Car (South Africa) - - TECH -


There are many rea­sons why diesel-en­gined ve­hi­cles achieve bet­ter fuel econ­omy than their petrol equiv­a­lents. The fact that diesel fuel is more en­ergy dense per litre is a good start, but the main rea­sons are that diesel has a higher com­pres­sion ra­tio, which is ther­mally more ef­fi­cient; and, un­like petrol en­gines that need to con­trol the in­take air vol­ume (throt­tle) to achieve the sto­i­chio­met­ric (ideal) air-fuel ra­tio of 14,7 to 1, diesel en­gines run “un-throt­tled” and that means there are less pump­ing loss during part-load con­di­tions to get fresh air inside the com­bus­tion chambers.

Another ad­van­tage of high fuel ef­fi­ciency is that less CO is pro­duced. If man­u­fac­tur­ers plan to meet the low fleet-av­er­age tar­gets in Europe of 95 g of CO per kilo­me­tre by 2020, but still use ICE, diesel will have to be part of the plan.


Refin­ing crude oil (the most com­mon method of diesel pro­duc­tion) cre­ates a mul­ti­tude of prod­ucts, in­clud­ing fu­els such as diesel and petrol. If diesel en­gines dis­ap­pear al­to­gether, there will be lit­tle de­mand for diesel fuel, which will mean an im­bal­ance in the cur­rent re­fin­ery process. It there­fore makes sense to use diesel as a fuel as long as petrol is pro­duced.

Per litre, diesel fuel is more en­ergy dense (its chem­i­cal po­ten­tial en­ergy) than petrol (around 36 MJ/L ver­sus 34 MJ/L) and is a lower fire risk be­cause it has a higher flash­point than petrol (above 52 °C ver­sus mi­nus-43 °C).


Be­cause of high in-cylin­der pres­sures during com­bus­tion, diesel en­gines (es­pe­cially tur­bod­iesels) are known for high torque out­puts across the en­gine-speed range. This re­sults in ex­cel­lent in-gear per­for­mance and en­hances driveability. Com­pared with a petrol unit, a diesel’s lim­ited en­gine-speed range is less of an is­sue with to­day’s pro­fi­cient au­to­matic trans­mis­sions.

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