An Engine in Your Pocket
By Alice Rhein (BA ’84, MS ’86)
When today's soldiers set off on a mission, they frequently carry payloads that include 40-pound battery packs—a lot of weight for even the broadest shoulders. But Aerospace Engineering Professor Werner Dahm, together with Mechanical Engineering Professor Jun Ni and their research team, are developing a small, portable, efficient power source that just might replace those hefty batteries and lighten the load: the micro internal combustion swing engine (MICSE).
And how small is the MICSE? No larger than a small stack of crackers—small enough to fit in your pocket. How powerful? It’ll generate 20 or more watts, Ni said—enough power to replace a 40-pound battery.
The Pressing Need for a Small Power Source
| CoE researchers have completed a prototype of the pocket-sized micro internal combustion swing engine (at right in photo) that can generate 20 watts of power continuously for three days using only seven cubic centimeters of fuel (about half the size of a tomato can). It takes 15 watts to run a laptop computer with a CD drive. |
“It’s a huge priority for the military to find a replacement for those batteries,” said Dahm, who is the project director and head of the Laboratory for Turbulence & Combustion (LTC).
And while much has been said about the potential capabilities of fuel cells, creating a practical system that can run on liquid hydrocarbon fuels like butane or JP-8 is still several years away, according to Rhett Mayor, assistant research scientist, Mechanical Engineering.
“The popular belief is that fuel cells can provide a near-term solution to micro power generation needs,” Mayor said, “but many of the required technologies aren’t ready now, and it may be at least several years before they come online. One of the advantages we have with this project is that internal combustion systems are the most realizable source of portable power for the next five-year window, and possibly longer.”
From Swing Engine to Small Wonder
A swing engine is a rotationally oscillating free-piston engine in which combustion occurs in four chambers separated by a single rotating swing-arm, with virtually no other moving parts. There are about three dozen swing-engine patents going back to 1903, but because no one has been able to harness oscillating-shaft motion to do any useful work, engineers have largely ignored the swing engine as a mechanical power source.
“When we took a look at the swing engine,” Dahm said, “the innovation we saw was that if you coupled it directly with a generator—a generator that’s optimized for oscillating motion—you’ve got a great electrical power generating device.”
When Dahm, Ni and Mayor and a handful of doctoral and graduate students couple the micro swing engine with this sort of generator, they will have developed the core of a revolutionary palm-power device. Using it to replace cumbersome battery packs for soldiers is an obvious use; however, the MICSE has potential for many other military and commercial uses.
| Diagram of MICSE generator. |
It’s obvious that for military and search-and-rescue operations, MICSE power systems are a big issue. And though it’s not likely that an MICSE would run cell phones or other everyday appliances, an engineer working in a remote field could potentially run a laptop for 10 days using the portable power source in development.
Having completed the first year of the three-year study, the MICSE team reports major progress. “When we had a site visit in October, we assembled the engine in about two minutes, let it run, and ran it continuously with very few moving parts,” Dahm said. The customized generator was completed in December 2002, so the researchers now have a prototype of the fully integrated system.
Problems and Solutions
“To achieve rapid progress, we split out the research efforts into parallel tracks,” Mayor said. “So, for example, we had simultaneous progress being made on the generator design, power conditioning design, actual engine design and thermodynamic simulation. And with that came the need for close communication between the various participants.”
Dahm said the project involves “fluid mechanics, combustion, electromagnetics, solid design, manufacturing and so forth. All of those issues are hugely important.”
The swing engine converts chemical energy into mechanical energy and drives a generator that converts the mechanical power into electrical power. While DARPA would ultimately like the MICSE to run on JP-8, initial testing is being done with butane.
“As with any combustion, there is exhaust,” Ni said, “and further fuel tests will look at the thermal signature and acoustic signature. But for research purposes, butane burns much cleaner, so the group is building both a butane/propane and a JP-8 fuel system to operate both two- and four-stroke devices. Ultimately the amount of fuel that a soldier would have to carry to run twenty watts continuously for three days would be equal to seven cubic centimeters—about half a can of tomatoes.”
The first phase of MICSE included a 165-watt power-generation system, but DARPA made the specific request for 20 watts. “Twenty watts can run night vision or a laptop or satellite communication for several days,” Ni said. Commercially, it takes 15 watts to run a laptop with a CD.
Dahm pointed out that there’s another reason for choosing 20 watts as the starting point. “Engines get harder as you make them smaller,” he said. “It’s pretty easy to scale up from twenty watts, but below that you enter into different physical regimes, and the design has to change fundamentally.”
The operation of an unmanned aerial vehicle (UAV) the size of a large radio-controlled airplane (larger than an MAV) requires approximately 200 watts. Preliminary testing has verified that the MICSE would not need fundamental changes in design to power a UAV.
“For UAVs, you like to be able to have them loiter for a long time and be reasonably silent,” Dahm said. “With a small internal combustion engine and a battery, you could carry much more energy than what is currently available using batteries alone to run electric motors.”
The simplicity of a swing engine’s mechanics translates into several benefits. With no “dead point” in its operating cycle, the swing engine doesn’t require an external starter like a car engine. Having few moving parts makes it highly reliable and easy to manufacture. The system adjusts quickly to large step changes in power demands, and a technique called Electrical Discharge Machining (EDM) produces very high tolerance parts, eliminating the need for seals between moving parts.
Where the MICSE is Going
Ultimately the CoE team would like to see the MICSE perform three modes of operation. In one mode, the MICSE drives the generator, and the generator in turn provides the electrical power directly to the load. In a second scenario, power is drawn from a battery that is recharged continuously by the MICSE generator. There’s a third possible function in which the engine directly powers a load and simultaneously recharges the battery.
“Of course, that takes a power management system,” Mayor said. “You need to have some sort of system intelligence in place to tell it when to do that.”
When Dahm, Ni, Mayor and their team succeed in developing that power management system and refining the MICSE, they will have developed a twenty-first century twist on a mostly forgotten twentieth-century technology, and they will have found a way to improve communication and safety for soldiers in the field, transform hazardous civilian applications into benign tasks and—in both cases—possibly save lives.—E
Alice Rhein (BA’84, MS’86) is a freelance writer and former newspaper editor. Her story on “Polymers for Life: Biomedical Engineering Research Offers Creative Options in the Treatment of Deadly Neurological Disorders” appeared in the fall/winter 2002 issue of Michigan Engineer.
