Designing a single cylinfer 4 stroke engine for a moped

[webster7]

Hey
I currently have a project to design an engine and I have chosen to design it for a moped. My research so far has brough me to this:

The highest engine rating of a moped is 130cc in Kansas, while most states require moped to be less than 50cc.1 For this project, we will consider a moped that needs an engine capacity of 125cc, which is 0.125 liter. The speed at which mopeds are allowed to run varies between 20mph(32.19 km/hr) and 35mph(56.3km/hr).2 Since the moped will need to go faster than this at times, we’ll take the maximum speed to be 70km/hr and the optimal speed to be 40km/hr. Basing the estimation on the Smart Scooter XM-5000Li from Wholesale Motorsports, the weight of a scooter is 363lbs (165kg) and the maximum rider weight is 400lbs (181.8kg).3 This means that two people of around 200lbs could ride comfortably. For this project, the weight of the scooter will be 150kg for easiness of calculations. Furthermore, the maximum allowable weight will be 200kg. With a safety factor of 1.5, which is appropriate for this application, the total weight that the engine will need to be able to power is 450kg. It will also need to be able to run at a maximum speed of 70km/hr.

Based on this, how would you guys recommend me proceeding to design the engine? Do I start with the links? From your perspective, what's the best link shape for this?
What type of space constraints am I looking at for this type of application?

Any input would be greatly appreciated.
Thanks

 

[jack action]

To estimate the power you need, look http://hpwizard.com/car-performance.html" .

Once you know the engine power you want, these are the basic equations for the engine (all in http://en.wikipedia.org/wiki/Unit_conversion" ):

Qm = P / bmep

Ab = spc X Qm / VE / MPS

S = Veng / Ab

D² = Ab / N / 0.7854

w = 3.14159 X MPS / S

T = P / w

Where:

Qm = volumetric flow rate (m³/s)
P = crankshaft power (W)
bmep = brake mean effective pressure differential (Pa) (2-stroke: 8.8-9.6 bar; 4-stroke: 10-11 bar; you should use the lower values for a typical moped engine)
Ab = total bore area (m²)
spc = # of stroke per cycle (2-stroke: 2; 4-stroke: 4)
VE = volumetric efficiency (no tuning: 2-stroke: 0.55; 4-stroke: 0.75-0.80) (int & exh tuned (2- & 4-stroke): 0.90-0.95)
MPS = mean piston speed (m/s) (typical moped engines are around 7 m/s, but this can be easily doubled if a faster revving (smaller) engine is needed)
S = engine stroke (m)
Veng = engine displacement (m³)
D = cylinder bore (m)
N = # of cylinders
w = engine angular velocity (rad/s)
T = engine torque (N.m)

The bore-to-stroke ratio (D/S) is usually between 1.0 and 1.3.

 

[webster7]

These are great links, tools, and equations you gave me here. Thanks a lot. I will start on these and let you know what I come up with.
Any more input is always welcome.
Thanks again for such an awesome post.