Understanding the Mechanics of a Car's Speed: Answers to Common Questions

[apples]

OK I have a few stupid questions, some of which I need to confirm answers too.

1. How does a car increase it's speed? When a driver accelerates, more fuel is injected so the explosion size increases, and the piston moves with more force downwards at a greater speed. Is that true?
So the intake of gasoline by the cylinder depends on how hard you press on the acceleration?
So a car's speed depends on the movement of the piston, which in turn depends on how much fuel is injected?

2. If what I said above is true, then how long does it take the piston to move from the bottom of the cylinder to the top when the car is at its lowest speed? How long does it take to move from the top to the bottom when the car is at its lowest speed?
How long does the piston take to move from the bottom to the top when the car is at its highest speed? From the top to the bottom?

3. What is the best way to reduce friction between two pieces of wood so that the friction between the two pieces of wood be at its lowest?

4. Is there anyway to block out the magnetic field of a magnet? What is it?

Thanks a lot.
And sorry mods if I posted this in the wrong section, I wasn't sure which section to post this in.

 

[Dr.Brain]

I guess this is the wrong section :p

 

[FredGarvin]

1. How does a car increase it's speed? When a driver accelerates, more fuel is injected so the explosion size increases, and the piston moves with more force downwards at a greater speed. Is that true?

TRUE.

So the intake of gasoline by the cylinder depends on how hard you press on the acceleration?

TRUE.

So a car's speed depends on the movement of the piston, which in turn depends on how much fuel is injected?

It depends on how much fuel and air are injected. The speed also depends on the transmission/drive train as well.

2. If what I said above is true, then how long does it take the piston to move from the bottom of the cylinder to the top when the car is at its lowest speed? How long does it take to move from the top to the bottom when the car is at its lowest speed?
How long does the piston take to move from the bottom to the top when the car is at its highest speed? From the top to the bottom?

That depends entirely on the engine being considered. What you need to know in particular is the "stroke" or the length of piston travel for each and every engine out there. They are not the same.

3. What is the best way to reduce friction between two pieces of wood so that the friction between the two pieces of wood be at its lowest?

It depends on what you mean by "best." If you mean the absolute lowest friction coefficient possible, then the best would be a very thin layer of air between the two pieces that provides just enough space as to not let the two pieces touch. However, that's not very realistic in practice. So the answer is most likely some kind of grease or similar fluid. What that fluid/grease would be would also depend on what one is doing.

4. Is there anyway to block out the magnetic field of a magnet? What is it?

Encase it in very thick lead. This is what is used for magnetic shielding in things like speakers and TVs.

 

[russ_watters]

1. How does a car increase it's speed? When a driver accelerates, more fuel is injected so the explosion size increases, and the piston moves with more force downwards at a greater speed. Is that true?
So the intake of gasoline by the cylinder depends on how hard you press on the acceleration?
So a car's speed depends on the movement of the piston, which in turn depends on how much fuel is injected?

Basically, yes. They try to keep a good mixture of fuel and air, though...

2. If what I said above is true, then how long does it take the piston to move from the bottom of the cylinder to the top when the car is at its lowest speed? How long does it take to move from the top to the bottom when the car is at its lowest speed? How long does the piston take to move from the bottom to the top when the car is at its highest speed? From the top to the bottom?

Ever been in a car with a tachometer? It tells you how fast the drive shaft is spinning. Most cars idle at 500-700rpm. That's a tenth of a second for a cylinder to go up and down. The red line can vary, usually from about 5,000-10,000 rpm Please remember that there is a clutch and gears, though, so you don't always have a direct ratio (or the same ratio) between speed and rpm.

3. What is the best way to reduce friction between two pieces of wood so that the friction between the two pieces of wood be at its lowest?

Sand them.

 

[Astronuc]

3. What is the best way to reduce friction between two pieces of wood so that the friction between the two pieces of wood be at its lowest?

Other than adding a lubricant, one can smooth the surface or reduce the surface 'roughness' and make the surface as 'flat' as possible. Sanding with fine grit, and polishing, is the method.


Magnetic shielding materials -

Mu-metal - http://www.mushield.com/
http://farside.ph.utexas.edu/teaching/jk1/lectures/node52.html

GIRON - http://www.lessemf.com/mag-shld.html

http://www.magnetic-shield.com/

http://www.amuneal.com/pages/magshield-intro.php

 

[apples]

Thanks a lot guys that really helped!
Yes, that's what I meant making the coefficient of friction as low as possible. I guess, the best method is sanding them and then polishing them. What about PTFE, FEP, PFA, acetal, is it possible to use them? But I doubt if I can get them from anywhere, so I think polishing will do the work hope fully.

And I need to know about a rough estimate of the time it takes for the piston to up and down. Maybe an average of sedan cars without not really really awesome specs (ferrari's, merc's etc.). Let's say an average car, suppose, err... a Honda civic.

