One thing we know. We can’t use fuel in liquid form during compression, because liquids are incompressible. Now we have option to use diesel in vapour form. At atmospheric pressure, boiling point of diesel is 250°c to 300°c. And diesel in fuel tank have temperature around 40°c to 50°c. So it is not possible to make vapors (as in carburetor of petrol engine).So, how can we convert diesel into vapour form?There are three options: 1. If we put liquid diesel into a medium having pressure less than vapour pressure of diesel, it means we create vacuum inside the cylinder. During suction stroke we have vacuum and we can convert the diesel into vapors, but during compression stroke we must avoid auto ignition due to high compression ratio. The one and the only way to avoid the auto ignition in such case is to reduce the compression ratio. Which is not possible (2nd law of thermodynamics, higher is the pressure more is the power) if we want to get power from engine. Another important reason for using higher compression ratio is to get higher expansion ratio (more piston displacement) thus more power with same fuel consumption.Work done = pressure (mean effective pressure) × piston cross-sectional area × piston displacement.
But still if someone manages this miracle, how he overcomes condensation of diesel during compression stroke where the pressure goes up again. So drop this option. 2. Increase the temperature of liquid diesel before putting into engine cylinder which is possible but not reliable. We have to use pre-heaters, which cost energy consumption and makes vehicles bulky. 3. Put liquid diesel into high temperature air (greater than saturation temperature of diesel) and provide some time for heat exchanging between them.
In practical air standard engines third option is perfect as we produce high temperatures by compression. But still we can't use spark plug for ignition, since mixture present in the cylinder is heterogeneous (somewhere mixture within ignition limits and somewhere not). Ignition by spark plug required homogenous air fuel mixture. Now we have only 1 option to ignite the diesel, that is; compresses the air and raised the temperature of air up to ignition temperature of diesel and inject the diesel in cylinder.