Why do internal combustion engines spray liquid gas in, when vapors are the flammable state?

Answer 1

It's easier to transport a liquid than a gas. A simple pump can move a liquid from point a (gas tank in this case) to point b (engine). It is difficult to reliably move a gas. Secondly, it is easier to store a liquid than a gas. The liquid takes less volume per pound of material. Therefore, from a practical sense, it makes more sense for the fuel to begin as a liquid in your car.

The next issue is the combustion process itself. As another has mentioned, you need a rather exact mixture of air and fuel for the combustion to occur properly. Too much air and the engine runs too hot. Too little air and you can flood the engine. In between, you can have detonation, carbon/"gunk" build-up, etc. None of these things are desirable. Therefore, a small range of air to fuel ratio is needed for different conditions (it's not always the same). However, while there is a small range that will work, there is an even smaller range that is the optimum. In order to maximize the fuel efficiency vs maintenance vs initial cost of building, etc, you want to have a very tight control over that ratio. To do that, you must have good control over the fuel injection rate. Just like it can be hard to move a gas, it is also hard to know how much of a gas you're moving. For a liquid, it is simple. mass = density x volume. Since the volume of a liquid changes only very slightly with temperature and almost not at all with pressure, you can be relatively sure of how much you have. (It would be prudent to point out that the proper combustion is determined by mass/moles, NOT by volume) So, for a liquid it is relatively easy to determine that you are moving the proper amount. However, for a gas, the amount of material is determined by a gas law. The ideal gas law (PV=nRT) shows us that the amount of material (n, moles) is:
n = PV/RT (to get mass from this, you multiply n x molecular weight)
So we can see that the amount of material is going to depend on pressure, temperature, and volume. If we can control volume, we still must know pressure and temperature to use the proper amount. Therefore, to get the proper amount of fuel in the engine, it is much easier and more exact to use liquids.

But, because we want to use a liquid, we have thus brought up the issue that it is the gas form of the petrol that burns. This is why it is injected in tiny droplets. Gasoline/Petrol will evaporate (eventually) under standard temperature and pressure. If you pour some out on the driveway (and I don't advise this, since it costs so much right now) you'll see it slowly disappearing. Evaporation, however, only occurs at the surface of the liquid. Thus, any method that increases the surface area of the liquid will increase the rate of evaporation and will increase the chance of fully utilizing the fuel. Spraying tiny droplets is one such method.
You can easily prove this mathematically:
If you have some volume of liquid petrol and you assume it takes the shape of a sphere. If you instead break that up into 100 smaller spheres of 1/100th the volume of the original, the surface area of all the spheres is 4.6x greater than the surface area of the one, large sphere. This is why the fuel is injected as a mist. The mist helps to ensure that the petrol will evaporate quickly and efficiently.

Answer 2

And it still burns fossil fuel hydrocarbons. Still releases sequestered CO2 into the atmosphere. Long term reason we need to get off oil isn't because gasoline doesn't burn that cleanly. We need to get off oil because it is finite and releases sequestered carbon causing global warming. "you realize don't you that you are undermining Algore and his lies, right?" No, none of those claims discuss CO2 emissions. Unlike "standard" pollutants it isn't toxic and won't build up in an engine. Still gets released. As long as you burn carbon you will get CO2. The only way to make this be a nonissue is to make sure the carbon source was already in the carbon cycle. Biofuels of some sort.

Answer 3

Not flammable until mixed with Oxygen (AIR), and it must be in the right proportions to support "Internal Combustion". An explosion must take place in each cylinder, you can't just pour flammable vapor into a cylinder and expect it to work, their must be an explosion for "ENERGY".

Answer 4

Caburettors and fuel injection pumps for liquids are very compact compared to equipmemnt designed for gaseous / vapor state, and finely divided fuel droplets do burn quite easily.