Engines

Introduction
Public summary: 

Have you ever wondered what an engine is? Learn about how various types of engines work from the ubiquitous Internal Combustion Engine found in almost every car ever made to a small engine which runs on nothing but hot water!

What are the different types of engine, and how do they work?
Useful information
Kit List: 

Internal combustion engine model, Jet engine model, Stirling engine model, soft pencils, kettle, mug.

WARNING: STIRLING ENGINE MUST NEVER BE LUBRICATED WITH OIL. It will gum it up. Lubricate bearings with graphite from soft pencil only.

Packing Away: 

Put two plastic models in box. CAREFULLY pack material around Stirling Engine and pack back in box.

Frequency of use: 
3
Explanation
Explanation: 

Engines Experiment - Explanation
Last Update - Gareth Funk, April 2015

************** INTRODUCTION **************
I like to start with the Stirling Engine whirring away whilst I give the following introduction. To do this, the Stirling Engine should be placed on a mug of freshly boiled water and given a push to get it going. It will only go one way for reasons which should become clear.

Before looking in detail at any of the models, it is worth asking the question “what is an engine?”, or alternatively, “what do you know that has an engine?” and as a follow-up “why does it need an engine?”.
Many of the children will not be able to give a good answer to the first, but can answer well enough the second two. The key thing about all engines is that they turn energy from one form into mechanical (or kinetic) energy and an engine is simply any device that does that.
Examples of engines are:
- Electric motors
- Jet Engines
- Steam Engines
- Internal Combustion Engine
- Stirling Engine

Excluding the Electric motor, all the above are heat engines: they use heat energy as their input. Obviously this energy usually comes from burning something, but thermodynamically the input to the engines is heat. It is worth noting that the jet engine is also, strictly speaking, an type of internal combustion engine. Similarly it is worth noting that the steam engine and the stirling engine are examples of external combustion engines.

The Stirling Engine is the best one to demonstrate because it best exemplifies the idea of a thermodynamic cycle and gives a real-life demonstration of an engine in action. The Internal Combustion Engine and the Jet Engine models are just models and a lot of children can’t see past the fact that a battery and an electric motor are what is making them turn. However, if there is time left after the Stirling Engine demonstration, feel free to move on to the others if you have time.

************** STIRLING ENGINE **************
The Stirling Engine we have is one of three commonly seen implementations of the Stirling Cycle. Out of interest, ours is a Gamma Configuration Stirling Engine. Most Stirling Engines used in practice are of different configurations but the thermodynamic processes they undergo are the same.

The following explanation requires that the children are familiar with the concepts of pressure and volume. A good, quick way to explain pressure is to get them to consider running around in a room with their eyes closed. They’d bump into walls every so often. They’d bump into walls more often if there were more of them in the room, or if the room got smaller.

Before we proceed, the following things need to be pointed out concerning the names we will use for the various different parts of the engine. Point out the two different pistons. Note that the black piston simply moves the gas to the hot space (when it lifts up) and to the cold space (when it comes back down). It is as such termed the “displacer piston” however during the demonstration I tend to stick to “black piston” but it is nonetheless worthwhile to show them that is simply moves the gas around and does NOT compress it. The glass piston at the top changes the overall volume of the chamber: when it lifts up, the volume increases (expansion) and when it descends, the volume decreases (compression). The hot space of the engine is the metal surface in contact with the hot steam (please stress that this is not a steam engine!) and the cold space is the upper metal surface in contact with the surrounding air. Also tell them what the flywheel is and point it out (it’s the large gold-coloured wheel which spins). The flywheel is there mainly to store the mechanical energy we extract but also to tip the pistons from one phase to the next.

Stop the Stirling Engine by keeping your finger on the flywheel so that we can slow the process down and step through the stages in turn. Let the engine get to the point where the black piston is about to come up and then proceed with the following explanation.

The idealised Stirling cycle consists of four thermodynamic processes acting on the working fluid (in our case, the air trapped inside the chamber):

Constant-Volume (known as isovolumetric or isochoric) heat-addition:
The glass piston is staying roughly still at the bottom of its stroke during this process which means that the volume of the chamber is remaining constant. The big black piston is moving up which moves the gas into contact with the hot space and so it heats up. During this process, the pressure will also increase.

Isothermal Expansion:
The small glass piston rises up thus expanding the hot fluid meanwhile the gas continues to be in contact with the hot space as the black piston is staying roughly still.
It is NOT important to stress the constant temperature; the expansion of the gas and the intake of thermal energy are the key points.

Constant-Volume (known as isovolumetric or isochoric) heat-removal:
The glass piston stays still (now at the top of its stroke), hence constant volume, and the black piston moves down moving the gas from the hot space and into contact with the cold space and so heat is lost.

Isothermal Compression:
The glass piston now moves down and compresses the gas. The black piston stays still so the gas continues to lose heat at the cold space.

The cycle repeats:
Now we are back where we started but in getting back to where we started we made the wheel spin! Now the cycle will continue over and over again until we run out of heat.
This is the really important point to stress: by doing those four thermodynamic processes we extracted some mechanical energy!

Note here that these processes have to happen in this order hence why the wheel only spins one way.

