4 x 4 Australia - - Contents -

DIESEL en­gines have come along in leaps and bounds in re­cent years thanks to a tech­nol­ogy rev­o­lu­tion. That rev­o­lu­tion is based on elec­tronic con­trol of ex­treme­pres­sure fuel in­jec­tion sys­tems and the ever-more-so­phis­ti­cated tur­bocharger, which is now uni­ver­sally em­ployed on diesels, some­times in mul­ti­ples on any one en­gine.

You prob­a­bly think that’s why a mod­ern diesel-pow­ered 4x4 will typ­i­cally use 20 or 30 per cent less fuel than a petrol-pow­ered equiv­a­lent, but the fuel-ef­fi­ciency ad­van­tage of a diesel runs much deeper than that.

Most im­por­tantly, diesel fuel con­tains more en­ergy than petrol. It’s a sim­ple as that. The ex­tra en­ergy in ev­ery litre of diesel will take you fur­ther than the lesser amount of en­ergy in a litre of petrol. The dif­fer­ence isn’t great but it’s sig­nif­i­cant, and runs be­tween six and 16 per cent, the ex­act dif­fer­ence be­ing de­pen­dent on the spe­cific grade and blend of the diesel and petrol in ques­tion.

Given the en­gine in your 4x4 merely con­verts the po­ten­tial en­ergy in the fuel into me­chan­i­cal en­ergy, the more en­ergy you put in the more en­ergy you get out. The trick is not to waste too much along the way through heat, noise, me­chan­i­cal losses and the like.

The other in­her­ent ad­van­tage of diesel en­gines over petrol en­gines is the me­chan­i­cal ef­fi­ciency of their higher com­pres­sion ra­tios. Mod­ern diesels run com­pres­sion ra­tios that are typ­i­cally 50 per cent higher than cur­rent petrol en­gines, and it wasn’t long ago that diesels had com­pres­sion ra­tios twice as high as typ­i­cal petrol donks. The idea here is that by squeez­ing more air/fuel mix­ture into a small space you cre­ate a more force­ful ex­plo­sion, which acts on the pis­ton to cre­ate more torque at the crankshaft. Higher com­pres­sion ra­tios gen­er­ate greater pump­ing losses than lower ra­tios, but the ex­tra en­ergy used in com­press­ing the air/fuel mix­ture to a greater ex­tent cre­ates a big­ger bang in the com­bus­tion cham­ber.

Why don’t petrol en­gines run high com­pres­sions ra­tios like diesels? The an­swer is again very sim­ple and down to an in­her­ent virtue of diesel fuel. Diesel is a far more sta­ble fuel than petrol and will stand much higher tem­per­a­tures be­fore it will ig­nite. High com­pres­sion ra­tios cre­ate a lot of heat and that’s what ig­nites the fuel in a diesel en­gine.

If you try to com­press a petrol/air mix­ture to the com­pres­sion ra­tios seen in a diesel the re­sult is an un­con­trolled ex­plo­sion in the com­bus­tion cham­ber called pre-ig­ni­tion. Also called ‘knock­ing’, this un­con­trolled ex­plo­sion can lead to cat­a­strophic en­gine fail­ure.

Given the ad­van­tage high com­pres­sion ra­tios bring, mod­ern petrol en­gines push the bound­aries of high com­pres­sion via com­bus­tion cham­ber de­sign, knock sen­sors and high-oc­tane petrol (among other things), but could never run the sort of ra­tios you see in a diesel.

Diesel com­pres­sion ra­tios have come down in re­cent years in or­der to im­prove en­gine re­fine­ment, given that lower com­pres­sion means less noise, harsh­ness and vi­bra­tion. There’s also an en­vi­ron­men­tal ben­e­fit, as lower com­pres­sion ra­tios lead to re­duced out­put of the var­i­ous ox­ides of ni­tro­gen (col­lec­tively known as NOX), a par­tic­u­larly nasty group of pol­lu­tants and ar­guably the big­gest stum­bling block for diesel en­gines in the fu­ture. Gree­nies are big on this!

So next time you marvel at the tech­nol­ogy in a mod­ern diesel en­gine don’t for­get about the in­her­ent ben­e­fits of diesel, a fuel that’s more en­ergy-rich yet at the same time less volatile than petrol.

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