When we talk about engines, there are a lot of things that come to mind. What they are, what they do, how they do it. Although the word engine is not restricted to just the automobile alone, it spans from people to other systems. Take for instance; a person or system that helps improve the economic status of a place can be referred to as an engine of economic growth.
Also, the CPU in computers is also an engine as it does some work to produce results. I wonder if we are in an English class. Well I just felt like passing that across. An engine is not always a mechanical device it is anything that is used for a particular purpose source.
Now talking about engines in the mechanical world, it is simply a machine that is used to convert energy into motion (or mechanical force) source. most of the time, the engines are named based on the type of fuel (or working fluid) they use to perform this energy to motion conversion. We have the likes of steam engines which from the name makes use of steam to drive the moving parts, the gas engines, petrol engines and diesel engines, even sometimes, some engines make use of a mixture of two fuels (I have seen a gas and diesel powered engine before) although those engines are usually large engines.
A step further, we have automobiles (my loved machine) classified into two based on the type of fuel they use. Yes, you guessed right – the diesel and petrol engines. A lot of people do not really know the difference between a petrol and diesel (apart from the obvious fact of the type of fuel they use) and many wouldn’t mind as far as they have a vehicle to drive around.
Actually, the petrol and diesel engines operate in quite similar cycles called the Otto and Diesel cycles respectively. The major difference being the mechanism of ignition. You can check out these posts by 
Maybe you have, maybe you haven’t noticed that heavy duty trucks and machinery never use petrol engines but diesel engines. Well, that’s about what I am going to be discussing today so sit tight and simply enjoy the ride read!
image source; WIKIMEDIA COMMONS under Creative Commons Attribution-Share Alike 2.5 Generic, 2.0 Generic and 1.0 Generic license.
Lets take a little detour shall we?
An experiment is what we will be doing here (as always). In this experiment, I would like you to get a wrench (preferably a wheel spanner) and head over to the wheel of your car (please don’t do it on another person’s car without permission) fix the spanner on the nut and attempt to loosen the nut but make sure your hand is positioned very close to the nut about 5cm from the nut. Sincerely, I do not expect you to be successful with the operation as the nut would be harder to set loose. Why? Wait lets proceed.
Now shift your arm a little further away from the nut and attempt to loosen it again with the same force you applied earlier. Now, I expect you to achieve a little success this time around but might not be as easy as required. Why? Be patient for a little while.
I know your instincts would have been telling you that shifting your hand to the extreme end of the wrench or even attaching an extension to the wrench would help loosen the nut quite very easily as a matter of fact, the longer the extension (or the distance from the nut), the easier it is to get it loosened.
Yeah you know that, but do you know the physics behind that? Well, if you don’t, its called torque!
TORQUE
This time around, it has not been too long I did physics.
In my post about the torque converter a while back, I defined torque as the turning effect of a force about a given point.
Torque, also referred to as moment is the defined according to Khan academy as
Well, what all that grammar means is that if you apply a force (such as you apply with your hands) to a body (such as the nut in the example) at a distance (such as the end of the wrench) you are simply applying torque to the body (the nut) to make it rotate about its axis (well, in the case of the example, to loosen it).a measure of the force that can cause an object to rotate about an axis.
Mathematically torque τ is the product of the force F and the perpendicular (or angular) distance L
That equation actually explains the reason why a longer extension would help you loosen the nut with ease. So basically to get the nut loose, its either you apply more force (which would not be as easy) or to simply increase the (perpendicular) distance.
See, physics is not as hard as you thought. maybe not
Have you been wondering what relates what we just discussed to the petrol and diesel engines?
Wonder no more.
A heavy duty truck. Source PIXABAY
The engine delivers torque to the wheels and that is what makes the vehicle overcome its weight and move. If the engine does not deliver as much torque that can overcome the weight, then the vehicle would not move an inch (just as the nut would not get loose when your hand is closer to it).
