My car, the design project

So I have come almost full circle and am now sponsoring a senior design project in the Mechanical Engineering department here at school. The project is to convert a Mercedes diesel car to burn straight vegetable oil.



As you may or may not know (or have guessed after that first sentence), I recently purchased a 1984 300SD turbodiesel Mercedes Benz. It's a very nice car still, with power leather seats and a sunroof and so on. Most importantly, it has the Mercedes OM617 diesel engine, which is widely regarded as one of the most durable diesel engines in the world. Ever.

Because of this fact it is one of the most popular for "grease car" conversions, the science of which I will briefly cover below.

Basically, the vegetable oil (VO) that is used as fryer grease in restaurants all across America is a perfectly viable fuel for most diesel engines as long as it is heated up to around 160 degrees F. This is necessary because at colder temps it is too thick to be successfully pumped through the fuel system or sprayed into the cylinder through the diesel injection nozzles. It may burn fine in the engine if you manage to get it there, but not efficiently and not without damaging the engine. This is mainly because the system has been optimized for the viscosity of petroleum-based diesel fuels. (The original diesel engine was developed to run on peanut oil by Rudolf Diesel). So you have to heat the VO. This isn't too hard to do really, since car engines produce tons of heat that we attempt to dissipate from the radiator via the coolant system. Some of this heat just needs to be used to heat the VO too. The system that the students are going to make for my car will have 2 tanks of fuel. When the car is first started it will be on regular diesel fuel or biodiesel. It will then run on this for a few minutes while the engine heat being generated heats the VO. Once a suitable quantity of VO has been heated, some valves will be switched and the engine will start burning heated vegetable oil and the exhaust will start to smell like popcorn.

The benefits of burning grease are large. For starters it produces more then 75% less carbon dioxide, which is good. It also produces around 80% less of things that cause cancer. Another important benefit is that fuel will cost between 0-$1.50 per gallon, depending on whether I scavenge and filter it myself or buy it from someone else. My VO tank will be 21 gallons, add half a dozen 8 gallon gas cans and that's a lot of traveling that can be done without having to buy any $3.20 dollar gallons of fuel. Sounds great huh? Well it's not the solution to out problems by any means, as there is only so much oil going to waste out there. Once the local supply is gone, the advantages become fewer.

There are a couple of drawbacks to burning vegetable oil, or biodiesel for that matter. One is that it has a slightly smaller amount of stored energy in it, which translates to around 5% fewer MPG then petroleum diesel. It is also a harsh solvent that likes to dissolve rubber, which in lesser engines then the OM617 can be a problem. I've been burning biodiesel in my car for a while now, the first thing it did was dissolve a bunch of the residue that the petro-diesel had left in the fuel system and clogged the fuel filter. After replacing that it's been trouble free. Back to the senior design project though.

There is more to the project then just my car. It is the first part of a larger project headed up by Kevin Cook and the local ASHRAE club that hopes to get a large federal grant to convert many of the MSU fleet vehicles to run on waste fryer grease from the MSU dining halls. (which produce about 200 gallons of waste oil a month that is currently being "recycled" into things like dog food on the west coast somewhere.) It makes a lot more sense to cart the grease 100 yards across campus and then run cars on it then to ship it in diesel trucks across the country. My car is the experimental one, where they try to make a system that is totally reliable even in the sometimes arctic Bozeman winter. I have faith they will, but I'm not going to leave them alone with it quite yet.