Calculating Torque & Power of a Single Piston Engine | Gaz

[gareth01422]

Hi guys

I'm new here and needed some help with an idea i have. I am trying to work out how much torque/power a single piston engine will have, after i have designed it. I know how much force there wil be on the piston (in lds) at all positions of the piston. I gather i need to work out the leverage on the crack.

Is there a website i can read about how to work this out?

Gaz

 

[xxChrisxx]

Do you have the geometry and weights of the components?

 

[berkeman]

Hi guys

I'm new here and needed some help with an idea i have. I am trying to work out how much torque/power a single piston engine will have, after i have designed it. I know how much force there wil be on the piston (in lds) at all positions of the piston. I gather i need to work out the leverage on the crack.

Is there a website i can read about how to work this out?

Gaz

Have you had basic trigonometry? What math classes have you had? What physics classes?

 

[gareth01422]

Have you had basic trigonometry? What math classes have you had? What physics classes?

I am studying with the open university at the moment only just touching on physics at the minute (studying a book called Energy and light). So i have basic maths skills but learning all the time.

Also I have a modle drawn up in CAD (solidworks).

Gaz

 

[xxChrisxx]

Good good, well you can use the following method.

You need to plot the gas pressure force vs crank angle. Then to find the inertia force of the piston going up and down, and plot that vs crank angle.

You can combine these two fo find the force acting down the cylinder axis. The graph should look like this:

You acn then use this figure to work out the torque, at the crankshaft using the folling equation.

From that you get a graph of torque vs crank angle. Fin the average torque and work out power from that.

 

[gareth01422]

Good good, well you can use the following method.

You need to plot the gas pressure force vs crank angle. Then to find the inertia force of the piston going up and down, and plot that vs crank angle.

You can combine these two fo find the force acting down the cylinder axis. The graph should look like this:

You acn then use this figure to work out the torque, at the crankshaft using the folling equation.

From that you get a graph of torque vs crank angle. Fin the average torque and work out power from that.

This is great stuff.

I can easily find out what presure is on the piston as it pushes down, on the up stroke it will be inertia from the fly wheel. Do I find out the presures for every 5 degrees or do i go for 10 degrees?

Gaz

 

[xxChrisxx]

This is great stuff.

I can easily find out what presure is on the piston as it pushes down, on the up stroke it will be inertia from the fly wheel. Do I find out the presures for every 5 degrees or do i go for 10 degrees?

Gaz

Thats a judgement call, as with any other numerical simulation, the smaller the timestep the more accurate but time consuming it is.

Only you can judge the level of accuracy required. Tbh I can't remember what I used, but just that I got to within 5% accuracy. I didnt have a plot of gas pressure force I had to construct one and had to estimate pumping losses.

Note that at high engine speeds, the torque graph looks very funny, this is because inertia forces dominate and the gas pressure makes no difference. The average torque should always be the same though, if the gas pressures are constant (they arent in realisty, but i made that assumption)

 

[gareth01422]

Thats a judgement call, as with any other numerical simulation, the smaller the timestep the more accurate but time consuming it is.

Only you can judge the level of accuracy required. Tbh I can't remember what I used, but just that I got to within 5% accuracy. I didnt have a plot of gas pressure force I had to construct one and had to estimate pumping losses.

Note that at high engine speeds, the torque graph looks very funny, this is because inertia forces dominate and the gas pressure makes no difference. The average torque should always be the same though.

Chris

Thanks for the help, I may call back and need more guidance if that's ok. I will put my pressures into a excel spreadsheet like yours to start of with, This will take me a while.

Gaz

 

[xxChrisxx]

No problem bud.

 

[gareth01422]

No problem bud.

Hi chris

So I've got pressures for every 10 degrees from TDC (top dead centre). apart from the up stroke,

How do i calculate the inertia of the fly wheel as that is what it will need to push the piston back up. Also will i need to include this in the pressures for the down stroke?

Gaz

 

[xxChrisxx]

You don't need to know the flywheel inertia.

You can work this out directly from the mathematical equation determining the acceleration of the piston.

 

[gareth01422]

You don't need to know the flywheel inertia.

You can work this out directly from the mathematical equation determining the acceleration of the piston.

Ok so where do i go from here?

