4-stroke engines are the power source of motorcycles, cars, generators, lawn mowers, and much more.
Regardless of whether you are currently employed in the field or are thinking about working as a technician one day, it’s important to understand 4-stroke engine technologies and how different types of engines operate!
The 4-stroke engine cycle consists of the following: intake stroke (where the air/fuel mixture enters the engine), compression stroke (the mixture is compressed), power stroke (the mixture is ignited to move the vehicle), and exhaust stroke (the engine expels the spent gas). The intake and exhaust valves are operated by camshafts.
Though advancements in technology support that 4-stroke engines and 2-stroke engines are about equally reliable and competent in efficiency, their designs and operation vary greatly. 2-stroke engines do not use valves; instead, they use ports and generate power more often per cycle, which means they can be more powerful, but also generally less efficient and more polluting in comparison to 4-stroke engines.
Continue reading and learn more! You’ll learn about how 4-stroke engines work and how they are different from or similar to 2-stroke engines.
What Is a Four-Stroke Engine?
A four-stroke engine is an internal combustion engine that creates power from four movements of the piston, otherwise called strokes. The four strokes are:
- Intake
- Compression
- Power
- Exhaust
In a single stroke, the piston moves from the top of the cylinder to the bottom (or vice versa). This design was conceived in the late 1800s by Nikolaus Otto, with the design leading to what we now call the “Otto cycle”.
Almost 150 years later, you’ll find four stroke engines everywhere. Everything from your car and motorcycle to general aviation aircraft, boats, and heavy equipment has a four-stroke engine.
Who invented the four-stroke engine?
Nicolaus Otto invented and patented the successful four-stroke internal combustion engine in 1876, despite prior publications by French engineer Alphonse-Eugène Beau de Rochas years earlier about the four-stroke principles, but he had built a working engine.
Otto, however, developed a practical and efficient working engine for use as a useful alternative to the steam engine, which became a popular means of heat for other means of locomotion.
Nicoluas Otto’s Contribution:
- “The Otto Cycle”: Otto made the first working four-stroke internal combustion engine in 1876, which was a known engine design that worked, forming the basis of the ensuing commercial success.
- Practical application: The Otto engine, upon which the principles of operation were based on earlier designs, was much more efficient, reliable, and quiet than the older steam engine.
- Commercial success: The Otto engine was commercially successful, with Otto’s company building thousands of engines for a variety of uses that produced vehicles and other machines in the years to follow.
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: They allow 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.
What are the four strokes in a four-stroke engine?
A four-stroke cycle engine is an internal combustion engine consisting of four different piston strokes (intake, compression, power, and exhaust) to complete one full operating cycle. Two complete passes in the cylinder are required to have a complete operating cycle.
A complete operating cycle amounts to two revolutions (720°) of the crankshaft. The four-stroke cycle engine is the most common in a small engine application. A four-stroke cycle engine completes five strokes of operation within one operating cycle, including intake, compression, ignition, power, and exhaust strokes.
#1. Intake Stroke.
The intake stroke occurs when the piston moves from TDC to BDC with the intake valve open, and the piston moving toward BDC creates a low pressure in the cylinder.
Ambient atmospheric pressure causes the air-fuel mixture to flow in the open intake valve to fill the low-pressure area created by the piston. The cylinder continues to fill past BDC as the air-fuel mixture continues to flow due to its inertia, while the piston begins to change direction.
The intake valve will be open for a few degrees of crankshaft rotation after BDC, depending on engine design. The intake valve then closes, which seals the air-fuel mixture inside the cylinder.
#2. Compression Stroke.
The compression stroke happens when the trapped air-fuel mixture is compressed inside the cylinder when the combustion chamber is sealed to form the charge. The charge is the amount of compressed air-fuel mixture trapped inside the combustion chamber, ready for ignition.
Compressing the air-fuel mixture allows greater energy to be released during the combustion process when a charge is ignited. The intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide a compression stroke.
Compression is the actual process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. The flywheel aids in maintaining the momentum required to supply compression.
When the piston inside an engine compresses the charge, an increase in compressive force supplied by work being done by the piston generates heat. The compression and heating of the air-fuel vapor in the charge causes the temperature of the charge to rise along with an increase in fuel vaporization. The temperature of the charge increases uniformly throughout the combustion chamber to create a faster combustion (oxidation of fuel) after ignition.
The increased fuel vaporization occurs as the small droplets of fuel vaporize more completely from the heat generated. The increased droplet surface area exposed to the ignition flame allows greater quantities of the charge to combust in the combustion chamber. Only gasoline vapor ignites, and as the droplet surface area increases, more gasoline vapor will be released rather than remaining in a liquid state.
The more the charge vapor molecules are compressed, the more energy is generated from the combustion process. The energy needed to compress the charge is considerably less than the gain in force developed during combustion; for example, the energy required to compress the charge in a very small engine is only ¼ of the amount of energy developed during combustion.
The compression ratio of an engine compares the volume of the combustion chamber with the piston at the bottom of the stroke (BDC) to the volume of the combustion chamber with the piston at the top of the stroke (TDC). This area, along with the design and style of the combustion chamber, will determine the engine’s compression ratio. Gasoline engines generally have compression ratios from 6:1 to 10:1.
