Steel Pistons Part 1: Is Steel a Better Material?

Aluminum isn’t the only modern option.

Steel-Pistons-1

Side view of an experimental steel piston for a big-bore, high-performance engine (Ducati Panigale?). Note how short the skirt is and how high up near the edge of the piston top is the fire ring.

There is another relatively small Italian piston specialist up near Turin, in addition to the fairly renown Borgo and Asso. Its name is PistAl Racing, from Pistoni Alluminio Racing, or Aluminum Pistons for Racing. The man in charge of R&D and production is Giorgio Casolari, a former NCR and Ducati specialist who became fairly well known around the motorcycling world when he started what became one of the leading Ducati specialist shops.

Then personal problems led him to quit the business, but still Casolari is a very capable technician and a man of great creativity. He led PistAl Racing into specializing in small production runs of custom high-performance pistons for most of the present and past motorcycle and automotive engines. Visiting him, I saw 116mm Ducati Panigale pistons, Moto Morini 350cc high-performance pistons, as well as racing slugs for vintage Alfa Romeo Giuliettas, and even Chevrolet Corvette LS3 pistons. The whole lot!

Its flexibility allowed PistAl Racing to challenge its much larger competitors. A few years ago, to further enhance its already outstanding level of specialization, PistAl Racing started investigating the possibilities offered by using high-tensile steel alloys for the construction of high-performance pistons. Steel pistons are obviously not a new idea. Iron and steel were once the standard of the industry at the very beginning of the internal-combustion motoring era, but were replaced by aluminum in the 1920s. The result was cooler-running pistons.

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This piston is sintered from 8600 series steel alloy, and its very accurate finishing of the top was achieved utilizing highly specialized diamond tools.

The position of aluminum as the go-to material coincided  with the development of high-silicon alloys and related forging methods. Yet, despite all of the dramatic evolution it went through in design, materials, and production methods, the piston still is the weak link of the energetic conversion process that happens in the internal combustion engine, due to the inherent limits of aluminum in terms of resistance to high thermal and mechanical loads.

In aerospace engineering we’ve learned that the mechanical characteristics of aluminum structures decline due to fatigue after a given number of working cycles. Remember the De Havilland Comet, the first civilian transport jet plane? It should have had an expiration date on the fuselage, like milk cartons; its fuselage would tear in half along the window line after many take-off/high-altitude cruise/landing cycles due to the strain applied by the sea-level pressurization and the consequent pressure difference between the inside of the cabin and the thin air outside at 30,000 feet.

Many of the early aluminum frames that the motorcycle industry proudly put into production in the 1980s and '90s would suffer from progressive loss of structural rigidity at their high-stress points after a given number of miles due to fatigue strain. One of the worst cases was Suzuki's air-oil cooled GSX-R1100 frame. Its frame would start to fail where it supported the swingarm pivot after as few as 10,000 miles in some cases.

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The inner face of the piston top looks incredibly thin and light, accurately ribbed to ensure total stability to the top under the most extreme thermal conditions. The pin is short, a stub of a pin that can be located higher in the heart of the piston, its supports are cross braced for maximum rigidity.

And the short-lived aluminum frame on the Kawasaki GPZ600 (first-gen Ninja 600) was even worse. It almost appears that the quest for maximum weight savings was at the expense long-term durability. The real advantage of an aluminum frame over a steel one is not its lighter weight, but rather that a larger-section (and therefore stiffer) tube can be made of it, given an equal weight.

On the thermal qualities side, even the best high-silicon aluminum alloy has a relatively low melting point (1220 degrees fahrenheit), and more important, it starts losing its tensile and yield strength at a mere 305 degrees. PistAl Racing investigated the feasibility of producing steel pistons on the light side of the present range of high tensile alloys and of the latest casting, powder metallurgy and machining technologies. The result, was a number of what I would call, fascinating prototypes featuring ultra short skirts, very thin tops, compact and light pins, and very precisely positioned rings. And the bigger the bore, the lighter the steel piston gets in comparison to its aluminum counterpart.

Test pistons were created in both micro-cast 4140 series chrome-moly and in powder metallurgy 8600 series nickel-chrome-moly steel alloys. In both cases, the pistons PistAL created are lighter than their aluminum counterparts, have very short skirts and, more importantly, locate the top ring much closer to the upper edge of the piston top (not more than 3 millimeters), about half of what is required on an aluminum piston.

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This is a 4140 series steel alloy vacuum micro cast piston of similar characteristics but featuring a slightly taller skirt.

Additionally, the piston pin is very short and has a smaller diameter, and therefore is positioned closer to the piston top, which allows the possibility of using longer rods within a given deck height (the longer the rod, the less side-thrust presses the piston against the cylinder wall to create friction). Piston-pin supports are cross braced for maximum rigidity and feature both fully circular oil grooves and holes that take lubricant to a thin line that runs from inside the support to the piston top and from there inside a stiffening rib that ends at the oil ring groove. Very accurate indeed.

The PistAl Racing steel pistons appear to be beautifully machined in every possible area to further reduce their weight. Tests have confirmed marked advantages in terms of both tensile and yield strength and the capability of operating at higher temperatures with total reliability, thus increasing the adiabatic efficiency of a piston engine, which translates to higher performance and a substantial reduction of fuel consumption. Additionally, steel offers superior retention qualities for a variety of both hardening and low-friction plating.