Gaz

 

[xxChrisxx]

It's time to dig out the maths books, and do some horrid derivation I'm afraid.

Or if you don't need to show derication, google piston acceleration. It'll give an equation in terms of crank angle and engine speed.

 

[gareth01422]

Chris

Once again, thanks for the help.

Ive found this website, Is this what i should be reading through and applying to my piston engine?

Gaz

 

[berkeman]

Chris

Once again, thanks for the help.

Ive found this website, Is this what i should be reading through and applying to my piston engine?

Gaz

You found what website?

 

[gareth01422]

You found what website?

Sorry I forgot to add the link.

http://www.marinediesels.info/2_stroke_engine_parts/Other_info/piston_movement_2.htm"

Gaz

 

[xxChrisxx]

I've just noticed something from another thread, my constants were lumped together differently, so my equation is different to many that are out thre. Until I can figure out exactly what I did (i've forgotten) and re done my derivations, please don't use my formula directly as it may very well be wrong.

I seem to have added a half to the second part, which I can't remember why. The method is correct however.

 

[usewf]

Good good, well you can use the following method.

You need to plot the gas pressure force vs crank angle. Then to find the inertia force of the piston going up and down, and plot that vs crank angle.

You can combine these two fo find the force acting down the cylinder axis. The graph should look like this:

You acn then use this figure to work out the torque, at the crankshaft using the folling equation.

From that you get a graph of torque vs crank angle. Fin the average torque and work out power from that.

how did you determine the gas forces..mine looks a bit differ..can give me the formula...already pm you..please help me chris X(

 

[xxChrisxx]

What are you trying to achieve, and what have you done already?

EDIT: It may be worth just starting a new thread on this.

 

[usewf]

here is my F vs theta...

 

[xxChrisxx]

You're gas forces look like they are taken directly from an ideal cycle PV diagram.

I did some tests and applied some assumptions to my ideal cycle PV to get a more representative graph.

 

[usewf]

im tryin to get ideal force vs theta..from my pv diagram..thus i multiply the gas pressure with area...however my PV diagram start from BDC at 180dgree..ive shifted the angle to get a graph like yours..however i just ended up like that...considering at intake and exhaust the pressure is equal to atm

 

[usewf]

did you mean that I've already found the answer? sorry i have limitation of references...now only you can guide me haha

 

[xxChrisxx]

Well whether you have found the answer depends on the reason for creating the graph.

If the idea is to just convert an ideal Otto PV diagram into force vs crank angle then your graph is exactly what I would expect to see.

 

[usewf]

cheers mate!
sorry for the stupidity lol, as i mention before I am really lack of references..tenx 4 for the view...btw do you interested in wankel engine?

 

[xxChrisxx]

No problem bud, the wankel is pretty cool.

 

[usewf]

damn rite! feel free to point your view at https://www.physicsforums.com/showthread.php?p=3092776#post3092776 if you had a free time XD...again tenx mate

 

[usewf]

hey chris tenx 4 your previous help i manage to get the ideal result..i've also done several graphical result for part load and volumetric efficiency...here is my result...is there any assumption that i can make so that my result will slightly comes like yours? I've thinking about making ignition at 20 btdc sadly i get stuck on the mathematical modeling...do you still have the spreadsheet? can you please send an email to me?

<<e-mail address deleted by Mentors >>
tenx!

 

[berkeman]

hey chris tenx 4 your previous help i manage to get the ideal result..i've also done several graphical result for part load and volumetric efficiency...here is my result...is there any assumption that i can make so that my result will slightly comes like yours? I've thinking about making ignition at 20 btdc sadly i get stuck on the mathematical modeling...do you still have the spreadsheet? can you please send an email to me?

<<e-mail address deleted by Mentors >>
tenx!

Please do not post your personal e-mail address in the open forums. It attracts spam bots.

 

[xxChrisxx]

I don't have the spreadsheets to hand any more, I made my assumptions to the PV diagram so they were there before I translated it to force vs crank angle.

I didn't model that bit I just put in the values manually, it was a bit of a fudge because I didn't have a running engine to test against. So I picked a peak pressure value of 75% theoretical maximum, as found from an old book. At set to peak to 13 deg aTDC, which is where the engine was tuned to give peak pressure.

Ended up giving a power value just over 5% over real value.