The magnetic shield is going to cost a lot! :p. Thanks a lot for the links.
P.S. All that stuff on those sites are confusing.:P

 

[russ_watters]

And I need to know about a rough estimate of the time it takes for the piston to up and down. Maybe an average of sedan cars without not really really awesome specs (ferrari's, merc's etc.). Let's say an average car, suppose, err... a Honda civic

The numbers I gave you were revolutions per minute. Flip them over and you get time. Ie, 600 rpm is 10 revolutions per second or one revolution every tenth of a second. Every revolution of the crankshaft takes each piston all the way up and down. A typical sedan that redlines at 6,000 rpm then, has its pistons go up and down 100 times a second at its max.

 

[apples]

How can something mechanical moves so fast?
How is the piston powered?
It's hard to imagine something going so fast.

 

[FredGarvin]

They do move quite quickly. The motions of a piston are very well studied and are part of a basic engineering dynamics class. The motion creates some pretty large accelerations which in turn create some large forces. So if you ever wonder why a connecting rod or crank shaft is so beefy, that's part of the reason why.

 

[Astronuc]

'Soft iron' is also used for magnetic shielding.
http://tpub.com/neets/book1/chapter1/1j.htm

How is the piston powered?

Mechanically. The explosion in the combustion chamber increases pressure/force on the piston. The piston transfers the force mechanically via the rod to the crankshaft, which in turn rotates and transmits force to the other pistons. The crankshaft is also couple to a transmission which transfers the mechanical force to the axles either directly as in front-wheel-drive (or with certain rear mounted engines like VW's) or indirectly through a drive train in a rear-wheel-drive system.

 

[DaveC426913]

1. How does a car increase it's speed? When a driver accelerates, more fuel is injected so the explosion size increases, and the piston moves with more force downwards at a greater speed. Is that true?

I believe the fuel amount is not increased, nor is the size of the explosion.

The only thing that increases is the speed at which the explosions happen.

 

[Averagesupernova]

The amount of fuel and air drawn into a gasoline engine is increased to increase the power output. Technically, they aren't even considered explosions.

 

[DaveC426913]

The amount of fuel and air drawn into a gasoline engine is increased to increase the power output. Technically, they aren't even considered explosions.

Yes, but unless I am wrong, the extra air/fuel goes into faster cycling of strokes, not more powerful strokes. (I may be wrong on this. ICEs may be more sophisticated than I thought.)

 

[russ_watters]

Force must accompany (cause) acceleration. The rpms cannot increase unless the force on the piston increases.

If you push down on the gas pedal, you start to accelerate - if you push down the brake at the same time to counter that, you keep the same speed, but are absorbing the acceleration force with a friction force. Your rpms and haven't changed, but your brakes are exerting a force and absorbing energy - extra energy that was put in via extra fual and air in each firing.

Now you know why cars run so far below their theoretical thermodynamic efficiency...

 

[DaveC426913]

Force must accompany (cause) acceleration. The rpms cannot increase unless the force on the piston increases.

Frequency of application of the force can accomplish the same thing.

eg. The guy on the office chair that throws a beanbag every second will accelerate and move faster than the guy on the office chair throwing a beanbag every ten seconds.

 

[russ_watters]

Frequency of application of the force can accomplish the same thing.

eg. The guy on the office chair that throws a beanbag every second will accelerate and move faster than the guy on the office chair throwing a beanbag every ten seconds.

I just said you aren't changing the frequency if you hold down the brake while holding down the gas pedal. Changing the frequency itself is an acceleration. Certainly, you can change the force simply by changing gears, but you can also accelerate without changing gears (and while the clutch is down, the engine is free to accelerate if there is a force to accelerate it).

 

[apples]

I still don't understand.
A piston moves down after the explosion, and the piston turns the crankshaft. How does the piston move back up?

 

[FredGarvin]

I still don't understand.
A piston moves down after the explosion, and the piston turns the crankshaft. How does the piston move back up?

When one piston is at it's lowest point, there is another piston just starting it's down stroke.

Take a look here:
http://library.thinkquest.org/C006011/english/sites/mehrzylinder.php3?f=2&b=50&j=1&fl=1&v=2

 

[ray b]

stored energy in the flywheel for a single cyl motor
returns the piston to begin a new cycle

 

[apples]

Thanks a lot!

 

[DaveC426913]

I just said you aren't changing the frequency if you hold down the brake while holding down the gas pedal

This is not true. If you depress the brake and gas pedal simultaneously you are certainly revving the engine. The engine is not directly linked to the driveshaft; the transmission has a slipdisk.

 

[russ_watters]

When one piston is at it's lowest point, there is another piston just starting it's down stroke.

And for a single piston engine, the momentum of the spinning shaft is enough to keep it moving.

 

[Danger]

The engine is not directly linked to the driveshaft; the transmission has a slipdisk.

You're assuming an automatic transmission. With a standard, applying the brake with the clutch out will either overpower the brakes or stall the engine.
By the way, if you guys don't mind a brief side-step... How does an ABS computer know to leave the brakes applied on a deliberate stop? I would expect the car to creep.
Feel free to move this or delete it if it's too far off topic.

