combustion occurring in the in-between space at the moment of greatest compression. Without the cylinder heads, the engine is lighter, and the dual-piston action means compression ratios are higher, so it gets more bang for the buck.
The concept of putting two pistons in the same cylinder is pretty old—manufacturers like Junkers and Fairbanks-Morse started using opposed-piston designs to power aircraft and submarines as long ago as the 1930s. But until Pinnacle came along, all opposed-piston engines (including the Ecomotors engine, and a diesel opposed-piston job from Achates Power in San Diego) were two-stroke engines, meaning there’s one powered stroke for each two movements of the piston. Diesels, motorcycle engines, leaf-blower engines, and other two-stroke engines are powerful but dirty. The problem is that the intake and exhaust functions happen simultaneously in the brief moment when the piston is at the bottom of the cylinder, which makes it difficult keep the burnt exhaust and the incoming air-fuel mixture from getting mixed up.
The Cleeves Cycle engine, by contrast, is a spark-ignited four-stroke engine, just like most conventional gasoline engines. The whole thing hinges on a clever sleeve-valve design that Cleeves says first occurred to him back in the 1970s. (While Cleeves always had a day job helping to improve semiconductor manufacturing methods for companies like Fairchild, Candescent, Cypress, Matrix, and Kovio, he says he spent most evenings in his garage at home, “wrenching on classic cars and hot rods.”)
In Cleeves’ arrangement, metal sleeves move independently on the left and right sides of the cylinder, activated by rocker arms on the camshafts. The cycle starts with Stroke 1; as the pistons move apart, the right sleeve slides open to expose the intake port, and fresh air and fuel are sucked into the cylinder. On Stroke 2 the right sleeve closes, the pistons move together, and the fuel-air mixture is compressed. At the very end of this stroke a spark ignites combustion. In Stroke 3, the only powered stroke, the pistons are pushed apart again; at the end of this stroke, the left sleeve opens and exposes the exhaust port. On Stroke 4, the pistons come together again, carried either by momentum from a flywheel or by power from adjoining cylinders. This squeezes the burnt exhaust out, clearing the way for the next cycle. (If you don’t have it all in your mind’s eye quite yet, just stare at this animated illustration on Pinnacle’s website for a few minutes.)
Under the four-stroke opposed-piston design, “There’s very little mixing of fresh charge with spent charge, the intake air temperature is very well controlled, and doesn’t get heated by residuals from the previous cycle,” explains Cleeves. On top of that, the geometry of the combustion chamber means the exploding vapor expands through a larger volume than in a traditional four-stroke engine. It’s “basic thermodynamics that with bigger expansion ratios you get more efficiency,” Cleeves says. In essence, the Pinnacle engine extracts more work from the same amount of combusting fuel before the exhaust is vented.
And there’s one more advantage to the opposed-piston design. Simply by changing the spacing between the two pistons, Pinnacle can adjust the compression and expansion ratios to accommodate different fuels. Diesel fuel requires a higher compression ratio than gasoline; ethanol higher still.
While he had his sleeve-valve inspiration early on, however, Cleeves says it took him another 30 years to come up with “the other seven or eight systems that have to be pulled together to make an engine that plays.” It’s all in the omnibus patent Cleeves filed in 2006, just before founding Pinnacle.
The company now has 16 employees, including 10 engineers. The core team works from a modest office and garage space in San Carlos, which is sandwiched mid-peninsula between Redwood City and San Mateo, while a few others are based nearly full-time in India. Hoge says Pinnacle’s venture funding will carry it well into 2013, by which time, it’s hoped, the Indian partner will have a product on the market. That, in turn, is envisioned as the stepping stone to