This is the positive side of steel pistons. Now the negative side. Tests run on engines featuring nikasil plated aluminum cylinder bores had the steel pistons carve into the aluminum despite their hardening plating. Steel or cast-iron sleeves are a must. Appropriate low-friction plating on both piston and cylinder sleeves might result in an efficient and reliable combination. Steel-piston manufacturing demands higher investment for the process, in particular the tooling and its maintenance due to the need for more frequent overhauling.

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The standard aluminum piston (left) and the experimental steel piston for the same diesel engine: the direct comparison tells all. Note how much more cropped the rings are on the steel piston and in particular how much closer (3mm vs. 10mm) to the piston top is the fire ring.

There is one more problem, and that is related to that “virtue” of reliably withstanding much higher working temperatures. The problem stems from the fact that steel does not disperse heat as quickly as aluminum. From the center of the piston, heat does not travel quickly enough to the cylinder barrel from where it would be transferred to the cooling fluid circulating around the cylinders. Therefore heat is concentrated up on the piston’s top, which is a problem for the lubricant. Even today’s excellent semi- and fully synthetic oils “crack” at the temperatures easily digested by steel.

The problem is more complex than it sounds. To fully exploit the advantages offered by steel pistons it is necessary to extract heat from them more quickly than we can do with customary piston cooling oil jets. It takes a more extended cooling procedure, which implies the use of a dedicated cooling circuitry that can hose the entire underside of the piston to uniformly remove heat, and then the lubricant must be circulated as quickly as possible to prevent it from overheating.

There is potential in steel pistons, but we have to figure out an effective solution that prevents their heat from killing the oil. Or, hope that the oil industry comes up with a super synthetic that does not breakdown under extreme heat. For now, it appears this solution is still a long way down the road.

In Part 2, Kevin Cameron gives us more background on the history of steel piston, and provides answers for why aluminum has been the go-to choice for 100 years.

Photo #1

Side views of an experimental steel piston for a big-bore, high-performance engine (Ducati Panigale?). Note how short the skirt is and how high up near the edge of the piston top is the fire ring.

Photo #2

Side views of an experimental steel piston for a big-bore, high-performance engine (Ducati Panigale?). Note how short the skirt is and how high up near the edge of the piston top is the fire ring.

Photo #3

This piston is sintered from 8600 series steel alloy, and its very accurate finishing of the top was achieved utilizing highly specialized diamond tools.

Photo #4

This piston is sintered from 8600 series steel alloy, and its very accurate finishing of the top was achieved utilizing highly specialized diamond tools.

Photo #5

The inner face of the piston top looks incredibly thin and light, accurately ribbed to ensure total stability to the top under the most extreme thermal conditions. The pin is short, a stub of a pin that can be located higher in the heart of the piston, its supports are cross braced for maximum rigidity.

Photo #6

The inner face of the piston top looks incredibly thin and light, accurately ribbed to ensure total stability to the top under the most extreme thermal conditions. The pin is short, a stub of a pin that can be located higher in the heart of the piston, its supports are cross braced for maximum rigidity.

Photo #7

This is a 4140 series steel alloy vacuum micro cast piston of similar characteristics but featuring a slightly taller skirt.

Photo #8

This piston has been sectioned along the centerline to show the oil lines running from the pin supports to the central rib at the top and then to the oil ring.

Photo #9

This piston has been sectioned along the centerline to show the oil lines running from the pin supports to the central rib at the top and then to the oil ring.

Photo #10

Finishing by specialized diamond tooling is the most demanding part of the job but the final result is beautiful.

Photo #11

This is the rod/piston assembly of an experimental steel piston for diesel engines. Diesel engines might be the first to go into production with steel pistons because their working temperatures are lower than in spark ignited engines, thus the oil cracking problem might be less imminent.

Photo #12

Top of the vacuum micro-cast 4140 series steel alloy experimental piston for diesel engines created by PistAl Racing.

Photo #13

As always in diesel engines, pistons are more massively structured than in spark ignition ones, but still this experimental steel piston shows remarkable compactness, particularly in the arrangement of the rings.

Photo #14

The skirt of this diesel engine piston is extremely light and it only runs down below the pin centerline to take care of the high center of gravity necessary with the depth of the toroidal combustion chamber.

Photo #15

The standard aluminum piston (left) and the experimental steel piston for the same diesel engine: the direct comparison tells all. Note how much more cropped the rings are on the steel piston and in particular how much closer (3mm vs. 10mm) to the piston top is the fire ring.

Photo #16

Moto Morini 350 of old never had forged pistons like this, featuring a high turbulence, ?heart? shaped Heron combustion chamber.

Photo #17

A forged piston for a Chevrolet Corvette LS3 V8, note the cross bracing at the pin supports.

Photo #18

A forged piston for a Chevrolet Corvette LS3 V8, note the cross bracing at the pin supports.

Chart 1

Comparison of graphs representing the decline curves of tensile strength (in mega-Pascal) in relation to temperature increase (centigrade)

Chart 2

Comparison of graphs representing the decline curves of yield strength (in mega-Pascal) in relation to the temperature increase (centigrade)