By Jan Ridders Model Engineer

This engine is actually a re-design of the first internal combustion engine that I ever built. That original design was by Kees Geverde.

The Atkinson engine has a very special system for linking the piston and the crankshaft. A characteristic of this design is that the piston makes all four strokes with only one revolution of the crankshaft and flywheel, and it is fascinating to watch.
Building this engine was relatively simple because there are no difficult parts and one can make it by turning and milling standard materials; it took me about 80 man-hours.
After a lot of attempts and experiments I succeed in getting the engine to run. However, it only ran at very high speeds (2000 to 3000 rpm) with the result that the engine’s behaviour was very turbulent due to the abrupt and multi-reversing of the special linkage system. The engine had to be fixed to the workbench and the enormous forces generated damaged several parts of the whole driving system.

To improve the performance I set about making four major changes to the original design. More recently I made further changes and made CAD drawings of the redesigned engine which are available below.

First change: piezo crystal ignition
In the original design the ignition was supplied from a high-tension coil as used in (classic) motor cars. The switch with capacitor for interrupting the primary current was mounted against an upright and was driven with a cam on the flywheel axis.
I got the idea to create the ignition spark using a piezo element from a lighter normally used for gas stoves. A push rod, driven by a cam on the flywheel shaft takes over the role of the human hand operating the gas lighter. A very compact and simple system without the need of a large high-tension coil and an external electrical supply! The piezo delivers a short row of sparks, which is even better for igniting the gas mix.
You may well have to adapt the design a little bit to suit the lighter you have available. Choose the biggest one you can get and it will work perfectly.

Second change: Strengthening the driving system
To prevent damage, I first strengthened the mechanical system, including hard soldering the crank onto the (thicker) 12mm flywheel shaft and making the main part of the linkage system out of a single piece of 6mm thick aluminium.

Third change: ‘taming’ the engine
I was convinced that I could only ‘tame’ this wild thing by reducing the revolution speed significantly. It turned out that this was not possible through any adjustment of the classic carburettor, because the engine always stopped running at speeds below 2000 rpm. There were no other adjustments possible such as changing the timing for the intake valve versus the exhaust valve, because the original design is based on a self-sucking system. That means that the intake-valve is not driven by a pushrod against a camshaft but that it has only a spring that closes this intake valve as long as the overpressure in the cylinder is higher than the spring force. The determination of the correct spring strength is therefore rather critical as well; too low means leakage, too high means no intake of the fuel mix.
So I decided to add a second pusher system for the intake valve, similar to that of the exhaust valve. There was just enough space for adding a second cam on the camshaft. The intake valve can now be 100 per cent certain of closing. The camshaft can easily open this valve.
Experiments provided me with the best cycle timing for the engine to run at much lower speed of about 500 rpm. I did the measurements with a protractor on the flywheel:
0º Upper position of the piston;
10º Inlet valve starts to open, also 10º after the upper position of the piston;
75º Inlet valve is completely closed. The gas mix in the cylinder is compressed after 90º;
185º Piezo spark, also 5º after the upper position of the piston. An earlier spark can cause a backfire with the relative low speed of this engine;
260º Exhaust valve starts opening; also about 10º before the lowest position of the piston;
370º(10º) Exhaust valve completely closed, also at the same moment at which the inlet valve opens.
With this change the engine behaviour is acceptable. It stays put on the table with revolution speeds between 300 and 800 rpm. But it is still somewhat tempestuous; it is still exciting to watch it running!

Fourth change: The carburettor
The carburettor is one of the most important parts of an I/C engine. It determines the performance and the behaviour of the engine to a large extent. Although the Atkinson engine did run with the classic carburettor I wasn't very happy with it and decided to develop a new and unique carburettor system.

New drawings and more changes

In January 2010 I made a re-design of this engine with new CAD drawings (click on miniatures below for full size drawings) for two reasons:
1. I had implemented a lot of changes compared to the original drawing plan (which wasn't mine, anyway) mainly to improve the original behaviour of this engine. I also added three-dimensional views and English text.

2. My original Atkinson is running well and is reliable, but its behaviour is still rather erratic. The reasons are the abrupt, reversing movements in the driving rods and probably also the valve tumbler system with its long pushers.

My latest design incorporates a compact overhead camshaft without tumblers as I did successfully with my Otto 4-stroke engine. This camshaft is driven by a flexible tooth belt around cogwheels on the flywheel axis and the cam shaft (1:1 ratio).
3. Instead of the classic carburettor I included the Petrol Vapour Carburettor which I now have successfully used for all my I/C engines (drawings can be found on this website).
4. There are some other small (cosmetic) changes such as a smaller exhaust muffler mounted directly on the cylinder head and the wooden base.

PLEASE CLICK ON DRAWINGS BELOW TO ENLARGE

AND ON ‘DOWNLOAD’ FOR LARGER STILL