SKYACTIV: Keeping the Zoom in Zoom-Zoom

July 31, 2014
I am amazed at the new technology and the twists taken on some old ideas you see in Mazda’s SKYACTIV technology, including much lower compression ratios. Looking into how they could do this and make it work was very interesting.

What is Mazda’s SKYACTIV technology? Let’s break it down. “SKY” is about preserving the environment by lowering CO2 emissions. “ACTIV” is about preserving the excitement of driving by making the driver an active part of a system Mazda disguised as a car.

I am amazed at the new technology and the twists taken on some old ideas you see in Mazda’s SKYACTIV technology. One of the ideas that really caught my eye was raising the compression ratio of the gasoline engine to 14:1 and the lowering of their diesel engines to 14:1 compression ratio. Looking into how they could do this and make it work was very interesting.

Background Mazda launched its SKYACTIV technology for model year 2011. Mazda took on the challenge to promote driving pleasure while being environmentally friendly and providing outstanding safety for its vehicles’ occupants. Mazda began the redesign around 2008 with the idea that by 2015 it could increase fuel economy by 30 percent over 2008 models, without decreasing driving pleasure or safety.

Mazda believes the internal combustion engine is still going be a major player in the future. It made the decision to position itself to seamlessly merge the internal combustion engine with present and future technologies such as electric drive, while keeping the driving experience alive. Mazda believes in the concept of having the engine, transmission, body and chassis all work as a system, including the driver. Some of the problems Mazda faced were improving already highly efficient engines, improving both automatic and manual transmissions and giving the vehicle a rigid body and chassis while maintaining safety from a lighter weight car for improved fuel economy.

We all know that an internal combustion engine is not efficient. Somewhere in the neighborhood of 70 to 80 percent of the energy produced is actually lost either through heat or friction. First, Mazda took a hard look at both its gasoline and diesel engines. It went back to the drawing board to figure out how to pull more useful energy from the engines. One of first things it considered was gasoline direct injection (GDI) technology. That was just not good enough, though; it wanted to improve it. All hot-rodders know that high compression engines typically put out more power, but the problem is that squeezing the air that tightly costs energy (pumping losses), just like the power it takes to run an air compressor. Mazda engineers also took aim at reducing internal engine friction and weight.

SKYACTIV-G (Gasoline Engine)
Engineers have been chasing Homogenous Controlled Combustion Ignition (HCCI) engines for a while. Gasoline Direct Injection (GDI) is the newest step toward getting more energy from combustion. GDI is a gas engine designed much like a diesel. The gas is injected under higher pressures (500 to 3,000 psi) directly into the combustion chamber allowing for a more complete burn. The air in the cylinder is directed toward the spark plug located in the center of the cylinder head where gas is sprayed just before ignition.

Mazda asked the question, “How do you get more power with less fuel?” The answer is to increase combustion expansion ratio. This means a smaller combustion area, thus increasing down pressure on the piston. This is why Mazda increased the compression ratio to 14:1. As you know, engine knock robs power, increases emissions and causes engine damage so they first went about lowering cylinder temps. Mazda decided to remove more residual exhaust gases from the cylinder.

As you know there is some exhaust back pressure with a major part being push-back (resonance) from the other cylinders. To overcome this problem, Mazda played on the idea of long tube tuned exhaust headers. Hot-rodders have used tuned headers to help extract the exhaust by making each tube length a specific length so one cylinder’s exhaust pulse helps extract another’s. They measured the timing of each exhaust pulse to determine the needed length to prevent push back at low to mid rpm range. The exhaust tubes needed to be 600 mm (about 23.5 inches) long between cylinders on opposite strokes to reduce exhaust gas push-back. To fit the exhaust manifold in a reasonable space, they used a wrap-around Y-type tube design to get the needed length between two cylinders. Then the two tubes run into one pipe dumping into the catalytic converter, leading to the 4-2-1 design. This cut the residual cylinder gases from 8 to 4 percent, thus lowering the cylinder temps to help defeat engine knock.

