B Negative pressure in a car to power it

[Bike Mi Vie]

TL;DR Summary

Powering a car trying "negative pressure". The force created by the action of sucking. How?

My father has an idea is powering vehicle using negative pressure. I'm trying to explain to him that I don't think that can work. He is saying that imagine you're in space and you try create negative pressure by keep on removing and the only thing that's preventing the collapse of the material is the strength of the material therefore, if you build a strong enough material to repel the action of the opposite of a compressing machine. The idea if this was trying to use air to power a car but compressing it would be a risk if explosion. Can we create negative pressure in a container in a vacuum. A adamantiun syringe in space that has the end sealed and the plunger is being pulled back 100 miles.

 

[hutchphd]

A gallon of gasoline contains about 40 million Joules of chemical energy. A vacuum tank at STP of size 25 liters will provide about (1)RT=2400 Joules. Probably not a great idea!!
I do not understand your syringe in space.

 

[Herman Trivilino]

TL;DR Summary: Powering a car trying "negative pressure". The force created by the action of sucking. How?

He is saying that imagine you're in space and you try create negative pressure by [...]

You cannot have a pressure that's negative relative to a vacuum.

 

[DaveE]

Your power source is based on the pressure difference between your tank and the outside. On Earth that pressure is about 15 PSI. So the best you can do with a vacuum is 15 PSI differential, when you've removed every molecule from your tank. If you go to your local hardware store you can by a cheap compressor that will easily make 100 PSI which would give an 85 PSI difference. So using positive pressure in your tank instead of 0 allows much more energy storage. Many of the tools at your local auto repair shop use this to power their tools. SCUBA tanks will hold 3000 PSI; I've seen them used to start diesel engines instead of an electric starter. You absolutely could power a car with SCUBA tanks; but it wouldn't go very far.

Still, it just isn't a great way to store a lot of energy compared to chemistry, like gasoline, batteries, or nuclear fuel.

 

[DaveE]

The "how?" answer is a pneumatic motor of some sort. They are commonplace. Basically turbines, pistons, or rotary vanes.

 

[Baluncore]

Can we create negative pressure in a container in a vacuum. A adamantiun syringe in space that has the end sealed and the plunger is being pulled back 100 miles.

On Earth, when you "pull a vacuum" by drawing a plunger or piston back, the work you do lifts the atmosphere of the Earth. It does that by increasing the volume of a vacuum chamber, that displaces air.
When you are in space, it takes no energy to draw the plunger back, it does no work, and it stores no energy. In the vacuum of space, the differential pressure across the piston is always zero.

Adamantine means; unable to be broken.
https://en.wikipedia.org/wiki/Adamantium

 

[Bike Mi Vie]

My dad think that if I have a vacuum that keeps pulling air it will collapse the container. So he's saying, so my idea is if I make a container that withstand atmospheric pressure but inside of it there is a space of

 

[Bike Mi Vie]

Sorry forget I said space. Do we have a material that can withstand atmosphere pressure?

 

[Baluncore]

Do we have a material that can withstand atmosphere pressure?

Finding that material is no problem.
If you pull a vacuum, you can only get a maximum pressure difference of 1 atmosphere, 1 bar, 15 psi, or 101 kPa.
If you compress a gas, you can get enormous pressures that will destroy the container.

 

[Ibix]

Do we have a material that can withstand atmosphere pressure?

Easily - glass jars will do. And you could open a hole in it and allow air to rush in and use the movement of the air to do useful work. However, once the jar is full of air, that's the equivalent of being out of fuel. And as @hutchphd calculations above show, you'd need a vacuum jar the size of a large articulated lorry to have an energy store similar to a normal car fuel tank.

It's not impossible - just not practical.

 

[russ_watters]

I think we had another thread once where someone asked a similar question, like using a vacuum cleaner for propulsion. Sure, a vacuum cleaner [pump] is just a really inefficient fan.

 

[jbriggs444]

I think we had another thread once where someone asked a similar question, like using a vacuum cleaner for propulsion. Sure, a vacuum cleaner [pump] is just a really inefficient fan.

It brings to mind Feynman's inverted sprinkler. Or the imagining of a jet engine: "It sucks itself along like a Hoover".

As it turns out, the propulsion is better ascribed to the exhaust stream, not the input stream.

 

[DaveC426913]

And think of the ancillary advantages of such a propulsion system! When it's fully "fueled", it's lighter than when it's empty! In fact, the more auxiliary "fuel" tanks you load it up with the lighter it becomes!

