Most people think engines are baffling but they really aren’t.
It is generally not what is going on inside the engine itself that is complex, just all the fancy electronic devices that are fitted to today’s modern engines.
Engines come in all different shapes and sizes but the principle of how an engine works remains the same.
Just about all modern engines are known as four-strokes.
However, there are also two-stroke engines, which were popular on older motorcycles and even some micro cars from the 1950s and 1960s.
The four strokes of the engine
A four-stroke engine works exactly as the name suggests – in four different strokes:
- The induction stroke.
- The compression stroke.
- The power stroke.
- The exhaustion stroke.
So how do the strokes fit together?
All engines feature valves at the top of a cylinder (the cylinder head) and pistons below them in the cylinder shaft (the cylinder bores).
The pistons are then connected to connecting rods (commonly called “con-rods”) which are connected to a crankshaft. The crankshaft rotates and provides the motion used to propel a vehicle.
The valves at the top are pushed up and down by a camshaft, which is connected to the crankshaft with a timing chain or timing belt and a set of gears or pulleys.
This keeps the valves and pistons synchronised.
Starting the engine
To start the engine, some sort of starter motor or winch needs to get the main engine rotating. This then allows the main engine to suck fuel and air into the cylinders, which starts the first stroke – induction.
The engine strokes
On the induction stroke, inlet valves at the top of the cylinders open up before the piston reaches the highest point – top dead centre (TDC) – in the bore (the cylinder tube).
As the piston starts to come back down the bore it draws the fuel and air mixture into the cylinder bore and the inlet valve then closes just before the piston reaches bottom dead centre (BDC).
The engine is now ready for the compression stroke. For this, the piston starts to make its way back up the cylinder bore, with the inlet valves still closed.
As it rises to the top of the cylinder it compresses the fuel and air mixture and the piston once again hits TDC.
It’s now ready for the power stroke – where the magic happens. Here, a spark plug is activated which ignites the fuel and air mixture.
This then forces the piston back down the cylinder bore to BDC and forces the crankshaft to turn. As the piston starts to rise back up the cylinder bore it enters the exhaustion stroke.
Here, the exhaust valve begins to open before TDC and the piston pushes the burnt fuel and air gasses up the cylinder bore out the exhaust valve and into the car’s exhaust system.
This process then repeats for each cylinder.
No matter how many cylinders your engine has, each cylinder will go though the four strokes mentioned above, just at different times.
You would think the engine would fire or ignite the fuel and air mixture in the cylinders in order, but it does not.
Because of the way the engine and the crankshaft are structured, the typical firing order for a four -cylinder engine is 1-3-4-2.
Here, if cylinder number 1 is on the power stroke or has just fired, cylinder number 3 will be on the compression stroke ready to go on to the power stroke and ignite the fuel and air.
Meanwhile, cylinder 4 will have finished the induction stroke and be ready to enter the compression stroke and cylinder 2 would have been pushed down by the power stroke ready to enter the exhaustion stoke.
Tips for working with a four-stroke engine in the real world
Some people, like myself, like to rebuild vintage or classic car engines themselves. This can cause some problems especially if you get the engine timing wrong so I have included some tips below to help you along the way.
- Before taking off the timing chain or timing belt, mark the crankshaft and camshaft pulleys then mark the engine block in the same place as the marks on both pulleys. I find that something white works well, like Tipp-Ex. This will help you keep the timing right.
- When taking off any auxiliary cables or wires within the engine bay, mark them with labels or different colored bits of tape. If using the label method write on the label what that cable or wire was connected to.If using the tape method then there are two ways this can be done. Either you mark the cable or wire with tape, and add some of the same color to the cable or wire where it came off.
If you run out of colors then use two bits of tape with a combination of different colors. Alternatively, you can mark the cable or wire with tape and write the color used and where it came from into a book, spreadsheet or database.