2 Stroke Engine Basics

2 stroke engines are far and away the most simple and straightforward of all the internal combustion engine configurations out there. 2 stroke engines are also incredibly efficient, though they aren’t all that powerful.

+++Composed of just a handful of moving parts (centered around the crankshaft and the piston), 2 stroke engines are ridiculously easy to maintain. They’re also a perfect “project engine” for those that want to dive into the world of auto/motorcycle/marine repairs or modifications without feeling completely overwhelmed.

+++If you’ve always wondered what separated 2 stroke engines from the rest of the pack, hopefully you’ll find that this quick guide and introduction to this engine useful.

Let’s dive right in!

two stroke engine diagram

What makes the 2 stroke engine so different?

+++As mentioned above, the most significant difference between this engine configuration and all of the other options out there (including the 4 stroke engine) is that this internal combustion set up uses only a handful of moving parts. The other important difference is that the valves remain static throughout.

+++Because the valves (both of them) remain stationary throughout the entire combustion process, the inlet and exhaust are always timed based entirely upon the current position of the piston in a cylinder. The only two strokes possible are the “Power Stroke” and the “Exchange Stroke”.

+++At the beginning of a brand-new stroke (the power stroke), both the exhaust and the inlet ports are going to be wide open. Air and fuel are going to enter into the cylinder, pushing the piston upwards (to actually complete the power stroke), and the exhaust port is going to be closed by the piston.

+++The compression process starts here, with a vacuum forming inside of the inlet port because of the pressure created by the compression process. This pulls the piston forward towards the inlet, closing it off while at the same time opening up the exhaust port. Then the process repeats as necessary until the engine is turned off.

2 stroke engine disadvantages

+++The main disadvantage of these engines is the oiling system. This style of engine doesn’t have a traditional oil pump and oil pan setup. Most of the oiling that goes on is due to the oil in the gasoline.

2 stroke engine oil

+++2 stroke engines need to have oil in their fuel. Usually the gas cap of the engine will state what ratio of oil to gasoline you should use. The cap might say something like 10:1 which means ten parts of gasoline per one part of oil. A 2 stroke engine uses oil called “two-stroke oil”. It is available at most auto and hardware stores. Once you have added the oil to your fuel it is called “mixed-gas”. At that point, the fuel can only be used in a 2 stroke engine. If mixed gas is run in a 4 stroke engine, it will foul the spark plugs, run poorly, and smoke a lot.

+++Another disadvantage of a 2 stroke engine is the exhaust. A two-stroke engine smokes a lot. The smoke is caused by the engine burning the oil that is mixed in the fuel. The smoke is usually white and varies in density based on the throttle position.

+++Because this style of engine is constantly spewing oil out of it along with the exhaust, its emissions are poor. A normal four-stroke engine has much cleaner emissions. Even a Wankel engine has better emissions than a 2-stroke.

2 stroke engine oil mixing chart

How come 2 stroke engines aren’t as powerful?

+++It’s true that 2 stroke engines aren’t quite as powerful as their 4 stroke brethren or other significantly larger gasoline and diesel internal combustion engines. That doesn’t mean that they aren’t going to be able to propel you at top speed.

+++The compression ratio is low (usually sitting around 6:1 or 8:1), but that is more than enough to provide power to dirt bikes, smaller motorcycles, and marine-based engines. This is the opposite of a diesel engine which has a very high compression ratio.

+++This also guarantees that the engines will deal with less wear and tear, extending the life cycle of these engines significantly. This makes them popular in smaller style applications and a dream for those that manufacture lawnmowers and other small landscaping equipment.

Why is the 2 stroke engine so popular?

+++You’ll be able to fly around on a smaller dirt bike or motorcycle/moped that’s running off of a 2 stroke engine for a lot longer than you would have been able to with a 4 stroke engine or a traditional gasoline powered internal combustion set up.

+++This makes vehicles and equipment that run off of this engine configuration a lot less expensive to run, especially in an industrial style application or a commercial one.

+++Cleaning, maintenance, replacements, and repairs are also pretty simple and straightforward when you’re talking about this particular type of engine. A lot of individuals that are interested in learning about internal combustion, the workings of an engine, and the world of mechanics and engineering start off with a 2 stroke engine because of the simplicity that it brings to the table.

Closing thoughts

+++For the right applications (the ones with mentioned above are merely the tip of the iceberg), it’s difficult to argue with the success of the 2 stroke engine. Powerful enough to get the job done while at the same time fuel-efficient and inexpensive to work on, repair, or replace. There are hundreds or maybe even thousands of different applications for this engine configuration – with more cooked up on a daily basis.

Filed in: Gasoline Engines
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