To finish off, take the stirling engine off the heat, let it stop, and then ask “what would happen if we were to spin the flywheel in the opposite direction to the way it normally spins?”. Think about the system we had before: Heat was added at the bottom, the wheel turned round. The answer to the question is that the opposite would happen if we were to spin the wheel in the opposite direction the bottom surface would heat up. This will not be practical the demonstrate as it’s difficult to spin the wheel sufficiently fast but it is nonetheless true. Some children will instinctively get this without much prompting but in my experience most struggle with the concept.

************** INTERNAL COMBUSTION ENGINE **************

Our particular model is of a four cylinder, four-stroke engine. The piston completes four separate strokes which together comprise a single thermodynamic cycle. A stroke refers to the full travel of the piston along the cylinder, in either direction. The strokes are as follows:
INTAKE: this stroke of the piston begins at top dead centre. The piston descends from the top of the cylinder to the bottom of the cylinder, increasing the volume of the cylinder. A mixture of fuel and air is forced by atmospheric (or greater) pressure into the cylinder through the intake port.
COMPRESSION: with both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air or fuel-air mixture into the cylinder head.
POWER: this is the start of the second revolution of the cycle. While the piston is close to Top Dead Centre, the compressed air–fuel mixture in a gasoline engine is ignited, by a spark plug in gasoline engines, or which ignites due to the heat generated by compression in a diesel engine. The resulting pressure from the combustion of the compressed fuel-air mixture forces the piston back down toward bottom dead centre.
EXHAUST: during the exhaust stroke, the piston once again returns to top dead centre while the exhaust valve is open. This action expels the spent fuel-air mixture through the exhaust valve(s).

Focus on one cylinder when explaining these but make sure you are pointing out the correct stroke! Check the valves to check you’re telling them the right stroke at the right time: If the valves are open then the stroke is intake or exhaust, depending on which way the piston is moving. If you get this wrong the spark won’t be at the right time and the valves will be open when you’re saying the fluid is being compressed etc.

Note that the cylinders are not all moving together and even those that are do not fire at the same time. This is in order to deliver the power more smoothly with four smaller bursts per two revolutions than one large burst per two revolutions. Most cars have 4 cylinder engines like our model.

Side Note: The model can be switched off and on such that it gets out of sync with the spark. If it does this, turn it off and on again, stopping at a different point in the cycle, until it gets back in sync.

************** JET ENGINE **************
As of this update, the Jet Engine model has not been built yet. This section will need updating once the model is finished and I have decided what can be demonstrated with it.

The following is from wikipedia:

A turbofan engine is a gas turbine engine that is very similar to a turbojet. Like a turbojet, it uses the gas generator core (compressor, combustor, turbine) to convert internal energy in fuel to kinetic energy in the exhaust. Turbofans differ from turbojets in that they have an additional component, a fan. Like the compressor, the fan is powered by the turbine section of the engine. Unlike the turbojet, some of the flow accelerated by the fan bypasses the gas generator core of the engine and is exhausted through a nozzle. The bypassed flow is at lower velocities, but a higher mass, making thrust produced by the fan more efficient than thrust produced by the core. Turbofans are generally more efficient than turbojets at subsonic speeds, but they have a larger frontal area which generates more drag.[11]
There are two general types of turbofan engines, low bypass and high bypass. Low bypass turbofans have a bypass ratio of around 2:1 or less, meaning that for each kilogram of air that passes through the core of the engine, two kilograms or less of air bypass the core. Low bypass turbofans often used a mixed exhaust nozzle meaning that the bypassed flow and the core flow exit from the same nozzle.[12] High bypass turbofans have larger bypass ratios, sometimes on the order of 5:1 or 6:1. These turbofans can produce much more thrust than low bypass turbofans or turbojets because of the large mass of air that the fan can accelerate, and are often more fuel efficient than low bypass turbofans or turbojets.

Risk Assessment
Date risk assesment last checked: 
Tue, 06/02/2018
Risk assesment checked by: 
jaredjeya
Date risk assesment double checked: 
Wed, 07/02/2018
Risk assesment double-checked by: 
bakrill
Risk Assessment: 
DESCRIPTION 4-stroke and Jet engine models: These are commercially available childrens' toys, so risks are minimal
RISKS
  1. Choking hazard from small parts
CONTROL MEASURES
  1. Keep away from very small children. If engines break, close experiment and put parts in box.
IN CASE OF ACCIDENT
  1. Call first aider.
DESCRIPTION Stirling Engine
RISKS
  1. Hot water may cause burns
  2. Water may be spilled
  3. Risk of burns from boiling water in kettle/on hotplate
  4. Some sharp edges if engine dismantled (flywheel runs on point bearings)
  5. Finger trap risk in fly wheel
CONTROL MEASURES
  1. Warn of hot water (and steam) before experiment starts, keep hot water away from easy reach of children's hands. Mark mug as "HOT".
  2. Use stable mug for water. Do not over-fill.
  3. Heat water in kettle or temperature-limited hot plate. Keep heating apparatus well away from rest of experiment/children
  4. Do not allow children to play with Stirling engine. If flywheel becomes detached, demonstrator to reassemble
  5. Flywheel is very light and has little angular momentum, even at high speed.
IN CASE OF ACCIDENT
  1. Rinse burnt area under cold water for at least 10 minutes; call first aider
  2. Close experiment; Mop up spilt water if cool enough to do so
  3. Rinse burnt area under cold water for at least 10 minutes; call first aider
  4. Call first aider
  5. Call first aider
This experiment contains mains electrical parts, see separate risk assessment.
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Images
Experiment photos: