This article focuses on the increasing commercial application of capillary force vaporization. Possibilities range from fuel oil burners to the sophisticated kerosene heaters that are popular in Japan. A new stove will introduce capillary force vaporizing as a way of atomizing fuel, stepping away from cartridge and metal-tank stoves that dominate the market. Researchers in the United States are also exploring the technology’s suitability to diesel and homogeneous charge compression (HCCI) ignition and engines. A capillary force vaporizer’s ability to vaporize low-volatility diesel fuel at atmospheric temperatures and pressures gives the technology an edge over air-assist or other methods that produce small droplets. A capillary force vaporizer could be applied to a Heel engine in the manifold between turbocharger and intake valves or a vaporizer could be installed in the combustion chamber itself. Jetboil Inc. of Guild, N.H., integrated a pot, burner, heat exchanger, canister, and insulator into a single unit to promote efficient fuel use. As a result, the stove uses less fuel in the field, which is about half that of a conventional pot-and-stove setup that lacks an engineered interface.
On January 1, 2001, a 39-year old systems engineer began walking north from Springer Mountain in Georgia, the southern terminus of the Appalachian Trail. Early into his journey, Brian Robinson picked up the moniker "Flyin' Brian." Like most other through-hikers who finish the trail in four to six months, Robinson reached the forest pathway's northern end in Maine before the calendar turned. Along the way, however, his path diverged from that of his fellow hikers by many million steps.
More than midway between Georgia and Maine, Robinson turned left, heading west by bus to walk the two other great north-south routes that gild the western ranges. In addition to hiking through 14 eastern states, Robinson, by the time he finished this first-ever calendar-year Triple Crown, had followed the Continental Divide Trail across the highlands of New Mexico, Colorado, Wyoming, and Montana. And he walked the Pacific Crest Trail over the mountains of California, Oregon, and Washington. Setting a blistering pace, he trod 7,371 miles in ten months, walking nearly the equivalent of 300 marathons in as many days. He slashed his winter pack weight of 19 lbs. to an ephemeral 10 lbs. for the summer, not including water or food.
A member of hiking's rarefied but growing long-distance community, Robinson carried a minimum of commercially manufactured gear. For this group, a 3 oz. propane stove is too heavy. Many members favor lithe running shoes to bulky boots. The fringe element cuts its own alcohol stoves from empty cat-food cans and parses every pack item for its multi-task functionality.
This rivulet of fast-and-light hiking has joined backpacking's mainstream who would prefer to skip, rather than trudge, its way across hill and vale. Mainstream members insist on camping comfortably, though, and even luxuriously. Equipment manufacturers happily oblige them.
For example, Mountain Safety Research of Seattle will introduce a new stove this month that could threaten traditional travel cook sets with its efficiency and simplicity.
According to MSR's vice president, Jeff Bowman, the stove will vaporize white gas fuel with a single device that combines capillary action and phase transition. For hikers, that means the traditional stout metal fuel tank disappears. The stove, which requires no tank pressure to feed its burner, instead uses a light plastic tank in its place, Bowman said.
Dubbed a capillary-force vaporizer by developer Vapore Inc. of Richmond, Calif., the tablet-size device at the stove's heart merges a high-porosity ceramic element with a heat-conducting orifice disc and an insulating capillary wick. According to Vapore's CEO, Robert Lerner, micrometer-scale holes make up 80 percent of the high-porosity element, or vaporizer. These pores carry the fuel to the underside of the orifice disc where heat-conducting posts expand the liquid through an array of fins and channels. These, in turn, direct the pressurized gas toward a central orifice where it escapes as a vaporized jet.
Although Mountain Safety Research's stove is expected to be the first commercial application of capillary force vaporization, other projects are under way, according to Vapore's chief executive, Robert Lerner. Possibilities range from fuel oil burners to the sophisticated kerosene heaters that are popular in Japan, he said.
Researchers in the United States are also exploring the technology's suitability to diesel and HCCI, or homogeneous charge compression ignition, engines, according to Rolf D. Reitz, director of the Engine Research Center at the University of Wisconsin. They've identified several areas on engines where a capillary force vaporizer could be used to mete out fuel or clean up exhausts.
