Mazda SKYACTIV Technology
Mazda SKYACTIV Technology: Redefining everything about cars.
The need to meet safety and environmental performance standards means that car makers around the world have been rushing to put out novel technologies. Mazda stopped to think about this trend and decided to go back to the basics. They revisited fundamental vehicle components and made some changes — the result is the SKYACTIV family of technologies.
The applications and numerical analysis of each technology depend on such models, grades and specifications of vehicle.
A new-generation highly-efficient direct-injection petrol engine that achieves the world’s highest gasoline engine compression ratio of 14.0:1
Merits and issues of a high compression ratio
The compression ratio of recent petrol engines is generally around 10:1 to 12:1. However, one of the reasons preventing the spread of high compression ratio petrol engines is the large torque drop due to knocking.
Knocking is abnormal combustion in which the air-fuel mixture ignites prematurely due to exposure to high temperature and pressure, creating an unwanted high-frequency noise. When the compression ratio is increased, the temperature at compression top dead center (TDC) also rises, increasing the probability of knocking.
Knock resistant technologies
Reducing the amount of hot exhaust gas remaining inside the combustion chamber is an effective way to lower the temperature at compression TDC. SKYACTIV-G adopts a 4-2-1 long manifold exhaust system to significantly reduce residual gas.
The major issue with the 4-2-1 exhaust system is that the long distance cools the exhaust gas before it reaches the catalyst, delaying the catalyst’s activation. Exhaust gas temperature can be increased by delaying the ignition timing, but too much retardation causes unstable combustion. For SKYACTIV-G, stable combustion was realized even when the ignition timing after engine-start is considerably delayed. This was made possible by adopting a piston cavity and optimizing fuel injection.
The engine in my own car. With the world’s lowest diesel-engine compression ratio of 14.0:1, this clean, highly-efficient diesel engine will comply with strict exhaust gas regulations globally without the aid of expensive NOx (nitrogen oxides) after treatment systems. A new two-stage turbocharger realizes smooth and linear response from low to high engine speeds, and greatly increases low- and high-end torque (up to the 5,200rpm rev limit).
Causes of NOx and Soot Formation
Due to the fact that diesel engines generally have a high compression ratio, the compression temperature and pressure at piston top dead center (TDC) are extremely high. If fuel is injected under these conditions, ignition will take place before an adequate air-fuel mixture is formed, resulting in the formation of NOx and, due to combustion with insufficient oxygen, the formation of soot.
Under strict emissions regulations, this makes it difficult to ignite the mixture at the optimal timing (TDC), leaving no other choice but to delay combustion until the piston begins to descend and lower the cylinder pressure and temperature, although this causes fuel economy to worsen.
Merits of a low compression ratio
When the compression ratio is lowered, compression temperature and pressure at TDC decrease. Consequently, ignition takes longer even when fuel is injected near TDC, enabling better mixture of air and fuel. This alleviates the formation of NOx and soot.
Due to its low compression ratio, the maximum in-cylinder combustion pressure for SKYACTIV-D is lower than the current diesel. It therefore became possible to change the cylinder block’s material to aluminium, which saved 25kg (vs. current non-SKYACTIV diesel). The cylinder head became 3kg lighter with thinner walls and an integrated exhaust manifold. As for the reciprocating parts, the weight of the pistons were reduced by 25%.
The crankshaft had its main journal diameter reduced from 60mm to 52mm, achieving a 25% weight reduction. As a result, the mechanical friction was greatly reduced to the same level as an average petrol engine.
There are two main problems that have been preventing the spread of low-compression-ratio diesels. The first is the fact that when the compression pressure is reduced, the compression temperature during cold operation is too low to cause combustion, preventing engine-start. The second is the occurrence of misfiring during warm-up operation due to lack of compression temperature and pressure.
Ensuring cold-start capability and prevention of misfiring during warm-up
Newly adopted multi-hole piezo injectors allow for a wide variety of injection patterns. Precision in injection amount and timing increases the accuracy of mixture concentration control, ensuring cold-start capability. Hardware-wise, the injector is a high-spec type capable of a maximum of 9 injections per combustion. Along with the three basic injections: pre-injection, main injection, and post-injection, different injection patterns will be set according to driving conditions. Definite engine-start even with a low compression ratio is attributable to this precise injection control and also the adoption of ceramic glow plugs.
