Fuel Efficient Cooker Instructor Materials

"The Stingy Stove"

INSTRUCTOR

OVERVIEW OF THIS PROJECT - WHAT TO EXPECT

This project involves the design, fabrication, and testing of a "cooker" which will efficiently burn fuel to cook food (for testing purposes, to boil water). Cookers are sought which will rapidly bring a small quantity of water to boil with a minimum amount of fuel. The cookers must be constructed using only a limited set of common materials. The project should help the students understand many principles of combustion, heat transfer, as well as increase their sensitivity to a serious world problem. This project has proven to be lots of fun and quite safe.

SPECIFIC PURPOSE OF THIS PROJECT

The purpose of this project is to give students an opportunity to think about and create solutions, using engineering principles (i. e., efficient combustion and heat transfer), to a problem related to those of minimizing the consumption of fuel wood for cooking in third world countries. Students should find this project challenging, creative, and fun along with having a successful experience in the process.

This project expects students to gain practical insight into the principals and difficulties involved in the serious, real problems of heating objects and burning fuel. They will gain an understanding of combustion principles and design for efficient heat transfer. As in any engineering design task, to meet required design goals engineers must make compromises to maximize performance while satisfying various practical constraints. Students will experience the same creative challenges as engineers when they take part in this project.

THE REAL-WORLD PROBLEM RELATED TO THE PROJECT

Deforestation is a critical current global problem, particularly in many third world countries. The rate at which deforestation is occurring is alarming. In fact, burning wood for warmth, light, and cooking accounts for more than 90% of the energy used by many poor countries. The use of wood as fuel is a major factor in the destruction of forests in these countries and in the associated severe problems of soil loss by erosion, lower crop yields (since there is a reduction in farm top soil), and reduced water supply (rain water runs off rather than being absorbed).

The traditional cooking arrangement in these countries is the "three-stone fire," with a cooking pot supported on three stones and a fire underneath as shown in figure 1. This system is very inefficient in its use of fuel. Typically, less than 6% of the energy in the wood is applied to food being cooked. A cheap, reliable, higher efficiency cooker could have a major impact on reducing fuel wood consumption and thus on reducing the severe problems associated with the destruction of forests. This project gives the students a chance to think about the design of an improved cooker, to create candidate cookers, and to test their ideas.

Figure 1: A Three Stone Cooking Fire

SPACE AND TIME REQUIRED/DESIRABLE

Design and fabrication activities are generally suited to an indoor tabletop situation. Space requirements are approximately nine square feet with enough space between student groups such that their materials and building processes do not interfere. Testing, while requiring little space, must be done outdoors in an open space to minimize the fire hazard. The main requirement for the testing location is a fire proof surface with nothing flammable nearby. Some shelter from the wind is also advisable. Obviously, the cookers will heat up, and the student teams might need room to feed fuel to their cookers during testing.

The amount of time involved is somewhat flexible. The minimum time is about four hours, which can be either in a block or in segments. This can be expanded to perhaps six to eight hours, which could include time at home to think about the project. We have found that this project can be repeated once with good results if six to eight hours are available. The first time through the students see many new issues, develop new questions, and generate ideas which they don't have time to explore. They also develop some fabrication skills which considerably improve a second try. Second generation results are usually much superior to first generation results. This gives the students additional satisfaction plus illustrates the importance of learning with each design cycle - an important real world consideration.

MATERIALS AND EQUIPMENT NEEDED

In this project the students are limited to a small set of materials. In particular, the cookers which they fabricate and test may use no materials other than aluminum foil (you may wish to limit this to perhaps 5 sq. ft per cooker), aluminum soda cans, tin cans, metal wire (limit to ten feet of wire per cooker), paper clips (a dozen maximum), bricks (optional), and newspaper (for fuel). We have found that coat hangers are sufficient to use as wire; however, coat hangers might be difficult to cut to size for some students. If available, we recommend using baling wire that is available at hardware stores. Baling wire is thinner, easier to cut, and more malleable than coat hangers. Paper clips also serve the same purpose as wire; however, the length is limited, and some groups might need longer lengths.

No equipment limitations are placed on the students. They definitely should have rulers, wire cutters, pliers, tin snips, awls, hammers, and church-key can openers (triangular head can opener) available. Awls let students punch small circular holes in metal. Screwdrivers, ice picks, or other pointed metal tools could substitute. If possible, making one each available for each group of students will speed building.

You will also need some means of measuring time, a measuring cup or beaker to measure the amount of water in each cooker, and plenty of matches. We would advise wooden, long stem matches, but obviously, any type of matches will do. For fuel used, you can simply count the number of sheets of newspaper.

Since the cookers will heat up during testing and students might need to steady their cookers while feeding fuel into them, you should have a set of oven mitts or pot holders to protect the hands of handlers. Having a bucket of water handy for putting out fires, cooling hot cookers, and supplying water for testing is advisable. After a cooker's fire has been extinguished for ten minutes, the devices usually have cooled enough to handle without protection. Alternatively, students can pickup their cookers with protective oven mitts and immerse them in the water bucket.

