2 STROKE VS. 4 STROKE Engines
A simple explanation by Jonas Borchert
Now we solve for the swept volume of the 2-stroke engine:
r = 66.4÷2r= 33.2mm
stroke=72.0mm
Vs = π x r2 x stroke
Vs = π x 33.22 x 72.0
Vs = 249320.8mm3Or ≈ 249.3 cc
Finally we can plug the new value into the previous formula to calculate the Clearance Volume:
Vc= Vs÷(Cr-1)
Vc= 249320.8÷((8.9 to 10.6) -1)
Vc minimum = 25970.9mm3
Vc maximum = 31559.6mm3
Now we do the same for the 4-stroke engine:Swept Volume:
r=77.0¸2r=38.5mm
Stroke=53.6
Vs = π x r2 x stroke
Vs = π x 38.52 x 53.6
Vs =249595.1mm3Or ≈ 249.6cc
Clearance Volume:
Vc= Vs¸(Cr-1)
Vc =249595.1¸(13.5-1)
Vc =19967.6mm3
To calculate the clearance volume of the cylinder we can re-arrange the Compression Ratio formula as follows:
Cr=(Vs+Vc)÷Vc
Vc x Cr= Vs + Vc
(Vc)Cr- Vc = Vs
(Vc)(Cr- Vc )= Vs
Vc= Vs÷(Cr-1)
The Calculated Difference
2-Stroke 4-Stroke
cc 249.3 249.6
Clearance Volume 31559.6 19967.6
Stroke Distance 72.0 53.6
Data Collection
The 2-stroke engine is fitted with a far greater Stroke Length, but has a smaller Bore Measure. The results therefore equal in the same amount of cc when put through the equation. Although both the engines have the same displacement (cc), the 2-stroke gains more torque. A longer stroke makes for greater leverage which equals more torque lessened acceleration, as the piston has to travel greater distance from BDC to TDC. On the other hand the 2-stroke engine has a greater Clearance Volume, which creates more room for a bigger explosion in the combustion chamber. This then enhances the 2-stroke engine’s acceleration. The torque of an engine depend on stroke length, which can be compared to a wrench: when the wrench is longer it is easer to loosen a bolt because you have more leverage. Once the screw is loose though, it is harder to go fast with a long wrench. With a shorter wrench it is harder to loosen the bolt, but once loose you can quickly unscrew it. From this we can conclude that the 4-stroke engine has less Torque and Horsepower but a better distribution of it at different revolutions per minute. The 2-stroke has more Torque and Horsepower, but at a lower RPM range.
Torque, Horse Power and RPM
We can compare the RPM at which the YZ250 and the YZ250F peak in their Torque and Horsepower: Horsepower equation:
HP= (RPM x Torque) ÷ 52525, where 5252 is a constant and RPM is the revolutions per minute done by the engine. Since we are given Horsepower(HP) and Torque(τ)in the specifications of the two engines we can now solve for RPM:
HP= (RPM x τ) ÷ 52525
(HP x 5252) = RPM x τ
RPM= (HP x 5252) ÷ τ
HP= 46.4
τ=30.6
RPM= (HP x 5252) ÷ τ
RPM=(46.4 x 5252) ÷ 30.6
RPM=7963.8
2-Stroke:
4-Stroke:
HP= 36
τ=20.1
RPM= (HP x 5252) ÷ τ
RPM= (36 x 5252) ÷ 20.1
RPM=9406.6
Data Collection:
2-Stroke 4-Stroke
Torque 30.6 20.1
RPM 7963.8 9406.6
HP 46.4 36
In the data collection above we see that the RPM at which the 2-stroke engine achieves its maximum performance level is about 1500 RPM lower than in the 4-stroke engine.
Solving for torque at different RPM
We rearrange the HP formula and solve for Torque to examine changes at different RPM’s.
HP= (RPM x τ) ÷ 52525
(HP x 5252) = RPM x τ
τ = (HP x 5252) ÷ RPM
Sample Calculation:
2-stroke
τ = (46.4 x 5252) ¸ 7000
τ = 34.8 ft-lb
4-stroke
τ = (36 x 5252) ¸ 7000
τ = 27.0 ft-lb
Revolutions per minute (RPM) 2-Stroke Torque 4-Stroke Torque
7000 34.8 27
6000 40.6 31.5
5000 48.7 37.8
4000 60.9 47.2
3000 81.2 63
2000 121.8 94.5
1000 243.7 189
Comparing torque vs. RPM
In the graph we see that the 4-stroke engine Torque values calculated do follow a similar pattern as in the 2-stroke engine. From 50 torques to 100 torques the lines diverge slightly. When at 1000 (low) RPM the 2-stroke has 60 more torques than the 4-stroke and therefore proves the mechanical advantage: the shorter stroke length results in greater leverage.