It's the KOH!

Making quality diesel ain't simple and one really needs a good grasp of the chemistry involved rather than just following a recipe.

It doesn't matter too much in an IDI aslong as the viscosity isn't too high so as to damage the pump.

In simple terms, you add methanol to the vegetable oil and heat the two together in the presence of potassium hydroxide. This is the process of transesterification.

Vegetable oil+methanol +(base catayst) = glycerol+methyl esters

The methyl esters are the wanted 'biodiesel'. They sit on top of the by product glycerols which are tapped off.

The resulting fuel is more 'diesel like' than vegetable oil, but it still isn't diesel. There are two main problems with b100 (100% biodiesel):

• It is very viscous when cold-and hence does not flow well and can damage high pressure fuel pumps. The boiling range of biodiesel is higher than diesel so post injections used for DPF regens tend to end up on your cylinder bores and not in the exhaust stream...
• Biodiesel contains oxygen. This is why biodiesel creates less soot than normal diesel. Unfortunately, it also reduces the energy content of the fuel and reduces the high temperature stability of the fuel which can undergo polymerisation in the accumulator rail and injectors.

Cost effective? Well starting at ~£1/litre for vegetable at the supermarket-don't even think about it! You won't break even.

For biodiesel to be cost effective, you need a source of waste vegetable oil-and that's a niche sector.

I ran commercially bought B80 in my Astra 2.0DTi for 20,000 miles, not a problem.

That would be B100!

How to run your car on chip oil: news.bbc.co.uk/1/hi/sci/tech/7047128.stm

It's like the log burning stove phenonemon: everyone likes the idea of a cosy stove with cheery carbon neutral flames beating off the winter blues. But then they realise just how much wood it takes to run one. And once more than a few people buy into the concept, old farmer MacTaggart isn't interested in giving you a year's supply of seasoned timber in exhange for a bottle of whisky at christmas. Then you get the unedifying reality of people stealing wood from other people's gardens and forestry commision land.

There just isn't enough waste veg oil to make a dent in diesel consumption. For a lucky few, it makes a nice pastime though!

Older pre cdi mercedes diesels run great on bio. In fact, any car with indirect injection should be fine. The viscosity of my bio is no greater than normal diesel in this weather. I save loads driving my landy round, as she only does 20 to the gallon!

P.S. If you buy ready made B100 on the basis of it being slightly cheaper than regular diesel, think twice. The calorific value of B100 is about 15% less than ordinary diesel-that means you'll be get about 15% less mpg.

9% lower calorific value, but higher cetane value, so it works out pretty much the same.

@unthrottled -- complete tanget, but here's a copy of the spec sheet for V-Power Diesel. Dates back to 2007, but one or two interesting differences. No bio content and very low particulates. http://www.vectra-c.com/forum/uploader/pics/picsNov2007/Shell%20VPD.JPG

How is cetane rating rating a substitute for energy?

Higher cetane means it burns quicker, so less energy is wasted as half-burnt reek going out the exhaust valve...