Don't forget to have a fire extinguisher handy!

SUGGESTIONS REGARDING STUDENT TEAMS

For this project we recommend small teams, two or three students each. We suggest that the teams meet together to discuss principles and ideas and then spend some time sketching, discussing, and evaluating candidate design forms before beginning fabrication efforts. For two or three person teams, one cooker per team is probably best. If there are larger teams, have them subdivide into groups of two or three with each group fabricating a cooker. Students should be urged to consider how their cooker designs will create and maintain a good fire. We have found that students focus on details and loose sight of principles. Specifically, they forget what makes an efficient fire and what is useful to know about good heat transfer properties. The section on important principles involved lists several common design flaws to be aware of in your students' designs.

PREPARING FOR THIS PROJECT - WHAT TO DO IN ADVANCE

The main tasks are finding a suitable place for testing and gathering the necessary materials and equipment. Although the project testing is quite safe, there are fires and hot objects involved. Designating an approved audience location and getting one of your students or another instructor to monitor for safety is a good idea.

The quantities of materials needed can be estimated based on the number of students. Allow one set of materials for each student group. You may wish to provide some or all of the materials. One compromise that works well is to have each student bring five or six empty tin cans, aluminum cans, several coat hangers, and some newspaper, while you furnish the other materials. If the bricks pose a problem, they can be eliminated completely without great harm to the project. Note that the amounts given are for practice as well as final versions of their cookers.

It is best to make sure that all the tools are available before the students begin fabrication. You will need aluminum foil (you may wish to limit this to perhaps 5 sq. ft per cooker), aluminum soda cans, tin cans, metal wire (limit to ten feet of wire per cooker), paper clips (a dozen, maximum), bricks (three or four, maximum), and newspaper (for fuel) for each student group. In addition, you will need a liquid measure, matches, a timing device, oven mitts, and fire extinguisher (or bucket of water). A stop watch or some digital watches can measure the time required. You will need a measuring cup or beaker to measure the amount of water in each cooker, and plenty of matches. We would advise wooden long stem matches, but obviously, any type of matches will do.

Students might need to test their cookers for burn efficiency, and their water containers for tightness. You might want to give each group some newspaper for testing, but for the final test, you can simply count the number of sheets of newspaper. We consider standard size newspaper (27.5in x 22in), a sheet of which contains four actual printed pages.

IMPORTANT STEPS IN MANAGING THE PROJECT

We recommend the following approach:

Divide the group into teams (two or three students per team preferred) and have the teams move so that they are sitting together.
Distribute the student documents, one set (common documents plus documents unique to this project) to each student.
Present the project to the students following the approach suggested in the common instructor documents (15-25 minutes).
Provide the student teams with about 60 minutes to read the materials, to discuss the principles, and to generate design ideas.
Give the students the working materials. (This is a good time for a break, or perhaps to go home for the evening.)
Give the students 90 to 120 minutes to design, sketch, fabricate, and test prototype cookers.
Formally test the cookers produced (one per team if the group is large or if time or space is limited, one per pair of students otherwise). Testing will take about 5 minutes per cooker, providing the students are ready to test. Ready to test includes preparing any newspaper fuel (cutting into strips, crumpling into balls, etc.), resetting the timer, and measuring water. Testing the cookers one at a time lets you supervise the fires, thereby decreasing the possibility of accidents. This also lets other students observe and learn from their peers. If time allows, let the students perform two to three tests per cooker. Some variation in times will occur due to amount of initial fuel, efficiency in lighting the fuel, how they prepare the fuel, the amount of wind, the initial temperature of the water, etc.. By doing a series of tests, the students will get a better feel for the combustion and heat transfer characteristics of their cookers.
Discuss the results of the testing, emphasizing the reasons for differences in performance (about 5 minutes per cooker). This should be done in a group setting. You might want to take notes about the different cookers during the testing phase as you write time times.
Allow the students from 30 to 60 minutes to redesign their cookers for improved performance. Students might need additional materials. You will have use you judgment depending on the amount of spare materials. We do not encourage students to do a complete redesign; modifying existing designs usually is faster with little or no addition material requirements. The students should have enough materials from their initial allotments to make any modifications. Limiting additional materials is also a real world issue that designers face.
Formally retest the new series of cookers (5 minutes per cooker). Again, if time allows, let each group perform a series of tests with their cooker.
Review the project, discuss the differences in performance from the first test to the final test, noting in particular significant improvements in performance between first and second generation cookers.

GUIDELINES FOR TESTING THE COOKERS

For safety, you should supervise individual testing of the cookers. Individually testing each cooker gives all students a better opportunity to learn from the performance of the various designs, and it gives you a better opportunity to supervise lighting and extinguishing the cooker fires.

There are two possible ways to handle the fueling and lighting of the cookers. Whichever way you choose, remember to be consistent with all the teams.

Allow all required fuel to be loaded into the cooker before lighting. Student designs using this parameter need not consider ways to load additional fuel during the heating process; however, these types of cookers require testing to determine the optimum amount of fuel. These types of cookers will boil water faster than those that need to be loaded during testing.