Diesels have not responded to NOx after-treatment the way spark-ignited engines have, mainly because of a high percentage of oxygen present in their exhaust, Reitz explained. Efforts at developing selective catalytic reduction systems for diesels could be helped by deploying capillary force vaporizers to inject controlled, homogeneous charges of diesel fuel or liquid urea onto catalyst surfaces. There, the diesel fuel and its partly reacted products, or the ammonia derived from urea, act as reductants, Reitz said.
Two tricks are necessary to make this work, he cautioned. The spray needs to be highly uniform. And the amount of spray must follow engine speed and load demands.
Achieving both conditions with conventional spray atomizers is difficult; capillary force vaporizers offer the hope of more precise control.
HCCl engines begin where diesels leave off. Rather than injecting a stream of fuel into the combustion chamber just before ignition, HCCI mixes fuel and air upstream of the intake valves and relies on the chemistry of the mix to time its ignition in the cylinder. The result is low NOx, high efficiency, and emissions that can be handled by conventional catalytic after-treatment. The hurdles include potentially damaging high-heat releases and the difficulty of timing the ignition.
According to Reitz, a capillary force vaporizer's ability to vaporize low-volatility diesel fuel at atmospheric temperatures and pressures gives the technology an edge over air-assist or other methods that produce small droplets.
A capillary force vaporizer could be applied to an Heel engine in the manifold between turbocharger and intake valves. Or, a vaporizer could be installed in the combustion chamber itself, Reitz said.
Fuel for the vaporizer comes up from the tank through a series of larger capillaries running through the insulator. A glaze binds together the entire ceramic stack, Lerner said.
Vapore uses a microfoam process to manufacture the high porosity vaporizer. It produces the fins and channels covering the bottom of the orifice disc with a process it calls micro-detail gel casting.
According to MSR's senior design engineer, Redwood Stephens, the stove makes an ideal commercial introduction of the new technology. Many hikers, climbers, and paddlers relish innovative designs that can enhance their adventures. Because it will pair canister-stove simplicity—no pumping or maintenance—with the refillable convenience of a white gas or liquid stove, the new design carries MSR's hopes of diverting stove sales away from both of these established categories. The stove will start without pumping or priming.
An important aspect of the new stove is the interface between the burner and the pot, Stephens said. It extracts much more heat from the burner than is possible with an open jet. A heat exchanger focuses heat on the pot bottom that would otherwise escape up the sides. The entire creation pays close attention to industrial design, Step hens added.
Forest Test Bed
Engineering of the pot/stove interface was a key element of another high-efficiency design introduced this past January. Jetboil Inc. of Guild, N.H., integrated a pot, burner, heat exchanger, canister, and insulator into a single unit to promote efficient fuel use, said chief technology officer, Perry Dowst. As a result, the stove uses less fuel in the field, about half that of a conventional pot-and-stove setup that lacks an engineered interface.
When the folks at Mountain Safety Research offered to lend Mechanical Engineering a Miox water purifier, "sure" was the only appropriate response. Hikers accept almost any handout. The purifier rode along on a brief visit to Vermont's Long Trail.
The slender micro plant, about the size of a C-cell flashlight, produces a mixed-oxidant cocktail with salt, water, and electricity. After pouring the cocktail into a measured volume of water, a thirsty travel er need only wait 30 minutes to kill bacteria and viruses, according to MSR's senior design engineer, Paul Smith. Bigger bugs such as Cryptosporidium, with their rugged, encapsulating sheaths, take longer to die—about four hours. They are easily filtered, however, and live mainly in still water.
Iodine and other chemical treatments have been around for years. They impart a taste to water that most hikers mask with drink mix. That means lugging along empty powdered calories.
Filters, too, are a popular way to sanitize water. Unlike in-line filters at home, though, most trail models require a user to force the water through by hand pumping. Try pumping a couple of gallons of river water through a filter to keep a group of canoeists hydrated, Smith suggested. You'll quickly discover it's hard work, he said.
Out on the trail, Jeff, a novice hiker, thirsted for a quenching mountain stream taken without the hose-play of filters or chemistry. His partner had spent a college summer years back traipsing a length of the Appalachian Trail. He drank his water straight up the whole way, even during the hot southern summer. No consequence had come of it.
This time, these guys were high up in northern Vermont, in late spring. The rain had just stopped. What the heck. Out came the cups.
