I've been working on some extrapolating the shape and size of the moving parts of an engine, given limited starting figures. All I know about an engine is number of cylinders, cylinder configuration, bore & stroke. From this I need to estimate the dimensions of many moving parts.

I've come up with some rules of thumb based on research, estimation and guesswork, but I'm still questioning how accurate they are. I'm assuming all engine parts are steel (I'm hoping this is normal for a road car?).

So rather than post my estimates and have people working from (potentially) bad figures, if anyone experienced enough with engines could post any ideas about the following, I'd be very interested:

* piston height, wall thickness and surface thickness (assuming the surface would be thicker than the walls?)
* con rod length, diameter, big & little end diameters
* bore spacing

Also, as an extra complication, how would any of these dimensions change if, for example, pistons were made of aluminium, or conrods made of titanium.

If you can't think of any such rules, then I'm still interested in more examples to work from.

Cheers for any help.

I'd have to look up some sizes to give you, but using my imaginary air-ruler I'd say most pistons are about 6 - 8 cm in total height, with a skirt thickness around 4mm and a crown thickness of about 10mm. There is quite a complex structure inside a piston to account for the expansion rates of the different thicknesses around gudgeon pins and what not.

They are usually, in road cars, cast aluminium. Cylinder heads are nearly always aluminium as well, and a lot of blocks/crankcases are aluminium now too, but the majority remain cast iron. Conrods and cranks are usually cast iron too, rather than forged or machined steel.

Rod length depends heavily on engine, but a range between 130 and 200mm would cover many. They have a varied cross section, so a singular diameter value can't be given. Usually I section overall (although the I section is routinely in the wrong plane, even on many 'race' rods).

Big and little end diameters also vary, but between 45 and 55mm won't see you far out. Little ends are smaller (duh!), at around the 20mm mark.

Bore spacing (minimum distance between the walls of two adjacent cylinders) can be as little as 5mm or as much as 30mm.

Quite a tricky, complex little project you've got going by the sounds of it.

Quote from tristancliffe :

Cylinder heads are nearly always aluminium as well, and a lot of blocks/crankcases are aluminium now too, but the majority remain cast iron.

Of the engines which do have aluminum blocks, don't they usually have steel/iron cylinder liners?

Yup. I think some very expensive race engines use a ceramic coating straight on the aluminium to give a liner literally microns thick rather than a thick steel liner, but I wouldn't be able to cite any examples. Aluminium on it's own wouldn't be durable enough to cope with the friction.

Wooden pistons are quite good though. And they work!

Regarding the difference between aluminium and iron parts I'd start my research at the specific yield strenght (yield strenght per density). That will give you the corresponding amount of aluminium you need to sustain the same amount of force as the original iron part. Add 5% due to technological difficulties with aluminium though (larger termical elongation, etc.). That'll give you an estimate until you have better data.

Try working out the necessary lenghts and widths for some parts of the engine like conrods. Your formulas should include the terms that are necessary for the part to work and also a part that accounts for all material added for engineering reasons.
In the example of a conrod you absolutely need:
- Twice the thickness of the small end bearing shell, determined by fatigue strenght of cast iron and the amount of centrifugal force generated by the cylinder
- The diameter of the small end bearing
- Stroke
- Height of cylinder minus distance from small end bearing to cylinder surface
- Twice the thickness of the big end bearing shell
- The diameter of the big end bearing, determined by friction and thermal properties of the bearing material and physical properties of the cranbk shaft
On top of that you need some space to account for other influences you can't estimate. When you start researching real part's dimensions you should have every part broken down into such necessary and optional dimensions. Then measure both necessary and optional dimensions from real parts. If your formulas are correct you should recieve a gaussian distribution for each dimension. If not, recheck.

I hope that helps.

Vain

That's a good way of thinking about it, cheers Vain.

I'll rework some maths tomorrow and see if the figures start moving in the way I'm hoping. Cheers for those estimates Tristan, I'm still interested in any hard figures you have to-hand. I take it you values would be most applicable for your typical four banger?

Well I think this proved sucessful, I got what I hope are much better numbers out from my maths using Tristan's figures.

More recently, I've worked out how thick a shaft needs to be to handle a specific maximum torque input, and I'd now like to do the same for gears, although I'm unsure where to start on that front.

If I have two different size gears meshing together, and I know the ratio of sizes (although not necessarily the exact number of teeth), and the maximum torque being applied to the drive gear, the maximum angular velocity of the gears, the material used for construction and all relevant material parameters, what maths do I need to perform to decide how big the cog should be? I can start with standard external gears as I presume that's easier, but it's bevel gears I'd ideally like to work with.

If anyone can shed some light, or at least start steering me, it will be much appreciated.

Cheers.

If you want to calculate the volume of a gear you also need to know wether it's straight cut or helix cut because that affects how many gears are engaged at any time (which affects the tension in the material caused by the torque etc.).
The angle of the helix thus has a lot of influence on the dimensions of the gear.

I have no idea how the angle of the helix differs between different gearboxes though. Ask a mechanic.