Modification of a BMW 525 TDS (E34) with engine M51 for use with vegetable oil (rape oil)
As can be seen from the drawing below, the system involves only one fuel tank. This is because of the hassle with two tanks.
The fuel is delivered by the pump of the tank via a non-return valve to a plate heat exchanger. The heat-exchanger is a sparepart
from the Audi A8 2.5 TDI, there it is used to cool the return line of the diesel injection pump.
The heat-exchanger gets its temperature from the engine coolant, which has about 80°C when the engine is warm.
A solenoid valve controls the temperature of the heat exchanger. If the temperature is to high the solenoid closes a short
circuit around the heat exchanger thus cooling the heat exchanger. This short circuit is also used in the start phase, here a water
heater heats up the water in the short circuit and thus the heat exchanger.
Compared to ecessive direct heating of rape oil this method provides for a gentle heating thus preventing problems with cracking oil.
The auxiliary water pump is used to force water flow through the heat exchanger. To save on operating hours the pump is only operated
After beeing warmed by the plate heat exchanger the rape oil is fed into an electrically heated diesel filter.
This diesel filter is a modified version of the series dieselfilter with build-in heater. In series state this heater is intended to heat up very cold diesel in the winter.
For this application a heater switch-off temperature of about 7°C is sufficient for the diesel-filter. For use with rape oil the switch-off temperature needs to be modified. In the section below this modification is described.
Now the rape oil is warm enought to be injected. So it passes a temperature sensor for process-control and enters the original Bosch injection pump. The return line of the injection pump is fed to a solenoid valve which can be switched to short the return line to the supply line thus
saving valuable heat in the start phase. Air that may be in the line is removed by the optional air escape valve "tigerloop" which can be found on many oil heating systems. A non-return valve stops the rape oil from traveling back to the tank via the supply line. To provide a save escape for pressurised oil the solenoid valve is bypassed by a pressure control valve. Thus ensuring a maximum pressure of 0.5bar in the system which prevents leakages.
The whole process will be controlled by a specially build ECU which will be designed later on, as the modification project advances.
Beeing greedy, the car was operated with a mixture of 50% rape oil and diesel in the summer time without problems.
Now the first steps, the modification of the electric diesel filter heater and the installation of the plate heat exchanger will take place.
A special ECU which will be build for the vegoil-project to control all solenoids and the pump.
The xfig file is here. There is also a pdf-file.
Lowering the viscosity of rape oil by heating
To be able to inject rape oil we need to heat it up to approx. 60 deg. C. This can be done using a plate heat exchanger using
the coolant of the car. Though beeing a economic solution this popular method does not work if the engine is cold.
So an electrical heater is needed. Some people tend to buy an electrical heater for that purpose.
Fortunately, my BMW (and the other models with M51 engine (325tds, 725tds, Opel Omega as well as other brands should have a similar
fuel filter heater) has an automatic filter heater which is designed to heat
up cold diesel fuel in the winter to allow for a trouble-free start.
As this filter heater has its own little ECU which is build into the filter cap, one can modify it for vegetable-oil usage.
The heater consists of a heater-element (which should be of ptc-type) and an electronic thermostat that switches on the heater if
the temperature of the fuel is low enough and if the battery is high enough.
The following photo shows the fuel-filter and the little ECU.
After removing the pcb from the filter-unit the small circuit has been reverse-engineered, and I mus say, it is a
very cost-optimized circuit that has some tricky properties:
May be, the last two properties were intended by the manufacturer, but even if not, this little circuit has a chance of scoring in the
contest for doing a lot of stuff with minimal effort.
- If the battery voltage is low, the heater will not be switched on
- Dependend on the battery-voltage the switch-off temperature of the heater varies (higher voltage higher swith-off temperature).
- The hysteresis of the heater controller (heater-on) lowers the trip-point of the battery-monitor
Here comes the circuit-diagramm of the little ecu with comments and modifications.
filter_heater.sch (gschem schematics file)
The following picture shows the reference-designators of the components chosen in the circuit diagram.
The fullsize-picture can be downloaded here.
Note: If you use the gschem-schematics file, be sure to download my modified component library in the