Designing a crankshaft; need help with stroke to bore ratio

[dpbird90]

Currently doing a 3D CAD drawing project for uni, and we've been asked to create a crankshaft for a 1600cc 4 cylinder engine. I know how the firing order and number of bearings etc determines the design, but they've given us a stroke to bore ratio of 1:1.5 in the briefing. I've googled it, wiki'd it (http://en.wikipedia.org/wiki/Stroke_ratio) and still cannot see how this is going to affect my design. So, I'm calling on the might of PH to help out. Anyone know what to do? Thanks

[Dr Derek Doctors]

Well your bore is your cyl' size and you stroke is how far it moves.... you can easily work it out from that.

Edited by Dr Derek Doctors on Tuesday 23 November 19:25

[egomeister]

Bore to stroke ratio gives you the relative sizes. You know the total swept vol, so you can work out the volume that must be swept for each cylinder. Then with a bit of maths you can figure out what throw and spacing you'll need on the crank.

[dpbird90]

Thanks guys, got the pen, paper and calculator out now. Curiously, I came across this http://www.burtonpower.com/technical_1/formulae.as... which gives an example bore of 81mm and a formula to work out the stroke; how accurate is this? It says in the brief that any measurements not specified are left to us to decide, presumably meaning I can choose my own bore (which wasn't specified; just the stroke to bore ratio was) and divide by 1.5 to get the stroke?

[TheEnd]

You can't just chose a bore size, as a larger bore will mean it has to have a longer stroke, which will give it a larger capacity, and put you over the 1.6l limit.

The calculations are correct, so you'll need to work backwards from the capacity to find the displacement in each cylinder, and then the exact bore and stroke in the ratio give that will match.

[Mrs Fish]

James here:

Simple GCSE maths. All you have to do is pick a bore or stroke. 400cc per cylinder therefore pick the shape of the cylinder ie circular therefore pie r sq is area etc etc

Maybe suggest an oval or squared cylinder to make a more compact engine and get more marks?

[dpbird90]

Thanks for the help guys, I've looked at the calculation for capacity on that engine formulae website and done the following;

capacity=0.7854*bore*bore*stroke*number of cylinders

I have let x be the stroke, and as I've been given a stroke to bore ratio of 1.5, this means the bore is 1.5x (or 3/2x if you prefer)

So, plugging numbers into that equation;

1600=0.7854*1.5x*1.5x*x*4

Gives 3.1416*(9/4)x^3=1600

Quick bit of rearranging gives x^3=226.3531675... meaning x (the stroke) is 6.09437...cm and the bore is 9.141555...cm

For the degree of accuracy required, I can round these to 60.9mm for the stroke and 91.42mm for the bore (converted to mm as standard units for drawings).

Please tell me I'm right

EFA: I was talking bks, that is the stroke I was after
Edited for not converting bore to mm properly

Edited by dpbird90 on Tuesday 23 November 19:58

Edited by dpbird90 on Tuesday 23 November 20:00

Edited by dpbird90 on Tuesday 23 November 20:04

[TheEnd]

You have double checked whether it was bore/stroke, or stroke/bore?

[dpbird90]

TheEnd said:

You have double checked whether it was bore/stroke, or stroke/bore?

It was definitely stroke/bore;

Design Brief said:

2/ Stroke to bore ratio 1:1.5

[WhoseGeneration]

Mrs Fish said:

Maybe suggest an oval cylinder to make a more compact engine and get more marks?

Only if the OP has greater engineering resources at his disposal, than, for example, Honda.

[Tango13]

The numbers add up but the Throw on the crank is half the stroke. I think you should go oval...

[dpbird90]

Pressing on with the design itself (I can go back and edit dimensions when I'm finished), I've been looking at this diagram http://www.motorera.com/dictionary/pics/c/cranksha... for reference. Just want to check (warning, potential Karl Pilkington style question coming up) are the main journals the same diameter and length as the crankpins?

[dpbird90]

Tango13 said:

The numbers add up but the Throw on the crank is half the stroke. I think you should go oval...

Cheers, so I'd have a throw of 45.71mm. Another (potentially) idiot question; does this mean I set the diameter of the counterweights at 45.71mm?

[Tango13]

The diameter of the main bearing and big end journals vary enourmously from engine to engine.

As a general rule the big end journal is smaller than that of the main bearing. The only exception that I've managed to find is the Mercedes Benz W125 inline eight from 1937 which had 63mm mains and 66mm big ends.

[dpbird90]

Tango13 said:

The diameter of the main bearing and big end journals vary enourmously from engine to engine.

As a general rule the big end journal is smaller than that of the main bearing. The only exception that I've managed to find is the Mercedes Benz W125 inline eight from 1937 which had 63mm mains and 66mm big ends.

Cheers, although I've read the design brief further (always a good start!) and they want the main bearing to be 40mm diameter x25mm, and the big end journal to be 35mm diameter x25mm

[Frik]

Which CAD program are you using dp? You might not need to use the paper and pen so much.

[dpbird90]

We're using Solidworks 2007, its not too dissimilar to ProE, its simpler, but I still would have preferred Catia, seeing as all the racing teams prefer you to be experienced in using it.

[Tango13]

dpbird90 said:

Tango13 said:

The diameter of the main bearing and big end journals vary enourmously from engine to engine.

As a general rule the big end journal is smaller than that of the main bearing. The only exception that I've managed to find is the Mercedes Benz W125 inline eight from 1937 which had 63mm mains and 66mm big ends.

Cheers, although I've read the design brief further (always a good start!) and they want the main bearing to be 40mm diameter x25mm, and the big end journal to be 35mm diameter x25mm

Sounds a bit small to me but if that's what they are asking for then go with it but I'd be looking for mains of at least 50/55mm and big ends of 45/50mm.

[davepoth]

Cylinder dimensions check out using Pi*(bore/2)^2 * stroke.

[dpbird90]

Tango13 said:

dpbird90 said:

Tango13 said:

The diameter of the main bearing and big end journals vary enourmously from engine to engine.

As a general rule the big end journal is smaller than that of the main bearing. The only exception that I've managed to find is the Mercedes Benz W125 inline eight from 1937 which had 63mm mains and 66mm big ends.

Cheers, although I've read the design brief further (always a good start!) and they want the main bearing to be 40mm diameter x25mm, and the big end journal to be 35mm diameter x25mm

Sounds a bit small to me but if that's what they are asking for then go with it but I'd be looking for mains of at least 50/55mm and big ends of 45/50mm.

Yeah I thought it was a bit small, but that's how they want it. Just in case I've missed anything, here's my design brief in full:

Design Brief said:

Using SolidWorks, design a basic crankshaft for a 4 cylinder, 4 stroke engine of the following specification.

1/ Cubic Capacity of engine 1,600cc

2/ Stroke to bore ratio 1:1.5

3/ Firing order 1,3,2,4

4/ Number of main bearings 5

5/ Main bearing journal size 40mm dia × 25mm

6/ Big end journal size 35mm dia × 25mm