Spray-on liner systems that coat aluminum cylinders with a thin layer of iron or steel, eliminating the need for a cylinder sleeve, have been popular with low-volume performance cars for nearly a decade. Automakers and machine tool providers expect the technology to make the leap from the specialty market to the mass market within the next few years because it reduces engine size and lowers engine friction – boosting fuel economy and engine power.
Applying thin coatings requires greater precision than inserting sleeve liners, requiring inspection and process monitoring at several stages. Jenoptik Automotive North America product manager Chris Vroman says optical scan systems developed by his company measure engines at three stages during production to ensure high-quality, durable coatings.
At the IMTS 2016 - the International Manufacturing Technology Show, Heller Machine Tools U.S. President and CEO Keith Vandenkieboom said the system his company developed, cylinder bore coating (CBC), will pass a 300,000-engines-coated milestone this year in commercially installed, high-volume applications. Throughout the past three years, Today’s Motor Vehicles has highlighted technologies from Heller, robotics and machine tool producer Comau, and tooling company Walter USA that apply coatings or prepare surfaces. Vroman says that as the technology jumps from niche applications to the mass market, quality engineers are studying new techniques to speed inspection.
“When you’re doing spray bores, it’s much more important to have high-quality castings. You’re putting on such a thin layer of material, so if you have any porosity or void, you’re going to have a failure in the process,” Vroman says.
Jenoptik’s primary cylinder inspection system, the IPS 100, uses an extreme fish-eye lens that feeds data to a complementary metal–oxide–semiconductor (CMOS) ring sensor. Typically, a machine tool inserts the scanner into the cylinder bore where it takes an uninterrupted, high-resolution, 360° image of the surface.
“We acquire the data at multiple angles simultaneously. It looks straight against the cylinder wall in one area. Then the CMOS sensor, a little bit lower, gives you a different angle,” Vroman explains. “This helps us process the data.”
Scanning at 37µm resolution, the system is searching for flaws that are typically about 100µm or larger. By taking angled, overlapping images, he says the system can detect flaws that are barely larger than the resolution of the system.
“A small porosity may land in the center of a pixel or at the intersection of four pixels. This is why you can’t have a single pixel detect a flaw. You need more than that to detect it. If you have a flaw that’s 40µm, you might not catch that with one pixel. But once you have two or three pixels across, if your optics match your sensor, it will be clear enough where you can easily detect it,” Vroman says.
Scanning each bore takes about 20 seconds – about half of that for the scan and half to process the 1.5GB collected into a manageable file – so scanning each cylinder in a V-8 engine would take nearly three minutes. Automakers can speed that inspection time by simultaneously scanning multiple bores. BMW has a system with six scanners for its V-6 engines.
“Manufacturers are always doing cost-benefit studies. It costs a lot to have eight sensors, and they don’t see a big need because this technology isn’t in high production yet,” Vroman says. He adds that as companies increase production rates of engines with spray-on liners, he expects the popularity of multiple-sensor scanners to increase.
Activating the surface
Once quality engineers have signed off on castings, declaring them free of porosity issues, manufacturers can prepare surfaces for the spray coating. Coatings stick best to roughened surfaces, so producers use different techniques to create a scratchy surface on the aluminum engine blocks. Cylinder pre-treatment strategies include:
Jenoptik scanners can measure post-activation surface quality using any of those treatments, but Vroman says mechanical activation is growing the fastest and requires the most precision measurement.
“If the tool starts getting worn, rather than cut dovetails, it can tear out the aluminum. If it hits a large porosity or something like that, it may also tear out the aluminum,” Vroman explains. Engineers can detect these issues with the IPS system and correct/contain them quickly. Another system that Jenoptik has developed for this stage of the process is the CCS-C100 system.
“This is a production gage, so you can measure your activation process and detect any problems with your tool before you start failing engines,” Vroman explains, adding that Jenoptik delivered its first inspection system to qualify dovetail cuts early in 2016. “We’ll measure the dovetails – the angles and the structures. We do it optically, so we can get down into the angles of the dovetails without destroying the engine block.”
