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* How Stuff Works - An Introductory


GIR

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In light of most people not knowing the basics about physics or automotive engineering I decided to write this article explaining some basic stuff about cars and engines.

A forum member named "All Wheel Drive" suggested these type of articles in this thread:

http://www.carpassion.com/en/forum/viewtopic.php?t=1181

This article is here to inform people about the basics of automotive engineering and the physics behind it. This is not a flame thread! We will respect your freedom of expression but keep in mind that flames and/or insults will be deleted. Lets keep these threads informative and friendly. If this one works out well you guyz will be seeing more of these articles in the future.

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The most important unit that drives a car is torque, this unit sais more about the car then any other. Torque combined with gear ratios and weight will give you an accurate indication on how fast a car can be, you will see that bhp (or rhp) is a useless unit and doesn't tell you much about the car excapt the amout of energy it generates at it's peak.

So what is torque?

Simply put torque is a rotational motion... uhu? well even simpler torque is a force that rotates or turns. For example if you unscrew a lugnut from your cars wheel with a wrench, you are generatin torque on the lugnut because it turns. You generate this torque by applying force on the wrench itself, which is not a rotational force.

So to understand torque we first need to understand force.

So what is force?

Force can be many things. Force is the amount of energy you excert on a certain object to get it moving ot pull it down. A type of force everybody knows is weight. Your weight is detimined by the amount of mass you have (the amount of atoms in your body) multiplied by the gravetational pull that the Earth excerts on you. The Earth pulls you down towards the center of the planet. So the Planet excerts energy on you pulling you downwrds so you don't fly off.

So why don't we all get sucked into the Earth? Well Newton's third law sais that for every object that is standing still there's a equal and counter directional amount of force. So if the Planet is pulling you down, the ground is excerting and equal and counter directional amount of force pushing you up.

There are many types of force which can be converted into each other. For example if you apply your weight onto a toycar, the toycar starts moving. In this scenario you have converted the energy you stored in your body (by eating) into mass which then is being converted into movement.

Good old sir Isaac Newton had his say in the matter aswell. His second law sais that the amount of acceleration is the force divided by the mass of the toycar. The more mass the car has the harder it;s going to be for you to push it forward. To honor Sr. Isaac Newton's great achievements force is indicated by the unit Newton (or the letter N).

So lets apply our force theory on a real car that's standing still and about to accelerate. When the car is standing still the Earth is pulling the car towards the center of the planet, this force is excerted onto the body of the car. In turn the gorund push the car upwards, this force is excerted into the wheels. These two cobined keep the car from sinking into the earth or flying off.

When the car starts to accelerate the wheel of the car push the car foward by excerting force onto the road, but the road also pushes back. This is known as drag. In the initial state there is alot of drag because the car was standing still (if there wasn't any then the car wouldn't stop or standstill). The main point while accelerating is to over come the drag. The faster you go the less the drag is, this is why while accelerating the car tilts back a little, it is because the drag from the road is trying to push the car back.

Once the drag has been overcome the car can accelerate freely, or so you might think. Here comes another factor and that is airresistance. Because the car is moving forawrd it has to push the air infront of it to the side. Pushing the air to the side requires energy and the faster tyou go the more air there is so the more energy you require. If you're moving slow you can accelerate all you want because the resistance from the air is that much but at some point the amount of energy needed to push the air aside is equal to the amount of energy created by the engine, hence the car wont accelerate anymore because all it's energy is going towards overcoming the airresistance. So for having topspeed low airresistance is a very important factor.

What does this have todo with torque?

Liek I sais torque is a force that is truning or rotating. It just happens to be that the crankshaft and the wheels on our cars turn. So torque is a kind of force but a special kind of force which makes it a little harder to understand. Torque is not only defined by the amount of force (energy) you put into a object but also the distance you are from the center of the object. In the English unit torque is defined as pound per feet or inch, in the SI torque is defined as Newton per meter. Notice that it has a energy unit and a distance unit.

