4-Stroke Engines: What Are They and How Do They Work?

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!

The 4-stroke engine cycle includes the intake stroke (air/fuel mixture enters the engine), compression stroke (mixture is compressed), power stroke (mixture is ignited to propel the vehicle) and exhaust stroke (expels spent gases), with the intake and exhaust valves being controlled by camshafts.

While modern advancements have made 4-stroke and 2-stroke engines comparable in reliability and efficiency, they differ significantly in design and operation. 2-stroke engines use ports instead of valves and can generate power more frequently per cycle, making them potentially more powerful but less efficient and more polluting than 4-stroke engines.

Understanding the differences between 4-stroke and 2-stroke engines is crucial for motorcycle, marine and small-engine technicians for effective diagnosis, maintenance and repair. Technical training such as that offered by Motorcycle Mechanics Institute (MMI) is essential for acquiring the necessary skills and knowledge to pursue a career in motorcycle mechanics.^1.1

MMI offers a 48-week Motorcycle Technician Training program that covers servicing motorcycle engines, including 4-stroke engines.⁷ Students are prepared for careers as motorcycle technicians by learning to diagnose, maintain and repair various types of motorcycle engines.

Continue reading to learn more! You’ll discover how 4-stroke engines work and how they differ from 2-stroke engines.

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.

Read: Motorcycle Ignition Systems: What Are They and How do They Work?

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.

What is a stroke in an engine?

In an engine, a stroke refers to the movement of the piston in the cylinder as part of the combustion process. 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.

1. Intake stroke

• The piston descends the cylinder bore from top dead center (TDC) to bottom dead center (BDC).
• The intake valve is open and the exhaust valve is shut.
• Downward motion of the piston creates a vacuum (negative air pressure), which draws the air/fuel mixture into the engine through the open intake valve.

2. Compression stroke

• The piston moves up the cylinder bore from BDC to TDC.
• The intake and exhaust valves are both closed.
• Upward motion of the piston compresses the air/fuel mixture in the combustion chamber.

3. Power stroke

• The spark plug fires and ignites the compressed air/fuel mixture just before the end of the compression stroke. This ignition/combustion forces the piston down the cylinder bore from TDC to BDC and rotates the crankshaft, which helps propel the vehicle forward.
• The intake and exhaust valves remain closed.

4. Exhaust stroke

• The 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.
• The intake valve is closed and the exhaust valve is open.
• This final stroke pushes the spent gases and exhaust out of the cylinder. The cycle is now complete and the piston is ready to begin another cycle.

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 pushrod 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.

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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.

Read: Motorcycle Suspension Systems Explained: Ultimate Guide

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 — one power pulse for every two engine strokes (versus one power pulse for every four strokes as in 4-stroke engines), potentially making 2-strokes almost twice as powerful as their 4-stroke counterparts.

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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.⁷

Read: Motorcycle Technician Job Opportunities & Outlook