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

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From motorcycles and cars to lawn mowers and generators, 4-stroke engines power a variety of different kinds of equipment. This type of engine utilizes four distinct piston strokes in order to effectively run.

Whether you’re currently working in the field or are considering becoming a technician, it’s important to understand exactly how this process works. Technicians regularly work with various engine types, and knowing their differences is key to success in the industry.

If you’re wondering, “What is a four stroke engine?,” this guide is for you. Keep reading to learn all about 4-stroke engines and how they work, as well as 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 very common variation of an internal combustion engine. Most modern internal combustion-powered vehicles are 4-strokes, powered by either gasoline or diesel fuel.

 During engine operation, pistons go through 4 events to achieve each power cycle. The definition of an event is an up or down piston motion. Upon completion of the 4 events, the cycle is complete and ready to begin again.

4-stroke engines deliver a good balance of power, reliability and efficiency. When it comes to emissions, 4-strokes separate each event mechanically, which reduces unburned fuel emissions. It also separates oil from fuel, which significantly reduces carbon monoxide emissions. This combination of desirable traits has earned the 4-stroke the top spot in passenger vehicles today.

Other Engine Types

2-stroke engines are also common, however they do not power cars or light trucks. 2-strokes are found on small engines, such as dirt bikes, chainsaws, outboard marine engines, lawn care equipment, scooters and mopeds, and so on. A 2-stroke engine is still an internal combustion gasoline powered engine, but they differ from 4-strokes in their design. More on 2-stroke engines below.

Diesel engines are 4-stroke, but they differ from their gasoline-powered counterparts in their method of combustion. Diesels rely on very high compression ratios to ignite the air/fuel mixture rather than a spark plug. More on diesel engine operation will follow in a separate blog post.

Wankel and Rotary engines are internal combustion engines, but they do not have “strokes” in the same sense that a 2 or 4-stroke engine has. We will cover Wankel and Rotary engines in more detail in another blog post.

What Are the Strokes of a 4-Cycle Engine?

In order to effectively power equipment, 4-stroke engines complete and repeat the following steps:

Intake stroke

  • Piston moves down the cylinder bore from top dead center (TDC) to bottom dead center (BDC)
  • Intake valve is open, the exhaust valve is closed
  • Downward piston motion creates a vacuum (negative air pressure) that draws that air/fuel mixture into the engine via the open intake valve

Compression stroke

  • Piston moves up the cylinder bore from bottom dead center to top dead center
  • Both the intake and exhaust valves are closed
  • Upward piston motion compresses air/fuel mixture in the combustion chamber 

Power stroke

  • At the end of the compression (previous) stroke, the spark plug fires and ignites the compressed air/fuel mixture. This ignition/explosion forces the piston back down the cylinder bore and rotates the crankshaft, propelling the vehicle forward.
  • Piston moves down the cylinder bore from top dead center to bottom dead center
  • Both the intake and exhaust valve are closed

Exhaust stroke

  • Piston moves up the cylinder bore from bottom dead center to top dead center. The momentum caused by the power stroke is what continues the crankshaft movement and the other 3 strokes consecutively.
  • Intake valve is closed, the exhaust valve is open
  • This final stroke forces the spent gasses/exhaust out of the cylinder. The cycle in now complete and the piston is ready to begin the intake stroke.

The below diagram gives a visual representation of how this process works:

 

four stroke engine 01

 

  1. Intake stroke: The intake valve (on the top left of each image) is open and as the piston travels downward, this suction pulls the air/fuel mixture into the cylinder.
  2. Compression stroke: Both valves are now closed and the piston compresses the air fuel into a much smaller volume, preparing the mixture for ignition.
  3. Power stroke: With both valves closed, the spark plug—located in the picture between the intake and exhaust valve will fire, igniting the air/fuel mixture. The resulting explosion forces the piston downward and rotates the crankshaft, which in turn propels the vehicle.
  4. Exhaust stroke: The exhaust valve (on the top right of each image) is now open, allowing the piston to push the spent exhaust gasses out of the engine as it rises. The 4-strokes (1 engine cycle) are now complete, and the process repeats.