The higher the compression ratio, the better the fuel efficiency will be generated by the engine. However, normally, a higher compression ratio will have a substantial increase in combustion pressure or force on the piston. Higher compression ratios will usually also require more operator effort to start the engine. Some very small engines have a system in place to relieve some of the pressure during the compression stroke.
Ignition Event
The ignition (combustion) event happens when the charge is ignited and then rapidly oxidized by a chemical reaction, thus releasing heat energy. Combustion is a rapid oxidizing chemical reaction in which a fuel chemically joins together with the oxygen in the atmosphere to release energy in the form of heat.
Perfect combustion takes a short but definite time to allow a flame to spread through the combustion chamber. A spark at the spark plug ignites combustion, BTDC, at approximately 20° of crankshaft rotation before TDC.
The flame front consumes atmospheric oxygen and fuel vapor. The flame front represents the wall at the boundary of the charge and the combustion by-products, which moves across the combustion chamber until all the charge has been burned.
#3. Power Stroke
The power stroke is an operation stroke in an engine in which hot expanding gases force the piston head away from the cylinder head. The force from the piston and its motion is transmitted via the connecting rod to create torque applied to the crankshaft.
This torque creates rotation of the crankshaft. The amount of torque produced is based on the pressure applied to the piston, the size of the piston, and the engine throw. During the power stroke, both valves are closed.
#4. Exhaust Stroke
The exhaust stroke occurs when the spent gases are forced out of the combustion chamber to the atmosphere. It is the last of the four strokes and occurs with the exhaust valve open and the intake valve closed. The piston movement evacuates exhaust gases to the atmosphere.
When the piston reaches BDC during the power stroke, combustion is finished, and the cylinder is full of exhaust gases. At that moment, the exhaust valve opens and the inertia of the flywheel and moving parts pushes the piston back up to TDC and drives the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC, and one cycle of operation has been completed.
How Four-Stroke Engines Work Together
Multiple Cylinders
Can’t we just have one cylinder? The answer is simple. To better power and smoother delivery.
A single-cylinder four-stroke engine only exposes one power stroke every two revolutions of the crankshaft. With big gaps in the power stroke, you feel the vibrations in chainsaws or small motorcycles.
With multiple cylinders, you can time the pistons of their cylinders to run to a lesser value. With more runs, you will get a better feeling of power delivery.
Firing Order
The firing order is the specific order in which each cylinder goes through its power stroke.
Engineers have carefully chosen the firing orders to ensure that the engine will run as balanced as possible. If the firing order of the cylinders is awkward or a bit unusual, it’ll cause the engine to run really rough, vibrate too much, or even destroy the engine.
As an example, a typical firing order for an inline-four engine is 1-3-4-2.
The reason for the firing order being arranged that way is that it places the power strokes evenly spaced and also opposing movements somewhat cancel each other.
If we were to fire, say, an engine with a firing order of 1-2-3-4 (like the numbers appear), you would have a much lumpier operation because you would have two cylinders next to each other firing successively, and then a long space, with no firing.
Engine Configurations
When multiple cylinders are involved, you need to think carefully about how the engine block is arranged. The arrangement will affect not only smoothness of operation but the size and fitment of the engine into a vehicle or aircraft.
- Inline engines have all the cylinders in a straight row, one behind the other. The inline-4 is extremely common in automobiles because of its compact and simple design. The only drawback is that as you keep adding cylinders in a straight line, the engine gets longer and longer, and may become too long to fit in an engine bay.
- V engines have two rows of cylinders arranged in a V formation. The bank of cylinders in either row is half of the total number of cylinders in the engine. For example, a V6 has two rows of three cylinders each, and a V8 has two rows of four, and so on. The angle between the two rows of cylinders also affects the balance of the engine. A 90° V8 can be very smooth, but takes up a lot of vertical height in the engine compartment.
- A flat engine is also called a horizontally opposed engine or “boxer” engine. In other words, the opposite rows of pistons are directly opposite each other on either side of a common crankshaft, and lay flat. To visualize a “V” engine where the “V” is opened out to 180°.
- In a boxer engine, each pair of opposite pistons is taking the exact same motion in and out, like boxing gloves punching towards each other, hence the name “boxer”. Boxer engines are extremely smooth because the opposing forces of the “punching action” can cancel each other out quite well. For example, the Cessna C172 uses a flat-four engine for smooth operation and compactness.
- Radial four-stroke engines were common in propeller aircraft up until the ’60s because of their excellent power-to-weight ratio. The specialty, however, was cooling power relative to the weight. Each cylinder is exposed to the airflow action in the radial arrangement. The drawback is that because of the lack of compactness and bulky nature of a radial engine, you will not see any radial engines on aircraft today.
4-Stroke vs. 2-Stroke Engines
Advancements in technology have made 4-stroke and 2-stroke engines comparable in reliability and efficiency, though one engine may be better to run under certain conditions.
However, mechanics will likely still work on older models, so it’s still important to understand the differences between the two styles of engines.
Ports instead of valves.
2-stroke engines operate with ports, not valves, to route the air/fuel mixture and exhaust through the engine. The piston controls when the ports open and close, as it travels through the two-stroke cycle (Top Dead Center (TDC) to Bottom Dead Center (BDC) and BDC to TDC).
The diagram above illustrates how 2-stroke engines utilize the space above and below the piston. Machining ports in the engine case and cylinder allows 2-stroke engines to operate without valves.
They do not need a camshaft to open or close a valve. A 2-stroke engine can be lighter and have a smaller overall footprint than a comparable 4-stroke due to the fact that it has fewer parts.
Power two times as often.
A 2-stroke engine is unique because it completes its entire engine cycle with one revolution of the crankshaft.
2-stroke engines, therefore, create power twice as often as a 4-stroke engine – a 2-stroke has one pulse of power for every two engine strokes (versus a 4-stroke having one pulse of power for every four strokes). In this sense, a 2-stroke can be almost twice as powerful as a 4-stroke engine.
Common Applications for 4-Stroke Engines
Four-stroke engines are the most common type of combustion engine by a wide margin. They are put to use in many different applications and across many different industries; their most common uses include:
- Automobiles: Four-stroke engines are perfect for use in cars, trucks, SUVs, and other on-road vehicles.
- Heavy Machinery: Four-stroke engines are ideal for construction, agricultural, and industrial heavy machinery.
- Generators: Four-stroke engines are used to power portable and stationary generators.
Advantages of Four-Stroke Engines
Here are some advantages of four-stroke engines:
- Four-stroke engines generate more torque at a low RPM while operating.
- Four-stroke engines are more fuel efficient than two-stroke engines (fuel is burned one out of every four strokes).
- The engine produces less pollution, since no oil is burned with the fuel.
- Four-stroke engines are typically more durable and last longer, and they can resist wear and fatigue.
- The engine vibrates and makes less noise while running.
Disadvantages of Four-Stroke Engines
Here are some disadvantages of four-stroke engines.
- They tend to be complex, so troubleshooting is challenging, and repair and maintenance are often higher cost.
- Four-stroke engines produce less power than two-stroke engines.
- You must provide necessary maintenance to ensure optimal engine performance.
4-stroke Engine Maintenance
As with the previous items, the instructions for this will vary by engine model. This instruction will certainly have similarities, but these are specifically for FR 651, 691, and 730V engines.
Here are some of the more standard maintenance tasks you may be required to do on your four-stroke engine.
Oil Change
An oil change should occur every 100 hours or once a year, whichever comes first.
Steps to conduct an oil change for an FR engine:
- You will want to run your engine to warm the oil
- Ensure the engine is level
- Stop the engine
- Remove the oil gauge (dipstick). Remove the drain plug and then drain the oil into a suitable container while warm (not hot!) (remember, engine oil can cause severe burns if too hot!). Allow the engine temperature to cool enough to be warm, but not too hot, before you drain and handle any of the operating engine oil.
- Install the drain oil plug
- Refill with fresh oil
- Check the oil level with the oil gauge (dipstick)
IMPORTANT: When disposing of used engine oil, exercise caution because engine oil is toxic. You can check with local authorities as to the best and approved methods or recycling options.
Change the oil filter
Change oil filter within each operating period of 200 hours; before removing the oil filter, allow the engine to cool to below 120°F (49°C). To change the oil filter:
- Dump engine oil into a suitable container
- Remove the oil filter by turning it counterclockwise
- Lightly coat the new oil on the new oil filter seal
- Install the new oil filter by installing it clockwise until the seal is seated against the mounting surface, then lightly tighten by hand for an additional 3/4 turn
- Refill the engine oil to the proper level
- Run the engine for about three minutes, then shut the engine down to inspect the oil filter for leaking oil
- Refill engine oil if necessary to compensate for filter capacity
Air filter paper element
If you have a FR engine using average power, you should clean the air filter paper element every 100 hours of use. You will need to replace the paper element after 200 hours of use or one year.
The paper element can be cleaned by gently tapping the seat of the paper element against a hard surface to knock the dirt/dust off; if the paper element is very dirty, you will need to replace it, as cleaning will not work.
Do NOT wash or oil the paper element, or use pressurised air to clean.
Spark Plug Maintenance
Clean or replace the spark plugs and reset the spark plug gap every 100 operating hours.
- Remove the spark plug caps; remove the spark plugs
- Clean the electrodes using a non-metal brush to remove carbon buildup.
- Check for cracked porcelain or wear and tear; if necessary, replace the spark plug with a new one.
- Check the spark plug gap and reset if necessary. The gap must be 0.75 mm: adjust only the ground electrode, using a spark plug tool. Install and tighten the spark plug to 22 N·m.
- Reinstall the spark plug caps.
Cleaning Your Cooling System
The entire cooling system should be checked and cleaned after every 100 hours of operation, but the rotary screen should be checked prior to every run.
To clean the system, check the cooling fins and the inside engine shrouds, and remove any grass, dirt, or debris. To clean, you will have to remove the air filter, loosen the bolts, and then remove the fan housing. Please be very careful, do not run the engine before you reinstall all cooling system parts.