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In this blog, you’ll discover:
- The basics of an internal combustion engine
- The 4 most common motorcycle engines
- Differences between 2 and 4-stroke (and what a “stroke” is)
NOTE: For this blog post, we’re going to focus solely on internal combustion engines. Electric engines are a topic for another day, but we’ll certainly cover them.
For now, however, how does an internal combustion engine work?
Prefer to get this information in podcast form? Listen to the 30 Minute Motorcycling Podcast episode about the basics of the internal combustion engine and the 4 most common engine layouts:
Internal combustion engine 101
At a basic level, an internal combustion engine works by burning fuel (usually petrol/gasoline) inside a place called the “combustion chamber.”
The combustion chamber is located inside a place commonly referred to as a “cylinder.” Inside the cylinder, there’s a component known as a “piston,” which moves up and down while the engine’s running.
While this happens, a few small spaces called “valves” open and close to let in fuel and air mixture and expel the exhaust gases.
No matter what design an engine has, it must complete the following stages to complete one combustion cycle:
1: Intake stroke: fuel and air mixture enters the combustion chamber through the inlet valve
2: Compression stroke: the piston moves upwards, compressing the mixture
3: Ignition stroke: Once the mixture reaches the top, the spark plug ignites the mixture, which also pushes the piston down
4: Exhaust stroke: the burnt gases are pushed out of the combustion chamber to the exhaust pipe through the outlet valve.
The whole process looks something like this:
Every motorcycle engine is separated into two ends – the “top end” and the “bottom end.”
The “top-end” is the part located above the crankcase assembly (where the crankshaft is located) and includes everything needed to control the fuel flow in and out of the engine.
Some of these components include:
- The cylinders
- The cylinder head
- The pistons
- The valves
- The camshaft (if the engine uses an “overhead camshaft design”)
For more information about what the camshaft is and how it works, read this article:
The bottom end is everything below the crankcase that delivers reliable and smooth power, including:
- The crankshaft
- The flywheel
- An oil pump (if the engine is 4-stroke)
- The oil sump
4 Common Motorcycle Engines
The most simplistic engine layout, a single-cylinder engine is common on:
- Dirt bikes
Since single-cylinder engines are so simplistic, they’re cheap and easy to make. Not only that, but they’re also easy to work on.
A single-cylinder engine can be powerful – at least at lower speeds. At higher speeds, it starts to lose much of its power.
On inline engine layouts, the pistons are positioned vertically in line with which other – hence the term “inline engine.”
Since the 1970s, the inline four-cylinder engine has been one of the most common engine designs.
You might hear “inline six” or “inline four” mentioned a lot. If someone says they’ve got an “inline four,” it means that:
- The engine’s cylinders are positioned in line with each other (hence the term)
- The engine has 4 cylinders
No matter how many cylinders an inline engine may have, it isn’t just powerful – it also has generally good fuel economy.
On the flip side, an inline engine has very uneven torque and can be a little too powerful for some.
Sometimes known as a “Parallel twin engine,” the cylinders in V-twin motorcycle engines are positioned side-by-side in a V-shaped pattern – hence the name.
Many cruiser motorcycles use the V-twin.
Compared to the inline engine, a V-twin has a much more even and flat power curve – making it easy to handle for a new rider.
But a V-twin is also not as powerful as an inline engine. Some of them also vibrate a lot.
Flat twin engine
Often known as a “boxer engine,” or a “horizontally-opposed twin,” the cylinders in flat-twin motorcycle engines are positioned on opposite sides of the crankshaft in a horizontal pattern.
BMW has used the flat-twin engine for their motorcycles since the 1920s (including the R68 pictured above).
Boxer engines are powerful but also expensive to make. As such, motorcycles with boxer engines tend to have a high price tag.
What is an Engine Stroke?
Prefer to get this information in podcast form? Listen to the 30 Minute Motorcycling Podcast episode about engine strokes and the differences between 2-stroke and 4-stroke engines:
Remember what I mentioned earlier about the combustion chamber, the cylinder and the piston?
Simply put, a “stroke” refers to one movement of the piston inside the combustion chamber.
Therefore, if an engine is 4-stroke, it means that it takes 4 movements to complete a full combustion cycle.
What is a 2-stroke Engine?
2-strokes are often found on single-cylinder dirt bikes, motorcycles, mopeds, and scooters.
These engines just require two strokes to finish a combustion cycle, which looks like this:
2-stroke pros & cons
The 2-stroke’s biggest advantage is that it’s simpler and lighter than a 4-stroke.
And just like a single cylinder engine, a 2-stroke engine can be very powerful – at least at slow speed.
However, at higher speeds – not so much.
Also, a 2-stroke usually doesn’t have a reservoir to store the engine oil – meaning that you’ll have to mix the engine oil in with the petrol yourself.
If that’s something you need to do, check out this video from the Tomos America YouTube channel:
What is a 4-stroke engine?
4-stroke engines are very common on street and racing bikes and have four stages to complete a full combustion cycle.
We talked about these four stages when we covered the basics of an internal combustion engine, but for the sake of revision, the four strokes in a 4-stroke engine are:
1: Intake (“Suck”)
2: Compression (“Squeeze”)
3: Ignition (“Bang”)
4: Exhaust (“Blow”)
4-stroke pros & cons
If you need a motorcycle capable of higher speeds, a 4-stroke engine is the best choice.
After all, have you ever seen a Honda Goldwing or any other touring motorcycle with a 2-stroke?
Also, unlike a 2-stroke, you don’t need to pre-mix engine oil with petrol or gasoline by hand – it’s all done by the fuel pump inside the engine.
So far, so good, right? Well, even a 4-stroke engine has its shortcomings. For one thing, it’s heavier.
Furthermore, since it has more parts than a 2-stroke, if the engine breaks down, it means more parts to replace.
Maintenance can also be more expensive.