Power delivered/Internal work done by a car's engine

[alkaspeltzar]

Attached is an example from my Tipler physics book of a determining the power delivered from a cars engine. I get the derivation(math), but am having some issues with the understanding/concept.

Mainly, isn't Power=Work done/time. If so, they define the system as Car-earth. There is no external work done on the earth-car system. Yet we are solving for the power done by the engine as if it was doing work on the car.

So my question are:
1. Is the engine doing internal work, taking chemical energy(fuel) and putting it into the car which become gravitational potential energy? So the power we are really finding is the internal work of the engine/time.
2. Externally, no work is done, as the system as a whole doesn't lose or gain energy, so net work on the system is zero correct?
3. In general, I understand external work needs to be done to add/remove energy from a system. But can internal work be done within a system to change energy forms right? This is an example of this. So would be walking, where its the work of our muscles that gives us kinetic energy at the expense of our chemical/food energy.

I wish the book explained more, its kinda vague on the subject of how chemical energy can be converted to mechanical energy, and I am just trying to understand the means in which this is happening

 

[anorlunda]

You must be careful when defining the boundaries of "the system".

A solar powered car uses external energy from the sun. Energy from the Sun also produced fossil fuels. So even a car using gasoline is powered by external energy.

A car could run on a battery, and the unspecified energy used to charge the battery in advance can be considered external.

I prefer to analyze using the idea of external energy in the abstract. I say that the origin of the external energy is unknown. Then I don't waste my time thinking about it.

 

[Cutter Ketch]

In thinking about energy or work you are welcome to define the boundaries of the system you are considering to fit the questions you are asking. In this problem not every statement and question they ask is about the same boundaries. In the statement of the problem when they say that the work done on the earth-car system is 0 that is justification and proof of the subsequent statement that energy is conserved. This does not imply that all of the subsequent questions are about that Earth car system as a whole. In the subsequent questions they are asking you to consider smaller systems which happen to be pieces of that whole.

Noting the energy conservation in the larger system is useful in the following questions where they ask about smaller pieces of that system. The energy is not conserved when they ask about the work done by the engine on the car etc. but you can use the knowledge from the larger system, that energy is conserved, to say that the pieces must all add up to 0.

 

[alkaspeltzar]

Cutter Ketch,

So are you saying they use the car-earth system to justify and calculate how energy is conserved as a whole, but later then break it down to show that the change in Chemical energy/time is really the work done by the engine. AKA this is the smaller piece of the system where there is work being done. Energy is not conserved, and in this portion of the problem we are focusing our attention to the engine?

But couldn't we look at it as internal work/power being delevoped by the engine as well which yields an increase in the cars potential energy? Both ways of defining the system work on the same right?

Before I get to far down the rabbit hole, can someone answer my three questions above to know if I am right or wrong? Thank you

 

[jbriggs444]

But couldn't we look at it as internal work/power being delevoped by the engine as well which yields an increase in the cars potential energy? Both ways of defining the system work on the same right?

I do not see a system defined there. All I see is a book-keeping entry -- chemical energy consumed, potential energy produced.

 

[alkaspeltzar]

If they did define the system as the car-earth, then wouldn't the work of the engine be internal work converting chemcical energy to potential.

Or another way of looking at it would be looking at the cars engine alone, which would be doing work/power equal to the rate of change potential energy. Both systems would achieve same answer right?

 

[alkaspeltzar]

I guess here is what I am wondering:

My books says that "sometimes a systems internal chemical energy is converted into mechanical energy with no work done by an outside agent"...this is the example of the car or of a person walking.

So that makes me question, does that mean then the internal work to the system(say the car) is being performed to convert and transfer the chemical energy to mechanical energy. Since the system is closed, most of time we ignore the work side of it, just looking at the energy balance? In this example the book looks to be doing this, doing an energy balance. Then relating the power back to the engine of the car itself.

This would be similar to how we look at gravity/potential energy converted to kinetic energy of a falling object. Really gravity is doing work in speeding it up, but we most of time ignore that.

 

[sophiecentaur]

I wish the book explained more

I wish the book acknowledged the power needed in just getting along the road against air resistance and friction. At 70mph, that is equivalent to a fairly steep incline yet the message in the exercise is about total energy.
Not surprisingly, they tend to specify a car engine power in terms of "brake horsepower" (or the SI equivalent) which looks at the power delivered to a shaft with a brake around a drum. That's the limit of the 'easy' region of study. Once you get to the wheels on the road, air resistance and inclines, you can get a whole range of answers (driving technique affects things too).
"self charging" Hybrid vehicles are an interesting case for this sort of discussion.

 

[alkaspeltzar]

I agree it is vague. I get it is just a problem, I just want to understand my questions in Post #1 and 7 if someone can shout out

 

[russ_watters]

I agree it is vague. I get it is just a problem, I just want to understand my questions in Post #1 and 7 if someone can shout out

I think you are overcomplicating all of this. The distinction between "internal work" and "external work" is rarely important/meaningful in my opinion.