If you have a lug nut and you have a wrench that is 1 meter. When you put 100 Netwon of force on the wrench you are generating 100Nm of torque. If your wrench is 2 meters long then you generate the same amount of torque but the amount of enegery required is just 50 Newton. This all has todo with rotational speed and distance. Look at these two circles.

circles.JPG

As you can see we have two circles. Look at the red traingles within the circles. At the center the traingles are both equaly wide but the firther you go outwards the wider the traingles get. This can be replaced by our wrench. The longer the wrench is the further you go outwards the wider the distance gets. So with a longer wrench to get the same amount of turn as a shorter wrench you have to travel a greater distance but because the supplied energy is constant more energy goes into a shorter distance. Because you travel a greater distance the amount of energy you actually require to turn the nut in the center is less.

Okay so how does this all translate into engines?

Let me first explain how torque is generated in a engine, look at this picture.

firststroke.GIF

Everybody knows how a engine works, the piston moves up, the gas is compressed, a spark is made, the explosion pushes the piston down, the piston in turn pushes the arm down and the arm pushes the connecting rod down.

The connecting rod is the key ellement here. As you can see in the picture it has been placed a great distance away from the crankshaft. This is because the amount of torque is the distance from the center times the force (remember?). This whole process generates torque, but the torque specified on an engine is not the torque it generates because torque is also dependent on the force (ie how far you push the gaspedal in). Instead the torque specified on an engine is the amount of torque it can deliver just before it stalls. So okay if we want more torque then why don't we place the connecting rod further away from the crankshaft? This can be done and is kinda upto the engineer where he wants to put it. Placing the connectingrod further away has some other side effects aswell which are sometimes not wanted.

If you place the connectingrod further away from the crankshaft then the enginespeed (the rpm count) is also reduced. So you would have more power but at cost of a higher rpm count which in turn translates into lower topspeed. But placing it further away from the crankshft also means a bigger engine. The crankshaft takes by far the most space. Finally it also means that you will need a somewhat bigger piston because the distance travveled by the arm is greater. This is to keep the engine alive, remember a two stroke engine needs 2 strokes to be operative so because the distance traveled is greater it needs more force to be able to travel that distance. If you don't apply more force your engine will have more viberation and evetualy kill itself.

So placing the connectingrod at exactly the correct distance from the crankshaft is crucuial to a engine. Mercedes engines have them at a somewhat greater distance then other manufacturers resulting in more torque over a greater lower rpm band but in the upper regions have nothing. This in turn results the car being able to get off the ground much faster and smoother but in cost of topspeed.

Japanese makers place them at a much shorter distance. This results in a much higher rpm and much smaller engine but at the cost of the lower rpm band. I like to think that this is a wrong engineering tactic because the the only thing it does is give the engine to run higher rpm counts which in turn translates into more bhp but the torque supplied by the engine (ie. the energy) is lower. What it does do is give the car a much higher topspeed because it supplies more energy at the higher ranges and that's where the airresistance is greater.

It's upto individual carmakers to sort out their own engineering tactics, for me to label one as wrong would be incorrect. It's upto the individual design goals which detirmine what the end results are going to be, but I feel that the only reason Japanese carmakers do this is because they know that they can specify a higher bhp count and most people just look at the numbers.

So where does the gearbox come into play

I'm planning on writing a seperate article on gearboxes and gearratios but here's a small explenation. Like I said while accelerating from a standstill the car has to overcome alot of initial drag which goes away after a while. So to get off the ground the car needs as much power (torque) as possible. So the start from a standstill we need as much power from the engine as we possibily can. But after a while the car is upto speed and we don't that much force anymore so we need something that will spread out the power of the engine into a much wider distance. This is what you do while shifting gears.

