Compression required to &quot;diesel&quot; petrol (gasoline) is <i>lower</i>, which is why some autos have the problem of continuing to run (&quot;diesel&quot;) after the ignition (and spark) are shut off.  Most now incorporate other controls and limiters, such as controlled fuel injection, to eliminate this.<p>Gasoline is comprised of a shorter set of hydrocarbons (mostly 6, 8, and 10 chain) than diesel (about 12-16 chain length, though 8-21 carbon are listed at Wikipedia).  Volatility (and ignitability) <i>increase</i> with shorter chains -- methane (C2H6) and butane (C4H10) -- that is, two and four chain hydrocarbons -- are both gaseous at or near room temperature.<p>Longer chains still get you heavy fuel oils, tars, etc.