Jeff Taylor boasts a 30-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Taylor is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.
I am sure that by now almost all of us have seen or heard the advertising of Mazda’s Skyactiv technology, but how many of us actually understand what it means? If you’re a full-time Mazda tech or just a Mazda aficionado, then this will just be a lot of review for you. But for the average tech who doesn’t specialize in just Mazda’s latest vehicles, I want to explain what is meant by Skyactiv technology, and what changes it has meant to the engine packages.
Mazda came up with the term Skyactiv to describe the methodology rather than technology to improve its products, by making them lighter, more efficient with reduced friction. Skyactiv was going to involve new technology, but it would come from the Skyactiv way of thinking... Skyactiv G for gasoline engines, Skyactiv D for diesel technology and other areas including Skyactiv transmissions, Skyactiv chassis, Skyactiv hybrids, etc. In this article I want to concentrate on the Skyactiv G technology. Introduced in late 2011 as the 2012 Mazda3, it has been around for a while and many of the vehicles are off warranty and starting to darken the doors of our shops.
Skyactiv G was to deliver an estimated 15% increase in fuel efficiency and torque over the similar engine before Skyactiv technology, and they did this using a number of unique to Mazda ideas and a few ideas that are shared across many platforms and manufacturers.
When the hood is first opened on a new Mazda the large plastic sound cover doesn’t allow you to see much of the technology that is underneath. But even when that shield is removed it still looks like a regular four cylinder engine. However, there are a lot of very significant differences in the way the air is brought in, fuel burned and exhaust exhausted.
The first and foremost idea was the use of the Miller cycle engine. The Miller cycle engine was developed by a U.S. engineer back in 1957 and the premise is to get more thermal efficiency out of the engine then the current Otto cycle four stroke engines. This isn’t Mazda’s first kick at the Miller cycle engine; they used it on the Millenia S V6 engine for a few years in the 1990s, but that’s where the similarity ends. The Millenia S was a supercharged engine and the currently used engine is naturally aspirated.
But before I get too far ahead, let’s refresh how the Miller cycle engine works. We all understand the Otto cycles engine’s four strokes; intake, compression, power and exhaust, but the Miller cycle modifies the way the intake stroke works and the timing of the exhaust valve opening at the end of the power stroke.
The intake valves stay open longer, into part of the compression stroke; allowing some of the fresh intake air to be pushed back into the intake. The initial design required a supercharger to counteract this flow to avoid a lack of power, but the idea is to lessen the power required to compress the air in the combustion chamber.
The next difference is to have a higher compression ratio then the standard Otto cycle (for more power) and finally to allow the gases to expand much farther into the power stroke before opening the exhaust valves to release the exhaust gases. This harvests more power out of the fuel and increases thermal efficiency. In the familiar Otto cycle engine the exhaust gases leave under lots of pressure, about five times more than a similar Miller cycle power plant.
Mazda’s Skyactiv philosophy is to make things lighter, to generate less friction and be more efficient, so they adopted the Miller cycle engine once again. To do this they made significant design and engine concept changes (they didn’t just recycle a past version).
There is no supercharger to increase the intake pressure, so they had to rely heavily on new technologies developed by the Skyactiv engineers to perfect a Miller cycle engine that not only gave incredible fuel mileage but was fun to drive as well.
There are some real highlights as to how they were able to achieve this and some interesting technology was embraced to get to the current point that the Skyactiv team has reached. Note that this engine is designed to be able to run as high as a 15:1 compression ratio, so a huge focus was on reducing the effects that high compression was going to have in the creation of pre-ignition knock and still reap the benefits of such a high compression ratio. In reality the compression ratio in the U.S. is maxed-out at about 13:1 so the vehicle can comfortably run on regular gasoline.
The main changes are in distinct areas of engine control and architecture; intake and induction, piston and internal engine designs, anti-knocking/pre-ignition technology and the exhaust system. Yes, there are a number of other technological advances that are included, but many of them were already being used by other manufacturers, but with a Mazda twist.
Starting with the induction system...
The intake cam is driven by a timing chain, but the phase of intake cam is controlled by an electric motor. This radical departure from an oil control phaser is part of the Skyactiv approach. The electric motor drives a planetary gear set that can advance or retard the intake camshaft with great precision under a much greater range of operating conditions.
During cold starts, for example, the cam can be much more easily fine-tuned to the best possible position without worrying about the oil’s viscosity. This cam timing system keeps the intake valves open longer, thus reducing pumping losses (again a principle of the Miller cycle engine). Under certain conditions such as light load or low speed cruise when the system is active and applying full advance gaining the full effect of the Miller cycle operation without a supercharger the throttle body can be held close to wide open to keep pumping losses to a minimum.
