2 Stroke engines are known for their high torque output but low RPM. These engines use a lower compression ratio, which limits their performance. Engines need spark plugs for creating combustion, so how can 2 strokes engines do without them?
Some 2 stroke engines have spark plugs. Gasoline engines have spark plugs that ignite the fuel-air mixture to continue powering the engine. In contrast, diesel engines do not have spark plugs but use glow plugs for ignition.
The rest of this article will explain how two-stroke engines work and the role spark plugs play in the process. Read on for in-depth details and insights into what diesel engines use in place of spark plugs.
How Many Spark Plugs Does a 2 Stroke Engine Have?
Most, but not all fuel injected 2 stroke gasoline engines have one spark plug per cylinder. The most common type has a single cylinder and only one piston to compress the air/fuel mixture before ignition. In some instances, these engines come with more than one plug per cylinder.
Most high-performance engines typically feature at least two spark plugs per cylinder. The main reason for this is that it provides two combustion points, which allows the engine to complete the combustion cycle twice as fast (source).
As a result of the faster burning, the engine delivers the following:
- Improved fuel economy: With the same amount of energy as a four-stroke engine, the quicker burn time means there’s less wasted fuel.
- Improved torque and power: More complete combustion leads to more torque and horsepower.
- Less smog: The cleaner, more efficient burn leads to fewer pollutants.
How a Two-Stroke Engine Works
Now that you know how many spark plugs a 2 stroke engine usually has, it’s time to look at how these engines work. To make things simple, I’ll break the process down into the following steps:
- The fuel-air mixture is drawn into the cylinder.
- The piston compresses the fuel-air mixture.
- The spark plug ignites the mixture.
- The hot gas pushes the piston down.
- The engine expels the exhaust gases.
Here’s a rundown of what happens in each of these stages:
1. The Fuel-Air Mixture Is Drawn Into the Cylinder
The piston draws in the fuel-air mixture through either a Carburetor or intake port. There are no valves on these ports: the only thing that restricts airflow is the piston itself. This means there can be some loss of power during this stage as some of the air escapes without igniting.
2. The Piston Compresses the Fuel-Air Mixture
Next, as the piston moves down within its cylinder, it compresses this fuel-air mixture more until it reaches its optimum compression ratio for combustion. Most modern engines feature between 9:1 and 12:1 compression ratios, but older or less efficient models typically have lower ratios than this.
3. The Spark Plug Ignites the Fuel-Air Mixture
Once the compression ratio has been reached, a spark plug—or glow plug— ignites the fuel-air mixture. Gasoline engines use spark plugs for ignition because gasoline is easier to ignite than diesel—more on that shortly (source).
Note: Glow plugs require the engine to draw current from its battery in order to heat them. Once they’re hot enough, they induce combustion throughout the rest of the cycle.
4. The Hot Gas Pushes the Piston Down
As the fuel-air mixture ignites, it creates a combustion reaction that pushes the piston back down. The piston then turns the crankshaft to provide the mechanical power needed to rotate the engine. At this point, you hear your engine starting: it turns over and starts to generate power.
5. The Engine Expels the Exhaust Gases
Finally, as the piston heads back up from being pushed by the explosive force, it expels these hot gases through an exhaust port. These gases are then collected by an external muffler or other elimination components. In some cases, this exhaust goes directly into a smog-catcher filter before exiting out an exterior vent.
The following video summarizes how a two-stroke engine works in 2 minutes:
Tip: If you have a machine that uses a two-stroke engine and need guidance on maintaining it, I recommend reading this Two-Stroke Engine Repair and Maintenance (link to Amazon). This is a guide by Paul Dempsey, a long-time mechanic who explains how to troubleshoot common two-stroke problems, ways to fix them, and how to safely overhaul these engines.
Why Does a Diesel Engine Use Glow Plugs Instead of Spark Plugs?
Diesel engines don’t need to use spark plugs to ignite the fuel-air mixture. Instead, these engines use something known as a glow plug. A glow plug is an electrically heated device that heats up when connected to a battery.
Diesel engines use glow plugs instead of spark plugs due to the higher heat demand in their compression systems. Spark plugs cannot produce enough heat to maintain combustion in these engines. However, glow plugs produce enough heat to ignite the fuel when needed.
In general, diesel engines contain much higher compression ratios than gasoline engines. While gasoline engines typically have a maximum compression ratio of 12:1, diesel can go as high as 25:1 or more. This is beyond the limits that spark plugs can handle without exploding. Glow plugs, however, can tolerate these extreme conditions and provide consistent ignitions.
See Will a Diesel Engine Run Without Glow Plugs?
FAQs
How Does a 2 Stroke Engine Get Spark?
2 stroke engines get spark with a spark plug that ignites the fuel-air mixture. The spark is created by an electrical current that passes through the spark plug. This current is created by a magneto, an electric generator that provides power to the ignition system.
Spark plugs wear out over time, so they need to be regularly replaced. A worn-out spark plug can cause your car to run poorly or not start at all. If you’re having trouble with your 2 stroke engine, make sure to check your spark plugs and replace them if necessary.
What Is the Difference Between a 2 Stroke and a 4 Stroke Engine?
The main difference between a 4-stroke engine and a 2-stroke engine is the number of piston stroke cycles used for power generation. A 4-stroke engine requires four strokes (up, down, up, down) to generate its power. Meanwhile, a 2 stroke only requires two (up and down).
Here’s a table that summarizes the key differences between 2 stroke and 4 stroke engines:
| Two-Stroke Engine | Four-Stroke Engine | |
| Number of revolutions per power stroke | One | Two |
| Torque | Higher | Lower |
| Power-to-weight ratio | Larger | Smaller |
| Thermal efficiency | Less efficient | More efficient |
| Weight | Lighter | Heavier |
The Bottom Line
Spark plugs in a 2 stroke engine provide the ignition needed for your vehicle to keep running.
However, in a diesel engine, spark plugs serve no purpose. Diesel is less volatile and doesn’t ignite from a spark as easily as gasoline does. Instead of using spark plugs for combustion, these engines use glow plugs that can be heated up with electricity or direct contact with a combustion chamber.
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