Let’s torque it through

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Diesel en­gines love tow­ing

Ev­ery­body knows that when you say “Break a leg,” you don’t mean it lit­er­ally. And ev­ery car­a­van­ner knows that the more torque your ve­hi­cle pro­duces, the bet­ter its tow­ing ca­pa­bil­ity will be. If you want to know ex­actly how hot or cold it is out­side, you use a ther­mome­ter, so if you want to know how much torque your ve­hi­cle can gen­er­ate, you have to know how many New­ton meters it de­liv­ers.

New­ton me­ter is a stan­dard unit used to mea­sure torque. The formula states that torque is equal to 1 N be­ing ex­er­cised at a right an­gle on an arm of 1m in length.

You can­not sim­ply use an en­gine’s ca­pac­ity as a yard­stick. There are 1.6-litre en­gines that put cer­tain 2.5 litres to shame, and diesel en­gines al­most al­ways have more torque than petrol en­gines of the same ca­pac­ity or less. Why is this so?


Let’s start at the be­gin­ning: Diesel and petrol en­gines are both in­ter­nal com­bus­tion en­gines and work on sim­i­lar prin­ci­ples. An in­ter­nal com­bus­tion en­gine is based on a four-stroke sys­tem that works as fol­lows: 1 Breathe in When a piston moves from its apex po­si­tion to the bot­tom, it draws air into its cham­ber. 2 Press The piston rises to the top of the cham­ber again and com­presses the air. 3 Watch out! The com­pressed air/fuel mix­ture ig­nites and the force presses the piston down again. 4 Breathe out The piston rises up the cham­ber again and the spent air is ex­pelled through the ex­haust man­i­fold.

The dif­fer­ence be­tween a petrol and a diesel en­gine lies in the ex­plo­sion at stages 1 and 2. In a petrol en­gine, the air that is ini­tially sucked into the cylin­der is a mix­ture of air and va­por­ised petrol. Af­ter the mix is com­pressed in the sec­ond stroke, it’s ig­nited by a spark plug. In a diesel en­gine, the air that is sucked in dur­ing the first stroke is clean air, which then gets com­pressed dur­ing the sec­ond stroke. At this point, an in­jec­tor shoots diesel into this high pres­sure en­vi­ron­ment, and due to the pres­sure and the chem­i­cal char­ac­ter­is­tics of the diesel, it ex­plodes spon­ta­neously with­out the need for an elec­tric spark. (Like your wife when you for­get your wed­ding anniversary – Ed).


Each piston is at­tached to the crank shaft by a rod and a bear­ing. When viewed from the side, the crank shaft has a zigzag shape, with a piston at­tached to each point of this up-and-down shape. When you turn the shaft, the points or­bit a cen­tral point. It is this ro­ta­tion of the crank shaft that causes the pis­tons to move up and down in­side the cylin­ders. The dis­tance from the point

What is the dif­fer­ence in tow­ing ca­pac­ity be­tween a diesel and petrol en­gine? What is the best gear to tow in, how do you pre­vent trailer sway, do you know how to prop­erly set your car’s rear-view mir­rors, and what is a safe tow­ing speed?

where the piston’s rod at­taches to the crank shaft and the crank shaft’s cen­tral pivot de­ter­mines how much torque the en­gine can de­liver. In other words: The larger the arc in the crankshaft, the more po­ten­tial torque. This arc works like a lever.

To un­der­stand it bet­ter, pic­ture lift­ing your ve­hi­cle with a jack. The longer the lever on the jack that you use to pump up and down, the eas­ier it is to lift your ve­hi­cle, and there­fore the more torque you have.

One of the more dis­tinct char­ac­ter­is­tics of a diesel en­gine is that it has a longer suc­tion stroke than a petrol en­gine. Put dif­fer­ently, the pis­tons in a diesel en­gine moves up and down over a longer dis­tance in its cham­bers than those in a petrol en­gine.

The dis­tance from the crank shaft’s zigzag points (as ex­plained be­fore) is also the rea­son why diesel en­gines ro­tate at fewer rev­o­lu­tions per minute than petrol en­gines.

The shorter this dis­tance, the shorter the stroke dis­tance of the piston, which in prac­ti­cal terms mean that the en­gine can ro­tate much faster.

This ro­ta­tion is ex­pressed in rev­o­lu­tions per minute, or rpm. In an en­gine that ro­tates faster (like a petrol en­gine) you there­fore sac­ri­fice torque, but be­cause the en­gine can ro­tate faster, with more rev­o­lu­tions per minute, it pro­duces more power, mea­sured in kilo­watt (kw) which en­ables you to drive faster.


Apart from pro­duc­ing more torque, diesel en­gines also have other in­ci­den­tal ad­van­tages that in­ad­ver­tently con­trib­ute to them be­ing more fru­gal than petrol en­gines.

Same, but dif­fer­ent. The dif­fer­ence be­tween the two types of fuel starts with their unique char­ac­ter­is­tics. Both are made from crude oil through a re­fin­ing process, but petrol is re­fined a lot more than diesel. Be­cause diesel is less re­fined, it is more vis­cous com­pared to the more flammable petrol. Diesel used to be cheaper than petrol in the past be­cause of the fact that its re­fin­ing process re­quires less en­ergy. Since 2004, the ta­bles have turned, how­ever, be­cause the world­wide de­mand for diesel is has in­creased. It’s also true that the qual­ity of crude oil has changed, and to­day the re­fin­ing process of diesel re­quires more en­ergy than in the past.

Diesel has more kick. Diesel has a higher en­ergy den­sity: 1 litre of diesel con­tains a lot more en­ergy – 40,1 MJ (mega joule) com­pared to 34,7 MJ in petrol. It’s also one of the rea­sons why fuel con­sumed by a diesel en­gine is less than that of an equiv­a­lent petrol. You need less diesel as you would petrol to cover the same dis­tance.

Un­der pres­sure. The pres­sure in a cham­ber at the sec­ond stroke dif­fers markedly in the two en­gine. When the piston in the petrol en­gine com­presses the air-and-fuel mix­ture, the ini­tial vol­ume in the cham­ber be­comes 8 to 12 times less, while the pres­sure in the diesel en­gine is much higher. It can be 14 to 25 times less than the orig­i­nal vol­ume. The diesel en­gine there­fore has a higher com­bus­tion point, which leads to a higher torque out­put.

Lost en­ergy. In diesel en­gines, about a third of the heat en­ergy of the com­bus­tion process is trans­ferred to the crank shaft, while in petrol en­gines it’s up to 25%. This ki­netic en­ergy even­tu­ally gets trans­ferred to the driven wheels, but some of the en­ergy gets lost due to fric­tion among the en­gine parts and the drag co­ef­fi­cient of the ve­hi­cle. In the­ory, only a quar­ter of the diesel fuel’s orig­i­nal en­ergy is used to pro­pel the ve­hi­cle. That fig­ure falls to al­most a fifth in petrol-driven ve­hi­cles.

go! Drive & Camp says Diesel en­gines re­lease less toxic gasses such as car­bon monox­ide and car­bon diox­ide, so they im­pact the en­vi­ron­men­tal less neg­a­tively. The down­side is that th­ese en­gines re­lease a large amount of ni­tro­gen-con­tain­ing pol­lu­tants, which con­trib­ute to acid rain, among other things.

A petrol en­gine uses spark plugs to ig­nite the petrol fumes, while a diesel ig­nites spon­ta­neously un­der pres­sure. While petrol en­gines use fuel in­jec­tion sys­tems or a car­bu­re­tor, diesel en­gines sim­ply work with a diesel in­jec­tor.

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