Pistons For Engines

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Written by Tim Lampe
RC Pylon Racing
As seen in the May 2021 issue of Model Aviation.

silicon is the most abundant element in the earths crust
Silicon is the most abundant element in the earth’s crust and is mined from sand/quartzite. It is mixed with carbon and reduced in an electric furnace at 2,700° to 3,600° F to make metallurgical-grade silicon with an approximate 98% purity. Photo credit: Wikipedia Commons; public domain.

BY NO STRETCH OF THE IMAGINATION am I purporting to be a model airplane engine expert, especially about the development of engines for Pylon Racing. I am fascinated, however, by the technology and metallurgy of all things, so through a conversation with Dub Jett and a little research of my own, I put together this presentation.

For those who might not know, Dub is a craftsman and figure in the model airplane glow-engine industry and manufactures his own brand of engines and accessories. Although he caters to the sport, Control Line (CL) Stunt, and CL Speed disciplines, I know Dub mainly for his Pylon Racing engines and his continued presence (and success!) on the circuit. I also can’t help but boast that I’ve had the pleasure of teaming and rooming with Dub a few times when his regular teammate, Mike Helsel, was not available.

When Dub first began manufacturing model aircraft engines, the critical materials and technologies that were required (specifically for pistons and cylinder liners) were not commonly known and could not easily be purchased. Glow-powered model aircraft engines—especially those intended for racing—require metals with unique properties. Pistons and liners were (and remain) the most important components because the fit between the two is critically related to performance.

"Back in the day," pistons were cast iron and liners were steel. These parts were durable and reliable, but the performance was relatively low because, among other things, the coefficients of expansion did not allow for the tight, precise fit we enjoy with pistons and liners made from today’s materials.

In approximately the early 1970s, SuperTigre was one of the first pioneers to incorporate aluminum pistons into its production engines. With this came ABC technology (aluminum pistons in a chrome-plated brass cylinder liner). But SuperTigre’s pistons weren’t just aluminum; they were an aluminum alloy (and other trace metals) mixed with the element silicon (Si on the element table—not silicone).

In my research, I’ve learned just how big of a deal silicon is and how greatly it enhances the properties of aluminum in all industries. I have found many papers about aluminum-silicon (Al-Si) pistons in the auto industry. It’s all over the place!

Aluminum is alloyed with silicon for many reasons. It improves the casting process by increasing fluidity, reducing the melting temperature, and decreasing the contraction associated with solidification as it cools. When it comes to performance in a model airplane engine (or any engine), silicon reduces piston weight but, most importantly and relevant to this article, it has two major effects on the finished part.

Silicon has low solubility in aluminum and therefore precipitates out or migrates to the surface as virtually pure silicon, which is super hard and therefore improves abrasion resistance (piston wear). The addition of silicon into aluminum also greatly improves the thermal stability.

Aluminum pistons and chrome-plated Al-Si cylinder liners do not expand nearly as much as standard alloy aluminum when the operating temperature is reached. This allows for closer manufacturing tolerances and a tighter piston fit, which is referred to as "pinch" in the Pylon Racing world.

Dub’s early attempts at aluminum pistons alloyed with silicon swelled too much. To combat this, he would put the pistons in an oven to "pre-swell" before machining. This had mixed results.

Coincidentally, the Chevrolet 2300 2.3-liter (139.6 cu. in.) inline four-cylinder, die-cast aluminum alloy engine block was in production for the 1971-1977 Vega and Monza cars. At that time, the hightech engine block featured an alloy with 17% silicon. This alloy was made by Reynolds Metal Co. and was specified as A390—look it up!

Dub and his colleague, John Shannon, actually went to junkyards to scavenge these A390-alloy Vega engine blocks. They broke them up with sledgehammers, melted and cast the chunks, then machined them into pistons, thus beginning the era of aluminum pistons with chrome-plated brass liners for Jett model engines. What Dub might lack in physical stature, he more than makes up for in determination, vision, resourcefulness, and creativity!

