Two cycle engines, also known as 2 stroke engines, have been used for many decades in small engine applications such as chainsaws, weed trimmers, outboard motors, motorcycles, scooters, and more. They offer some advantages over 4 stroke engines including mechanical simplicity, lighter weight, and higher power to weight ratio. However, 2 cycle engines have always been notorious for being more difficult to start, especially when cold, compared to typical modern 4 stroke engines. There are some fundamental reasons for this.
Higher Compression Ratio
One of the main reasons 2 cycle engines are harder to start is their higher compression ratio. The compression ratio is the ratio of the volume of the combustion chamber from its largest capacity to its smallest capacity. It is a specification relating to how much the fuel/air mixture is compressed before ignition. Higher compression ratios result in more power, but require more effort to squeeze the mixture during starting.
Typical compression ratios in 2 cycle engines range from 7:1 up to 14:1 for high performance models. By comparison, most modern auto engines have compression ratios in the range of 8:1 to 12:1. The highest compression ratios tend to be found in chainsaws, string trimmers, and high performance motorcycles where power to weight ratio is optimized.
With more compression, there is simply more resistance for the starter motor to overcome when cranking the engine during starting. More battery power and starter torque is needed compared to a lower compression 4 stroke engine.
Greater Tendency to Flood
Another inherent issue with 2 cycle engines is their greater tendency to flood during starting. This can make cold starting problematic. Flooding occurs when too much fuel accumulates in the crankcase/combustion area prior to startup. The spark plug then fires in an over-rich mixture which cannot properly ignite.
In a 2 cycle engine, fuel is directly injected into the crankcase as the piston moves upward. Some of this fuel can accumulate and puddle rather than atomizing properly with the incoming air. Flooding is more prone to occur with extended cranking or choke use during cold startup when fuel condenses more easily. This leads to wet, fouled spark plugs and poor or no ignition.
In contrast, 4 stroke engines rely on their intake stroke to draw fuel mixed with air into the cylinder, and are less prone to fouled plugs and flooding during startup cranking. However, 4 strokes are not immune to flooding if over-choked.
Lean Idle Mixture
Many 2 cycle engines also suffer from weak idle and low speed fuel mixture. At idle speed, there is less crankcase pumping action to move fresh fuel/air mix into the combustion chamber. This can lead to a lean mixture and unstable idling after cold startup. By contrast, 4 stroke engines utilize their separate intake stroke to keep pulling mix into the cylinder at low rpm levels.
Modern fuel injected 2 stroke engines have largely solved the weak idle mixture issue. But many legacy 2 stroke designs still struggle with this. The weak idle mixture contributes to cold starting difficulties and stalling if the choke is released too early before the engine fully warms up.
Smaller Engines and Lower Inertia
Being smaller displacement than most 4 stroke engines, 2 cycles suffer from lower inertia which can make cold starting more difficult. Less rotating and reciprocating mass in a small 2 stroke engine allows it to slow down quicker during the compression and power strokes while cranking. This makes it harder for the next rotation to build up enough momentum to complete the compression stroke and start the engine.
Heavier 4 stroke engines with greater rotating mass tend to be easier to spin through compression and have an easier time starting as a result. Their greater inertia helps carry them through the compression stroke.
Short Burn Time
The shorter combustion burn time in a 2 cycle engine also contributes to cold starting difficulties. In a 2 stroke, the compact combustion chamber needs to fully burn the fuel air mix in the brief time the piston travels from bottom dead center to top dead center. This gives a very narrow window for complete combustion. By comparison, 4 stroke cycle burn time is much longer with a dedicated compression stroke followed by a full power stroke.
When cold, the shorter burn time makes it harder for 2 stroke combustion to properly develop when cranking. Weak or incomplete burn can lead to misfires, poor starting, and low initial power until the engine fully warms up. Modern direct injection 2 strokes have made some improvements here.
Small Spark Plugs and Ignition Components
Being generally smaller displacement engines, 2 cycles utilize smaller spark plugs and ignition components. This can negatively impact cold starting in some cases. Smaller plugs and ignition coils provide lower voltage spark energy. Combined with already high compression, a weak spark can struggle to ignite the mixture when cold and cause misfires.
Many modern 2 stroke engines now use multiple spark plugs per cylinder and electronic ignition systems to improve cold start performance. But some legacy 2 stroke designs still utilize traditional single plug magneto ignition systems.
