Pre-Heating System

      The pre-heating system is a two stage system where waste heat from the exhaust is used to provide the bulk of the heating and an electrical heating element is used to maintain the desired input temperature range.   Figure 5 shows a flow chart for the system and how the various components function together.

Pre-Heating Flow Chart

Pre-heating System

      A plate heat exchanger (Shirron Brazed Plate Heat Exchanger) is installed to use waste heat from the engine exhaust as a source of heat for the fuel (jatropha or canola).  The pre-heating loop is comprised of a closed loop of 7/16th inch copper tubing filled with propylene glycol, which is insulated with rubber.   Part of the copper tubing loop is wrapped in coils around the exhaust pipe of the engine and covered in insulation creating a heat exchanger. Propylene glycol is chosen because of its resistance to burn at temperatures reaching as high as 185°C 2 its high boiling point and “is generally recognized as safe”3 in terms of handling and fume exposure.   

      A simple ball-valve is used to bleed the propylene glycol into the tubing.  The heating loop is also equipped with an expansion tank to allow for expansion of the propylene glycol as it heats up.  An Omega Engineering Commercial Pressure Gage 232-01 (160psi max) is also attached for safety reasons to allow the user to see if there is a large pressure build up that could indicate a failure of the expansion tank.  A military grade bearingless Facet Electric Pump operating on 12V DC circulates the propylene glycol. 
A custom shelf setup is created to keep the heat exchanger system compact and manageable.  The shelf has room for the plate heat exchanger, the pump, the expansion tank, the pressure gage, and allows for logical placement of the inputs and outputs for the fuel and the propylene glycol.

Copper Coils around Exhaust Pipe

      Propylene glycol was chosen due to its resistance to burn at temperatures reaching as high as 185°C 3 and “is generally recognized as safe”4 in terms of handling and fume exposure.  As the propylene glycol circulates through the coils wrapped around the exhaust, heat from the exhaust is transferred to the liquid.  The coils are covered in ceramic fiber insulation to allow the maximum amount of heat to enter the propylene glycol.  As the liquid passes into the plate heat exchanger, alternating plates of propylene glycol (heated) and SVO (from the tanks) pass each other and heat is transferred from the propylene glycol to the SVO. 

      The maximum amount of heat passed from the exhaust to the SVO is proportional to the number of copper tubing coils wrapped around the exhaust and the distance the heated propylene glycol must travel to get to the heat exchanger.  The distance traveled by the propylene glycol is fixed at the minimum possible distance given the engine configuration.  Presently, we have allowed for an adjustable number of coils by separating the “coil section” of the tubing from the rest of the loop using compression fittings at both ends. 

      If the amount of heat transferred to the SVO from the heat exchanger is too low once normal operating temperatures are reached, the coil section of the loop can be disassembled and a new set of coils can be added.  Preliminary estimates can be made via heat transfer analysis; however, only experimentation can determine the final setup.  Originally, five coils were used, but that was changed to 22 coils.  The increased number of coils led to drastic improvements in heat transfer to the fuel.