Require that no fuel be loaded before lighting and students must initially light the fuel before introducing it into the cooker. This requires that students design their cookers to allow loading fuel during the firing. These types of cookers will not boil water as fast; however, there is more excitement.

Students might have difficulty lighting matches on windy days. Suggest that they cup their hands around the match and make sure it is burning well before lighting the fuel. Do not let students light more than one match at a time! We have found that if you do not limit the number of matches for each team that they will use all the matches. A good number is four matches for each test with only one match being lit at any time. Students should not be allowed to add matches since this increases the amount of fuel and initial heat given to the fire. On windy days, you might need to increase the available number of matches.

Start the timer when the designated student strikes the first match and stop the timer when the water in the cooker comes to a rolling boil. What constitutes a rolling boil is subjective so be careful that you are consistent during your observations. You will also need some plan for counting the number of sheets of newspaper each group uses during the formal cooker testing process. We have relied upon each group of students accounting for the quantity of fuel used.

Neither smoke nor fire is comfortable. Students tend to build cookers that do not breathe well and thus are smoky. Remind your students of this and try to keep them upwind of the cookers. During testing, groups tend to ignore their own safety and will be more concerned with properly lighting and feeding fuel to their cookers. You will need to closely supervise the students, constantly reminding them to stand out of the smoke and be conscious of the generated heat. Student teams should have designated assignments during testing such as: one person to light the fire, one person feeding fuel, one person placing the water container, etc..

IMPORTANT PRINCIPLES INVOLVED

This project is very empirical in nature. Combustion principles are simple to understand, but they are difficult to quantify. Students tend to underestimate the amount of fresh air required and the importance of a good flow of hot air away from the fire.

When wood or paper burn, the energy is released mainly as very hot gases (there is some energy released in the form of radiation). Since these hot gases are lighter than air, they tend to rise. The energy from the hot gases is then transferred to a cooking container which in turn heats and thus cooks the food. One way of inducing a good flow of fresh air is by having air vents below the fire. The hot exhaust gases, as they rise, will draw cooler fresh air in from the vents below the fire.

Good burning is primarily dependent on a good supply of fuel with lots of surface area, on a good supply of air (really oxygen) being available to the fuel, and on a free flow of the hot gases away from the fire.

The rate at which the energy is transferred from the hot gases to a container depends largely on the volume and temperature of the gases, on the surface area of the container that they contact, on the length of time, and on the speed with which the gases are moving past the surfaces.

The bottom line of all this is that each cooker will want to have a good fire, a good supply of fuel to maintain the fire, and have the hot gases flowing freely in such a way as to contact lots of surface area of the water container.

The following pitfalls are common to student designs.

The amount of air flow feeding the fire is underestimated.
The size of the exhaust opening is underestimated.
Densely packed fuel does not burn as effectively as loosely crumpled newspaper.
Water containers made out of crumpled aluminum foil tend to leak and act as insulator for the water since a large volume of trapped air exists within the crumpled aluminum.

PROVIDING SUCCESSFUL CLOSURE

We encourage you in this project to emphasize that effective design is the application of relevant principles to achieve specified goals. Depending on the application, many design goals involve a tradeoff. For instance, the size of the combustion chamber can be either too small or too big. Too small a chamber does not allow enough fuel to be loaded nor good air flow. On the other hand, a chamber that is too large may not direct the flow of hot gases effectively around the water container. These principles may not be evident to the students. Reviewing the principles listed above should prove helpful to the students when given in the context of their designs.

This project illustrates to students that engineering design requires the incorporation of knowledge from multidisciplinary fields, conflicting objectives, and multiple goals. This is what makes engineering challenging; however, when they loose sight of what they are doing and begin to become obsessed with design particulars, their designs will become less successful. Even the brightest and most creative students can fall into this trap.

Most designs, even those that are not as successful, have strong points. Taking note of those strong points along with some constructive criticism helps make a learning point for students. Many groups can dramatically improve their cookers by slightly altering their designs. As long as the students realize why their designs improved, they will have gained a great deal from this project. A group discussion following testing in which each team critiques the strong and weak features of their design has proven fruitful.

Finally, please make a special point of reminding the students that this kind of creative activity is at the heart of engineering and that, if they found the project enjoyable, they should consider engineering as a profession.

SPECIAL SAFETY CONSIDERATIONS

There are three main safety concerns in this project. These are:

Sharp edges can cut students during cooker fabrication or testing (mainly when feeding fuel into the cooker).
Hot parts of the cooker can burn students during development or final testing.
Be aware of the danger of setting fire to the surroundings (watch out particularly for wind carried sparks).

We recommend warning the students about these hazards, encouraging them to be careful, thinking before acting, not getting into a rush, and having someone designated as safety monitor during all testing. Wearing gloves or masking sharp edges with aluminum foil will help in preventing cuts. Using gloves and letting things cool adequately before touching them prevents burns. The most important safety measure to prevent unwanted fires is to choose a wind free location without nearby combustible materials. Of course, you should have a working fire extinguisher present and know how to use it. This project was developed by John Garcelon.