Fuel system diagram

A few people have asked me to publish a diagram of the fuel system used. Well, here it is. My apologies for the rather rudimentary sketch. I'll try to improve it one of these days.

There are a number of possibilities available for a twin tank conversion such as this, and there is no such thing a the "perfect system". There is always a compromise of some sort. I’ve found that my system works very well. Initially, I was concerned that this particular system would be prone to air intolerance, but as it turned out, it is very good in this respect. In fact there are NO air leaks at all, and even after the car has been left standing for a few days, the system remains perfectly primed.

Initially, this was not the case, and I was going crazy trying to eliminate the air when bleeding the lines. I refitted all the fuel lines and retightened the hose clips, so I was satisfied that the air leak was not from poor attention to the plumbing work. The source of the air leak turned out to be the design of the 3-way return line banjo fitting on top of the stock Mercedes fuel filter. The fitting has a hole through the middle of the screw so that air was entering from the canister. It was easily fixed using some metal epoxy to block the hole. Once that was done, priming using the stock priming pump on the side of the IP was a cinch!

Note that the spill line for the diesel circuit goes back to the diesel tank, whereas the the spill line for the UCO/WVO circuit re-enters the UVO/WVO line just before the heat exchanger. This helps to conserve a small amount of heat.

A separate filter for each fuel system ensures that changeover times are minimised. A one-filter system would require that the entire contents of the filter would need to be emptied before effective changeover occurred.

No electric heaters are employed at this stage. I wanted to avoid placing additional load on the alternator, as a decent electric heater would draw something like 200-300watt.

You will also notice that I have not used a separate fuel pump in the WVO system. I’ve found that the stock lift pump is quite capable of the task.

One disadvantage of my system design is that the vehicle must be started and shut down on diesel fuel. There is always the possibility that the driver could forget to switch over before stopping for extended periods, thus leaving UCO/WVO in the IP and injectors. While I've found that this is not really a problem and re-starting from cold is still possible on WVO, albeit a little more cranking is involved, this is not really ideal because it can cause coking of the combustion chambers.

Overall, I’m quite happy with it. I’ve found that the 300D seems to reach operating temperature quite quickly, and that I am able to switch over to WVO fuel after about 3 minutes. Change overs are not noticeable at all…

It all comes together... Conversion complete!

The final stage of the veg-oil conversion of the 300D has been completed! This stage was almost all plumbing work. It now runs on

• Diesel,
• any new vegetable oil,
• any used vegetable oil,
• biodiesel or
• any combinations of the above!
  

The veg fuels are filtered to at least 5 micron before filling the tanks and then again by a pre-filter and an on-board water-removing CAV cartridge type filter.

In operation, I start the car on diesel. It takes 3 minutes for the 3litre Mercedes OM617 diesel motor to heat up the coolant suffiently to enable burning of the cooking oil to commence. I flick the on dash fuel selector switch and about 15 seconds later I’m driving along smelling like a mobile Maccas! When I stop and turn off the engine, if I’m not intending to restart within a couple of hours I’ll shut down on diesel to make starting easier. However, it will start OK on the cooking oil, but to avoid any damage to the injectors, a proper shut-down is preferable. I’m thinking of the possibility of using a turbo-timer to automate the shut-down.

Here are the final shots of the installation:

This one shows the brass tee into the heater circuit to take off coolant for the heat exchanger. I’ve discovered that quality brass fittings are expensive! This tee routs the coolant to the 26 plate heat exchanger, from where it returns it to the cooling system via another tee before the thermostat on the other side of the engine. As you can see, I can still access the oil filter quite easily, so servicing is not a problem.

This shot shows the brazed plate heat exchanger (imported from Taiwan via the US) with all plumbing connected. The coolant enters from the top and exits at the bottom of the exchanger. Fuel (waste veg oil or “wvo”) enters at the bottom and exits at the top. This is called a contra-flow setup, and makes the most efficient use of the heat from the coolant. I found that it was not necessary to move the windscreen washer tank or the self-levelling suspension system’s hydraulic oil tank, as all the hoses fitted comfortably around and under the tanks.


From the heat exchanger, the heated WVO then goes through the Delphi CAV filter and from there to the Pollak valve (US made) for distribution to the lift pump on the IP. The Pollak valve is controlled from a switch on the dash.


This shot shows the connection from the lift pump to the injection pump, or IP. The line is transparent to allow me to see if there are any air leaks in the system. Air leaks will immediately be revealed by bubbles appearing in the line. Normally this supply line goes to the stock Mercedes filter before being routed to the IP, but the fuel is now filtered before it enters the lift pump, so this is no longer necessary. The glass pre-filter on the diesel supply line is just to the left of the transparent line. This transparent pre-filter is important as it also helps to detect air in the system. Additionally, it is important as it monitors any algae (“diesel bug”) build-up in the tanks which is a natural occurrence in oil fuels, but of course must be eliminated.


It was convenient to utilise the stock three-way return-line banjo fitting on the stock filter housing to route the overflow lines back to the Pollak valve. However, to eliminate air and facilitate priming this particular set-up required that the venturi hole in the three-way fitting had to be blocked off. I did this using metal epoxy. The rear of the CAV truck-style filter for the WVO fuel circuit is in the foreground. I have been asked why I have the two filters, and not just the one? After all the Delphi does have 4 fuel ports. The simple reason is that fuel changeovers are more rapid this way, as the fuel in the filter does not have to be used up before full changeover occurs.


The WVO tank is in the boot. I have two of them for easy changeovers and handling. Honda bayonet fittings ensure easy, no-mess changing of the tanks. They are restrained by “occie” straps and a bar that slips into a recess under the tank. In due course, I may decide to use these outboard tanks for the diesel fuel and put the WVO in the main tank. At the moment, it has all to do with the ease of fuel handling, and this system allows me to refuel the tanks out of the car. It also allows me to use a brew-heater under the tank overnight in mid-winter to prevent the possibility that cold temperatures could gel the WVO.