The Boring TruthEverybody's got ideas about treating cylinder bores; strongideas

Wow. Little did we know what a fire-storm we'd start with a seemingly innocuous mention of aluminum engine cylinder-bore treatment alternatives in last September's materials issue (see WAW - Sept. '98, p.61).

There were plenty of letters and calls to point out we'd muffed our facts regarding the Nikasil treatment process - and we'll straighten out that matter directly. But several merely used that mistake as an excuse to expound their opinions - often vociferously - about what cylinder bore-coating strategies the industry really should be employing.

We don't need to be hit over the head to recognize controversy, so for the last few months we've talked to numerous supplier and OEM sources to develop an overview of the aluminum-engine cylinder bore-treatment options used throughout the industry - and to convey the remarkable polarization of opinion on the subject.

Most sources were delighted to discuss their convictions at great length - provided they wouldn't be named, highlighting the submerged political nature of this seemingly innocent subject. Suppliers don't want to rankle customers, OEM sources don't want to openly fuel already contentious philosophical battles within their powertrain groups.

All over cylinder-bore treatments, for gosh sakes.

First, Why? With the now-continual need to drive weight out of passenger vehicles, converting the heavy mass of engine blocks from their traditional cast iron to aluminum is a sure-fire method to deliver a major curb-weight cut; aluminum engine blocks can weigh 40% to 50% less than a comparable cast-iron block. And aluminum's excellent thermal conductivity often permits smaller-capacity cooling systems.

"I doubt there will be very many new passenger-car engine programs in the near future that don't specify an aluminum block," asserts one casting industry source. "And probably not too many (light)-truck engines, either."

But aluminum is an inherently "soft" metal, and cylinder bores of an engine block crafted purely from aluminum wouldn't long withstand the constant, grinding friction of pistons and piston rings scraping their way up and down the bore surface. Most aluminum engine blocks actually are fashioned from an aluminum-intensive alloy that contains other metals, primarily silicon. That helps, but that alone isn't nearly durable enough.

"It's pretty simple," says one OEM powertrain engineer. "You don't want aluminum-to-aluminum contact. You've got to have a bore with a high wear surface, and aluminum has poor wear characteristics."

Thus, the primary reason cylinder bores can't be aluminum. Once it's agreed the bore must be protected, the question is: "How?" Other factors such as cost, manufacturing consequences and performance requirements then must be squeezed into the equation. That's where the cylinder bore-treatment "factions" start to dig in their heels.

The Methods

Broadly, there are two ways to protect aluminum-engine cylinder bores: Either install iron liners or find a way to make the bore surfaces more wear-resistant, usually with some type of coating or treatment of the aluminum. Some approaches use rather exotic and elaborate processes to accomplish this feat.

Iron Liners: The utility infielder

Installing cast iron liners - or "sleeves" - equates to what might be called the industry "default" to answer the cylinder bore-treatment matter. Iron liners have numerous advantages:

n They are probably the most inexpensive method.

n They are delightfully durable.

n They are easily and inexpensively integrated into the manufacturing process.

Cheap and durable - the two words the industry holds most dear, right?

Well, there are problems with iron sleeves.

Sticking iron into your fancy new aluminum engine obviously negates some of what you're trying to do in the first place. Iron is heavy - that's why you switched to aluminum!

Perhaps more importantly, iron liners take up space. A common iron liner is roughly 3 mm thick. Multiply that by the number of cylinders you're dealing with and the engine starts to grow; there has to be a certain amount of block "webbing" between each cylinder to ensure structure, so the room that liners require can't always simply be chopped out of the space between each cylinder.

To now, that hasn't been a big deal in the U.S. One foreign castings supplier, who asks anonymity because he's wooing domestic business, explains: "In the U.S. you have the 'luxury' of displacement. Engines are large, so there is no particular need to use modern (bore treatment) methods. The car companies here always think first of cost and high volumes."

This source's emphasis on the word "modern" is inescapably scathing.

W. Gregory Wuest, vice-president-research and development at Sulzer-Metco, a New York company espousing cylinder-bore spray coating technology, agrees, noting that engines in Europe and Japan must be inherently smaller and more energy dense because fuel prices are so high. "In Japanese engines, for example, there's no room for a liner. They're driven (to other methods) by that factor."

Mr. Wuest notes that spray coatings usually can be applied in thicknesses of no more than 100 microns - one-thirtieth the space each 3 mm iron liner demands.

Iron liners typically are cast into the block as it's being formed. General Motors Corp. employs this method with its Premium V-6- and 8-cyl. OHC engines. Saturn simply presses in the liners. Ford Motor Co., for its Intech all-aluminum V-8s, heats the block and presses in the liners; when the block cools, it "shrinks" around the iron sleeves.

Iron and aluminum exhibit different thermal properties, though, which can be troublesome. And aluminum blocks and iron liners don't completely "bond," regardless of the joining method. That leaves gaps between the liner and the cylinder wall. The bonding and added-weight issues can be improved by using aluminum sleeves instead of cast iron - DaimlerChrysler AG likes aluminum sleeves for Chrysler's 4-cyl. and new V-6 engines - but aluminum liners are tough to cast directly into the block.

