How engines work- A good article with a clear explanation

The clearest explanation of how an engine works in my opinion
from a lawn mower company...
I'm no engineer. Although I could understand separate functions, how engines work is still something I couldn't put together in my head.
Up until I read this article.
Hopefully that'll be helpful for the van and further if I get into boat as I'm thinking of.

This link is an article about " 4 common small engine problems and how to fix them" from the same website. That should be useful with the boating as I will probably start off with a cruiser's 4 strokes than a narrowboat with a diesel engine...

Understanding Small Engines

What are the essentials of a four-stroke engine?

The biggest difference between small engines and other types of engines is their small capacity and simplicity of design. Small engines generate very modest amounts of power - generally no more than 25 HP - compared to a typical family car (up to 200 HP). And a small engine's size makes it easier to maintain and repair.
Since small engines are designed for simple tasks like cutting grass and turning soil, their designs are fairly uncomplicated. Unlike cars and other vehicles that frequently accelerate, slow down or idle for long periods, small engines either run at constant speed or change speed slightly to handle modes changes in the "load," such as when a lawnmower hits a patch of thick grass.
Also, unlike car engines, small engines don't have to fit under a hood or make room for countless computers and other devices. This makes small engine parts easier to install, adjust and remove. You can reach most small engine parts with a few turns of a wrench.

Essentials Of The Four-Stroke Engine

Here, in its simplest form, is how a four-stroke engine works:
When you pull the rope, known as a rewind cord, or use your electric starter, precise amounts of fuel and filtered air mix in the carburetor. The mixture rushes into the engine to be compressed, ignited and burned in a controlled process known as internal combustion. Hot gases are produced. As the gases expand, they push a smooth, well-lubricated cylindrical component, known as the piston. The piston, in turn, drives the crankshaft, the arm that spins a blade or performs other work. Valves let air and fuel into the combustion chamber above the piston and allow spent gases to exit through the muffler.
The whole process involves four piston strokes and is designed to become self-sustaining from the time the engine starts until the moment it stops. Timed electrical surges cause the spark plug to fire repeatedly inside the combustion chamber, igniting each fresh supply of air and fuel and producing gases that continually drive the piston and crankshaft.
All the while, oil from the crankcase and air circulate to keep engine temperatures within an acceptable range, and a governor monitors changes in the workload and adjusts engine speed accordingly.

Five Basic Systems

There are five systems at work in every small engine: Fuel Supply, Compression, Ignition, Lubrication/Cooling, and Governor (speed control).
Two other common systems are also discussed in detail. Some engine models - especially heavier-duty ones - may contain a starter motor, which requires an electrical system to charge the battery.
Most small engines sold in the past 10-15 years include a braking system as well. This is designed to protect you and others by stopping the engine quickly if you let go of the controls.
In short, five systems generate the power to spin a blade, turn a wheel or perform other work. Two others may be included for safety and convenience. The following will familiarize you with the major parts in these systems and the essentials of how they work.

Engine Components And Their Function

Here's how the components in your engine interact:

1. The rewind cord is pulled to start the combustion process. On some models, a starter motor replaces the rewind, drawing on battery power to start the engine.
   
2. Revolving magnets work in conjunction with the ignition armature and spark plug to produce a spark in the combustion chamber.
   
3. The carburetor draws in fuel from the fuel tank and outside air to form a combustible vapor that is fed into the combustion chamber.
   
4. Intake and exhaust valve open and close at precisely timed intervals to let air and fuel enter the engine and to let spent gases exit.
   
5. The piston is pushed through the cylinder by the force of expanding gases. The piston's motion causes the crankshaft to turn. Momentum then carries the piston back toward the top of the cylinder.
   
6. Oil stored in the crankcase circulates through the engine to lubricate key components like the piston and crankshaft and to provide generalized cooling by drawing away heat from internal engine surfaces.
   
7. A flywheel brake and stop switch are included on engines for equipment such as mower that require constant supervision. The two components are designed to stop the engine if you release the controls.
   
8. An air vane or flyweights monitor engine RPMs so the governor can maintain the selected engine speed.
   
9. Cooling fins help reduce engine temperatures when air circulate across the hottest engine surfaces.