Murphy’s Law showed up with the 4-2-1 design. The engineers now faced a major issue with the catalytic converter’s light-off time due to the distance from the exhaust ports. They adapted another old idea of retarding ignition timing to increase exhaust gas temps to heat the catalytic converter fast enough to prevent any increase in exhaust emissions. But this causes cold run misfires, so they made a deep cavity in the piston specifically shaped so the air and fuel are directed toward the spark plug creating a stratified charge (rich enough for combustion) around the spark plug. The specially shaped piston cavity had a benefit Mazda took advantage of which prevented the combustion flame from coming in contact with the piston causing combustion cooling loss, which would prevent flame growth. Another trick Mazda used was to map the engine for the shortest time the air and fuel spend in the chamber before ignition and intensify air flow turbulence in the chamber for better mixing.

All of these combustion chamber changes allowed them to reduce the weight of the pistons by 20 percent, reduce connecting rod weight 15 percent, and reduce piston ring drag 37 percent. The engineers didn’t stop there; valve train friction losses were next on the list. The engineers used another hot-rodder trick of roller rockers in the form of roller followers on the cam. Mazda still was looking for more energy reduction, so it reduced the oil pumping losses 45 percent by electronically controlling the oil pump. All of the weight and friction reductions allowed Mazda engineers to increase fuel economy by 15 percent and add 15 percent low to mid range torque over previous models.

SKYACTIV- D (Diesel Engines) Mazda’s engineers defied conventional wisdom by lowering their diesel engine to a 14:1 compression ratio. This lowered emissions enough to do away with expensive exhaust after-treatments like Selective Catalyst Reduction (SCR) or Lean NOx Trap (LNT), but increased performance and fuel economy. This idea and some other improvements give the Mazda diesel 20 percent better fuel mileage while meeting Euro Stage 6, Japanese Long Term, and US Tier2 Bin5 emissions. Obviously, lowering compression ratio equals lower pumping losses, but this also allowed Mazda to optimize injection timing for better air/fuel mixture combustion while lowering engine weight and rotating mass.

Obviously lowering compression ratios reduces combustion heat thus lowering NOx. This also allows more advanced injection timing giving the air and fuel molecules more time to better mix before combustion. This does two things. First, it lowers combustion flash temperatures which lowers NOx and the better mixing gives a more complete burn greatly reducing particulate matter (PM or soot). Most diesels delay injection timing so the piston is starting down to lower emissions but Mazda knew combustion close to TDC makes for a more efficient engine.

It went after that expansion ratio, which is greater at top dead center (TDC). But again, if you change something usually other problems arise and it did. Lower compression causes lower heat so how do you start and idle a cold diesel with low compression plus get any power for acceleration? The answer required several new ideas like using multi-hole piezo injectors capable of rapid response times (durations in the 300 micro-seconds) and 9 injections per combustion stroke.

This allowed them to not only do the pre-, main and post-injection most diesels do, but to modify the injection times per cycle for operating conditions and allow for precise high pressure injections for cold starts. The highly atomized fuel helps cold starts, but also Mazda adopted super-fast heating ceramic glow plugs to insure quick starts. To overcome cold run misfires, Mazda adopted Variable Valve Lift (VVL) on the exhaust. Knowing that only a single combustion cycle was needed to make exhaust temps rise, they open the exhaust valve on the intake cycle to pull exhaust back into the chamber for rapid cylinder heating. Most manufactures use the exhaust gas recirculation system for this, but that takes too much time for heating to occur. This technology is instant.

Most every diesel today has turbochargers, but Mazda went to new lengths again looking at performance engines that use a two-stage turbo charger design adapting these ideas to meet needed technology for clean/efficient operations under all conditions. Most diesels have some form of turbo lag, so the two-stage turbocharger enabled them to have smooth responsive torque output while decreasing emissions.

A two-stage turbo is one small and one large turbo mounted together sharing the same exhaust and intake pipes. This allows for switching between turbo profiles giving the best air charge for the operating conditions. A small turbo will spool up using less exhaust to give good boost at low rpms, and the large turbo produces boost at higher engine speed giving a great torque curve over the entire driving range. This turbo design also gave Mazda better boost control helping to lower both NOx and soot output. 

Without the need for the expensive and fuel robbing regeneration of after-treatment systems Mazda is way ahead of the game. All of the advances Mazda did to their diesel engine allowed them to change to an all aluminum block and a thinner cylinder head, cutting a whopping 62 pounds (28kg) of weight plus shaving the pistons’ weight by 25 percent.

