A 4-stroke engine is a type of small internal combustion engine that uses four different piston strokes to complete one operating cycle. During this cycle, the crankshaft rotates twice while the piston goes up and down twice to fire the spark plug.
In order to understand how these two engines are different, you first need to become familiar with the basics.
How Do Combustion Engines Work, and What Is A “Stroke” Anyways?
In an engine, a stroke refers to the movement of the piston in the cylinder as part of the combustion process.
During an engine’s combustion cycle, the piston moves up and down within the cylinder. The terms “top dead center” (TDC) and “bottom dead center” (BDC) refer to the piston’s position within the cylinder.
TDC is its position nearest to the valves, and BDC is its position furthest from them. A stroke is when the piston moves from TDC to BDC, or vice versa.
There are typically four strokes involved in the operation of a four-stroke engine: intake, compression, power, and exhaust.
During the intake stroke, the piston moves downward, drawing in a mixture of fuel and air into the combustion chamber. In the compression stroke, the piston moves back up, compressing the fuel-air mixture to prepare for ignition. The power stroke follows, where the ignited mixture expands rapidly, exerting force on the piston to drive it down.
Finally, in the exhaust stroke, the piston moves back up while expelling the burnt gases from the combustion chamber. Understanding the concept of strokes in an engine is essential for comprehending the internal combustion process and the mechanical operation of different engine types.
What Is a 4-Stroke Engine, and How Does a 4-Stroke Engine Work?
A 4-stroke engine is a common type of internal combustion engine. Most modern internal combustion engines are 4-stroke engines that run on gasoline or diesel fuel.
4-stroke engines provide a good balance of power, reliability and efficiency. When it comes to emissions, 4-strokes mechanically separate each event, reducing unburned fuel emissions. They also separate oil from fuel, which significantly reduces carbon monoxide emissions.
These characteristics have propelled 4-stroke technology to the top of the passenger vehicle market today!
Compression
Air/fuel compression before ignition improves combustion efficiency. The compression ratio describes the relationship between total cylinder volume and compressed volume.
For example, a compression ratio of 10:1 (also known as “10 to 1”) means that the chamber squeezes 10 parts air/fuel volume into 1 part of that volume at the end of the compression stroke.
In some applications, a higher compression ratio can provide more power. But higher compression ratios can reduce durability and require more expensive higher-octane gasoline.
engine 4-stroke cycle
To power your equipment, the overhead valve engine completes a repeating 4-step process detailed below.
Element that Allows Internal Combustion Engines to Run
- Air
- Fuel
- Compression
- Spark
#1. Intake Stroke.
Air and fuel enter the small engine through the carburetor. It’s the job of the carburetor to supply a mixture of air and fuel that will allow for proper combustion.
During the intake stroke, the intake valve between the carburetor and the combustion chamber opens.
This allows atmospheric pressure to force the air-fuel mixture into the cylinder bore as the piston moves downward.
Key points
- Inlet Valve – Open
- Outlet Valve – Closed
- Crankshaft Rotation – 1800
#2. Compression Stroke.
Just after the piston moves to the bottom of its travel (bottom dead center), the cylinder bore contains the maximum air-fuel mixture possible. The intake valve closes and the piston returns back up the cylinder bore.
This is called the compression stroke of the 4-stroke engine process. The air-fuel mixture is compressed between the piston and cylinder head.
Key points
- Inlet Valve – Closed
- Outlet Valve – Closed
- Crankshaft Rotation – 1800 (Total 3600)
#3. Power Stroke.
When the piston reaches the top of its travel (top dead center), it will be at its optimum point to ignite the fuel to get maximize power to your outdoor power equipment. A very high voltage is created in the ignition coil.
The spark plug enables this high voltage to be discharged into the combustion chamber. The heat created by the spark ignites the gases, creating rapidly expanding, super-heated gases that force the piston back down the cylinder bore. This is called the power stroke.