Higher torque, clean emissions, and better fuel economy with two-stage turbocharger
It goes without saying that turbochargers greatly contribute to the diesel engine’s high torque, but they are also indispensable in reducing emissions and fuel consumption. SKYACTIV-D utilizes a two-stage turbocharger in which one small and one large turbo are selectively operated according to driving conditions. This technology achieves high torque and response at low speeds, and high power at high speeds.
A smart technology that saves fuel by switching off the engine when the car stops, and restarts it again when the driver is ready to start moving.
i-stop and petrol engines
While conventional idling stop systems rely solely on a starter motor to restart the engine, Mazda’s i-stop restarts the engine through combustion; fuel is directly injected into a cylinder while the engine is stopped and ignited to generate downward piston force. The starter motor is operated for less than a second to assist engine restarting, but using mainly combustion power for restarting requires less time and reduces power consumption. The result is a quick and quiet engine restart compared to other systems and a significant saving in fuel.
To restart the engine by combustion, the compression-stroke and expansion-stroke pistons need to be stopped at exactly the correct positions to create the right balance of air volumes. Mazda’s i-stop ensures precise control over the piston positions during engine shutdown. With all the pistons stopped at the optimum positions, the system then identifies the initial cylinder for fuel injection. It injects fuel and ignites it to restart the engine. Even at extremely low rpm, cylinders are identified for sequential ignition, making the engine quickly pick up to idling speed.
These technologies enable the system to restart the engine with exactly the same timing every time, to enhance fuel economy, and to deliver smooth and comfortable acceleration for the driver at restart. The restart takes place in a mere 0.35 seconds (internal measurement on vehicle with automatic transmission), which is about half of the time taken by conventional starter-motor idling stop systems.
i-stop and diesel engines
Unlike petrol engines, which use a spark plug to ignite the air-fuel mixture, diesel engines compress the mixture until it spontaneously combusts. Therefore, to restart a diesel engine, sufficient compression is required.
While conventional diesel engine idling stop systems require two engine cycles to restart, Mazda’s unique i-stop needs just one cycle, thanks to its precise control of the piston positions. As a result, i-stop achieves the world’s fastest diesel engine restart time of approximately 0.40 seconds (internal measurement on vehicles with automatic transmission) and its operation is barely noticeable.
Mazda fits a larger battery, with 12 plates per cell rather than the conventional six, on vehicles equipped with i-stop. That makes it more resistant to wear, and the battery is rated at something like 180,000 restarts.
Conditions when i-stop is not operable
Engine idling does not stop in the following conditions:
- The engine is cold.
- The vehicle is stopped but the engine is kept idling.
- The air-conditioning is operating with the airflow mode dial in the windscreen position.
- (Automatic air-conditioning)
- The temperature setting dial for the air-conditioning is set to the maximum cooling (A/C ON) position.
- There is a large difference between the cabin temperature and the set temperature of the air-conditioning.
- The ambient temperature is extremely high or low.
- The atmospheric pressure is low (when driving at high altitudes).
- (Automatic transaxle)
- The vehicle is stopped on a steep incline.
- The steering wheel is not in the straight-ahead position while the vehicle is stopped.
- (SKYACTIV-D 1.5)
- Particulate matter is being removed from the diesel particulate filter (DPF).
- Fuel injection amount learning is being performed automatically.
Conditions when engine restarts automatically
There are a number of conditions that will cause an engine to restart automatically while engine idling is stopped. See page 4-16 & 4-17 of the Mazda CX-3 Owner’s Manual.
A new-generation highly-efficient automatic transmission that achieves excellent torque transfer efficiency through a wider lock-up range and features the best attributes of CVT, dual clutch and conventional automatic transmission types.
Improvement in fuel economy and achievement of direct feel with full range lock-up
The torque converter transfers engine power to the transmission through fluid, making a smooth start-up and gearshifts possible. The drawback is that fuel economy worsens due to the loss of power transfer through the fluid, and slippage during rapid acceleration, which causes vehicle speed to lag behind engine speed. Therefore, a torque converter with a lock-up clutch was developed, which locks the torque converter’s turbine to the impeller to improve fuel economy and direct drive feel.
To improve fuel economy and direct drive feel the lock-up range has to be maximized. However, in order to do so, Mazda had to ensure noise, vibration and harshness (NVH) performance and clutch reliability.
Mazda say that with extensive analysis, they overcame NVH by improving the engine, mounts, exhaust, body, and system control as well as the transmission itself. Furthermore, improvement in clutch response and precise lock-up control ensures reliability by preventing heat build-up due to slippage.