Solutions for Easier Starting of 2 Cycle Engines
While 2 strokes will always be a bit more challenging to start than 4 strokes, there are some techniques to make the process easier:
- Use fresh, clean fuel – Old, dirty fuel is harder to ignite
- Check and gap the spark plug – Replace if worn or fouled
- Use proper choke settings – Don’t over-choke which can cause flooding
- Allow proper warm up time before high revving
- Consider pre-heating the engine if available
- Install an electric starter motor – Makes starts much easier than pull rope recoil starting
- Multiple spark plugs per cylinder can help on some engines
- Upgrade the ignition system – Electronic instead of magneto ignition
- Higher compression 4 stroke engines now face similar cold start issues which are mitigated via fuel injection, pre-heating, and electronic ignition.
Conclusion
In summary, the primary inherent factors that make 2 cycle engines more difficult to start cold compared to 4 strokes include:
- Higher compression ratios
- Greater tendency to flood the combustion chamber
- Lean low speed fuel mixture
- Lower inertia of smaller engines
- Shorter burn time window
- Small spark plugs and ignition components
These challenges can be mitigated to some degree via proper maintenance, allowance for sufficient warm up, upgraded ignition components, and optimized fuel/air mixing. However, the characteristics which give 2 cycle engines advantages like simplicity, compactness, and power to weight ratio are directly related to their cold starting disadvantages. As emission regulations continue to tighten, 4 stroke technology will likely continue displacing 2 strokes in most engine categories. But 2 cycles will retain their cold starting issues in applications where such characteristics necessitate their use.
Frequently Asked Questions
Why do 2 stroke engines need more choke?
2 stroke engines require more choke during cold starting because they are more prone to having a weak/lean fuel mixture enter the combustion chamber at low rpm. The choke provides additional fuel enrichment to compensate for this issue inherent in the 2 stroke cycle.
Why do 2 stroke engines burn oil?
2 stroke engines inherently burn some lubricating oil along with the fuel mixture. This is because the crankcase is used to pump the fuel/air charge into the cylinder, so it must be lubricated. Some oil inevitably escapes through the exhaust port along with combustion gases.
Do 2 stroke engines have more torque?
Yes, 2 stroke engines typically produce much higher torque relative to their displacement than equivalent 4 stroke engines. The power stroke comes each revolution in a 2 stroke allowing greater torque, whereas 4 strokes have only one power stroke for every two revolutions.
Are 2 stroke engines better?
2 stroke engines have some advantages including:
- Simpler design
- Lighter weight
- More compact
- Higher specific power
- Higher torque
However, they also have some disadvantages that limit their use:
- Poor fuel efficiency
- Greater emissions
- More frequent maintenance
- Poorer durability
- Difficult cold starting
So 2 stroke engines are better for applications favoring power to weight ratio and simplicity, while 4 strokes tend to be better for fuel economy, emissions, smoothness, and reliability.
Why do 2 strokes smoke so much?
2 stroke engines emit more visible smoke and exhaust emissions because lubricating oil is directly injected into the combustion chamber along with fuel. This oil burns incompletely and leads to smoking exhaust. Newer direct injection 2 strokes help reduce this issue.
Comparative Table of 2 Stroke vs 4 Stroke Engines
| Engine Characteristic | 2 Stroke Engine | 4 Stroke Engine |
|---|---|---|
| Compression Ratio | 7:1 to 14:1 (higher) | 8:1 to 12:1 (lower) |
| Power Stroke Frequency | Every revolution | Every other revolution |
| Torque Production | Higher | Lower |
| Specific Power | Higher hp/lb or hp/liter | Lower |
| Emissions | HigherHC, CO, NOx | Lower |
| Fuel Economy | Poorer mpg or kpl | Better |
| Idling Stability | Poorer | Better |
| Cold Start | More difficult | Easier |
| Lubrication Method | Fuel/oil mix | Separate oil reservoir |
| Durability | Generally lower | Higher |
Conclusion
In summary, 2 stroke engines inherently face greater challenges during cold starting compared to 4 stroke engines. Their basic operating principles dictate higher compression ratios, increased tendency to flood, short burn time, and weak low speed mixture. These factors hinder ignition and initial combustion when an engine is cold. Improvements in fuel injection, electronic ignition systems, pre-heating, and multiple spark plugs per cylinder have helped mitigate cold start issues on more advanced 2 stroke engines. But ease of cold starting remains an inherent disadvantage the 2 cycle design faces versus 4 cycle engines in most applications.
