Boiling temperature of Acetylene is -84°C and that of Petrol is 95°C (at Atmospheric Pressure). So the spark is efficient to ignite the compressed Acetylene and run the Engine. But why wont that work ? Is any other factor that I miss or misunderstood ?
There are certainly a few reasons why acetylene is not a very practical fuel for an IC engine.
Perhaps the most important is that it is inherently unstable and tends to explode under pressure, this makes it much more difficulty to store as it must be dissolved in acetone in an inert matrix, unlike say propane which can just be liquefied. This means that refueling would probably involve physically swapping containers and is exacerbated by the fact that it is quite aggressive on things like hoses and seals. These added complications with safe storage also take a big bite out of it's practical energy density.
With any Otto cycle fuel the requirement is not just that it can be ignited but that it can be ignited at the right time and a lot of chemistry goes into formulating gasoline which ignites reliably when a spark it applied but not before. Premature ignition (ie. during the compression phase)can wreak havoc on a piston engine, even worse is premature ignition in fuel pumps or injectors. Acetylene also burns significantly hotter than gasoline which presents a problem for combustion chamber design (hydrogen presents similar problems). Similarly piston engines require controlled combustion rather than violent detonation.
There is also the consideration that carrying pressurised acetylene around in a vehicle would represent a serious hazard, far more so than a tank full of gasoline. Gasoline is very unlikely to actually explode in a fire whereas acetylene tanks have been known to level entire buildings.
There is also the fact that acetylene is expensive and energy inefficient to produce as it is obtained by decomposing calcium carbide, which itself must be manufactured, as opposed to fuels obtained by fractioning petroleum.
So, while it might not be impossible to develop and engine which ran on acetylene there isn't really any reason why you would want to. Bearing in mind that the main reason acetylene is used is for its very high flame temperature with oxygen and it doesn't have significantly greater energy density than other liquid or gaseous fuels.
As an aside to this one niche use for acetylene is in lights for things like caving where it is generated on demand by dripping water into a supply of calcium carbide (not carbonate as I originally wrote) and burned to produce bright white flame, as this is a reasonably compact way to produce a long duration, bright light. Although the advent of LED lights and lithium batteries have rendered this, if not entirely obsolete then at least less attractive in many circumstances.
The main application for acetylene is in oxy/fuel welding. Here alternative fuel gasses such as propane can't provide the correct flame chemistry for the process to be self shielding to exclude oxygen from the base metal ie it is relatively easy to set up an oxy/acetylene flame to consume all of the available oxygen while maintaining a flame temperature hot enough for welding. However for similar processes such as brazing and flame cutting propane is often preferred as it has adequate flame temperature but is significantly cheaper as well as being safer and easier to handle.
It is also worth noting that hydrogen presents some of the same problems as acetylene in terms of storage, handling and combustion. But is of more interest as hydrogen potentially represents a non-petrochemical alternative to fuels like gasoline and natural gas which can be produced either from surplus electrical power or bio-reactors.
Due to the dangers involved with storing and transporting compressed or liquid acetylene, you would probably have to make use of an onboard acetylene generator, fueled with calcium carbide.
Acetylene generators are safe, but generally produce hot gas, which reduces specific power, and has a tendency towards preignition and even backfiring.
So, it can be done, and the barriers against doing so are fairly minor.
Obviously you wouldn't be able to mass produce your car, as the automobile industry is, in some ways, extremely conservative, but there is nothing at all to prevent you from building one or two such cars.
We prepared a bike propelled on acetylene for studying such problems. We found it was possible to do do, but some of the factors that govern the operation of acetylene powered engines are: cooling of the gas, proper mixing of gas and air and satisfactory cooling of the engine.