Liquid Nitrogen Engine

A spinning bottle fueled by boiling liquid nitrogen!

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Teachable Topics:

• Ideal Gas Law
• Newton's Third Law
• Work & Energy

Theory:

The liquid nitrogen engine is a simple engine that is made possible by the low boiling temperature of liquid nitrogen (about -196°C) and the nitrogen gas that is created. It is the pressure from the expanding nitrogen gas that keeps our motor in motion. 

When the liquid boils, it becomes nitrogen gas and takes up more volume. In a closed container, such as a water bottle for instance, the boiling gas will cause an increase in pressure. Since the capacity of the water bottle is not changing, and the volume of the gas is increasing, the pressure will go up. If enough liquid nitrogen has been poured into the bottle, and there is no way for the gas to escape, the bottle might eventually explode if there is enough pressure built up.

The water bottle used for this engine is not a closed container. In fact, there are two straws on either side of it to let the nitrogen gas out. However, these two openings are not enough to keep pressure from building up inside the bottle. Some pressure will build up and it is this pressure that causes the nitrogen gas to fly out the straws at such a high speed. According to Newton's third law, the action of the nitrogen escaping through the straws will have an equal and opposite reaction. It is this reaction that causes the bottle to spin around.

To aid in the creation of the gas, the liquid nitrogen filled bottle is placed in a beaker of lukewarm water. This water, compared to the liquid nitrogen, is very hot. The heat energy from the water will begin to be absorbed by the cooler liquid nitrogen. When this happens, the liquid nitrogen will boil very fiercely, creating more gas and more pressure.

Apparatus:

• liquid nitrogen
• water bottle engine (see below for building instructions)
• beaker or bowl
• warm water

Building Instructions for Water Bottle Engine:

1. Get an empty water bottle or pop bottle (with a cap).
2. Use a nail or drill to make a hole in the centre of the bottle cap.
3. Get some kind of handle, such as a PVC tube or long piece of wood to attach your bottle to.
4. Punch or drill another hole of the same size in your handle.
5. Find a threaded bolt that has a shank thin enough to loosely fit through the holes you have made in your bottle cap and in your handle.
6. Fit the bolt through your handle and then your bottle cap
7. Screw a nut onto the bolt just until the underside of the bottle cap (NOTE: Make sure the bottle cap is still able to spin around the bolt and is not too tight).
8. Take your water bottle and make two small incisions on either side of the bottle (preferably near the top) with an exacto knife or with scissors.
9. Cut straws so they are about 1 inch in length and fit them in the two holes you just made. Angle them so that when the gas comes out, they will be pushing in the same direction
10. Tape the straws in place.
11. Screw the cap (which is now attached to the handle) back on to the bottle and you are now done the bottle engine!

Procedure:

1. Fill your beaker or bowl with warm water (doesn't have to be very warm, room temperature is fine).
2. Screw the cap off the bottle engine.
3. Carefully fill the bottle about 1/3 to 1/2 of the way up with liquid nitrogen.
4. Promptly screw the cap back on.
5. Lift your bottle engine up by the handle and dip it in the water.
6. The bottle engine should start to spin very fast in the water.
7. Once it is up to speed, raise it out of the water and watch it spin!

Safety:

• Be very careful when pouring the liquid nitrogen, wear safety goggles and oven mitts.
• Exacto knives are sharp, don't cut yourself!
• Make sure your bottle (while loose enough to spin) is secure and won't come off during the experiment.