Air is compressible. When the air/fuel is compressed before ignition, combustion efficiencies are improved. Compression ratio is the relationship of total cylinder volume to compressed volume. For example, a compression ratio of 10:1 (verbally spoken as “10 to 1”) would indicate that the chamber squeezes 10 parts of air/fuel volume into 1 part of that volume at the end of the compression stroke.

Higher compression ratio can offer more power in some applications. The downsides to higher compression ratio can be decreased durability and the need to run higher-octane (expensive) gasoline.

Valves

Air and fuel enter the cylinder via the intake valve, and spent exhaust gasses exit through the exhaust valve. Use of valves is one of the major differentiating characteristics of a 4-stroke engine compared to 2-strokes. The total number of valves per cylinder will vary based on engine design (2, 3, 4, 5) but each valve can only be either an intake or an exhaust.

Valves open and close at predetermined times in relation to the piston, allowing for reliable and efficient power generation. Engine and valve train configurations vary, but the goal remains consistent—a camshaft is precisely timed to the crankshaft, and they work together to manage the mechanical operation of the engine.

In push rod valve actuation, the camshaft gear is driven off the crankshaft:

four stroke engine 02

 

In direct valve actuation, the camshaft is driven by the crankshaft via a timing chain or belt, also known as an overhead cam engine:

four stroke engine 03

 

While the engine crankshaft controls the up and down motion of the piston inside the cylinder, the camshaft is responsible for opening and closing the valves.

The Difference Between 2-Stroke and 4-Stroke Engines

2-stroke engines differ from 4-strokes in three key areas:

  • They do not use valves
  • They burn oil in the combustible mix of air and fuel
  • 1 power pulse for every 2 engine strokes (versus 1 power pulse for every 4 strokes as we see in 4-stroke engines)

Ports, Not Valves

2-strokes flow air, fuel and exhaust through the engine without the use of valves. Rather, they use ports. 2-strokes also take advantage of the airspace below the piston. Each piston stroke is pressurizing and acting upon 2 chambers simultaneously.

four stroke engine 04


The above diagram demonstrates how 2-stroke engines take advantage of the space above and below the piston. The uses of machined ports with the engine casing itself allows 2-strokes to avoid using valves.

Additionally, there is no camshaft required to open or close the valves—fewer parts means that 2-strokes are lighter and more compact than 4-stroke engines. 

Mixing Oil and Gas

Mixing oil and gas can be either a manual process where the operator physically mixes both components in a gas can, or an automatic process using oil injection systems. Either way, 2-stroke engines burn oil to provide lubrication to moving parts.

Power Twice as Often

The final major differentiating characteristic of a 2-stroke engine is its ability to complete each engine cycle and have a power pulse twice as often as 4-strokes. Similar displacement 2-strokes can be almost twice as powerful as their 4-stroke counterparts.

2-Stroke Disadvantages

If 2-strokes are lighter, smaller and more powerful than 4-strokes, why aren’t they more common? 2-strokes have a few distinct downsides, including:

  • Less fuel efficient
  • Noisier and more prone to vibration
  • Considerably more emission pollution
  • 2-stroke oil is expensive to purchase and mixing to the correct ratio can be challenging

What Does All of This Mean for Your Career?

While performing daily job activities, motorcycle, marine and small engine technicians regularly work with various engine types. Understanding the differences and characteristics is key to diagnosing and repairing vehicles and equipment.

Certain parts are more prone to wear out or fail. As technicians gain knowledge and experience, they will be able to quickly and efficiently remedy the situation for the customer. 

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In MMI’s 42-week Motorcycle Technician Training Program, you’ll learn the foundations of motorcycle, ATV, side-by-side and personal watercraft to prepare for a career as a motorcycle technician.

To learn more, visit our program page and request information to get in touch with an admissions representative today.

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