Oh I need some help.

**gareth01422** · Nov24-09, 02:31 PM

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 web site i can read about how to work this out?

Gaz

**xxChrisxx** · Nov24-09, 02:43 PM

Do you have the geometry and weights of the components?

**berkeman** · Nov24-09, 02:47 PM

Originally Posted by 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 web site 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** · Nov24-09, 02:54 PM

Originally Posted by berkeman

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** · Nov24-09, 03:12 PM

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** · Nov24-09, 03:17 PM

Originally Posted by 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.

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** · Nov24-09, 03:21 PM

Originally Posted by gareth01422

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 becuase 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** · Nov24-09, 03:23 PM

Originally Posted by xxChrisxx

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 becuase 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 thats ok. I will put my pressures into a excel spreadsheet like yours to start of with, This will take me a while.

Gaz

**xxChrisxx** · Nov24-09, 03:35 PM

No problem bud.

**gareth01422** · Nov24-09, 04:02 PM

Originally Posted by xxChrisxx

No problem bud.

Hi chris

So Ive 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** · Nov24-09, 05:18 PM

You dont need to know the flywheel inertia.

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

**gareth01422** · Nov24-09, 05:19 PM

Originally Posted by xxChrisxx

You dont 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** · Nov24-09, 05:25 PM

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

Or if you dont need to show derication, google piston acceleration. It'll give an equation in terms of crank angle and engine speed.

**gareth01422** · Nov24-09, 05:30 PM

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** · Nov24-09, 07:12 PM

Originally Posted by 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

You found what website?

**gareth01422** · Nov25-09, 05:06 AM

Originally Posted by berkeman

You found what website?

Sorry I forgot to add the link.

http://www.marinediesels.info/2_stro...movement_2.htm

Gaz