I take it that a conventional engine cannot be converted to biodiesel?
The short answer is no — you cannot use biodiesel in a conventional, unmodified gasoline engine. However, I’ve learned my lesson about giving car advice in this space, so the longer answer is: Anything can be done if you set your mind to it. If you want to change out a gas engine for a diesel engine, it may be possible. And it may be possible to replace various integral parts of your gasoline engine so that it runs on diesel fuel. You could also apparently do a conversion to use another alternative fuel (electricity, natural gas, etc.).
Contrary to my advice from several years ago, putting an efficient modern engine in an old muscle car is also within the realm of the feasible. I also have heard of using horses to pull a non-motorized wheeled contraption, of using billy goats to pull a sub-compact cart (often done when the mockingbird will not sing), and of replacing diesel tractors with a team of oxen.
I’ve written about how gasoline and diesel engines work multiple times, and I felt a sense of personal victory in understanding the concepts enough to be a convincing descriptor. But boy, plastics recycling questions start to look pretty enticing once we get into these “What kind of crazy thing can I do with my engine now” kind of questions.
To review: Gasoline and diesel are separate products of petroleum refining, and diesel has a higher energy density than gasoline. In both gas and diesel engines, there is a chamber where the fuel explodes. But the design of the two combustion systems and the design of the engine and transmission are fairly different and not interchangeable (read more Umbra ramblings on diesel and gas engines here). This is in part due to the difference in energy density, as the engines are built to cope with the properties of their fuels, explosive and otherwise.
All that said, it is apparent from what I’ve read that if you are handy and crazy enough with cars, almost anything is within the realm of the possible. However: For us regular folks with regular, unmodified cars, it’s best to run diesel fuels in diesel engines, gas in gasoline engines, and alternative fuels in the engines that suit them.
On the positive side, if you undertake a conversion and it doesn’t work, that would be one less car on the road, which is of course our ultimate goal.