Basic Automotive Maintenance Course Chapter 1: The Four Stroke Engine
Four Stroke Gasoline Engines
Most of today's automobiles still run on gasoline and use four stroke engines. There are also diesel engines in some cars, but, today we will focus on the four stroke gas engine. Additionally, there are two stroke gas engines, but, those are mostly in motorcycles and lawnmowers and the like.
To better understand what the four stroke engine does, I must tell you the parts of a basic engine so you will know what they do.
- The crankcase. This is where the crankshaft, the connecting rods, the cylinders, and the piston are housed. The camshaft is also housed here in non overhead cam engines. The crankcase is also known as the engine block.
shown with no manifolds for clarity
The crankshaft. The crankshaft is what turns the linear motion of the piston into circular motion and turns the transmission, which drives the car.
The pistons. The pistons are pushed down by the small explosions taking place in the cylinders, transferring the force through the connecting rods to the crankshaft.
The connecting rods. The rods connect the pistons to the crankshaft.
The cylinder head. The head sits on top of the cylinders and holds the valves. It contains the explosions and controls the gas flows to the engine. On overhead cam engines, this is where the cam is mounted. The cylinder head also houses the combustion chamber. This is where the explosions happen. The spark plugs are located here as well.
The cylinders. The cylinders hold the pistons in line so they can move up and down.
The valves. There are two types of valves in a four stroke engine.
a. The intake valve, which lets the air/fuel mixture into the combustion chamber.
b. The exhaust valve, which releases the burnt gasses to the exhaust system.
The camshaft. The camshaft is responsible for opening the valves at the correct time. The camshaft is driven by chain, belt or gear by the crankshaft.
The intake manifold. This lets the air and or air/fuel mixture into the engine. The intake manifold mounts to the cylinder head.
The exhaust manifold(s). This directs the burnt gasses to the exhaust system. The exhaust manifold mounts to the cylinder head as well.
Carburetor or fuel injectors. The carburetor or fuel injectors meter the fuel flow to the engine.
The four strokes are:
The intake stroke:
The piston starts at the top (top dead center, TDC) and travels down to bottom dead center (BDC), pulling in air and fuel through the open intake valve.
The compression stroke:
The valves are closed. The piston travels up from BDC to TDC, compressing the air/fuel mixture.
The power stroke:
The valves are still closed. The spark plug ignites the air fuel mixture, causing a small explosion, which drives the piston down to BDC
The exhaust stroke:
The exhaust valve opens and the piston travels back to TDC, pushing the exhaust gasses out of the cylinder.
A good way to remember the strokes is to think of them as: suck, squeeze, bang, blow ;)
These four strokes happen thousands of times a minute inside the engine. The more cylinders, the smoother this cycle is. These continuous cycles create heat and vibration, wearing on the engine components. Dirt is also the enemy of your engine and related components. This is why oil changes and replacing your air filter are very important.
I hope this helped you understand how your engine works. I will get into system specifics in the next installment, where we will go over the fuel system and the ignition system. I will also be doing an oil and air filter change to my van in a separate episode.
My goal with this project is to help people be more self sufficient and to save them money. Working on your own car is like an income all to itself!
Illustrations by @matthewtiii are not to scale or complete they will improve!