You may see this show up in scan tool data as a throttle position PID of 15% yet the actual throttle value on the throttle body may be 80%, but don’t worry, this is expected and the PCM is controlling the engine operation and rpm with valve opening and fuel injection timing. But having the intake valves open so long, and the throttle body open so far creates another issue that has to be addressed:There is little to no vacuum in the intake. It is this fact that necessitates that this engine has a vacuum pump to operate the vacuum accessories, like the brake booster. The vacuum pump is driven off the end of the exhaust camshaft. The intake cam can run in three different modes, normal or feedback mode where the PCM has full control, default or full retard if there is a control or electric phaser issue, and phase holding mode to aid in start ability.
Moving inside the engine...
A number of areas were addressed to reduce friction, prevent pre-ignition and increase efficiency. The combustion chamber was reshaped to form a compact pent roof design that holds four valves (two intakes, two exhausts) per cylinder in a cross flow orientation aiding in intake and exhaust flow. The spark plug and direct injection fuel injector are placed in the combustion chamber roof at the optimal position for combustion efficiency. The intake port shape increases cylinder pressurization and provides the needed tumble to speed up combustion and help prevent pre-ignition.
Needle roller bearings are now used in the rocker arms, and a special oil shower bar for lubrication is added to reduce friction in the cylinder head’s other moving parts. Moving deeper into the engine, specialized smaller bored domed pistons have a very unique shape with a cavity on top. A homogenous fuel charge is supplied by the multi-hole direct injection fuel injector aimed at this cavity. This provides for rapid and even flame front propagation when the charge is ignited by the spark plug.
Because the initial flame starts in the cavity, less heat is transferred into the piston surface (another pre-ignition prevention method) and greater thermal efficiency is achieved. Similar to Ford Ecoboost engines, there is now an oil jet that is aimed at the bottom of the piston to remove excess heat and cool the piston. The oil pump is chain driven to reduce the amount of force needed to drive it and the oil pressure is now controlled electronically. There is a low pressure and a high pressure mode, and the modes can be switched by the PCM, depending on the driving conditions.
The exhaust system...
The chain driven variable exhaust cam still uses oil pressure control for phase control, adjusted for the best possible fuel economy, torque and still control NOx production acting like an EGR valve. The exhaust cam control can operate in full PCM control, a unique cleaning mode that operates during a deceleration fuel cut on a warmed up engine and flushes out any foreign material from the oil control valve and finally advance mode to stabilize the engine during cranking, at idle, and when the engine is cold (below 140 degrees Fahrenheit). It’s after the exhaust leaves the engine that yet another very unique Skyactiv approach shows up.
The use of a specialized header that takes the exhaust ports away individually to a specific length, then pairs them up (again a very specific length) and then pairs them up again in a 4-2-1 design. The explanation is simple to any drag racer but for the average person the reason is a bit more complex. When each cylinder’s exhaust valves open, the exhaust is forced out into the exhaust manifold. This pulse of exhaust can be forced into neighboring cylinders if their exhaust valves are open and the manifold has a common runner. This robs efficiency by forcing hot gases back into the combustion chamber of the cylinder with the open valve.
These hot gases can cause pre-ignition by raising the actual combustion chambers initial operating temperatures, and with a high compression engine the last thing that the engineers want is pre-ignition. This unique exhaust system takes up a fair bit of space and places the catalytic converter a long way from the engine, so during cold starts the engine will actually retard the ignition timing, and change the injector’s strategy (stratified charge) to aid in the warming up of the catalytic convertor. This warm up mode can be heard on a cold start, with the engine changing tone and seems to labor for a short while, but this is a normal designed condition.
Many immeasurable hours where spent making things lighter, smaller and efficient as possible throughout the engine and still retain the durability and power needed for the average customer (the engine is 10% lighter and generates 30% less friction then previous designs) and still provide fabulous mileage and lower emissions. They even found a way to lighten the tension on the drive belt by reducing engine vibrations using an oil tensioner on the timing chain (less belt tension, less friction, more efficient).
They are using many other manufacturer-common ways to enhance efficiency, from computer controlled charging systems, lightweight water pump components, low viscosity engine oil (0w20) and numerous light weight plastic/composite parts. They are even reducing the amount of weight in the hardware used to assemble the engine without compromising any bolt strength.
I have only touched on the major unique-to-Mazda Skyactiv technologies in their gasoline engines, but they really didn’t leave many stones unturned in the quest to develop a new power plant that is fun to drive and provides good fuel economy. The other manufacturers are watching and paying attention. The recent Toyota/Mazda partnership offers a Toyota Yaris equipped with Skyactiv technology (it’s really just a re-badged Mazda2).
Hopefully Mazda keeps the Skyactiv technological advances moving forward as they embrace diesel engine, chassis and body and hybrid technology and apply the Skyactiv principles to these areas. Masamichi Kogai, the president of Mazda, recently announced that he expects the next generation of Skyactiv engines to be 30% more efficient by the time they roll out in 2018, so this should get interesting. ●