If liners are cast into the block - as in the GM method - scrappage becomes an issue. If the entire engine is built, only then to be discovered to be defective, the iron liners must be ripped out and are useless.

"We have some relatively high scrap costs," admits one powertrain engineer.

The Coatings: You pays your money, you takes your chances

This story was born when we first conveyed BMW AG's woes with Nikasil. So let's examine the competing cylinder-coating processes.

n Nikasil: An aluminum engine is dunked in an electrolytic "bath" of free-floating nickel, silicon and other junk. The electrolytic action causes these hardy substances to adhere to the aluminum surfaces.

"It does work very well in a lot of applications," admits one engineer philosophically behind iron liners. But Nikasil's main drawbacks are serious.

First, says Achim Sach, of VAW Motor GmbH, a part of the VAW Group aligned with Mexican casting giant Cifunsa SA, "Nobody wants to have nickel in the plants anymore." Also, as noted in September, high-sulfur fuels eat away at the coating, eventually rendering it useless. Result: ruined engine. And Nikasil has "throughput" issues: The block has to be labor-intensively "masked" before it takes a Nikasil bath, so that the particles cling only to the bore surfaces. And the block has to soak for more than an hour, claim some skeptical sources. Nikasil appears to be on the skids for these reasons. BMWhas abandoned the process. Jaguar Cars and Ferrari SpA still like it, though.

n Alusil: The engine block is fashioned from high-silicon content aluminum alloy. The block undergoes initial machining, then, similar to Nikasil, is dipped in an acidic bath that etches away the aluminum on the bore surfaces, exposing the durable-wearing silicon.

Again, however, there are considerable problems. Alusil blocks must be made in a slow, low-pressure process, says Mr. Sach, and the original alloy itself is more expensive. He believes Alusil is good for low-volume use where cost and manufacturing speed are not the priorities.

Alusil's cost might be bearable even for mainstream vehicles, but a foreign automaker engineer insists, "Throughput time is not acceptable for high-volume lines. We would never consider this process."

n Lokasil: Promoted largely by casting-kahuna Kolbenschimdt Pierburg AG, Lokasil is a "sacrificial" bore liner comprised of silicon fibers in a binding that, when inserted into the block mold, burns out the fibers, leaving the high-content silicon surface directly in the bores.

The Lokasil process is acutely effective. But it also is laborious - slow squeeze-casting is required - and expensive. Currently, Kolbenschmidt's sole customer for the process is Porsche AG.

n Finally, there is spray coating - thermal or plasma - and laser etching. Thermal and plasma spray coating, as a technique, has knocked around for some time; Sulzer Metco believes it eventually can be a prime force in the bore-coating industry. In one spray-coating method, a wire comprised of the material with which you'd like to coat the bore surface is heated and the "droplets" produced essentially are blown in a controlled fashion onto the bore surface.

Better yet, the bore coating, in powdered form, can be heated and blown into the cylinder, where it adheres to the bore surface. Ford, incidentally, owns the rights to the powdered materials themselves.

But Sulzer-Metco's Mr. Wuest is open about the process' drawbacks, particularly the high-velocity oxygen fuel (HVOF) method, which GM, he says, "has been working on for years, but they've never brought it to production." The problem with HVOL is the thermal loadings transferred to the block. One OEM engineer opposed to spray coating says, "That's one thing the spray coating promoters rarely talk about. You've got to bring the blocks up to a pretty high temperature. That's not easy; you've got to have a big investment to do all this, and pump heated coolant around and all that. There are large investments required to get going with that process."

But spray coating is attractive: no space-eating iron liners, the coating material selection is extensive - and it provides an iron-like durability and a "very natural" tribological surface.

But Mr. Wuest says two aspects currently mitigate against spray coating. "From what I've seen, it's cost - and the fact that it's a relatively new technology," he admits. There's plenty of industry dissension about the true cost of spray coating, but Mr. Wuest is frank with his figures: He reckons iron liners cost about $1.50 to $2 per bore; plasma coating probably ranges from $3 to $5 per bore.

Meanwhile, Nikasil runs from $5 to $10 per bore, the cost largely dependent on volume. Alusil and Lokasil cost even more.

VAW's Mr. Sach firmly believes laser etching - where an aluminum/silicon powder is fed into a laser that "etches" the material into the bore surface - is a promising technology. "We're very high on this for the future," he claims, particularly for near-term high-performance applications like direct-injection engines.

Finally, one domestic OEM engineer proposes a radical compromise: "My proposal for a new family of aluminum engines would have aluminum wet liners," the source says. "Wet" liners, around which coolant circulates, could be spray- or otherwise coated then simply dropped into the block - perhaps with the entire piston/con rod assembly already matched to the liner.

"I think people are afraid of wet liners because the design got a certain bad rap. Machining wet liners used to be a problem, but that was 20 years ago. And with separate aluminum liners, any type of treatment process could be more easily incorporated. Using wet liners isn't a lost art."

That might be true. As recently as 1994, GM was still employing wet liners - and for a high-profile engine, the 405-hp Corvette ZR-1 V-8.

Wet liners might have a bad "rap," but it's a reasonable suggestion. One that might bring some pacifism to this "boring" battle.