SKYACTIV-Drive (automatic transmission) Mazda took aim at combining all the best advantages of each transmission type to make each one better. They looked at the advantages of a conventional (step shifting) automatic transmission, continuously variable transmission (CVT) and a dual clutch transmission (DCT). It improved fuel economy 4 to 7 percent by reducing slip, cutting weight and reducing friction losses. It wanted a smoother shift, but also wanted to keep the fun in driving no matter which transmission choice the driver wants.

Everyone knows that a torque converter gives smooth starts and shifts but robs mpgs. Mazda wanted to get rid of slippage between shifts, so it improved the torque converter. To reduce the slippage, it locked the converter up just after takeoff. This required tighter electronic controls of both the engine and transmission plus better cooling of the multi-disc lock up clutch. The engineers controlled Noise, Vibration and Harshness (NVH) by providing a damper and compact torus (vane) in the torque converter, but also did a redesign of everything from the engine to the exhaust system.

To improve transmission oil pressure control they combined the hydraulic controls with the electronic controls into a Mechatronics module and located in the transmission. To get better shift quality, Mazda engineers came up with direct linear solenoids for quicker, more precise pressure control response giving better shifts.

SKYACTIV-MT (manual transmission) Mazda came up with some different slants on manual transmission shifting and internal redesigns to lower weight by 16 percent and reduce friction by 1 percent. Mazda wanted to develop two levels of manual transmissions, a large and mid-size to handle different types of power plants or combinations of. Space and weight were a major concern along with that driving experience theme.

It first looked to improve shift feel and shorten the shift stroke. This required redesigning the internal stroke by adding a small module spline to move the synchronizers. It adopted lock ball type synchronizers, shift load cancellers and ball bearing on the slides to give an easier but positive feeling shift.  It rearranged the gears, moving first gear to the top shaft, using a common 2nd/3rd input gear, and doing away with the reverse idler shaft. This allowed for a more compact and lighter weight yet tough, fun shifting transmission.

SKYACTIV-Body
Mazda wanted to keep a crisp looking, aerodynamic body that would turn heads. It was determined to lower the weight by 8 percent, improve rigidity by 30 percent and still meet worldwide safety standards. Some of the ideas it went for were “straightening” and “continuous framework.” Straightening refers to removing or decreasing underbody curves giving a straighter frame-like platform. Continuous framework means a completely bonded upper and lower structure combined into a dual brace configuration to spread any forces over the whole body. This gives the ride a tighter feel by absorbing impacts from bumps and improves crash safety. 

In this body redesign, even the door hinges were improved. The redesign also included weld-bonding of the roof as an assembly before welding it to the body structure. Body materials were also changed to a higher tensile-strength steel material.

SKYACTIV-Chassis Last but certainly not least, Mazda engineers knew the chassis needed to be revisited with all the changes made to the powertrain and body. Remember they wanted to be able to adapt to any type of power plant and lighten the overall weight. If the engineers were to control the ride and keep the fun in driving, the suspension needed to be lighter, but offer more rebound control.

Mazda improved low to mid-speed agility and high speed stability by adding electric power steering to increase yaw gain. The steering ratio was increased for better low speed response and better road feel at highway speeds. Through the use of electronic controls the turning effort is speed adaptable. It also increased both caster angle and caster trail for a better straight ahead driving feel.

The rear suspension was reworked to increase road grip and reduce rear wheel bounce. The rear suspension dampeners were moved rearward giving a straighter line for rear wheel impact control. To add to this rebound control for absorbing road bumps and keeping the wheel in contact with the road, the rear trailing arm was moved upward above the wheel centerline. This allows the wheel to swing up to roll over a bump rather than the impact pushing backward on the wheel axle causing more energy to be absorbed by the body. The engineers incorporated the old idea of “box channel frame” construction for a more rigid cross member while lowering the weight 14 percent. The front member was extended reducing the offset of the front lower control arms. The rear cross member was also given a bigger front to rear span to reduce the link offset for better wheel to body control.

Mazda engineers went back to the drawing boards to redesign their vehicles to blend with future technology while still keeping the fun in driving. They went against the normal thinking processes and made it work.  Zoom on Mazda!

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