Dub had his connection for metallurgy analyze this "homebrew," which led to further understanding, development, and refinement. The actual performance was close to the Nelson pistons of the period, but the silicon particles in his pistons were larger and not uniform, causing machining problems. Still, these pistons "wore better than anything," and Dub reports that he still sees pistons of this era with no wear.

this is a modern al si alloy piston from a jett qm40 pylon racing engine
This is a modern Al-Si alloy piston from a Jett QM-40 Pylon Racing engine. Pistons in these engines normally operate at roughly 29,000 rpm and can do so flight after flight.

To improve the machining quality, Dub added pure phosphorous to the molten aluminum alloy right before pouring. This exploded and dispersed the silicon in the aluminum, allowing for much finer machining. During these years, his engines had good success but were still not as good as the Nelson engine.

A customer of Dub’s, who was also a competitor on the Pylon Racing circuit, had connections with Mercedes-Benz factories in Germany that were using another Al-Si alloy. Dub obtained some of this material and made pistons from it. Testing revealed that his engines gained an additional 500 rpm. He had this material analyzed for composition, along with a piston from a Nelson engine.

(In speaking with Dub for this article, he stipulated that he never had components from competitors’ engines analyzed for the purposes of copying. In this instance, he was already pursuing a discovery of his own and was curious how this new material stacked up.)

When the paperwork from the analysis came back, Dub had difficulty deciphering which alloy was which. In other words, the alloy from the German supplier and the alloy of the Nelson piston were the same. The composition of Nelson’s secret piston alloy had been inadvertently discovered.

This new super material was not cast but sintered. My read-up on sintering revealed the world of powder metallurgy (PM). PM is synonymous with sintering, which is the process of blending fine-powdered metals, compacting them into a desired shape, and heating just until the particles adhere to each other. Sintered aluminum-powder alloys can have properties quite different from those of aluminum fabricated by conventional casting. Sintering can also be controlled to enhance properties such as porosity, density, strength, and resistance to corrosion and high temperatures.

As a side story, the German supplier of the sintered alloy telephoned Henry Nelson to inform him of Dub’s discovery. Henry then proclaimed, "He finally found it!"

Dub continued to test, develop, and discover other alloys, settling on material from a supplier in Holland. The alloy used in pistons today is composed of as much as 30% silicon, while in the SuperTigre days, it was just 13%.

As another side note, the disparity in these two figures (30% vs. 13%) gives me pause to describe another term that came up in my research. Silicon is normally soluble in aluminum up to approximately 12%. This maximum saturation is called the eutectic point. However, special molding and cooling techniques can be used to get even more silicon to mix with aluminum. The alloy is then called hypereutectic.

Hypereutectic pistons are stronger than more common, cast-aluminum pistons, have an even lower coefficient of thermal expansion to allow even tighter tolerances, and are used in many high-performance applications. I love this technology stuff!

Brass cylinder liners machined and chromed beautifully, but were heavy and not as good of a match for the newer piston materials with higher silicon content. When Dub switched to aluminum liners of 11% to 13% silicon, he was able to match thermal coefficients even more precisely, allowing for the use of the higher silicon pistons.

Back in the days of cast iron pistons, steel liners, and castor oil, the highest percentage of nitromethane that would mix with the castor oil-based fuel was approximately 40% (the balance being the castor oil and methanol). When ABC technology and synthetic oil arrived, up to 80% nitro could be mixed. Fortunately, contemporary engine technology and sensible rules limit nitro content to 15%, but our engines are still much faster, extraordinarily more reliable, and competitive right out of the box!

Do you want to discuss connecting rods? In the beginning, rods were easier to make. But tighter-fitting pistons with ABC technology imposed more load on the rods. FAI engines were no problem because they were pretty loose at the time, but as Quarter Midget (QM) engines evolved, with even tighter pistons, the quest for better connecting-rod bushing materials was on. A connecting rod that would last a long time in FAI engines of the day would last only a couple of runs in the QM engines that were in development.

Maybe another day I can coax Dub into talking to me more in-depth about connecting rods.

SOURCES:

National Miniature Pylon Racing Association (NMPRA)

www.nmpra.net

Dub Jett Model Engines and Accessories

(713) 680-8113

www.dubjett.com

WikiChip: Metallurgical-Grade Silicon (MGS)

https://bit.ly/3qdNWfp

Wikipedia: Silicon

https://bit.ly/376IfZf

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