Motors are in use everywhere. You can see large engines in trucks as well as buses and forklifts, you can find small engines within motorbikes and lawn mowers. It is because they are actually useful inventions. They may have started out as being steam powered and also unwieldy, but even small engines by today's standard are big related to the requirements of past years.
Both large and small engines come in two fundamental types. Petrol and diesel driven. The main difference is primarily in how the fuel is ignited. Diesel is much more difficult to ignite than petrol but will be used more effectively. Diesel-powered motors can be air or water cooled, particularly when talking about commercial size motors. Petrol engines are usually air cooled.
Whenever you take a look at vehicles you will notice that many of the faster cars, focused on horse power, are petrol powered. It is because the engine that's able to utilize petrol is generally smaller as well as lighter when compared to a diesel powered engine. Your larger automobiles, particularly the ones which are intended for towing normally use diesel-powered engines.
Household generators use small engines, usually form about 10kW and will only cost you a few thousand bucks. Manufacturing plants use engines which are considerably larger and cost within the tens if not hundreds of thousands. This just gives you an idea on the number of different sizes of engine there actually are.
The power that the motor creates is measured in terms of horse power. It was originally used as a way to compare the power output of an engine with the average power output of a typical draft horse. A single horse power (1hp) is equivalent to 745.7 watts. There are actually one thousand watts within a kilowatt so it would take 1.3410 horse power to give you one kilowatt of energy.
The rate that the engine's pistons move at is measured in revolutions per minute (rpm). This is basically the number of completed movement cycles the piston does in one minute. This is what provides the power to the engine. Small engines will need more revolutions than a bigger motor to put out the same amount of power as they can burn up much less fuel in each revolution.
This is all very simplified and doesn't give you any kind of idea of the complexness of a motor, regardless of how big or small it is. The engine is a wonderful bit of machinery and it has taken a long time to get to the stage it is at now. It is very likely that the large engines we use now will be replaced by small engines which are much more energy-efficient in a few short years.
Technology advances in leaps and bounds every years so who knows where engines will go next.