Heating Houses and Engines

Hot water (red) flows to the convector,
and the cold water (blue) flows back
to the heater to be heated up again

Today, heating houses is mostly done by heating a fluid and then transporting it to each room. Hot water heating works by heating water with a burner and moving it to convectors that are in the house. The heat from the water moves throughout the room. When the water loses enough of its heat, it is transported back to the hot water heater to be heated up again. Hot air heaters work in a similar way. Air is heated by a burner and distibuted throughout the house. 
Another type of heater codenses and evaporates gas. When the liquid is evaporated, it gets colder because it is under less pressure. The gas is then sent outside. The air outside heats up the cold gas, which then flows back inside to be condensed. When it is condensed, the heat from the outside that heated up the gas is tranferred in to the house and the liquid is evaporated in order to be heated by the outside again.

Two-stroke engines and four-stroke engines work in the same basic way. The engine intakes fuel, a spark plug ignites the fuel, exhaust from the combustion is released, and the process repeats itself. In a two-stroke engine, fuel enters and is pushed to the top by the piston moving down. Then the fuel is compressed by the piston moving back up. The spark plug ignites the fuel and the combustion of the fuel pushes the piston down, allowing the exhaust from the fuel to be released.The piston in a four-stroke engine moves up or down four times in one cycle. The low pressure caused by the piston moving down the first time results in the fuel valve opening and fuel entering the cylinder. The piston then moves up and compresses the fuel. A spark plug ignites the fuel and the combustion causes the piston to be pushed down for the second time. The piston then moves up for the second time and the exhaust from the combustion is pushed out when the exhaust valve opens.