Here one mnute, gone the next

The ignition load on an SI engine isn't "that" significant - you can get a battery operated Tazer that'll produce quite a significant spark for a fair length of time off of a few cells.

You can flick the magneto on a small engine and get two significant sparks with only a "few" joules of energy.

Higher calorific values, higher compression ratios and more effective operating temperatures of CI engines and diesel fuel are more significant.
 
Think I will stick with my cougar then as I only do 5000k a year as I would have died before I break even with a diesel lol

To be fair, that's probably the right choice.

:edit: Using my baseline figures from above and assuming you paid a grand for your Cougar, it's 84.5 years. Which may as well be eternity.
 
Diesel's days in the sun are coming to an end. The modern small engined petrol turbos are coming on leaps and bounds for those whose first consideration is MPG.