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| Know Your Car : The Car Engine |
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 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.
For a more relevant example, assume you took a big piece of plastic sewer pipe, around three inches in diameter and three feet long, and you put a cap on one end of it. Then say that you sprayed a little compressed gas into the pipe, or put in a tiny drop of petrol. Then assume that you stuffed a potato down the pipe. 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!
The potato cannon uses the basic principle behind any reciprocating internal combustion engine : If you put a tiny amount of high-energy fuel (like petrol) 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 then if you can harness that energy in a useful way, what you have is the core of a car engine!
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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 honor of Nikolaus Otto, who invented it in 1867. The four strokes are illustrated in the figure shown here. They are :
- The intake stroke
- The compression stroke
- The combustion stroke
- The exhaust stroke
You can see in the figure that a device called the 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."
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A - Intake Valve
B - Valve Cover
C - Intake Port
D - Head
E - Coolant
F - Engine Block
G - Oil Pan
H - Oil Sump
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I - Camshaft
J - Exhaust Valve
K - Spark Plug
L - Exhaust Port
M - Piston
N - Connecting Rod
O - Road Bearing
P - Crank Shaft
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Here's what happens as the engine goes through its cycle :
Step 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 petrol. This is the intake stroke. Only the tiniest drop of petrol needs to be mixed into the air for this to work.
Step 2 : Then the piston moves back up to compress this fuel / air mixture. Compression makes the explosion more powerful.
Step 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.
Step 4 : Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to exit through the tailpipe.
Now the engine is ready for the next cycle, so it intakes another charge of air and petrol ...
Notice that the motion that comes out of an internal combustion engine is rotational, while the motion produced by a potato cannon is linear. In an engine the linear motion is converted into rotational motion by the crank shaft. The rotational motion is required because we need to rotate the car's wheels in order to run the car.
There is such a thing as an external combustion engine. The steam engines in the older trains and steam boats are examples of external combustion engines. 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. As it turns out, internal combustion is a lot more efficient (takes less fuel per kilometer) than external combustion, plus an internal combustion engine is a lot smaller than an equivalent external combustion engine. This is why almost all cars today use internal combustion engines.
Now let's look at all the parts that work together to make this happen.
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