Although automotive tech has changed significantly over the years, four-stroke and two-stroke remain the two main combustion engine designs. Although the pre-owned cars at the NJ State Auto dealership in Jersey City feature four-stroke engines, many people can’t differentiate between two-stroke and four-stroke engines.

In this guide, our car experts discuss two-stroke and four-stroke engines and how they work. After reading this, you’ll have a better idea of how engines work.

Getting Down to Basics

Before we even distinguish these two types of engines, it’s best to explain the basics first. As fuel combusts, the piston moves up and down in the cylinder. “Bottom dead center” and top dead center” are two terms that refer to the piston’s movement and position in the cylinder. 

TDC is the piston’s position closest to the valves, while BDC is its position furthest from the valves. So, a stroke refers to the back and forth movement of the piston from the BDC to the TDC. Likewise, a combustion cycle/revolution is the entire process of air and fuel getting sucked into the piston, thus igniting to power the motor and expel the exhaust gases. Also, it’s best to distinguish these key terms: 

• Intake: Intake refers to the movement of the piston down the cylinder to allow the air-fuel mixture to enter the combustion chamber. 
• Compression: This is the movement of the piston up the cylinder. During compression, the intake valve closes off, thus compressing the gases within.
• Combustion: It occurs when a spark from the spark plugs ignites the gas.
• Exhaust: This is the process through which waste gases get expelled from the engine. The piston moves up the engine cylinder during exhaust, thus opening the exhaust valve to let out the gases. 

Two-Stroke vs. Four-Stroke Engines: What’s the Difference?

There isn’t a significant structural difference between two-stroke and four-stroke gases. The only notable difference is the speed at which the combustion cycle occurs and how often the piston moves up and down the cylinder during each cycle. 

Two-Stroke Engines

As the name suggests, the power cycles get completed with only two piston strokes in a two-stroke engine. This happens during one crankshaft revolution. However, the entire combustion process is completed with one piston stroke. Typically, this is a compression stroke followed by the ignition of the compressed fuel.

The exhaust gases are let out in the return cycle, and a fresh air-fuel mixture is allowed into the cylinder. In two-stroke engines, the spark plugs ignite the fuel-air mix once every single revolution. Similarly, power gets produced only once with every two strokes of the pistons. Therefore, for combustion to occur in two-stroke engines, the oil needs to get pre-mixed with the fuel. 

Two-stroke engines are primarily found in the lower-powered motors of dirt bikes, lawnmowers, jet skis, mopeds, and chainsaws. 

Pros of Two-Stroke Engines

• Simple and easy-to-understand mechanism.
• A high power-to-weight ratio provides a significant power boost.
• The lack of valves negates the need for a complex valve actuating mechanism.
• Lightweight.
• It doesn’t require an oil sump for lubrication.
• Low maintenance costs. 
• Uniform turning moment since there’s a power stroke in each revolution.

Cons

• Poor combustion leaves carbon deposits on the exhaust port and piston head.
• Unstable idling.
• Poor fuel economy, since part of the unburned charge gets rejected during the transfer phase.
• Faster wear and tear resulting in shorter life.
• Noisy operation and high vibration.
• Lower power band than four-stroke engines.

Four-Stroke Engines

We find these engines in different machines, from motor vehicles to generators. All the gasoline and diesel-powered cars in our Jersey City dealership feature internal combustion engines with four strokes. 

As the name suggests, four-stroke engines utilize four different piston strokes to ignite and run. During engine operations, the piston undertakes two strokes during every revolution. The first revolution involves one compression stroke and an exhaust stroke. A return stroke then follows the strokes. Here’s a deeper look at what happens at each of the four strokes: 

Intake Stroke

• The piston moves down the engine cylinder bore from the TDC to the BDC.
• The intake valve remains open while the exhaust valve closes.
• The downward piston motion creates a vacuum (negative air pressure). This draws the air-fuel mixture into the motor through the open intake valve.

Compression Stroke

• The piston moves up the cylinder to the TDC from the BDC.
• The intake and exhaust valves remain closed.
• The upward piston motion compresses the air-fuel mixture within the combustion chamber.

Power Stroke

• Towards the end of the compression stroke, the spark plugs fire, thus igniting the compressed air-fuel mixture. This forces the piston back down the cylinder and rotates the crankshaft to propel the vehicle forward.
• The piston moves down the cylinder from the TDC to the BDC.
• Both the exhaust and intake valve remain closed.

Exhaust Stroke

• The piston moves up the cylinder from the BDC to the TDC. The resultant momentum helps to maintain the crankshaft movement and the other three strokes.
• The intake valve closes while the exhaust valve opens.
• Spent gases and exhaust is forced out of the cylinder. This completes the cycle, thus readying the engine for the intake stroke.

In a four-stroke engine, the spark plugs only fire once in every revolution. Likewise, power gets produced with every four strokes of the piston. In addition, the engine doesn’t require fuel and oil to pre-mix since there’s a separate oil compartment. 

Pros of Four-Stroke Engines

• Better fuel efficiency since fuel gets consumed only once every four strokes.
• They create a higher torque at significantly lower RPM.
• The engines are quieter and more durable.
• More eco-friendly since burnt oil gets released with the exhaust.

Cons

• Complex design.
• Low power-to-weight ratio.
• Costlier than two-stroke engines.

Final Words

If you’re looking to hit the Delaware and Raritan Canal State Park Trail, one question will run through your mind; do you choose a machine with a two-stroke engine over one with a four-stroke engine? Hopefully, this guide will help you to make the best decision. 

If you’re looking to upgrade your ride to a clean-preowned unit, we have all car models in our inventory. Contact us today or schedule a visit to check what we have in store for you.