What is 4 stroke engine? 4-stroke engines power everything, from motorcycles and cars to lawn mowers and generators.
Whether you’re currently working in the field or are considering becoming a technician, it’s critical to understand how different types of engines work – including 4-stroke engine technologies!
This guide will give you a brief, effective overview of 4-stroke engines. If you want to learn more, consider enrolling at the Motorcycle Mechanic Institute (MMI)! With MMI’s technical training curriculum, students can take motorcycle mechanic courses to develop skills for servicing motorcycle engines – including those with 4-stroke engines.
What Is 4 Stroke Engine?
The four-stroke engine is the most common types of internal combustion engines and is used in various automobiles (that specifically use gasoline as fuel) like cars, trucks, and some motorbikes (many motorbikes use a two stroke engine). A four stroke engine delivers one power stroke for every two cycles of the piston (or four piston strokes).
The thermal efficiency of these gasoline engines will vary depending on the model and design of the vehicle. However in general, gasoline engines convert 20% of the fuel (chemical energy) to mechanical energy—in which only 15% will be used to move the wheels (the rest is lost to friction and other mechanical elements).
One way thermodynamic efficiency can improve in engines is through a higher compression ratio. This ratio is the difference between the minimum and maximum volume in the engine chamber (seen as TDC and BDC on figure 2). A higher ratio will allow a larger fuel-air mixture to enter, causing a higher pressure, leading to a hotter chamber, which increases thermal efficiency.
How Does a 4-Stroke Engine Work?
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!
Air/fuel compression before ignition improves combustion efficiencies. 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.
What Are the Strokes of a 4-Cycle Engine?
4-stroke engines have four valves so that pistons can go through four events to complete one 4-stroke cycle. A piston motion, either upward or downward, is defined as an event. After these four events, the cycle is complete and ready to restart.
The intake and exhaust valves regulate the air/fuel mixture that enters and exits the cylinders. During the 4-stroke cycle, the intake and exhaust camshafts open and close the intakes and exhausts at precise times.
- Piston descends the cylinder bore from top dead center (TDC) to bottom dead center (BDC).
- Intake valve is open and the exhaust valve is shut.
- Downward motion of the piston creates a vacuum (i.e., negative air pressure), which draws the air/fuel mixture into the engine through the open intake valve.
- Piston moves up the cylinder bore from BDC to TDC.
- Intake and exhaust valves are both closed.
- Upward motion of the piston compresses the air/fuel mixture in the combustion chamber.
- Spark plug fires and ignites the compressed air/fuel mixture just before the end of the compression (i.e., previous) stroke. This ignition/combustion forces the piston down the cylinder bore and rotates the crankshaft, propelling the vehicle forward.
- Piston moves down the cylinder bore from TDC to BDC.
- Intake and exhaust valves are both closed.
- Piston moves up the cylinder bore from BDC to TDC. The momentum created by the power stroke keeps the crankshaft moving and the other three strokes going in sequence.
- Intake valve is closed and the exhaust valve is open.
- Final stroke pushes the spent gases and exhausts out of the cylinder. The cycle is now complete and the piston is ready to begin the intake stroke.
The intake and exhaust valves are opened and closed by cams on a camshaft, which is driven by a timing belt or chain connected to the engine’s crankshaft.
For a pushrod engine, the camshaft is driven by the crankshaft during push rod valve actuation:
The crankshaft drives the camshaft via a timing chain or belt in direct valve actuation, also known as an overhead valve engine:
While the engine crankshaft controls the piston’s up-and-down motion within the cylinder, the camshaft opens and closes the valves.
4-Stroke vs. 2-Stroke Engines
Modern technological advancements have rendered 4-stroke and 2-stroke technologies comparable in terms of reliability and efficiency, though one type of engine may outperform the other according to certain operational conditions.
In any case, mechanics are still likely to have to service older models alongside newer ones, so understanding the differences between these two engines is critical.
Ports, Not Valves
2-stroke engines use ports, not valves for the flow of the air/fuel mixture and exhaust through the engine. The piston controls when the ports are opened and closed as it travels through the two strokes (TDC to BDC and BDC to TDC).
The diagram above shows how 2-stroke engines use the space above and below the piston. Machined ports within the engine case and cylinder allow 2-stroke engines to avoid valve usage.
No camshaft is required to open or close the valves. With fewer parts, 2-stroke engines are lighter and more compact than 4-stroke engines.
Power Twice As Often
One major distinguishing feature of a 2-stroke engine is its ability to complete each engine cycle with one revolution of the crankshaft, thus generating power twice as often as 4-strokes — 1 power pulse for every 2 engine strokes (versus 1 power pulse for every 4 strokes as in 4-stroke engines), potentially making 2-strokes almost twice as powerful as their 4-stroke counterparts.
Applications of 4-Stroke Engines
- Automobiles: The 4-stroke engines are most commonly used in cars, trucks, and other motor vehicles. These have high efficiency and reliability.
- Construction equipment: They are commonly used in construction equipment such as loaders, excavators, and bulldozers.
- Marine engines: The 4-stroke diesel engines are used in boats and ships to power their propellers.
- Lawn and garden equipment: Four-stroke engines are used in different leaf blowers, lawn mowers, and other outdoor power devices. These engines are easy to start.
- Generators: These engines are used in stationary and portable generators to transform fuel into electricity.
What Does All of This Mean for Technicians?
Motorcycle, marine and small engine technicians work with various types of engines every day. Understanding the distinctions and characteristics is essential for diagnosing and repairing vehicles and equipment. Certain components of each type of engine are more likely to wear out or fail and different types of engines require different maintenance standards and procedures.
Anyone considering becoming a motorcycle mechanic should consider technical training, which will provide the necessary knowledge and experience to perform motorcycle engine servicing correctly.
This training can be obtained through MMI’s 48-week Motorcycle Technician training program, which features motorcycle servicing courses that teach technicians how to properly diagnose, maintain and repair various types of motorcycle engines.
What is meant by a 4-stroke engine?
An engine that completes a power stroke in four strokes of the piston is known as a 4-stroke engine.
What are examples of a 4-stroke engine?
Four-stroke engines are most commonly used in heavy applications such as trucks, buses, dirt bikes, vans, tractors, and other heavy vehicles.
Which engine produces less pollution, 2-stroke or 4-stroke?
A 2-stroke engine produces more pollution than a 4-stroke engine. This is because a 2-stroke engine uses ports for the suction and discharge of the fuel.
Which is faster, 2 stroke or 4 stroke?
A 2-stroke engine has lower parts than a 4-stroke engine. In comparison, a two-stroke engine completes a working cycle (in just 2 strokes of the piston) faster than a four-stroke engine. Therefore, a 2-stroke engine is faster than a 4-stroke engine.
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