A new-generation manual transmission with a light shift feel, compact size and significantly reduced weight.
Technical aims & concept
Automatic transmissions dominate the North American and Japanese markets, but in Europe, manual transmissions are widely used. To meet global demand, two types of manual transmission (“Large” and “Mid” sizes) have been newly developed. The development concept was to create a “light and compact manual transmission with improved shift feel and better fuel economy”. The goal was to achieve a Mazda MX-5-like sporty and brisk shift feel.
For a quick shift feel, both a short shift-lever stroke and light shift effort had to be achieved. By pursuing the ideal structure of manual transmissions, 16% at maximum of weight reduction was achieved. Furthermore, internal friction losses were significantly reduced to achieve a 1% improvement in fuel economy.
Quick and crisp shift feel
To achieve lighter shift effort with a short shift lever stroke, the lever ratio must be increased. However, a larger lever ratio reduces the internal stroke. To achieve precise synchronizer and torque transmission even with a short internal stroke, a small module spline is used.
Also, the shift effort gradually reduces through the stroke, providing reassuring resistance as the lever is first pushed, then getting lighter so it feels as if the shift lever is automatically moving into gear. With the current shift lever, shift effort in the select direction (right-left direction) increases with the movement of the shift lever, but with the new shift lever, a stable shift effort is achieved. Binding is minimized when shifting diagonally.
Light and compact
The triple-shafted gear train with a common gear for 2nd and 3rd was selected from approximately 30 different configurations due to its potential to achieve lightness, light shift effort, high efficiency, and a wide gear ratio. Based on this selection, the lightest structural specifications were selected from over 10,000 alternatives.
A common 1st and reverse gear made possible the removal of the reverse idle shaft. As a result, by re-examining gear configurations, the number of components decreased and the gear train weight was reduced by roughly 3kg.
Excellent rigidity supporting Mazda’s fun-to-drive feel, with a lightweight body to achieve outstanding crash safety performance.
Straight and continuous basic framework
For the underbody area, curves were removed as much as possible to create a straight frame in a continuous configuration from the front to the rear. For sections of the frame that still require some curvature, Mazda implemented continuous bonding with the horizontal frame to make the structure a closed section, thus contributing significantly to weight reduction while at the same time achieving rigidity.
The upper body also functions as a constituent part of the continuously bonded framework. Specifically, the suspension mounting positions at the front and rear of the upper body are directly bonded with the underbody framework as a “dual brace”.
In addition, by creating four ring structures for the upper body that includes the roof rail and B-pillar, and the entire reinforcement area of the underbody, the overall rigidity of the body has been further enhanced.
Multi-load path structure
To improve crash safety performance, Mazda adopted a multi-load path structure. The structure efficiently absorbs the load at the time of a crash by dispersing it in multiple directions. For example, energy received when a frontal collision occurs is absorbed by being dispersed along three continuous routes (paths): from the front frame to the B-frame, from the front frame to the side of the body, and from the front frame to the A-pillar. In particular, the upper branch frame, which diverts the load to the A-pillar, is a multi-functional part that also works to cancel the upward motion of the front frame.
To create this kind of path, parts such as door hinges, which do not normally play a role in absorbing shock, are important elements in the design. Naturally, the multi-load path structure is adopted for lateral collisions and rear collisions as well to function in the same way, thus greatly improving safety performance.
The multi-load path approach was also adopted for individual parts, focussing on directing the crash energy mainly along the ridge lines of the parts, moulding the front tip of the front frame into a cross shape. In a conventional square section, there are four ridge lines, but when a cross is created there are twelve ridge lines, and the shock is dispersed more widely. By doing so, the energy is then absorbed more efficiently, the space in the engine room is more effectively used, and there is also greater freedom in exterior design.
Optimizing engineering processes
To create a circular structure for the reinforcement, weld bonding is used for the roof rail section. Previously, this structure was separated from the rear frame due to the body assembly process. To bond this section directly, Mazda adopted a method whereby the parts are bonded together in advance using the weld bonding method and then sent on to the assembly process as a bonded unit. By adopting this method, they have achieved continuous bonding, at the same time greatly increasing the number of spot weld points, which contribute to the excellent body rigidity.
Suspension and steering functions have been thoroughly revised to achieve the ‘driving pleasure of oneness between car and driver’.
Improvements to comfort and security.
Newly-developed front and rear suspension systems and electric power steering. Functional improvements are combined with reduced weight.