2D Two Stroke Engine Display and Fidget Toy

The 2 stroke engine can be difficult to understand how it works, for such a simple device there is a lot going on, so this display project is designed to help you get your head around it. It's about the size of a credit card and is fun to play with so also makes a great fidget toy.

The Display is cheap and easy for students to make their own and can fit in your pocket to show your friends.

You will need the laser cut parts, which you can download and cut out in 3mm clear acrylic and another colour.

• Some 0.9mm MIG welder wire I use stainless but steel wire will also work
• Some 1.75mm 3D printer filament.
• A tiny drop of glue
• Sandpaper
• Side cutters
• Somewhere clean to work, as the acrylic sheet is a bit of a dust magnet

Once you have your laser cut parts grab some MIG wire and clean out all the holes, as sometimes the material gets stuck in the hole, especially the .9mm holes. You can also remove the protective coating from the acrylic but make sure the area is clean as the static build-up will attract dust I like to leave the coating on the outside of the 2 cover pieces to protect from scratches and fingerprints during assembly.

Remove the coating from the wheel and the crank and insert a short length of MIG wire into the thumbwheel. Place a drop of glue on the crank and fit it onto the MIG wire and press down firmly place to one side and allow the glue to cure.

The filament is a tight fit and can put up a fight, also I noticed that some colours and brands can be tight or loose. You could use nuts and bolts if you wish, but the filament keeps the cost down. Push the filament through the holes on the side cover and cut them off much longer than you need. It also helps if the filament are all different lengths so that when the next pieces are fitted your not trying to align several filaments at once.

The next piece is the clear part with the large hole at the bottom. Align the holes and fit the filament and push them togther.

Next push the MIG wire through the thumbwheel and drop it into place. The black crankcase and the 2 small pieces that form the intake exhaust, and transfer ports are next.

The sharp corners on the piston need to be sanded round as they will catch the ports and cause the model to jam if assembled straight out of the laser cutter. The piston and connecting rod can be fitted and the outer case fitted.

Test the mechanism and check it moves freely without jamming and then the excess filament can be cut flush with side cutters. If all is well remove the final two protective films from the outside and you're done!

Two-stroke engines have two important advantages over four-stroke engines: They are simpler and lighter, and they can in theory produce about twice as much power. They are widely used in chainsaws and go-karts, the engine in the photo is a Lepard kart engine is rated at 27 horsepower, from 125cc which incredible from such a small light package

There are variations in 2 stroke design. The diagram is the same as the display model is a piston port, the Gif is a reed valve version. There is also a rotary valve version. Their operation is very similar.

In a four-stroke engine, each of the four essential steps of the power-producing cycle is given its own piston stroke:

1) Compression

2) Power

3) Exhaust

4) Intake

A 2 stroke engine performs all the same steps but in just two piston strokes. The simplest 2 stroke engines do this by using the crankcase and the underside of the moving piston as a fresh charge pump. 

As the two-stroke piston rises on compression, its underside pulls a partial vacuum in the crankcase. An intake port opens, allowing air to rush into the crankcase through a carburettor.

As the piston nears Top Dead Center, a spark fires the compressed mixture. As in a four-stroke, the mixture burns and its chemical energy becomes heat energy, raising the pressure of the burned mixture to hundreds of psi. This pressure drives the piston down the bore, rotating the crankshaft.

As the piston continues down the bore, it begins to expose an exhaust port in the cylinder wall. As spent combustion gas rushes out through this port, the descending piston is simultaneously compressing the fuel-air mixture trapped beneath it in the crankcase.

As the piston descends more, it begins to expose two or more transfer ports, which are connected to the crankcase by short ducts. As the pressure in the cylinder is now low and pressure in the crankcase higher, the fresh charge from the crankcase rushes into the cylinder through transfer ports. These ports are shaped and aimed to minimize the direct loss of fresh charge to the exhaust port. Even in the best designs, there is some loss. This process of filling the cylinder while also pushing leftover exhaust gas out the exhaust port is called “scavenging.”

While the piston is near Bottom Dead Center, the mixture continues to move from the crankcase, up through the transfer ports, and into the cylinder.

As the piston rises, it first covers the transfer ports, and then the exhaust port the mixture is compressed and the cycle starts again.

Two-stroke engines produce a lot of pollution The pollution comes from two sources. The first is the combustion of the oil. The oil is required for lubrication and is totally lost as it is burnt as the engine runs The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit huge clouds of oily smoke.

Each time a new charge of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port. That's why you see a sheen of oil around any two-stroke boat motor. The leaking hydrocarbons from the fresh fuel combined with the leaking oil is a real mess for the environment.

2 stroke engines don't last nearly as long as 4 stroke engines. The lack of a dedicated lubrication system means that the parts of a 2 stroke engine wear a lot faster. They are also less fuel-efficient, so for environmental reasons they are been phased out and have been banned in some parts of the world.