The mechanics are the similar as with the casting-qualification inspection system, but the pre-treatment inspector uses a confocal chromatic sensor that scans the bore twice – once angled clockwise to measure half of the dovetail groove, and then a counterclockwise pass to get the other half. Software combines the scans to create a 2D image of the grooves. He adds that with multiple passes, it’s a slower scan than the qualification system, but manufacturers can make up for that with other processes.
The system effectively measures the performance of a tool, so 100% inspection isn’t necessary. Scanning the final cylinder on each engine should show if the tool is still cutting properly. If the tool has become worn or broken, that failure will be obvious in the scan from one engine to the next, notifying technicians that it’s time to replace the tool at the first failure.
“What you’re doing is verifying the entire process, making sure it’s still performing as it should,” Vroman adds.
Following inspection of activated cylinders, producers spray on thin layers of steel or iron to give engine blocks the slippery surfaces they need (aluminum is too abrasive to work without some coating or sleeve) and machine the coated surface to the proper ID. Jenoptik cylinder sensors enter the bores one more time to verify that process.
They’re still looking for porosity, just as they did to verify aluminum castings, but this time engineers want to see tiny holes.
“Large porosity is a failure, but engine makers intentionally incorporate micro-porosity into the finished surface,” Vroman explains. “That’s where you get your oil retention. With the right porosity, you can reduce your spring tension, and you reduce your friction.”
By taking advantage of the flame spray lubrication properties, one supplier claims a 30% friction reduction at low engine speeds, improving fuel efficiency.
Like any in-process quality system, the post-spray inspection system looks for flaws – areas where cylinder walls may have been damaged in post-spray machining. Vroman notes the torches that melt wires for the spray-on coatings wear down just like any machine tool. So, the inspection system monitors that process to identify when to replace consumable products.
A recent trend in powertrain design has been to vary piston and cylinder properties based on their locations within the combustion process. Toyota, for example, unveiled engine designs in 2014 that focus water-jacket cooling at the top and bottom of the cylinder, allowing the middle to stay hotter, increasing thermal efficiency. Spray-on technology could further that customization trend by allowing engineers to vary lubricity by controlling coating thickness and surface finishes at different locations within the hone.
“Where your piston goes up to the top of the cylinder, you have the reversal area where the piston goes from its upstroke to its downstroke,” Vroman says. “The engineers want more porosity there for more lubrication.”
Manufacturers are already controlling the properties of the cylinder coatings to get a uniform thickness, Vroman explains. Air flow within the cylinder varies from top to bottom, so as spray-on torches apply the material, machine tools must manage gas-flow rates, torch temperatures, and wire-feed rates to get consistent results. Controlling that process for varying levels of coverage will require some study, but there’s nothing within the process that should prevent it.
Making the leap
With multiple machine tool companies investing in cylinder-spray technology and mass-market automakers studying it for niche vehicles (Ford’s GT350 Shelby Mustang uses it), engineers say it’s a matter of when, not if spray-on systems will start showing up in mainstream vehicles.
For that to occur, the application and inspection systems must get faster and less expensive – something at officials at Jenoptik and several machine tool producers say is already happening. Vroman likens it to the mainstream adoption of aluminum engine blocks. In the early days, the engines leaked, were far more expensive than cast iron to produce, and they didn’t perform well. Within a decade, those problems disappeared, and aluminum has become the dominant block material for gasoline powered vehicles (diesels still use cast iron).
“This will eventually replace liners on the bulk of your engines. I see this as mainstream in the not-very-distant future, especially with emission and fuel economy requirements,” Vroman says. “As the technology matures, it becomes less expensive and more efficient. We’re going to be ready when the producers move this into mass production.”
Jenoptik Automotive North America
About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or firstname.lastname@example.org.