How to calculate PSI created in an internal combustion engine

[Brian James]

Hi, working on a project and I need some assistance due to my lack of education in mechanical engineering. With that said I will be as specific as possible.

I need to figure out how much PSI is exerted on a piston during the power stroke in an external combustion emgine. I'm not sure how to calculate that figure. The information I do know is my theoretical 6 cylinder engine has a cylinder diameter of 135 mm and a stroke of 150mm and produces 1200 ftlbs of torque. I would imagine that with this information I should be able to calculate the amount of force produced by each cylinder, and that number would be my PSI exerted to the piston during the power stroke, correct? A little humiliated by my lack of knowledge for I never studied mechanical engineering so I apologize for my incompetence. But any help with this matter would help exponentially and if I need to give anymore information please tell me specifically what you need. Thank you!

 

[Ronie Bayron]

Hi, working on a project and I need some assistance due to my lack of education in mechanical engineering. With that said I will be as specific as possible.

I need to figure out how much PSI is exerted on a piston during the power stroke in an external combustion engine. I'm not sure how to calculate that figure. The information I do know is my theoretical 6 cylinder engine has a cylinder diameter of 135 mm and a stroke of 150mm and produces 1200 ftlbs of torque. I would imagine that with this information I should be able to calculate the amount of force produced by each cylinder, and that number would be my PSI exerted to the piston during the power stroke, correct? A little humiliated by my lack of knowledge for I never studied mechanical engineering so I apologize for my incompetence. But any help with this matter would help exponentially and if I need to give anymore information please tell me specifically what you need. Thank you!

It's not a topic that can be learned with one set-up or response via forum. Mechanical engineers passed through rigorous academic studies & test in Physics, Chemistry and Math before comprehending the close to complete image of things. Anyhow, I came across this software recently Engineering Equation Solver, they have a demo version. There you can find a simulation of internal combustion, diesel and gasoline engine.

You may download their demo version and by exploring it, you might find the answer of your quest.
I prefer you find a friend whose got background in Physics & Mech. eng.

 

[Brian James]

It's not a topic that can be learned with one set-up or response via forum. Mechanical engineers passed through rigorous academic studies & test in Physics, Chemistry and Math before comprehending the close to complete image of things. Anyhow, I came across this software recently Engineering Equation Solver, they have a demo version. There you can find a simulation of internal combustion, diesel and gasoline engine.

You may download their demo version and by exploring it, you might find the answer of your quest.
I prefer you find a friend whose got background in Physics & Mech. eng.

Thank you for the response! I was hoping it would be as simple as a plug and go equation but I'll see what I can accomplish with the information you gave me and see what I come up with and try to post amy update.

 

[MRFMengineer]

Maybe this helps: https://en.wikipedia.org/wiki/Mean_effective_pressure

 

[Brian James]

That's new information to me but doesn't quite help, or at least I don't think. To be more specific and re word my question a little better; I need to figure out how much down ward force exerted on the piston is necessary per cylinder to produce 1200 ft lbs of work of my original 6 cylinder theoretical engine.

Similarly, my question could be compared to asking if I have just a single cylinder engine, and I know I can exert (let's just use random numbers for sake of the example) a constant 500 psi for 30° of the rotation of the crank (I say this because we all know that during the combustion cycle, the increasing volume caused by the piston moving down the cylinder effectively reduces the cylinder pressure, thus reducing the power exerted to the piston, so 30° seems to be a safe number to pick for duration of constant force.) for every 1 of 4 total rotations of my crankshaft, how much working force could I expect my engine to produce?

I think asking it that way would help answer my question more effectively. Any more links anyone wants to send my way would be helpful too, I've done days of extensive searching with numerous key words with no luck.

 

[billy_joule]

That's new information to me but doesn't quite help, or at least I don't think. To be more specific and re word my question a little better; I need to figure out how much down ward force exerted on the piston is necessary per cylinder to produce 1200 ft lbs of work of my original 6 cylinder theoretical engine.

The question doesn't make sense.
Any engine can do 1200 ft lb of work.
Any non-zero force can perform any arbitrarily large quantity of work given enough time.
In other words, one horse can do the work of a million if you give it a million times as long to do it.
What really matters is power: the rate of doing workhttps://en.wikipedia.org/wiki/Power_(physics)
https://en.wikipedia.org/wiki/Work_(physics)

Or maybe you meant lb ft of torque?
https://en.wikipedia.org/wiki/Pound-foot_(torque)
https://en.wikipedia.org/wiki/Torque

Similarly, my question could be compared to asking if I have just a single cylinder engine, and I know I can exert (let's just use random numbers for sake of the example) a constant 500 psi for 30° of the rotation of the crank (I say this because we all know that during the combustion cycle, the increasing volume caused by the piston moving down the cylinder effectively reduces the cylinder pressure, thus reducing the power exerted to the piston, so 30° seems to be a safe number to pick for duration of constant force.) for every 1 of 4 total rotations of my crankshaft, how much working force could I expect my engine to produce?

The force is found via
F=PA
Force = Pressure* Area
https://en.wikipedia.org/wiki/Pressure
So for a given pressure a larger engine produces a greater force

I think what you're really interested in is power output?
P = Fv
Power = Force*velocity
So for a given force, a higher RPM will produce more power, that's why F1 engines spin at 18k rpm

 

[Brian James]

Like I said I apologize for my lack of knowledge I know that makes this difficult. I'm collecting that what I thought would be a simple a+b=c formula to discover what I want, isn't so. Joule you're correct; power output is was I'm most interested in discovering. I understand that even your typical lawnmower can produce any given number of work load if given enough time. It's the second equation that helps me most I think. I understand that an ICE produces more power as the engine speed increases and has a power band close to where the hp and ftlbs of torque cross at the 5454 mark on a dyno sheet. And their ratios and power output change dramatically based on displacement, stroke, compression, fuel, cam size, you know where I'm going with it. But I was trying to look at it more simple, without all those factors, along with friction loss and heat loss and so on. I just wanted a simpler look at it. I was hoping there was a theoretical equation that if given a specific force of 500 psi over an area of 9.5 inches^2 during a duration of 30° via a 3" stroke, how many ftlbs of torque should I expect to produce at 2,000 rpms at the flywheel. Is it easier to answer my question when given that way or am I still not making any since?

 

[Brian James]

And if it helps any I could try uploading a picture of what I'm trying to figure out so you have a better idea of where my head is at if that might help?

 

[Nidum]

Read some of this

 

[Brian James]

Thats it! It's going to take me a little bit to understand some of the terminology but that's what I need to get what I'm looking for. Thanks!

 

[Brian James]

Can someone help explain this a little better for me?

 

[billy_joule]

The slide uses the definition of work and some geometry to show how work done by torque is defined.

"If a force is allowed to act through a distance, it is doing mechanical work. Similarly, if torque is allowed to act through a rotational distance, it is doing work."
https://en.wikipedia.org/wiki/Torque#Relationship_between_torque.2C_power.2C_and_energy
See post #6 for info on work.