After that first dip, doing the right thing felt about as pointless as trying to recover a good reputation. So, they dipped from there on out. A day later, when the veteran's knee said, "You're finished," Jeff ambled solo on up the trail for a few more days, unprotected by the purifier. Neither party has reported any ill effects—yet.
The lightweight purifier in the pack never made its presence felt. Had the water quality been even slightly questionable, there would have been no reason not to commission the treatment plant right then and there.
Early in the development process, Dowst and the company's co-founder, Dwight Aspinwall, realized that many travel stoves—and kitchen stoves as well—lose a great deal of thermal efficiency during the heat transfer from burner to the cooking vessel. They also realized that the most popular form factor in the outdoor industry-the thin, tall Nalgene bottle—was the shape most pack pockets were sewn to accommodate. The heat transfer characteristics of a tall, slender cooking pot were lousy, at best.
Dowst, whose background includes heat exchanger design and combustion research and development, said he and Aspinwall decided an integrated system would be the best way to increase efficiency while staying within the unusual confines of the popular form factor.
Although they relied on some theory, the stove builders took a mostly empirical approach to developing a heat exchanger for the cooking vessel—a cup, really. In about a year's time, the pair had developed a system that almost doubled the heat-exchanger efficiency of most commercial backpacking stoves. The heat transfer efficiency of a typical stove under laboratory conditions fell between 35 and 45 percent, Dowst said. Jetboil brought that efficiency up to between 70 and 85 percent.
The heat exchanger reduced the temperature of the exhaust gas to about 250°F, Dowst said. That opened a design possibility that would be immediately evident to users. The benefit of reducing fuel use by half, though big, was "sort of invisible," he explained.
But the reduction in exhaust gas temperature opened the possibility of a high aspect ratio pot that could be held by the user. Wrapping a Neoprene jacket around the cooking vessel furthered its ability to hold heat and be held at the same time. That eliminated the need for pot grippers and handles. The pot secures to the stove during cooking to prevent spills.
Though Dowst and Aspinwall both are fervent mountaineers who know their market, the duo relied on usability tests at several key junctures in refining the prototype. Setting up a trailside table in New Hampshire's White Mountains, they asked the first hiker coming by to test the stove. He said, "I come up here to get away from guys like you." But the surveys proved invaluable; after three such sessions the design direction had changed dramatically, Dowst said.
They also engaged instructors in the Peterborough, N.H.-based Eastern Mountain Sports mountaineering school to carry the stove prototypes aloft. The testers brought comments and critiques back from the hills that helped refine the design.
Manufacturability played an important role from the beginning, and brought about major design diversions from the initial prototypes, Dowst said. Jetboil worked with local suppliers, many of whom responded quickly to pleas for help during last-minute changes to the design. That kind of support would have been difficult to come by if the company's suppliers had been located many time zones away, Dowst said.
Out of Cat Food Again
Variants abound on the Web, but the basic homemade stove jams together the bottom ends of two aluminum beer cans, stuffing between them a wad of building insulation or similar wicking agent. A series of holes, produced using a fine drill bit or needle and pliers, rims the top of the inner can, which is slotted along its side to provide interference for the assembly. Half-filled with denatured alcohol, then lit, the stove fires full blast until it burns through its fuel.
The primitive stove dovetails appealingly with the minimalist approach mahy long-distance hikers adopt for their meals: Boil water. Add rice, pasta, or another quick-cooking staple. Do not simmer. Do not saute. Top with tuna or canned chicken.
Unlike fuel canisters or white gas, alcohol stove fuel is readily available at many stores in trailside towns, so hikers don't have to stock up on the stuff. It travels readily in lightweight, cheap plastic bottles, too.
Once a trip extends beyond a certain duration, the advantage of the homebuilt's near weightlessness bogs down in alcohol's low heat content, compared to white gas or isopropane (11,500 vs. 20,000 Btu/lb.). Hikers relying on alcohol end up paying a fuel-weight penalty if they can't resupply every four to five days.
Low tech as they are, homemade stoves sum up for many outdoor enthusiasts the very reasons they head into the back country. Simpler living. More views. Less news.
Many hikers are closet gear heads, however. They spend hours evaluating the latest equipment designed to bring them lighter and faster into the wilderness, and then safely home. Manufacturers are always trying to give them less with hopes that they can do more.