Most petrol engines inject the fuel into the air supply prior to entry into the combustion chamber and only need low pressure to do this since the air in the intake is only around atmospheric pressure - just over 1 bar. They still require some additional pressure to atomise but but not as much because. - The diesel engine injects fuel directly into the cylinder when the piston has already compressed the air from the intake stroke. The pressure will be in the order of 15 bar (dependant on the compression ratio). The diesel fuel must overcome this pressure, spread to the extremes of the piston area and ignite in a fraction of a second. Atomisation is the key to increasing the surface area of the diesel fuel, millions of tiny particles of fuel. Anyway that's what I believe to be the case.