Lets compare a 12 liter diesel truck engine with a 4 liter gasoline car engine. Both engines can generate the same amount of horsepower but the diesel engine does this at 1200 rpm while the gasoline engine does this at 6000 rpm. Both supply the same amount of power but only the truck engine is suitable to pull alot of weight. This is because it is geared towards putting down alot of torque at once. Now we could help the gasoline engine along a little bit by putting a much longer gearatio in there, but this would mean that the car got upto speed very slowly and it would be destroyed after a while because it needs to generate alot of force at 6000 rpm. Truck engine micht generate more firce but also are built for durability. You could put a gasoline engine in a truck but that engine wouldn't last long.

Conclusion

BHP sais nothing about a car or how fast it is. It's all about force resistance and gearratios. There are several engineering tactics, one doesn't make the other wrong but from a consumer pov it could be better if the consumer knew more about engines. It is not only good to have alot of torque but also a very areodynamic body. A combination of lots of lowend and midrange torque combined with a very areodynamic body (like the KoenigsEgg) is the best combination for fast acceleration and topspeed.

Don't worry if you don't get it all. I barely get it myself. If there are any questions... ask them below and I will do my best to answer them.

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proffessor GIR, the man just explains it so well, i try explaining that stuff to people, and it just doesn't sound as cogent as his explanation. it's a really cool thing that you did that, and car people should be up to snuff on the physics of their car if they really want to understand it.

Thanks GIR; I've read all ur post and I've cleared my mind upon many things. I have some things to ask, still...

1. Can u add some torque/rpm diagramms to visually suport ur explanations? (I'll read the other pages put as links in the thread, but I don't realy have the time right now.)

2. Are u sure torque is measured in "newton per meter"? My school knowledges tell me it's rather "newton multiplyed by meter" (Nm).

Thanks again and I hope this is just the beggining... :roll:

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Hallo GIR,

 

schau doch mal hier zum Thema Zubehör für Verschiedenes über Autos (Anzeige)? Eventuell gibt es dort etwas Passendes.

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a newton meter is one newton's force extended over a leverage of one meter, so it is a newton times a meter.

Then, there are some parts that need to be rectified... a 100N force applied to an arm that has 2 meters delivers 200Nm of torque not 50Nm, isn't it?

I think you're reading it wrong or are confused with something else. Here's the full text once more:

If you have a lug nut and you have a wrench that is 1 meter. When you put 100 Netwon of force on the wrench you are generating 100Nm of torque. If your wrench is 2 meters long then you generate the same amount of torque but the amount of enegery required is just 50 Newton.

So to pull it apart even more:

Length from center = 1 meter

Force = 100 Newton

Torque = 100Nm

Length from center = 2 meter

Force = 50 Newton

Torque = 100Nm

So the end result, the amount of torque, is the same but you need to apply less force on the wrench.

to answer a previous question about air resistance, rolling resistance is something else, and often it is far less than air drag. Air drag is the resistance from the car relocating the air in the space it travels through, like when you stick your hand out the window while driving and the air tries to push it back, it does the same thing to the body of your car, and it gets exponentially stronger as you get faster.

i am well aware of what air resistance is, but what is the drag that is mentioned in the first post? Maybe GIR can elaborate a bit!!!

It is correct that if you double the length of the spanner you do need only half the force to achieve the same torque, but you require the same energy to do so (conservation of work or energy). This is because torque and energy have the same units Nm (if you keep torque constant the energy required will be the same).

Although i said that torque and energy have the same units this does not mean they are equal!!!

For some reason it is also mentioned that force (Newton N) is equivalent to energy(Newton Metres Nm or Joules J) which is not correct.

Yeah yeah yeah bla bla bla, sighs there's always somebody who has to complain about some technicality. Yes the total energy is the same (because nobody can create energy) but the distance travelled is longer hence the amount of force required is lower. I should really get somebody to proof reed these to avoid these situations.

The point of this article is to give a basic introduction to simple things not to explain to whole workings of the physics system.