It is left as an exercise for the motivated reader to calculate how many fuel tanks one could add before one's car becomes a balloon and floats away over rush hour traffic.

 

[jbriggs444]

the more auxiliary "fuel" tanks you load it up with the lighter it becomes!

For style points, make the containers spherical, construct their walls from lead and make them extremely thin, paint them in bright colors and tie them to a lawn chair.

 

[russ_watters]

It brings to mind Feynman's inverted sprinkler. Or the imagining of a jet engine: "It sucks itself along like a Hoover".

As it turns out, the propulsion is better ascribed to the exhaust stream, not the input stream.

Yes, that's true of a jet, but not a fan. For a jet*, the ram air pressure helps in the compression, so you have a high pressure area ahead of it. A fan creates a low pressure area ahead of it, so from a gauge pressure perspective it truly is both pulling+ and pushing.

*Turbojet or ramjet at least - a turbofan i'd expect pulls a low pressure area ahead of the fan.

+Some people really hate gauge pressure and hate it when I say this. But not as much as when I quantify vacuum as positive.

 

[Bike Mi Vie]

Easily - glass jars will do. And you could open a hole in it and allow air to rush in and use the movement of the air to do useful work. However, once the jar is full of air, that's the equivalent of being out of fuel. And as @hutchphd calculations above show, you'd need a vacuum jar the size of a large articulated lorry to have an energy store similar to a normal car fuel tank.

It's not impossible - just not practical.

Ok this right here is what I'm trying to prove. I want to show my father numbers and illustrations. I want to show him. How much energy it takes to do move a car or a mass of x pounds.

I was talking to my father again and we watched a video where a guy made a vacuum between the size of a baseball but cut in half And the person trying to pull it apart. What's the energy required to pull that apart. ? Right we'd need to measure the force it takes? What's the relationship between force and energy, if there was no loss been conversion.

If the ball was x inch in diameter it would take y amount of force to pull apart. Using that force and convert it into energy, it would take that amount of energy to move a to mass of z pounds. You are in the right track by saying a large articulated lorry, if that is true.

My dad is essentially saying using compressed air to power a car, but in reverse. So essentially, if there is an equation, flip all the numbers that are positive to negative ( that how I understand him). I want to be able to show him numbers by using equations. If you ever saw a episode of 'because science" that is what I'm trying to do to him.

How much energy does it take to move a 100 lbs person.

I think explaining to me ,large articulated lorry, would help me show him it's impractical

 

[hutchphd]

I was talking to my father again and we watched a video where a guy made a vacuum between the size of a baseball but cut in half And the person trying to pull it apart. What's the energy required to pull that apart.

The 3inch cut ball will have a "face" of <10 square inches and can provide at most (15 lbs/square inch)(10 square inch)=150 lbs. for a distance of maybe 1/4 inch. as you pull. Lots of force, very little energy. For the next quarter inch of motion you need another ball !!!!! Energy = Force x Distance
How many vacuum balls to go a mile?? A truckload, like @Ibix said

 

[Klystron]

You and Dad might be interested in a series of 'vacuum sphere' experiments conducted in the 17th Century called the Magdeburg hemispheres.

The Magdeburg hemispheres are a pair of large copper hemispheres, with mating rims. They were used to demonstrate the power of atmospheric pressure. When the rims were sealed with grease and the air was pumped out, the sphere contained a vacuum and could not be pulled apart by teams of horses in a famous first public demonstration in 1654.

The wikipedia entry veers into local politics, but one can find this famous experiment described in many physics and aerodynamics textbooks.

 

[Bike Mi Vie]

You and Dad might be interested in a series of 'vacuum sphere' experiments conducted in the 17th Century called the Magdeburg hemispheres.The wikipedia entry veers into local politics, but one can find this famous experiment described in many physics and aerodynamics textbooks.

Yeah the video I saw mentioned that same experiment lol.

 

[Vanadium 50]

Pressure has dimensions of energy density - the available energy will be approximately atmospheric pressure times the volume of you tank. That will work out to about 0.3% as much as gasoline.

You will run out of "gas" (pun intended) after a mile or so. Put the other way, a 20 mile range (like a bad EV) would take a tank that's hundreds of gallons.

 

[Klystron]

Yeah the video I saw mentioned that same experiment lol.