 

[alkaspeltzar]

Can anyone read and answer my original questions and explain why. This would save a lot of back and forth

 

[russ_watters]

Can anyone read and answer my original questions and explain why. This would save a lot of back and forth

Why WHAT? What problem are you actually trying to solve here? Unless I missed it, the problems you attached do not mention "internal" vs "external" work.

 

[alkaspeltzar]

Why WHAT? What problem are you actually trying to solve here? Unless I missed it, the problems you attached do not mention "internal" vs "external" work.

Please see post one and the questions I asked. That is what I was trying to understand.

 

[alkaspeltzar]

It's not the math I don't get, I trying to understand the concepts. I listed 3 questions at the beginning of this post I was looking for clarification on, that was all. Thank you

 

[russ_watters]

It's not the math I don't get, I trying to understand the concepts. I listed 3 questions at the beginning of this post I was looking for clarification on, that was all. Thank you

The questions are badly worded and not really answerable. The best answer I can give without dissecting and rewording them (which seems pointless to me) is they are mostly wrong; the answers are "no". But I think you already know this since they contradict the correct answers you actually know to the problems in the book!

 

[alkaspeltzar]

No answerable or you don't understand?

 

[russ_watters]

I'll try the first one:

1. Is the engine doing internal work, taking chemical energy(fuel) and putting it into the car which become gravitational potential energy? So the power we are really finding is the internal work of the engine/time.

You haven't clearly defined what system you are talking about (where the boundary is), so I can't tell what is internal or external. But if I had to guess, I'd say you are moving the system boundary around and losing track of the question you are really trying to answer.

 

[alkaspeltzar]

I'll try the first one:

You haven't clearly defined what system you are talking about (where the boundary is), so I can't tell what is internal or external. But if I had to guess, I'd say you are moving the system boundary around and losing track of the question you are really trying to answer.

Say it's the car with the engine. I didn't make the problem it's from my book. So I'm just wondering myself which system they are using and how to interpret it. I assume the entire car would be the system but it's like the book then goes back to looking at the power of the engine itself. Curious as to how others see the problem

 

[russ_watters]

Say it's the car with the engine. I didn't make the problem it's from my book. So I'm just wondering myself which system they are using and how to interpret it. I assume the entire car would be the system but it's like the book then goes back to looking at the power of the engine itself. Curious as to how others see the problem

Well, we already told you we don't see the problem discussing internal vs external work, but you didn't accept that! But whatever: If you define the system as the car, just the car and the entire car, is mechanical work crossing the boundary of the system?

 

[alkaspeltzar]

What? You don't have an issue with internal and external work...how is that an answer? And if you read the posts, most answers get off subject, talking about solar panels and electric cars. Nevermind.

 

[russ_watters]

What? You don't have an issue with internal and external work...how is that an answer? And if you read the posts, most answers get off subject, talking about solar panels and electric cars. Nevermind.

The very first sentence of the very first answer pointed out that the issue here is being specific about the system boundary. If you really want to think about "internal vs external work" all you have to do is draw a clear system boundary and ask yourself if work crosses the boundary. That's how you use the concept of a system.

But that doesn't mean the concept is useful in answering the problem in the book.

 

[zanick]

Attached is an example from my Tipler physics book of a determining the power delivered from a cars engine. I get the derivation(math), but am having some issues with the understanding/concept.

Mainly, isn't Power=Work done/time. If so, they define the system as Car-earth. There is no external work done on the earth-car system. Yet we are solving for the power done by the engine as if it was doing work on the car.

So my question are:
1. Is the engine doing internal work, taking chemical energy(fuel) and putting it into the car which become gravitational potential energy? So the power we are really finding is the internal work of the engine/time.
2. Externally, no work is done, as the system as a whole doesn't lose or gain energy, so net work on the system is zero correct?
3. In general, I understand external work needs to be done to add/remove energy from a system. But can internal work be done within a system to change energy forms right? This is an example of this. So would be walking, where its the work of our muscles that gives us kinetic energy at the expense of our chemical/food energy.

I wish the book explained more, its kinda vague on the subject of how chemical energy can be converted to mechanical energy, and I am just trying to understand the means in which this is happening

Could a more simple explanation be just that the chemicals in the fuel tank be looked at as potential energy, and the mechanical energy is what the engine produces as a product of the combustion, their forces on the internal engine parts and unltimately torque at the rear axles... this energy is work. a force over a distance... and the power is the rate of doing that work... a rate of change of Kinetic energy. So, if you have accelerated your car from one velocity to another (KE at speed 1 minus KE at speed 2) in 1 second and it took 500,000 Joules (KE changed by 500,000J) to do it... then you would know that the HP required by the vehicle was (500,000J / 746w) or 670hp