Key points
- Inlet Valve – Closed
- Outlet Valve – Closed
- Crankshaft Rotation – 1800 (Total 5400)
#4. Exhaust Stroke.
When the piston reaches bottom dead center again, the exhaust valve opens. As the piston travels back up the cylinder bore, it forces the spent combustion gases through the exhaust valve and out of the exhaust systems.
As the piston returns to top dead center, the exhaust valve closes and the intake valve opens and the 4-stroke engine process repeat.
Key points
- Inlet Valve – Closed
- Outlet Valve – Open
- Crankshaft Rotation – 1800 (Total 7200)
Ever repetition of the cycle requires two full rotations of the crankshaft, while the engine only creates power during one of the four strokes. To keep the machine running, it needs the small engine flywheel.
The power stroke creates momentum that pushes the flywheel’s inertia keeps it and the crankshaft turning during the exhaust, intake and compression strokes.
Parts of a Four-Stroke Engine
- Piston: In an engine, a piston transfers the expanding forces of gas to the mechanical rotation of the crankshaft through a connecting rod.
- Crankshaft: A crankshaft is a part that converts the reciprocating motion to rotational motion.
- Connecting Rod: It transfers motion from a piston to a crankshaft, acting as a lever arm
- Flywheel: The flywheel is a rotating mechanical device that is used to store energy.
- Inlet and Outlet Valves: It allows us to enter fresh air with fuel & to exit the spent air-fuel mixture from the cylinder.
- Spark Plug: It is a device that delivers electric current to the combustion chamber, which ignites the air-fuel mixture leading to the abrupt gas expansion.
Common Applications for 4-Stroke Engines
Four-stroke engines are the most common combustion engines. They are used in a wide range of different applications across multiple industries, with the most common uses including:
- Watercraft
- Motorcycles
- Automobiles and trucks
- Riding and push lawnmowers
- Off-road vehicles and dirt bikes
Advantages and Disadvantages of Four-Stroke engine:
Advantages of 4-Stroke engine:
- More torque
- More fuel economy than a 2-stroke engine
- Less consumption of lubrication oil
- Less pollution because of proper burning of fuel
- More durability
- Well-managed cooling system, so that longer engine life
- Less noisy
- Scavenging happens really well, the Thermal efficiency and HP of the engine is high
Disadvantages of 4-Stroke engine:
- There are many moving parts and hence there is more friction, therefore the engine has less mechanical efficiency.
- The design is complicated
- The weight of the engine is more because of heavier flywheel construction.
- Due to more parts, maintenance required quite often.
Comparison Between a 2-stroke engine vs 4-stroke engine
So, which is ‘better’? Here are a few of the pro’s and con’s to both engine designs:
- A 4-stroke engine is more efficient than a 2-stroke engine because fuel is consumed once every 4 strokes.
- A 4-stroke engine weighs 50% heavier than a 2-stroke engine.
- A 2-stroke engine creates more torque at a higher RPM, while a 4-stroke engine creates a higher torque at a lower RPM.
- The 4-stroke engine is also much quieter, a 2-stroke engine is significantly louder and has a distinctive, high-pitched “buzzing” sound.
- 2-stroke engines tend to wear out fast because they are designed to run at a higher RPM.
- 2-stroke engines are easier to fix because of their simple construction. 4-stroke engines have complex designs with more parts, making them more expensive, and repairs cost more.
- Two-stroke engines require pre-mixing of oil and fuel, while the 4-strokes do not.
- Four-strokes are more environmentally friendly; in a 2-stroke engine, burnt oil is also released into the air with the exhaust.
Two-stroke engines are typically found in smaller applications such as remote-controlled cars, lawn tools, chainsaws, boat motors and dirt bikes.
Four-stroke engines are found in anything from go-karts, lawnmowers and dirt bikes, right up to the typical internal combustion engine in your vehicle. It’s up to you to decide which engine you prefer, and for what purpose.