As a cyclist you may be interested in the evolution of the bike tire from a solid rim similar to a wagon wheel, to a pneumatic tube inflated by an air pump. While an example of positive air pressure, the cushioning shock absorbing effect of the rotating tire varies in interesting ways depending on the mass of the bike plus rider, the terrain, tire pressure and bike velocity.

In the US credit for improving biker comfort often resides with the Wright Brothers who owned a bicycle store in Dayton, Ohio, where they also designed wind tunnels and aircraft. The brothers gave credit to their Dad for their early ideas.

The brothers gained the mechanical skills essential to their success by working for years in their Dayton, Ohio-based shop with printing presses, bicycles, motors, and other machinery. Their work with bicycles, in particular, influenced their belief that an unstable vehicle such as a flying machine could be controlled and balanced with practice.[13]: 169  This was a trend, as many other aviation pioneers were also dedicated cyclists and involved in the bicycle business in various ways.

I mention wind tunnels because some designs use pusher fans to increase air pressure in front of the model and also puller fans to lower pressure behind the model that combine to simulate air flow at varying altitudes and automobile/aircraft velocities.

Some very high pressure blow-down tunnels also have large strong near-vacuum spheres at the end of the tunnel that rapidly lower the air pressure in the test chamber while also collecting the superheated air from the blow down (in some designs).

Picture of a hypersonic blow down wind tunnel test section with a human technician for scale.

 

[jbriggs444]

How much energy does it take to move a 100 lbs person.

One pithy answer is that it takes no energy at all.

If you have a 100 pound person over here and you want him over there you can grease up the floor (no energy required) give him a small push toward the goal (requiring as little energy as you please), wait for him to slide where you need him and then give him a push so that he stops moving (recovering the energy you used in the first push).

Zero energy used.Another answer is that it depends on how fast you need him to get there. If you want to get your 100 pound (call it 50 kilograms) person to move 10 kilometers in 1000 seconds and you are not planning on reclaiming any energy at the end of the trip then the most energy-efficient way to go about it is to accelerating him to 10 meters per second as fast as you can and then let him coast the rest of the way.

The required energy is given by $KE=\frac{1}{2}mv^2$ where $m$ = 50 kg and $v$ = 10 meters/second. That comes to 2500 Joules -- enough energy to run a 100 watt incandescent bulb for just over 4 minutes.

2500 Joules energy used.The above assumes an unrealistic road with zero friction in a weightless vehicle with no air resistance.

A quick trip to Google (see table IV on page 148) suggests a ballpark figures of 50 pounds of rolling resistance and 125 pounds of wind resistance for a sedan cruising at 50 miles per hour for a total of about 175 pounds.

A pair of 3 inch diameter evacuated hemispheres can provide 105 pounds of vacuum force ($F=Pa$ and $a=\pi r^2$ with $r$ = 1.5 inches and $P$ = 14.7 PSI). This is not quite enough to push the [rather ancient] cars contemplated in the Google reference at 50 mph, but it is close.

Let us re-imagine the pair of 3 inch diameter hemispheres as a 3 inch diameter evacuated cylinder and a 3 inch diameter piston with a 3 inch stroke. The piston will have 21.2 cubic inches of displacement ($v=\pi hr^2$ with $r$=1.5 and $h$=3.0) the hemisphere pair totals only 14.1 cubic inches ($v=\frac{4}{3}\pi r^3$ with $r$=1.5).

Roughly speaking, that means that the vacuum energy in 21.2 cubic inches of vacuum is good for only 3 inches of automotive motion at 50 mph. Meanwhile, the piston would take about 3.5 milliseconds to complete that stroke.

 

[russ_watters]

The 3inch cut ball will have a "face" of <10 square inches and can provide at most (15 lbs/square inch)(10 square inch)=150 lbs. for a distance of maybe 1/4 inch. as you pull. Lots of force, very little energy. For the next quarter inch of motion you need another ball !!!!! Energy = Force x Distance
How many vacuum balls to go a mile?? A truckload, like @Ibix said

150 lb is on the right order of magnitude. From memory I've previously calculated that a car going 60 mph and getting 30 mpg is outputting about 17 hp. That's about 100 lbs of propulsion force.

Figuring out the power of a vacuum pump to provide this is much more difficult. You're basically talking about powering the compressor section of a small jet engine.

 

[hutchphd]

Yes absolutely. The major misunderstanding in the OP is the equating of size of a force with work done by that force. The rest is (interesting) detail.