KEY CONCEPTS
• Conventional pistons have limitations that reduce their effectiveness and efficiency.
• A new piston design uses a combination of flexible, soft membrane materials and compressible rigid structures.
• A first generation, soft-skin prototype exhibited three times greater output forces and a 40% higher energy efficiency.

In the continuing effort to improve machinery efficiency, pistons perform important functions. For the internal combustion engine, a piston moves up in a cylinder, compressing the air fuel mixture, which upon ignition creates an explosion forcing the piston down and transforming the energy into linear motion that moves the automobile. Other applications for pistons include reciprocating pumps, gas compressors and pneumatic cylinders. 

Friction between the piston and the chamber has been found to be the source of a considerable energy loss in an internal combustion engine. One of the approaches for reducing friction has been the use of a skirt that helps with guiding the piston as it moves up and down in the cylinder. In a past TLT article (1), a new piston skirt coating based on graphite, molybdenum disulfide and carbon fibers was described. Evaluation of this coating led to an 18% reduction in friction and a substantial reduction in wear. 

Dr. Shuguang Li, Postdoctoral Fellow at Harvard University in Cambridge, Mass., says, “The function of a piston is to convert fluidic pressure to mechanical work. Conventional pistons were invented over 300 years ago in the late 1700s.”

The pistons originally developed utilized two fluids placed in two different chambers. Li says, “The conventional piston contains a rigid chamber with a piston inside that can slide along the chamber’s inner wall. A piston head divides the two chambers. When there is a pressure difference between the two chambers, the piston will slide in the direction of the chamber with the lower pressure, which enables a power conversion to some device that does physical work.”

Conventional pistons do have some limitations, which have reduced their effectiveness and efficiency. Li says, “At very low pressures, a conventional piston cannot move until the pressure reaches about 20 kilopascals. There is a dead zone where no operation is possible.”

The close contact between the piston and the chamber walls can lead to an increase in friction, breakdown of the seal and leakage of the fluids. This can then result in piston failure.

A new approach to improve the efficiency of a conventional piston and reduce friction is needed. After 300 years, a new piston made with flexible materials has now been developed.

Tension piston
A research team lead by Harvard University’s Wyss Institute Founding Core Faculty member, professor Robert Wood also co-lead of the Wyss Institute’s Bioinspired Soft Robotics Initiative has developed a new piston using a combination of flexible, soft membrane materials and compressible rigid structures. The new concept is known as a tension piston.

The tension piston contains six components that include a fluid-tight chamber, flexible and fluid-tight piston skin, compressible piston skeleton, output connector and two fluids, (a high-pressure medium and a low-pressure medium). Figure 2 shows an image of a linear tension piston. 


Figure 2. The linear tension piston shown is representative of a new piston design that exhibits superior performance to a conventional piston. (Figure courtesy of Harvard University.)

Li says, “A tension piston is a different structure compared to a conventional piston. The compressible skeleton structures are covered by a skin and the piston head is different. Instead of two chambers, a tension piston has two different spaces that contain the inside-piston fluid and the outside-piston fluid. A pressure difference created by pressurizing the outside-piston fluid or by depressurizing the inside-piston fluid is needed to produce a compressional force that an output connector can use to have a device perform work.”

The researchers produced a first generation soft-skin prototype. Li says, “We prepared the skin from a thermoplastic polyurethane coated nylon-fabric. For the tension piston to be successful, this skin needed to be sealed to the skeleton, display sufficient flexibility and generate no holes in the material.”

An object-crushing test (such as crushing a pencil or a plastic bottle) was used to evaluate a conventional piston versus a tension piston. The researchers determined that the tension piston broke the same object at a lower pressure. Under these conditions, the researchers found that the tension piston developed more than three times greater output forces and achieved a higher energy efficiency by 40%. 

The better performance results are generated because the skeleton of the tension piston contracts to produce the force needed to drive a device to do a specific task. In addition, this new design minimized potential friction that could occur between the piston and the chamber wall.

The researchers recognize that their initial design provide only limited performance. Li says, “We will be trying different materials that can enable operation of the tension piston under higher pressure and high temperature conditions.” One other need is to develop a tension piston that can be driven reversibly by fluid pressure. 

Additional information can be found in a recent article (2) or by contacting Wood at rjwood@seas.harvard.edu. 

REFERENCES
1. Canter, N. (2011), “Advanced piston skirt coating,” TLT, 67 (2), pp. 10-11.
2. Li, S., Vogt, D., Bartlett, N., Rus, D. and Wood, R. (2019), “Tension Pistons: Amplifying Piston Force Using Fluid-Induced Tension in Flexible Materials,” Advanced Functional Materials, 29 (49) 1901419.