 

[DaveC426913]

You're assuming an automatic transmission. With a standard, applying the brake with the clutch out will either overpower the brakes or stall the engine.

Good point. Conceded.

By the way, if you guys don't mind a brief side-step... How does an ABS computer know to leave the brakes applied on a deliberate stop? I would expect the car to creep.

I don't follow your logic. The computer will only override the braking if the wheels begin to slip, such as in a skid.

 

[Mech_Engineer]

You're assuming an automatic transmission. With a standard, applying the brake with the clutch out will either overpower the brakes or stall the engine.
By the way, if you guys don't mind a brief side-step... How does an ABS computer know to leave the brakes applied on a deliberate stop? I would expect the car to creep.
Feel free to move this or delete it if it's too far off topic.

I suspect the ABS will only turn on with the speed sensor registering over a certain value, like 5 or 10 mph... Also, it will only engage if one wheel begins to slip. If all four wheels are traveling at the same speed, the ABS stays off.

 

[Danger]

If all four wheels are traveling at the same speed, the ABS stays off.

Oh, okay. I didn't know that. W's van has ABS, but I've never owned one myself and never really studied up on it. Thanks.

The computer will only override the braking if the wheels begin to slip, such as in a skid.

That's sort of what I meant; how it would know that they're slipping as opposed to not rotating because of a deliberate stop. That still applies to the first part of Mech's response too, though, since the speedometer won't work with the drive wheels locked up.

 

[DaveC426913]

Also, it will only engage if one wheel begins to slip. If all four wheels are traveling at the same speed, the ABS stays off.

I am not convinced this is true.

That's sort of what I meant; how it would know that they're slipping as opposed to not rotating because of a deliberate stop.

I think the car has an independent way of knowing that it's moving. Thus, all it needs to know is that the car is moving and that the wheel/wheels are not.

 

[Mech_Engineer]

I am not convinced this is true.

I think the car has an independent way of knowing that it's moving. Thus, all it needs to know is that the car is moving and that the wheel/wheels are not.

What's not to be convinced of? How do you suggest that a car knows it is moving, GPS? While this could possibly be an option on new cars, 4-wheel ABS has been around a long time.

The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information.

http://auto.howstuffworks.com/anti-lock-brake.htm

4-Wheel ABS works through speed sensors at each wheel. When one sensor stops moving while the others are under braking conditions, the ABS activates (take note that these same sensors can be used for traction control under acceleration to detect a slipping wheel). All it needs to see is if there is a discontinuity with the speed sensors when the brakes are applied (the discontinuity could be a difference in speed, rather than a zero somewhere). Sometimes you can accidentally activate the ABS system when taking a sharp turn and applying the brakes. The system will see a difference in the wheel speeds, even though it is from differences in turning radius, not from a locked wheel.

The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

This is not a perfect system, and there are a few ways that it can fail.

1) One, is if a speed sensor is faulty. If a sensor is damaged and not sending a signal (or sending an erratic signal), the ABS will always engage, because it will think that a wheel is locked up. Quite annoying when driving.

2) All four wheels lock up at the exact same time. I have encountered this situation a few times while driving on ice. If you press on the brakes and all four wheels lock up the computer thinks you are stopped, when in fact you're sliding down the road on a sheet of ice. This seems to be a rare occurence however as many times one tire will lock up before the others, and once the ABS system is active it can see that there are wheel slip issues.

 

[russ_watters]

I don't know if they actually do this, but I'd think it would be able to detect a rapid deceleration of a wheel and know that that isn't just a normal stop.

If the manufacturer measures a maximum deceleration of, say, .5g with outstanding tires on a dry surface, they could set 1g as the threshold for ABS. If then the wheel's speed dropped, say, from 21mph to 0 in .5 seconds, the computer could calculate 2g and conclude it locked-up. It isn't a complicated control logic problem.

 

[Danger]

Thanks for the extended explanations, guys. I can't believe that I never saw that article, considering that I haunt 'How Stuff Works' with disturbing regularity.
Apples, sorry for the hijack.

 

[Averagesupernova]

There are many variations of ABS. Early GM systems that only had rear wheel ABS simply relied on ONE sensor at the output shaft of the transmission that determined how quickly the drive shaft decelerated. As far as I know, there is no method independent of the wheels for the vehicle to know how fast it is traveling.

 

[ray b]

I don't know if it is used
but a simple decelleration detector could be used
like a weight on a string on a micro scale
our GM suburban would kick in ABS at low speed on crossing white lines
as the ABS kicked in at AS LOW as 2-3 mph when stoping at a trafic light
kinda spookie when it happend

a radar system could KNOW speed independant of the wheels
or the quick cycle of the system may defeat creep at a stop

 

[apples]

Apples, sorry for the hijack.

LOL, I asked about the speed of a piston, and it ends up with ABS.

 

[Danger]

And it'll probably have worked its way around to limited-slip differentials by next week.