Car (South Africa) - - TECH -


If we con­sider dif­fer­ent pow­er­trains driv­ing a sim­i­lar ve­hi­cle at the same speed, should the fuel con­sump­tion not be equal, as the en­ergy needed to pro­pel the ve­hi­cle stays con­stant? This would in­clude petrol, diesel, nat­u­rally as­pi­rated and tur­bocharged units of vary­ing ca­pac­i­ties. In my opin­ion, a smaller-ca­pac­ity en­gine is not al­ways more fuel ef­fi­cient in day-to­day driv­ing. Do you support my views? JEREMIAH MNISI Hazyview

Great ques­tion, Jeremiah. When you con­sider the move­ment of a ve­hi­cle from an en­ergy point of view, you are cor­rect. It takes a set amount of en­ergy to pro­pel a ve­hi­cle at a set speed. If the ve­hi­cle spec­i­fi­ca­tion and mass is fixed but the pow­er­train is var­ied, as you de­scribed, the en­ergy re­quire­ment does not change.

What’s left out of the equa­tion is the ef­fi­ciency of the pow­er­train in con­vert­ing the en­ergy from petrol, diesel or bat­tery to the use­ful mo­tive en­ergy needed (which is the same amount, as ex­plained).

An ef­fi­ciency of 30% means that only 30% of the avail­able en­ergy in the fuel is con­verted to mo­tive force. De­pend­ing on the tech­nol­ogy used for each type of pow­er­train, the ef­fi­cien­cies can vary dra­mat­i­cally within each type but, for the pur­pose of this dis­cus­sion, we’ll gen­er­alise.


Petrol en­gines run at the ideal sto­i­chio­met­ric ra­tio of 14,7:1 air-to-fuel ra­tio. This means that, dur­ing part-load con­di­tions, the in­com­ing air needs to be throt­tled by a throt­tle valve re­sult­ing in pump­ing losses. By re­duc­ing the en­gine ca­pac­ity, the en­gine needs to work harder to pro­duce the re­quired power (en­ergy) and it means the throt­tle plate is open wider more of­ten, re­sult­ing in con­sid­er­ably less pump­ing losses. Smaller-ca­pac­ity en­gines also have less en­gine fric­tion. Ad­ding a turbo in down­siz­ing ap­pli­ca­tions in­creases the ef­fi­ciency fur­ther by har­ness­ing the nor­mally wasted ex­haust en­ergy to com­press the in­take air. Ob­vi­ously, this also re­sults in a per­for­mance ben­e­fit to off­set the ca­pac­ity down­siz­ing.


Diesel en­gines get a head start when it comes to fuel con­sump­tion be­cause diesel fuel has slightly more en­ergy per litre than petrol. Fur­ther­more, a diesel en­gine can run with ex­cess air as a fixed air-fuel ra­tio is not needed and there­fore lit­tle pump­ing losses are present. The com­pres­sion ra­tio is linked to ef­fi­ciency and diesel en­gines run at higher ra­tios to achieve the tem­per­a­tures and pres­sures needed in the com­bus­tion cham­ber for com­pres­sion ig­ni­tion (in con­trast to a petrol en­gine, which re­lies on spark ig­ni­tion via the spark plugs). Ef­fi­cient tur­bocharg­ing and down­siz­ing is pos­si­ble with diesel en­gines, too.


Electric mo­tors are by far the ef­fi­ciency win­ners when it comes to con­vert­ing chem­i­cal po­ten­tial en­ergy in the bat­tery (elec­tric­ity) to mo­tive force. Fig­ures higher than 90% are com­mon. The draw­back is the en­ergy den­sity of a bat­tery pack does not com­pare to the en­ergy den­sity of fos­sil fuels, mean­ing driv­ing range will al­ways be an is­sue be­cause bat­ter­ies with enough en­ergy to com­pete with fos­sil-fuel ve­hi­cles are heavy and ex­pen­sive.

The above is a good in­di­ca­tion of the di­rect run­ning costs of a ve­hi­cle with dif­fer­ent pow­er­trains. In gen­eral, electric ve­hi­cles have the low­est en­ergy cost fol­lowed by diesel ve­hi­cles and then petrols.

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