Diesel engines deliver more torque than the petrol engines and that is why they are used in heavy duty machinery such as towing trucks, trailers, industrial generators and the likes.
The next question is, how do the diesel engines get to deliver more torque than the petrol engines?
Here I am delivering you the answers straight up!
Why more torque?
- COMPRESSION RATIO
In my post about how internal combustion engines work, I explained the strokes one after the other and one of the strokes is the compression stroke. This stroke is when the piston moves towards the Top Dead Center (TDC) and thus compresses the fluid that has been sucked into the combustion chamber.
Compression ratio is simply the ratio of the volume of the cylinder before compression (at BDC) and after compression (at TDC). If you are wondering what TDC and BDC mean, they are simply the two extreme positions of the piston in the cylinder.
Now back to the diesel engines, the compression ratio of the diesel engines is higher than that of the petrol engines because of the type of ignition it operates with (compression ignition) which requires the air to be compressed to a very high pressure cum temperature which means the piston would have to travel a little further than that of the petrol engines. Theoretically, compression ratio is directly proportional to the efficiencysource and thus the resulting output is more torque.
STROKE LENGTH
With the required compression more than that of the petrol engine, the piston travels further and thus there is a longer stroke (the length travelled by the piston). Now recall the experiment we did earlier, a longer distance from the pivot helps us generate more torque.
Therefore, the longer stroke length of a diesel engine also contribute to the resultant higher torque generated.DURATION OF COMBUSTION
The compression ignition system is designed in such a way that the fuel is injected into the cylinder during the power stroke and as a result of the temperature of the compressed air already present in the chamber, it ignites almost instantly. This means the fuel would release all the energy of combustion at once which would deliver more torque.
This is in contrast to the petrol spark ignition system where a spark is responsible for starting the combustion. This means that the combustion starts from the point where the spark is and spreads which then means that it would take the fuel more time to burn up in the chamber.
Take for example, you need a weight of 50kg to drive a nail into a wood. Using a 50kg weight at once would do the job faster than dropping two 25kg weights one after the other.TURBOCHARGING
Diesel engines are usually turbocharged in contrast to their petrol counterparts that take turbocharging (or supercharging) as an extra. The effect of turbocharging is that the air is available in a good quantity and judging from the fact that quite coincidentally, air is what is needed at the suction stroke.
This availability of air and the fact that diesel engines naturally tend to burn lean (i.e. more air than fuel in the charge) means there is more air in the chamber to be used for power delivery and thus, more torque delivery.
WEIGHT OF COMPONENTS
Because diesel engines are usually turbocharged (as stated earlier) the components are built for this purpose and they are heavier. The weight of these components also contribute to the torque build up which is a win for the diesel engines.
- ENERGY DENSITY
According to Science, diesel has a higher energy density than petrol. In simple terms, this means that if you have one liter each of petrol and diesel, the diesel would contain more energy than the petrol which is of the same volume.
The higher energy density also then results in more force delivered to the piston and if you recall my illustration, increasing torque requires either increasing the length or the force applied. In the case of a diesel engine, increasing both presents you a win-win situation!
So there you have it, the heavy duty trucks are doing a lot of work (obviously) and this is made possible by the high torque delivered by the engine that powers them – the diesel engine.
One downside to the delivery of more torque is that the RPM (speed at which the crank shaft rotates) is limited but this factor can be easily overcome by gearing. Petrol engines on the other side have the ability to rev up to higher RPMs and that’s why you would find them in race cars or smaller cars that do not perform so much work as the heavy duty vehicles.
Simply put, a petrol engine powered vehicle tends to be able to go faster than a similar diesel engine powered one.
CONCLUSION
Diesel and petrol engines both have their individual advantages over one another and they both thrive in their individual markets. You wouldn’t want to use a simple petrol engine to power heavy machinery while you wouldn’t also want to use a diesel engine to power a race car.
When next you see an industrial generator humming away and delivering power to your industrial complex, take some time to say thank you to it because it took a bold step to choose torque over speed.
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