As I stated in my original article the original drag you have is the drag from (among other things) the road. A object that's standing still always wants to stand still. In the initial phase it's point to overcome this initial drag because while pulling out from a line it's the most. After you've overcome this drag another law comes into play and that is that an object that's moving always wants to stay in that motion.

You can vefity this by doing the following. Get a cardboard box and fill it with some very heavy stuff. Then just give the box a little push. The box won't budge. Then push it constantly. You'll notice that at first you'll have alot of trouble getting the box to move but once you get it moving it becomes easier the faster you go.

GIR, you could have just said it easily back to him that he should have known that work (or energy) is simply force times distance, so torque (force) times distance travelled is work done on moving the vehicle.

I think i now know what you mean, friction in the bearings amongs otherthings etc, the effect of the frictionional difference between sliding surfaces and non sliding (kinetic and static to do with cardboard box more specifically).

Though at low speed i think rolling resistance is more significant than any other effect apart from the handbrake.

A car tyre pumped up to the proper pressure will roll easier and hence be more fuel efficient than a softer tyre, as would steel wheels if they were practicle for a car.

No not quiet. All the moving parts cause drag aswell, that is true and this drag has to be overcome aswell but it's infinitely small compared to the drag cause by the road itself.

You can view it like this. Your car wants to move forward so it excerts a force onto the road. But the road doesn't like this so it tries to hold the car in place as much as possible. The faster you go the smaller this drag becomes. Well the amount of force excerted onto the road is infinite times bigger then the drag cause by the road so we can say that it's actually not there.

If you want it in equation form look at it like this:

y(x) = x/(1+x)

the 1 represents the drag

Now lets say x = 1 (accelerating from standstill)

y(1) = 1/(1+1) = 1/2 = 0.5

Now lets say x = 10 (still acellerating but somewhat further)

y(10) = 10/(1+10) = 10/11 = 0.909

now lets jump a little forward and lets say x = 10000

y(10000) = 10000/(1+10000) = 10000/10001 = 0.99999999999999999

When x = 10000 then we can say that the 1 (representing the drag) is so small compared to the x that we can infact leave the 1 out and say it's not there anymore. While infact it's still there but it's so much smaller then the total energy you won't feel it anymore.

Your car wants to move forward so it excerts a force onto the road. But the road doesn't like this so it tries to hold the car in place as much as possible. The faster you go the smaller this drag becomes. Well the amount of force excerted onto the road is infinite times bigger then the drag cause by the road so we can say that it's actually not there.

When the car exerts a force on the road doesn't the road apply an equal but opposite force to the car?

The only thing that can stop it moving is some force in the opposite direction (to the force the road exerts on the car).

I don't see by what mechanism/law of physics this force you call drag arises from?

I'm not trying to be a pain, but maybe i already know what your talking about, and its simply just that your using a different word/explanation to describe it!!!

Drag is caused by the moving of friction areas. Like you said turning parts of the car cause drag aswell, but that's not nearly as much as the drag caused by the road on the tires.

No that's only when the car is standing still. When the car is standing still all forces are equal and opposite to eachother. That's what's keeping the car in place. When you start moving the force that was keeping the car in place is still there but at some point it becomes so small (infinitly small) that we say it's not there anymore.

Like in the math example when x = 10000. In that situation the answer should be 0.999999999 etc. For convinience we say that it's not 0.99999999 but 1. It doesn't make much of difference for the final answer.

Same goes for the drag. At some point it's so small in the view of the energy output of the car that it doesn't matter much anymore wether we do or don't take it along in our equations so we say that it's not there anymore.

simpy put, when a car travel through any given space, it has to move the air in it out of its way, so the force of the car pushing the air out of the way has an opposite but equal, the air pushing back on the car.

most of the rolling resistance is just friction, molecules in the tire rubbing up against each other, turning energy created by the car into heat instead of forward momentum.

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