average-priced cars. Plus, we got some time behind the wheel of a prototype sedan. This new-tech engine delivered turbocharge-like torque from low in the rev-range and spun freely into the upper reaches of the band. It's still early in the development process, but the engine showed enormous potential. And that's where we technically get into the Miller cycle world. And that's cool, but this is literally cooler: The Sky-G can inject fuel during both the intake and the compression stroke, during the same cycle (something that Mazda is calling "split injection"). Injecting the gas during the intake stroke means injected fuel will actually cool the air, making the molecules more dense—and that means room for more air. Then close the intake valve, inject the rest of the fuel, and bang! Ridiculously efficient combustion.
A relatively small bore (83.5 mm) and a correspondingly long stroke (91.2 mm) help to ensure that fuel is evenly ignited under these high-compression conditions, while a piston featuring a deep cavity allows room for a proper, even burn. The current MZR 2.0-liter engine, by contrast, features a bore and a stroke of 87.5 mm and 83.1 mm, respectively.
There's also been a reduction in overall friction of between 15 and 20 percent through the use of lighter, more friction-resistant parts, such as the pistons, the connecting rods and even the crankshaft itself, according to Mitsuo Hitomi, General Manager of Powertrain Development. The valvetrain now uses silky smooth roller rockers instead of steel buckets, too. Finally, engineers designed a more efficient water pump, and the oil pump is now capable of variable displacement. Traditional oil pumps pressurize much more oil than is necessary for light load operation,and dump the excess back into the pan, which is—you guessed it—inefficient. In the case of the Sky-G, the variable pump delivers no more oil than necessary at low loads, but plenty when you pin the gas pedal and nail the redline.
As it happens, having an abnormally low compression ratio is as uniquely beneficial as having an abnormally high one—and it seems high and low are both 14:1 at Mazda these days. But while a 14:1 compression ratio is high in a naturally aspirated gasoline engine, it's barely a heartbeat in a modern turbodiesel engine. Unless that engine is Mazda's new all-aluminum Sky-D diesel engine, that is.
There are a few inherent benefits in having such a low compression setup. Typically, diesel engines require some form of after-treatment to meet emissions standards. Mazda engineers reckon that this is because of the high compression ratio of a typical diesel engine, in which ignition occurs before air and fuel are sufficiently mixed, leaving a large amount of NOx and soot in the chamber. By contrast, with a lower compression ratio, there's more time for stratification, which results in a clean burn. Mazda tells us that when its sequentially twin-turbocharged engine comes to the United States in the 2012 financial year, it will meet Tier II Bin 5 regulations with no urea after-treatment.
Also, you get more power from each downstroke, because ignition can occur sooner after the piston reaches top dead center. When all is said and done, the result is a 2.2-liter inline Four that whips up 310 lb-ft of torque at 1,800 rpm. Horsepower is 167 at 4,500 rpm. The engine will rev all the way to 5,200 rpm, and it does so with a willingness that's almost gasoline-like. Thanks to the lower compression ratio, the rotating masses have been significantly reduced, as they don't need to be strong enough to sustain sky-high cylinder pressures. The reduction in friction and rotating mass has more than compensated for the inherent power loss from the lower compression, say Mazda engineers (and as we see from the numbers).
Mazda engineers have taken the lessons learned in exhaust-gas evacuation from naturally aspirated gasoline engines and essentially applied them in the inverse to optimize cold start in the turbodiesel. Rather than scramble to suck hot air from the combustion chamber, the variable valve setup acts on half of the exhaust valves to retain hot air until the engine reaches normal operating temperature. It's not all about emissions, though—the new engine is said to return 20 percent better fuel economy than the current diesel engine, at 43 mpg highway on our cycle in the new Mazda6.
With its aluminum engine, the diesel prototype we drove weighed only 2850 pounds. Needless to say, the actual car will likely weigh more, but we could tell that the diesel's throttle response revealed a steadily mounting wave of torque becoming available from as low as 1,500 rpm or so. The lightness of the internals is also immediately evident—we were almost able to execute smooth heel-toe downshifts on exit ramps. As lightweight as it is, the mule was an absolute rocket, blasting off to more than 130 mph without so much as a hiccup. We hope that Mazda sticks to its plan to bring this engine stateside.
This article originally appeared in .