Let’s torque it through
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?
RECIPROCATION NATION
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).
PRIMED FOR LOCOMOTION
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
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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.
OTHER FACTORS
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.