The concept is very simple, it involves using the heat released from the exhaust flue gases of cars, through a conductive flow.
Step 1: Construct the Heating Element
We must make two holes in the trunk of the car to pass through the copper tubes.
On the heating element projecting into the trunk (U-shaped) shall be constructed fins to enhance heat transfer can be made only by winding copper wire or bare aluminum, smaller diameter.
Step 2: Union of the Heating Element (copper Tubing)
If we want a greater heat transfer, then we can add some water inside the copper tube, something like 1 cc, so we can leverage the latent heat of steam generated.
When speaking of steam in a closed system always is implicit the idea that the system can explote , but in this case we have two mitigating that preclude the case.
If we heat a tube inside which there is water vapor, it began go through of the tube's length and if it is not insulated, heat will transfer from the inside to the environment so that if the tour long enough we will have a temperature profile, where lower temperature, and by by handling the tube length can stabilize the temperature in a range between 40 -50 ºC. therefore in the road although of the tube the vapor will be condensing anger without even coming to overheat, condense into droplets that gravity will return to the hottest area of the copper pipe and then return to vaporize maintaining the temperature profile.
The connection between the ends of the brass tube are joined by a rubber stopper, which is a factor of safety if the vapor pressure is greatly increased, the union will disintegrate letting out steam.
In the pic, the black zone represent the rubber union.
Step 3: The Lunch Box
the lunch box can be a simple recipient isolated for food preservation but must be modified so that the U-section must fit in this as a lego, so that a sheet of metal is in contact with the inner, but in turn the back is insulated so the heat will be transferred through the sheet metal (aluminum, stainless steel, etc.) and polystyrene or another insulating material avoid that the heat scape. In the picture, the grey area represent the metal sheet.
Step 4: The Blocker
If we don´t want use the lunch, then we can place a blocker, that is a plug that fits into the U-shape, is a thermal insulator of any adiabatic material such as clay, certain plastics, porcelain. In this drawing the transparent gray area represents the trunk