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Martin's guide to running a Diesel Engine on Straight Vegetable Oil (SVO)

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Me and car.jpg (112443 bytes)

This is me, in the summer of 1998 with my 1989 Citroen BX DTR Turbo. Since this was taken, the car has had a new front bumper, a 75% respray (including the bumpers), added about 20,000 miles (making a total of 174,000), a hefty accident, an almost complete loss of value, and been replaced by my lovely 1999 Xantia HDi Exclusive. Oh, and I have a better pair of sunglasses.

The accident involved an enormous impact to the gearbox which shattered the casting around the engine mount, and snapped the nearside driveshaft. Apart from this and a few minor problems, the car was fine, so I decided not to scrap it, but get it back on the road, and use it as a second car. After 8 months immobile on my parents driveway, it finally turned a wheel on its own at the end of March this year (2001). I now intend to modify it to run a dual-fuel system between normal Derv from the petrol station, and SVO.


The Engine:

It's a 1769cc intercooled turbo-diesel XUD engine found in both Peugeots and Citroens. Independantly tested as one of the most reliable engines ever produced. It's been made in 1.7, 1.7TD, 1.9, 1.9TD and 2.1TD flavours. Basically any Citroen or Peugeot produced in the late 80's and throughout the 90's will have one of these, EXCEPT the new HDi's.

The engine is supplied with fuel by an injection pump which runs off the cam belt. This wholly mechanical pump does the work of drawing the fuel from the tank and pressurising it through the injectors. It also returns non-required fuel back to the tank (more later). It's a gloriously simple arrangement and the only way to stop it is to cut the fuel.


How can it run on Oil?

Vegetable oil is composed of a substance very similar in chemical composition to petrochemical diesel, and glycerine. Chemically, you can remove the glycerine and be left with the methyl ester, which we know as Bio-Diesel. However, with some not-too-complicated precautions and modifications, the engine can be made to burn the oil, glycerine and all. Obviously I can only speak with authority on this engine, but any indirect injection (preferably mechanically) engine is suitable for conversion.


The Rules:

1) The oil must be heated to 80 degrees celcius or higher before being allowed to the fuel pump.

2) Oil must not be allowed to cool inside the pump or injectors. The engine must be started and warmed up on derv.

3) As cold oil is considerably more viscous than derv, you must not burden the fuel pump with the work of pumping the oil round the car.



1) The oil must be heated to lower the viscosity to within the pump and injector design parameters, and to give complete combustion of all glycerine content without waxing up the cylinders. At 80 degrees, SVO has about the same viscosity as derv at 10 degrees. Methods discussed include electrical heating, and using exhaust heat. However, the most favoured involves routing engine coolant through a heat exchanger in the fuel line. I personally employ an auxiliary electrical element to assist.

2) A solenoid-controlled changeover valve is used to switch between derv and vegetable oil when the vegetable oil is hot enough. The supply is switched back again for a time before shutting off the engine to allow derv to flush through the pump and injectors. The fuel pump runs from the cam belt and therefore pumps fuel in direct proportion to the revs of the engine. Unfortunately the amount of fuel required by the injectors is relative to the speed of the engine AND the position of the accellerator, so the fuel pumps more fuel than is required, and returns unwanted fuel back to the tank.This function also serves to rid the fuel system of any air bubbles. Unfortunately you MUST NOT send any SVO return to the derv tank. However the opposite is acceptable.

3) This is the difficult part some have chosen to ignore. Due to the bleeding function of the return pipe, you should not send the return strait back to the supply. The following is courtesy of a Vegoil-Diesel Mailing List member and he certainly has me convinced!

The tail of the knackered pump made me wonder if I was hurting my
pump so I put a vacuum guage on the input of my injector pump here's
what I found. I let the engine come up to normal temp and hit the
switch. On diesel, with the return line going back to the tank, the
guage read -1.25 psi. When I switched to vegy oil the vacuum jumped
to -2.5 psi.. When the injector pump reached 175 F the vacuum dropped
back to normal. This was with the return line going back to the front
of the pump. With the return line in "return to vegy tank mode" and
the system just getting warmed up the vacuum jumped to -10 psi. and
kept climbing. With the injector pump at 175 F and return line in
"return to tank mode" the vacuum was at -5 (4 times the norm) and
wouldn't get any better. The injector pump sends about four times as
much as the engine uses, back to the tank through the return line.
Pumping all of this fuel to and from the tank obviously works the
pump very hard and probably requires a helper pump. I hereby proclaim
a vacuum guage to be required for all "real" SVO burning vehicles. It
can tell if there is a clogged fuel filter or line and if one is
running out of vegy oil. I don't know how I got along without one for
so long. Luckily I wasn't sending the return line back to the tank or
I would probably be replacing my knackered pump.



I don't know exactly yet, but I'll post details of my conversion as I perform it. In the mean time, please join the Mailing List and get talking to other SVOers!