Petrol Engines


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    OVERLAND 4WD CLUB Tech TorqueHOW PETROL ENGINES WORK   The Basics  The purpose of a gasoline car engine is to convert gasoline into motion so that your car can move. Currently the easiest way to create motion from gasoline is to burn the gasoline inside an engine. Therefore, a car engine is an internal combustion engine  -- combustion takes place internally. Two things to note: ã   There are different kinds of internal combustion engines. Diesel engines are one form and gas turbine engines are another. Each has its own advantages and disadvantages. ã   There is such a thing as an external  combustion engine. A steam engine in old-fashioned trains and steam boats is the best example of an external combustion engine. The fuel (coal, wood, oil, whatever) in a steam engine burns outside the engine to create steam, and the steam creates motion inside the engine. Internal combustion is a lot more efficient (takes less fuel per Km) than external combustion, plus an internal combustion engine is a lot smaller than an equivalent external combustion engine. This explains why we don't see any cars from Ford and GM using steam engines. Inside a typical car engine    Almost all cars today use a reciprocating internal combustion engine because this engine is: ã   Relatively efficient  (compared to an external combustion engine) ã   Relatively inexpensive  (compared to a gas turbine) ã   Relatively easy to refuel  (compared to an electric car) These advantages beat any other existing technology for moving a car around.    OVERLAND 4WD CLUB HOW PETROL ENGINES WORK   Combustion Is Key  To understand the basic idea behind how a reciprocating internal combustion engine works; it is helpful to have a good mental image of how internal combustion works. One good example is an old Revolutionary War cannon. You have probably seen these in movies, where the soldiers load the cannon with gun powder and a cannon ball and light it. That is internal combustion, but it is hard to imagine that having anything to do with engines.  A more relevant example might be this: Say that you took a big piece of plastic sewer  pipe; maybe 3 inches in diameter and 3 feet long, and you put a cap on one end of it. Then say that you sprayed a little WD-40 into the pipe, or put in a tiny drop of gasoline. Then say that you stuffed a potato down the pipe. Like this: I am not recommending that you do this!  However, say you did... What we have here is a device commonly known as a potato cannon . When you introduce a spark, you can ignite the fuel. What is interesting, and the reason we are talking about such a device, is that a potato cannon can launch a potato about 500 feet through the air! There is a huge amount of energy in a tiny drop of gasoline.    OVERLAND 4WD CLUB HOW PETROL ENGINES WORK   Internal Combustion The potato cannon uses the basic principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel a potato 500 feet. In this case, the energy is translated into potato motion. You can also use it for more interesting purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and if you can harness that energy in a useful way, what you have is the core of a car engine!  Almost all cars currently use what is called a four-stroke combustion cycle  to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle , in honour of Nikolaus Otto, who invented it in 1867. The four strokes are as follows: ã   Intake stroke ã   Compression stroke ã   Combustion stroke ã   Exhaust stroke  A device called a piston  replaces the potato in the potato cannon. The piston is connected to the crank shaft  by a connecting rod . As the crankshaft revolves, it has the effect of resetting the cannon. Here's what happens as the engine goes through its cycle: 1. The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke . Only the tiniest drop of gasoline needs to be mixed into the air for this to work. 2. Then the piston moves back up to compress this fuel/air mixture. Compression  makes the explosion more powerful. 3. When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes , driving the piston down. Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust  leaves the cylinder to go out the tail pipe. 4. Now the engine is ready for the next cycle, so it intakes another charge of air and gas. Notice that the motion that comes out of an internal combustion engine is rotational , while the motion produced by a potato cannon is linear   (straight line). In an engine, the linear motion of the pistons is converted into rotational motion by the crank shaft. The rotational motion is nice because we plan to turn (rotate) the car's wheels with it anyway. Counting cylinders  The core of the engine is the cylinder, with the piston moving up and down inside the cylinder. The engine described above has one cylinder. That is typical of most lawn mowers, but most cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine, the cylinders usually are arranged in one of three ways: inline , V  or flat  (also known as horizontally opposed or boxer), as shown in the following figures.    OVERLAND 4WD CLUB HOW PETROL ENGINES WORK   Figure 2. Inline - The cylinders are arranged in a line in a single bank.   Figure 3. V - The cylinders are arranged in two banks set at an angle to one another.   Figure 4. Flat - The cylinders are arranged in two banks on opposite sides of the engine.  
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