Doxford engine model

[sjam986]

Just for fun! My first attempt with FC

A simplified animation of one unit (cylinder) of a Doxford marine propulsion engine. The engine is a large 2-stroke opposed piston cross-head diesel engine, about 800mm bore. The centre crank operates the main piston which open the scavenge ports at the bottom of the cylinder while the two side cranks operate the upper exhaust piston which opens the exhaust ports at the top of the cylinder. This type of engine would normally have between 3 and 9 units but could be extended to 14 units.

This was modelled using Assembly4 in FC-0.19 on Windows 10 with much help from Zolko! (thanks!!)
Part production was straightforward. Main problem was getting geometry that was not moving in the XY plane to rotate without swapping orientation during rotation. My rotation axis was the X-axis, hence rotation was in the YZ plane. I also learned that all the parts of a sub-assembly need to be drawn in their correct orientation, otherwise linking the sub-assembly to the master sketch was a problem.

Video link for the animation is at https://1drv.ms/v/s!Ar_GKH8F5ZlYswuYeIB ... W?e=Z0SXzL

Doxford.JPG (52.81 KiB) Viewed 2727 times

p.s. for developers - It would make life much easier if the software did not change the orientation of moving parts when not rotating in the XY plane.

[chrisb]

p.s. for developers - It would make life much easier if the software did not change the orientation of moving parts when not rotating in the XY plane.

Use FreeCAD 0.19.

[sjam986]

p.s. for developers - It would make life much easier if the software did not change the orientation of moving parts when not rotating in the XY plane.

Use FreeCAD 0.19.

I am using FC 0.19.

[chrisb]

I am using FC 0.19.

In that case I had a different issue in mind. Can you please explain with a simple example which rotation is wrong?

[chrisb]

I had a look at the animation. It looks as if the cylinders with the controlling slots don't move. Was it forgotten, on purpose or postponed?

[kisolre]

cylinders with the controlling slots don't move.

Cylinders don't move Pistons move and when their combustion ends pass the slots they open/close them to the combustion chamber.

[chrisb]

Thanks, I see now that it is a two stroke engine. I was wrong assuming that it needed controlling the slots like a Knight sleeve valve engine.

[kisolre]

Knight sleeve valve engine.

First time I heard of that. Good read
https://en.wikipedia.org/wiki/Knight_engine

[chrisb]

It was the reason why I started with FreeCAD, I wanted to understand how it works. So it was my very first FreeCAD project, animation included. It took a whole week from downloading the first FreeCAD version to finish the animated engine. The piston is still missing as you can see at the bottom dead center.

[sjam986]

Yes, it is a 2-stoke engine. Static cylinder with scavenge ports at the bottom and exhaust ports at the top.
To demonstrate my problem I have stripped off the engine parts to leave the 3 sketches that control the motion.
With Zolko's help I have managed to get LCS_crank to rotate correctly. This was necessary because the crankshaft is asymmetric. If you observe the LCS_conrod, LCS_side1 and LCS_side2 during the animation, they swap orientation during the revolution and I have not found a way to correct this. In this engine it is of no consequence because the conrod and siderods are symmetric, but it may be a problem for future projects.

dox_master_sketch.FCStd

(13.08 KiB) Downloaded 62 times