First let me describe the history /origins of the combustion engine briefly. Someone had the idea of harnessing a series of explosions of a gas to power an engine. The engine was designed and it was found that petrol supplied the fuel for the explosions. That is grossly over simplified I will concede.
What I want to know is :
What was petrol doing lying around waiting for the invention of an engine to use it.
if not how could an engine be designed if a fuel did not already exist?
Did the fuel dictate the engine? or did the engine demand the fuel?
Discuss.

Actually the first internal combusion engines used a mixture of hydrogen and oxygen, after the model of internal combustion was proven it was then a case of improving the model, and yes the use of petrolium as a propellant did dictate the new designs. So it's kind of half and half to your last question

In other words - they evolved in a symbiotic relationship

Animavore wrote:In other words - they evolved in a symbiotic relationship

Indeed, I find it ironic that the first model used hyrdrogen, and yet here we are a century later coming full circle and using hydrogen again.

Amergin wrote:What was petrol doing lying around waiting for the invention of an engine to use it.

if not how could an engine be designed if a fuel did not already exist?
Did the fuel dictate the engine? or did the engine demand the fuel?
Discuss.[/quote]
I am most confused.

Whats the problem?

It was the chicken!

Natural oil existed in the ground long before humans did. Gasoline/petrol, a refined product derived from that natural oil, did not. Humans found the natural crude oil and started tinkering with it to find out what it would do, including modifying it and separating & extracting its ingredients from each other. Then they found uses for some of the products they could make from it. It or its extracts & derivatives were being used to slow or stop bleeding (in the form of petroleum jelly, like Vasoline), and in asphault for large buildings, and for waterproofing of wooden boats (in the form of pitch), and in burning for heat and light, all by at least centuries, if not in some cases millennia, before the alleged birth of Jesus.

Candles apparently were easier & safer to store & handle in a low-tech world, and were probably easier to make in a society where most people are farmers, so crude-oil-derived lighting oil wasn't particularly common for a long time, but it was known to work. By the time the whale-oil lighting industry went looking for an alternative to whale oil in the mid-1800s, it had already been known for ages that a decent lighting oil can be made from crude oil with the right refining process, so it was just a matter of tinkering with it some more to find out whether an even better one could be found for mass production for an industrializing society. So it wasn't long before a handful of similar but still somewhat different kinds of fuel derived from crude oil, such as kerosine and gasoline and diesel along with some others, had been produced and studied to see which would work best as lighting oil where whale oil had been used before.

By the time internal combustion engines came along, plenty of data about exactly how these various crude oil derivatives burn and how to re-create them at will had been recorded, including not only the one they had finally selected for lighting but also the similar-but-different rejects. So the engine's inventor(s) only needed to know about the experiments that had been done by the lighting industry and think "I'll bet one of those would be pretty close to what I/we need here..."

The first drivers had to stop at pharmacies, and buy the stuff in bottles.

Wasn't the external combustion engine aka the Sterling Engine reasonbly popular in the early days of engines?

Steam was of course the big success story before the internal combustion engine took off.

I believe the Diesel engine would have been the next big thing if it weren't for the much heavier construction required due to the high compression ratio, it was designed to run on all sorts of biofuels, not the oil based fuels we call 'diesel' now.

Petrol took off because it made for a lighter, higher revving, more flexible engine, thus better for mobile engines rather than stationary or semi-stationary engines.