The problem, low internal combustion efficiency. The solution, Mazda’s SKYACTIV.
Wayne Gerdes -
CleanMPG - May 8, 2012
SKYATIV - It is more than just a slogan or badge. It stands for fuel efficiency.
Mazda’s range of SKYACTIV technologies are designed to improve the efficiency and sustainability of the company’s latest and future generation of vehicles.
Mazda’s trademarked SKYACTIV moniker is not some “pie-in-the-sky” elixir guaranteed to solve all the world’s problems but it does one thing better than any other OEM internal combustion engine and that is provide greater fuel economy at lower cost through optimized internal combustion engineering.
SKYACTIV-G 2.0L Gasoline Engine
Mazda has advanced the state of the internal combustion engine to an art form thanks to a range of entirely new technologies in order to produce its highly-efficient direct injected SKYACTIV-G 2.0L gasoline engine. The engine takes the compression ratio to an entirely new level while solving most of the issues that has hindered high compression engines from providing normal drivability until now.
SKYACTIV-G 2.0L Highlights- Exceptionally high 13:1 compression ratio in North America (14:1 in other markets due to a higher fuel octane).
- Extraordinary compression ratio made possible thanks to a 4-2-1 exhaust system, redesigned piston cavity, new multi-port injectors as well as other innovations to avoid abnormal combustion (knocking).
- Continuously variable sequential valve timing (dual S-VT) on the intake and exhaust minimizes pumping losses.
- Internal engine friction was reduced by 30 percent.
- Overall weight was reduced by 10 percent.
- Approximately 15 percent lower fuel consumption and 15 percent more torque at lower and mid-range RPMs than the previous Mazda 2.0L.
120 years of non-stop development on the internal combustion engine has yet to overcome the waste of 60 to 90 percent of the energy contained in the fuel that keeps it running. Since this energy loss is primarily thermal and can be attributed to the exhaust, cooling system, and engine and transmission surfaces, Mazda’s R&D focused on improving the engine’s thermal efficiency and reducing internal engine friction.
The six controllable factors at the heart of this approach are:
- Compression ratio
- Air-to-fuel ratio
- Combustion duration
- Combustion timing
- Pumping losses
- Mechanical friction loss
The goal was to optimize each of these factors to create the current best available internal combustion engine. Ultimately, the compression ratio would end up playing a central role among these factors in both gasoline and diesel engines.
Higher Compression Ratios Rather Than Downsizing
Most automakers are looking to improve the average fuel economy of their internal combustion engines by reducing displacement and offset the loss of power and torque with turbochargers or superchargers.
Although this is an effective approach, Mazda has chosen a different approach, namely to optimize and raise the compression ratio.
By rethinking common thermodynamic principles, Mazda engineers have succeeded in building an engine with an extraordinarily high 13:1 compression ratio that can run on regular 87 Octane gasoline. This is a level only seen thus far in non-diesel high-performance race car engines on high octane racing fuels and most certainly not intended for everyday use.
While raising the compression ratio in a gasoline engine increases its thermal efficiency and its fuel economy, high compression in conventional engines leads to unwanted abnormal combustion known as knocking and the associated reduction in torque. A richer mixture and delayed ignition timing are used to avoid knocking, but these also come at the expense of fuel economy and torque. So how was this issue overcome?
Ingenious Thinkers
Knocking takes place when the fuel-air mixture ignites prematurely because the temperature and pressure are high enough within the cylinder to self ignite mixture before it was intended by the spark plug. This can be countered by reducing the quantity and pressure of hot residual gases in the combustion chamber.
Exhaust Manifold - In response, Mazda developed a special 4-2-1 exhaust manifold, which, due to its relatively long runner structure, prevents the exhaust gas that has just moved out of the cylinder from being forced back into the combustion chamber. The resulting reduction in compression temperature inhibits knocking.
Combustion Duration was also reduced. Faster combustion shortens the time the unburned fuel-air mixture is exposed to high temperatures, which enables normal combustion to conclude before knocking occurs.
