Simulation of a four-axis CNC machine holding an engine block-like casting. Tool head "a" moves up and down, in and out. Fixture "b" can rotate block horizontally clockwise or counterclockwise to align with tool head. Platform "c" can move tool head horizontally left to right.
The “all-new” engines that are introduced may have a lot that’s really new, but unless they’re built on an all-new line, they have an important carryover characteristic: the same cylinder bore centers as their predecessors. Bores may be slightly larger or smaller, strokes may be longer or shorter, but the fixed design of the machining equipment forces the manufacturer to maintain bore-center dimension for all engines with bores in a size range, even V6 from V8, or in-line four from "half a V8." The cost of a new line as an alternative is hard to justify unless volumes are sufficiently high.
Carmakers “pre-design” engines to accommodate substantial future upgrading, including changes in displacement, but this comes at the expense of larger size and more weight in some applications. The need for a low-volume engine that doesn't fit on an existing line is a particular problem—particularly for smaller manufacturers such as Mazda. Joint ventures to share production have become increasingly common.
A flexible machining line can introduce a new level of engineering efficiency for the lower-volume applications, and Mazda now is using just that for the new Skyactiv engines: four stations, each with a CNC (computer numerical control) machine. The machine is a four-axis type that can move the tool head side to side, up and down, in and out; and it can automatically change the machining tools and rotate the block. With a specific jig mounting of the block at each station, it can machine all sides. It also can cover engines of different size bores, bore centers and strokes, and also completely different types of piston engines, such as in-line four (gasoline or diesel) to V6.
Mazda-modified CNC machines
The flex system was designed by Mazda around “generic spec” CNC machines it purchased, for which it developed and installed automatic tool storage and deployment mechanisms. The four-axis machines hold up to 60 tools, so multiple machining operations can be performed on each side of the block. And because one axis is rotation, the tool head can be aligned for V6 bores.
And there’s more: unique reaming of the crankshaft main bearing bores in the block.
Mazda's conventionally machined (MZR series) engines use a single-purpose machine with one long reaming tool for each engine. Because of the length of the tool, it requires an engine-specific plate-type guide bushing to keep it aligned. Once the bore is reamed enough to hold the long tool in alignment, the tool becomes self-guiding through the rest of the operation.
The new reaming process uses another CNC machine and tool set that requires no guide and is applicable to all Mazda engines. First, a short tool reams through to the second main bearing. Then it automatically is replaced by a longer tool that is aligned within the just-reamed bore and (like the old process) is self-guiding to complete the job.
Mazda’s new system starts with robotic transfer of engine blocks. This saves time and incorporates camera identification of each block, just as soon as the block enters the machining and assembly sequence. So every following station in the process “knows” exactly what tools, machining procedures, and parts must be used.
This includes a computer-commanded parts kit for those operations performed by assembly line workers. So each worker has exactly the parts needed, accompanying the engine at the work station. A four-cylinder gas engine may be followed by a V6, by a diesel, etc. Result: Mazda has production flexibility to match any changes in the vehicle sales mix.
The ability to perform on-the-fly changes enables Mazda to make the new Skyactiv 2.0-L four a “Xerox-like reduction” of the 2.5-L, saving a significant amount of weight. Example: 2.0-L Skyactiv block/pan assembly weighs 26.1 kg (57.5 lb); conventional 2.0 ("MZR" series) block/pan weighs 31.8 kg/70.1 lb. Block/pan on a 2.0 MZR with direct injection, sold outside the U.S., weighs 33.4 kg/73.6 lb.
Savings: 5.7-7.3 kg (12.6-16.1 lb), although a factor is Skyactiv's split crankcase block, which is inherently slightly lighter than the MZR with a main bearing ladder.
The flexible process costs 70% less than a conventional fixed line, Mazda said. Equally impressive: The process reduces total machining time for a block from 6 h on an MZR line to 1.3 h for the new Skyactiv line.
Why it's fast
A comparison of all the block machining processes shows why Mazda is able to reduce total time by 4.7 h. Much time is spent clamping the MZR block to a jig, unclamping it, and moving it to the next station, plus the specific station machining. Each of Mazda's four engine families (small and "large" four-cylinder gasoline, one diesel, one V6) goes through a total of 45 machining stations in the old system. The flex system reduces that number to four stations and block mountings, and it covers Mazda’s entire engine lineup. If not for the V6, only three machining stations would be needed.
The flexible system and "Xerox-like" engine design means that for a smaller displacement from the same block, Mazda doesn't have to slightly reduce bore and primarily shorten the stroke, as was done on the MZR engines. Instead, it can use the longer strokes and bore ratios wanted for highly efficient gas flows and combustion process, including high compression ratios (up to 14:1).
Examples: the MZR 2.0-L has a bore and stroke of 87.5 x 83.1 mm (3.44 x 3.27 in)—ratio 1.05, whereas the Skyactiv 2.0-L is 83.5 x 91.2 mm (3.29 x 3.59 in)—ratio 0.92. This is very close to the 89 x 100 mm (3.50 x 3.94 in)—0.89 ratio—of the 2.5-L, which with a 100-mm stroke still permits the Skyactiv to maintain the 6500-rpm redline. This same block design and machining system also work for Mazda’s small fours, 1.3-L and 1.5-L.
The use of a single, optimized combustion process was one of Mazda’s priorities. Advantages include use of the same sensors—providing parts standardization and simplifying development of the computerized engine control system. Mazda also has been able to “scale” the performance of the engines, so although they differ according to displacement, the torque curves are very similar and the power output is reasonably proportional between them.