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Benz Veggie Conversion: The Nuts and Bolts


With the domestic auto industry collapsing around us (and keeping bloggers typing away madly), I've had little time to report on the host of mechanical changes I've been making to the 1982 Mercedes 300TD Wagon I'll be driving to L.A. in December. I've been underneath the oily old diesel every weekend making mechanical updates and preparing for my cross-country trek.

First up were fixes to get the 330,000-mile stalwart wagon operating at its best before setting sail from Atlanta to Los Angeles. Tightening the steering box eliminated most of the steering play and made the car much more pleasant on the highway. Replacing one of the rear struts on the self-leveling suspension eliminated a loud clunking from the right rear (and ensured proper self-leveling operation). And removing the dash, replacing the heater core, rewiring the HVAC controls, and ensuring proper vacuum to the entire system (and then putting it all back together--ugh) gave me heat and ventilation for the first time since buying the car in 2006.

Making the car run on used vegetable oil is remarkably simple. Pictured and described below are the six main mechanical items needed to turn my ride from a petroleum-fueled vehicle into one that will run on waste vegetable oil (WVO), biodiesel, or traditional diesel fuel.

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Colin Veg Conversion #4

Colin Veg Conversion #4

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#1) 16-plate brazed copper heat exchanger. Vegetable oil only needs to be heated to be burned in a diesel engine. It is too viscous at room temperature to be properly vaporized by the injectors, so heating it to about 180 degrees Fahrenheit does the trick to make it thin enough for proper burning in the engine. The unit above takes hot coolant from the Benz's radiator, cycles it through a metal matrix of tiny passageways, and returns the coolant to the radiator. Pressed right next to the hot coolant passages is another matrix of passageways, these with vegetable oil flowing through. Heat is transferred from coolant to vegetable oil, heating it quickly and preparing it for duty as fuel for the turbodiesel inline five.

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Colin Veg Conversion #6

Colin Veg Conversion #6

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#2) Heat Exchanger + Fuel Filter. Right before feeding it to the engine's injection pump, it is wise to heat the vegetable oil once more to ensure full atomization in the combustion chambers. Additionally, filtering the oil again ensures that no impurities enter the engine. The unit above functions similarly to the 16-plate heat exchanger above (#1), but features a smaller heat exchanger and a large-capacity, 4-micron oil filter that filters any impurities out of the oil. Engine coolant from the radiator is again used in the lower metallic chamber, and vegetable oil enters and departs at the base of the unit through the two copper fittings. After passing through this unit, the waste vegetable oil is ready for combustion.

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Colin Veg Conversion #5

Colin Veg Conversion #5

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#3) Fuel and Coolant Hose. To route vegetable oil from the gas tank, through both heat exchangers, and on to the injection pump, a 3/8-inch fuel hose (smaller spool in the middle) is used along with hose clamps to ensure the system is sealed. All coolant to and from both heat exchangers is routed through 5/8-inch coolant hoses (the larger hose on the outside).

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Colin Veg Conversion #1

Colin Veg Conversion #1

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#4) Continuous Duty Solenoid. A purge tank system is used so that the engine may be started and shut down on diesel or biodiesel. As the radiator is not hot upon startup, diesel or biodiesel must be used on a cold start until the radiator is sufficiently warm enough to heat the vegetable oil through the heat exchangers. This solenoid is triggered by a dashboard switch operated by the driver. When the switch is flipped on, the solenoid sends power to two fuel switchover valves that route fuel from the main fuel tank where the waste vegetable oil is stored. When the switch is turned off, the valves route fuel instead from a purge tank that has approximately two gallons of diesel fuel or biodiesel.

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