Turbines

Two of the earliest sources of power for driving machinery were the waterwheel and the windmill . The waterwheel has little paddies or buckets mounted around the edge. When it is placed in a stream , the  flowing water spins it around . A windmill  has large sails, which turn when the wind blows through them.

Waterwheels and windmills are no used for dri-ving machinery today. But we do find uses for modern versions of them -water and wind turbines . Water turbines are the most useful. They are used to harness the energy in flowing water to drive electricity generators in hyroelec- tric power schemes.
Q What does the word "hydroelectric" literally mean?

The four- stroke cycle

to the cylinder, certain actions are repeated in a regular cycle so that the piston can deliver power. These actions are repeated every four piston strokes (movements), so the cycle is called the four stroke cycle. The stages in  the cycle are shown below.

The engine cycle

The     diagram        show       in       more   detail     the  piston / cylinder / crankshaft set -up. The inlet  valve opens to allow fuel mixture (gasoline and air) into the cylinder. A spark from the spark plug sets the mixture burning. The outlet valve opens to let the burned gases escape. The pis- ton rings from a tight seal between the piston and cylinder walls. The connecting rod joins the piston to a crank on the crankshaft.  

2 Ebgines

Practical machines are much more complicated than the simple machines discussed in the first chapter. If you look of parts that are simple machines, particularly levers, wheel and gears. All these parts work together to perform a particular task, such as making a car run. We can think of a car as being two machines in one. The first machine is the engine. This changes the energy in a fuel (usually gasoline ) into motion . The second car machine is the transmission, which transmits the motion produced by the engine to the wheels that drive the car. Machines, like a gasoline  engines ,that produce motion are called prime movers. We use several other kinds of prime movers, such as steam engines, jet engines, and tur- bines . Electric motors are also very  widely used to drive machines. They work by electricity produced by prime movers in power plants.

Variable gears

Car engines always need to turn at a high speed , whether the car is crawling or traveling fast . They therefore have a system of variable gears so that the engine can drive the car at different speeds. These variable gears are house in a gearbox. As the driver changes gear, different sets of gears are brought into mesh to change the high engine speed into a speed suitable for driving the car on the road.

Changing sped

If two meshing gear have the same number of teeth, they turn at the same speed, if the driving gear has more teeth than the driven gear, it will drive the driven gear faster. On the other hand, if the driven gear has more teeth, them it will be driven around more slowly. Gears that increase speed are called multiplying gears. Those reduce speed are reducing gears. Reducing gears are used in the transmission system of cars, for example (see page 47). When a car is traveling at about 55 mph (88 km/h), the wheels male about 750 revolutions per minute (rmp). A car engine turn at about 3,000 rmp. Q  what is the missing number in the following statement? The gears must reduce the speeds transmitted by a ratio of -:1.

Kinds of gears

There are several kinds of gears, which can transmit power to shafts that are parallel or at different angles to each other. The simplest kind of gear is the spur gear, which has straight teeth. A helical  gear has curved teeth, and it is smoother and quieter in operation tham a spur gear. Bevel gears have teeth that slant at an angle, and they transmit motion between shafts at an angle each other.