Special piston crowns and injectors - The new engine also received special piston cavities, which allow the initial combustion flames to propagate without interference, and new multi-hole injectors, which enhance fuel spray characteristics. Together with the 4-2-1 exhaust manifold, these innovations resulted in a substantial 15 percent increase in torque and fuel economy over Mazda’s older 2.0L gasoline engine.
Reduced Pumping Losses - To improve engine efficiency, it is also necessary to reduce the pumping losses that occurs at lower engine loads when the piston draws in air through a tight restriction called the throttle plate while moving downward during the intake stroke.
The amount of air going inside the cylinder is controlled by the throttle plate located upstream of the intake pipe. At lower engine loads, only a small amount of air is necessary. The throttle is nearly closed, causing a vacuum inside the intake. As a result, the piston has to overcome a strong vacuum through a process known as pumping loss, which negatively affects efficiency.
Mazda managed to minimize pumping loss with a continuously variable dual S-VT (sequential valve timing) on the intake and exhaust valves. This changes the opening and closing timing of the valves, enabling the air intake quantity to be controlled by the valves rather than the throttle.
During the intake stroke, the throttle and intake valves are kept wide open while the cylinder moves downward. The intake stroke finishes when the piston reaches the cylinder bottom (bottom dead center or BDC). But if the intake valves close here, there is too much air inside the cylinder when only a small amount of air is needed at lower engine loads. In order to push out the excess air, the intake S-VT keeps the intake valves open when the piston starts to move upward during the compression stroke. The intake valves then close when all unnecessary air is pushed out.
This is how an S-VT minimizes pumping loss, making the overall combustion process more efficient.
A drawback to this process is destabilized combustion. Since the intake valves are kept open even when the compression stroke starts, the pressure inside the cylinder decreases, making it difficult for the fuel-air mixture to combust. Thanks to the relatively high 13:1 compression ratio, combustion chamber temperatures and pressures are somewhat elevated at the point of optimal ignition combustion so the combustion process remains stable despite reduced pumping losses.
2013 Mazda CX-5 and the 2.0L SKYACTIV-G
The technoolgy's inside.
What good is the engine if it drives a cacophony of gear parts in the transmission?
Mazda came up with a redeveloped, high-precision six-speed manual transmission. With a remarkably compact and lightweight design along with diminished internal friction resistance, the SKYACTIV program has provided another multiplier to reduced fuel consumption and emissions.
SKYACTIV-MT – The ultimate six-speed manual transmission
Mazda built the innovative new SKYACTIV-MT six-speed manual transmission to improve fuel economy by lowering its weight, making it smaller and more efficient without compromising drivability. Mazda benchmarked the Miata which incorporates one of the slickest shifting manuals ever to be installed in an automobile.
The SKYACTIV-MT was optimized for front-engine, front-wheel-drive vehicles with easy yet precise shifting patterns. It was also re-engineered with a considerably smaller and lighter design and provides better fuel economy thanks to reduced internal friction.
The Result
As posted above, other manufacturers are using advanced direct injection while others still are using direct injection and turbo charging with downsizing to achieve acceptable performance and the highest high fuel economy.
As of this writing however, none of those approaches comes close to providing the efficiency that Mazda’s SKYACTIV technology placed within the 2013 Mazda CX-5 compact Crossover can. That is its 35 mpgUS highway rating when equipped with the 6-speed manual transmission. We have featured the 155 HP SKYACTIV as far back as the 2011 NYIAS and highlighted it as a hidden gem at both the Detroit and Chicago Auto Shows earlier this year. Now it’s time to put up or shut up.
2013 Mazda CX-5 and the 2.0L SKYACTIV-G
$21,490 including D&H as she sits in the drive. What is that tree growing out of it all about
70.4 mpg on its first drive over 8.4 miles. I do not believe any other compact crossover can do that?
Now that we are finally driving one, “put up” indeed! Not bad Mazda, not bad at all.
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