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How Does an External Combustion Engine Work? http://science.howstuffworks.com/transport/engines-equipment/steam1.htm Steam engines were the first engine type to see widespread use. Steam engines powered all early locomotives, steam boats and factories, and therefore acted as the foundation of the Industrial Revolution. These steam engines are called external combustion engines because the fuel is burned outside the cylinders. In a boiler, coal or other fuel is burned to create steam. This steam is then transferred to the engine. The Boiler: The high-pressure steam for a steam engine comes from a boiler. The boiler's job is to apply heat to water to create steam. A fire-tube boiler was common in the 1800s. It consists of a tank of water perforated with pipes. The hot gases from a coal or wood fire run through the pipes to heat the water in the tank, as shown here: In a fire-tube boiler, the entire tank is under pressure, so if the tank bursts it creates a major explosion. The Engine: The steam is carried to the engine. The diagram on the next page shows the major components of a piston steam engine. This sort of engine would be typical in a steam locomotive. The engine shown is a double-acting steam engine because the valve allows high-pressure steam to act alternately on both faces of the piston. -You can see that the slide valve is in charge of letting the high-pressure steam into either side of the cylinder. The control rod for the valve is usually hooked into a linkage attached to the cross-head, so that the motion of the cross-head slides the valve as well. (On a steam locomotive, this linkage also allows the engineer to put the train into reverse.) You can see in this diagram that the exhaust steam simply vents out into the air. This fact explains two things about steam locomotives: • It explains why they have to take on water at the station -- the water is constantly being lost through the steam exhaust. • It explains where the "choo-choo" sound comes from. When the valve opens the cylinder to release its steam exhaust, the steam escapes under a great deal of pressure and makes a "choo!" sound as it exits. When the train is first starting, the piston is moving very slowly, but then as the train starts rolling the piston gains speed. The effect of this is the "Choo..... choo.... choo... choo choo-choo-choo" that we hear when it starts moving. On a steam locomotive, the cross-head normally links to a drive rod, and from there to coupling rods that drive the locomotive's wheels. The arrangement often looks something like this: How does the internal combustion engine work? http://www.wisegeek.com/how-does-an-internal-combustion-engine-work.htm The internal combustion engine is used to power nearly all land vehicles (like cars) and many water-based and air-based vehicles (like boats and planes) as well. In an internal combustion engine, a fuel, such as gasoline, fills a chamber and then is ignited by a spark plug, causing a small explosion, which generates work. The thermal energy is converted to kinetic energy. The superheated expanding gas created by the explosion pushes a piston, which drives a crankshaft usually connected to an axle. The axle is connected to wheels which turn to drive a vehicle, such as an automobile, forward. piston The entire assemblage of a chamber, spark plug, piston, crankshaft, and valves that allow in fuel and air is known as a cylinder. Whereas small appliances such as chainsaws only use one cylinder, automobiles generally use four to eight. Historic aircraft have had as many as 28 cylinders to provide the energy to drive their propellers. The internal combustion engine is distinct from external combustion engines (such as steam engines), in that the energy generated from the combustion of fuel is efficiently contained within a cylinder. In steam engines, fuel is used to transform water into steam which then moves through a mechanism and provides work. Internal combustion engines took some time to perfect because the cylinder must be able to withstand the wear and tear of many thousands of explosions over the course of its operating lifetime. Refrigerators Excerpts from an article by Chris Woodford. Last updated: May 18, 2011. Now here's a cool idea: a metal box that helps your food last longer! Have you ever stopped to think how a refrigerator keeps cool, calm, and collected even in the blistering heat of summer? Food goes bad because bacteria breed inside it. But bacteria grow less quickly at lower temperatures, so the cooler you can keep food, the longer it will last. A refrigerator is a machine that keeps food cool with some very clever science. All the time your refrigerator is humming away, liquids are turning into gases, water is turning into ice, and your food is staying deliciously fresh. Let's take a closer look at how a refrigerator works! How to move heat with a gas Let's look at how gases behave. If you've ever pumped up the tires on a bicycle, you'll know that a bicycle pump soon gets quite warm. The reason is that gases heat up when you compress (squeeze) them. What happens if you release a gas that's stored at high pressure? When you spray an aerosol air freshener, you've probably noticed that the spray is really cold—for exactly the opposite reason that a bicycle pump gets hot. When you release the gas, it gets cooler. Photo: Right: Liquids can turn to gases (and gases cool down) when you let them expand into more volume. That's why aerosol sprays feel so cold. The heating and cooling cycle By compressing gases, we make them hotter; by letting them expand, we make them cooler. How can we use this handy bit of physics to shift heat from the inside of a refrigerator? Suppose we made a pipe that was partly inside a refrigerator and partly outside it, and sealed so it was a continuous loop. And suppose we filled the pipe with a gas. Inside the refrigerator, we could make the pipe gradually get wider, so the gas would expand and cool as it flowed through it. Outside the refrigerator, we could have something like a bicycle pump to compress the gas and release its heat. If the gas flowed round and round the loop, expanding when it was inside the refrigerator and compressing when it was outside, it would constantly pick up heat from the inside and carry it to the outside like a heat conveyor belt. And, surprise, this is almost exactly how a refrigerator works. There are some extra details worth noting. Inside the refrigerator, the pipe expands through a nozzle known as an expansion valve. As the gas passes through it, it cools dramatically. The compressor outside the refrigerator is not really a bicycle pump! It's actually an electrically powered pump. It's the thing that makes a refrigerator hum every so often. The compressor is attached to a grill-like device called a condenser (the thin black wire curving back and forth on the back of the refrigerator) that expels the unwanted heat. Finally, the gas that circulates round the pipe is actually a specially designed chemical known as the coolant or refrigerant. Photo: Right: Here's the compressor from a typical refrigerator. Note the pipes carrying the coolant in one side and out the other. You can't see this unit unless you pull your appliance away from the wall, because it's tucked away around the back and at the bottom. Heat Pumps http://home.howstuffworks.com/home-improvement/heating-and-cooling/heat-pump1.htm There are many different kinds of heat pumps, but they all operate on the same basic principle -- heat transfer. This means that rather than burning fuel to create heat, the device moves heat from one place to another. There's a key to making this all happen -- heat naturally flows downhill. This means that it tends to move from a location with a high temperature to a location with a lower temperature. Pretty simple. What a heat pump does is use a small amount of energy to switch that process into reverse, pulling heat out of a relatively low-temperature area, and pumping it into a higher temperature area. So heat is transferred from a "heat source," like the ground or air, into a "heat sink," like your home. They take heat from the air outside your home and pump it inside through refrigerant-filled coils, not too different from what's on the back of your fridge. The air source variety is pretty basic, and you'll find two fans, the refrigerator coils, a reversing valve and a compressor inside to make it work. The key to allowing the heat pump to also cool is the reversing valve. This versatile part reverses the flow of the refrigerant, so that the system begins to operate in the opposite direction. So instead of pumping heat inside your home, the heat pump releases it, just like your air conditioner does. When the refrigerant is reversed it absorbs heat on the indoor side of the unit and flows to the outside. It's here that the heat is released, allowing the refrigerant to cool down again and flow back inside to pick up more heat. This process repeats itself until you're nice and cool.

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