Engine Building Formulas






Formulas
Predicting Power
 
BHP = PLAN/33,000
P is brake mean effective pressure, in PSI
L is piston stroke, in feet
A is the area of one piston, in square inches
N is the number of power strokes per minute
Piston Speed
 
Cm = .166 x L x N
Cm is mean piston speed, in feet per minute
L is stroke, in inches
N is crankshaft speed, in RPM
Brake Mean Effective Pressure (BMEP)
 
2-Stroke BMEP = (HP x 6500)/(L x RPM)
4-Stroke BMEP = (HP x 13000)/(L x RPM)
L = Displacement in Liters
 
i.e., 80 cc = .08 Liters
1 ci. = 16.39 cc
Piston Acceleration
 
Gmax = ((N^2 x L)/2189) x (1 + 1/(2A))
Gmax is maximum piston acceleration, in feet per second squared
N is crankshaft speed, in RPM
L is stroke, in inches
A is the ratio of connecting rod length, between centers, to stroke
Piston Stroke Motion
S = R cos X + L cos Z
S = the distance piston wrist pin is from center of crankshaft
R = the radius of the crankshaft wrist pin
L = the length of the connecting rod
X = the angle of the wrist pin
Z = the angle of the connecting rod
or
sin X = R/L sin Z
Piston Travel vs. Crank Rotation
 
d = ((S/2) + L) - (S/2 cos X) - L sin[cos-1 (S/2L sin X)]
S = Stroke (mm)
L = Connecting Rod Length (mm)
X = Crank Angle Before or After TDC (deg)
Note: (L) Rod Length is usually 2 times the (S) Stroke
OR
For Spreadsheets and some Calculators
HT = (r + c) - (r cos (a)) - SQRT(c^2 - (r sin (a))^2)
r = s/2
dtor = PI/180
a = d x dtor
HT = The height of piston
r = The stroke divided by 2
c = The rod length
a = The crank angle in radians
d = The crank angle in degrees
dtor = Degrees to Radians
Exhaust Systems Tuned Length
 
Lt = (Eo x Vs) / N
Lt is the tuned length, in inches
Eo is the exhaust-open period, in degrees
Vs is wave speed in feet per second (1700 ft/sec at sea level)
N is crankshaft speed, in RPM
Length of Curved Pipe
L = R x .01745 x Z
L is length
R is radius of the pipe bend
Z is the angle of the bend
Diffuser Proportions
D2 = SQRT( D1^2 x 6.25 )
D2 is the diffuser outlet diameter
D1 is the diffuser inlet diameter
6.25 is the outlet/inlet ratio constant
Baffle Cones
Lr = Le/2
Lr is mean point of the reflection inside the baffle cone
Le is the length of the baffle cone
Port Open Time
 
T = ( 60/N ) x ( Z/360 ) or T = Z/( N x 6)
T is time, in seconds
N is crankshaft speed, in RPM
Z is port open duration, in degrees
Compression Ratio
 
CR = ( V1 + V2 ) / V2
CR is compression ratio
V1 is cylinder volume at exhaust closing
V2 is combustion chamber volume
Carburetor Throttle Bore Diameter
 
D = K x SQRT( C x N )
D is throttle bore diameter, in millimeters
K is a constant ( approx. 0.65 to 0.9, derive from existing carburetor bore)
C is cylinder displacement, in liters
N is RPM at peak power
Crankcase Volume
 
Primary compression ratio =
Case Volume @ TDC / Case Volume at BDC
or
CRp = V1 + V2 / V1
CRp is the primary compression ratio
V1 is crankcase volume @ BDC
V2 is piston displacement
Resonance Effects
 
F = Vs / 2 * the square root of A / Vc (L + 1/2 the square root of   A
Vs is the sonic speed Uusually about 1100 ft/sec)
A is the cross-sectional area of the inlet
L is the inlet pipe length
Vc is the flask (crankcase) volume
Average Exhaust Temperature
 
Determine the exhaust gas temperature in Kelvin
(k = C + 273.15). This is usually a function of the engine's BMEP.
Torque
 
1.00 lb-ft = 0.138 kg-m = 1.35 N-m
1.00 kg-m = 7.23 lb-ft. = 9.81 N-m
1.00 N-m = 0.102 kg-m = 0.737 lb-ft
Mass
 
1.00 lb = 0.454 kg = 4.45 N
1.00 kg = 2.20 lbs = 9.81 N
1.00 N = 0.102 kg = 0.220 lb
Distance
 
1 in = 2.54 cm = 0.0000158 mi = 0.0000254 km
1 cm = 0.394 in = 0.00000621 mi = 0.00001 km
1 ft = 30.5 cm = 0.000189 mi = .000305 km
1 mi = 63,360 in = 160,934.4 cm = 1.609 km
1 km = 0.621 mi = 100,000 cm = 3281 ft
Pressure
 
1.00 bar = 14.5 psi = 1.02 kg/sq-cm = 100 kPa
1.00 psi = 0.069 bar = 0.070 kg/sq-cm = 6.89 kPa
1.00 kg/sq-cm = 0.980 bar = 14.2 psi = 98.1 kPa
1.00 kPa = 0.010 bar = 0.145 psi = 0.010 kg/sq-cm
Temperature
 
F = 9 / 5 x C + 32
C = 5/9 (F - 32)
K = C + 273.4
Area / Volume
 
1.00 sq-in = 6.452 sq-cm
1.00 sq-cm= 0.155 sq-in
1.00 cu-in = 16.387 cc
1.00 cc = 0.0610 cu-in
Power
 
1.00 HP = 746 W
torque (lb-ft) = 5252 x hp / rpm
hp = rpm x torque (lb-ft) / 5252
The Weight of Air
14.7 lbs. per sq. inch at sea level.
Air Density Calculation
StdAirDensity = 1.22556 and is defined at 59.0F degrees, 0.0% humidity, and 29.92 inches on the barometer.
 
Temp_c = (Temp-32.0) * 5.0 / 9.0;
Temp_k = Temp_c + 273.0;
Baro_mb = Barometer / (29.92 / 1013.0);
Baro_pa = Baro_mb * 100.0;
SaturationPressure_mb = 6.11 * pow(10,(7.5*Temp_c)/(237.7+Temp_c));
VaporPressure_mb = Humidity * SaturationPressure_mb / 100.0;
TempVirtual_k = Temp_k / (1.0 - (VaporPressure_mb/Baro_mb)*(1.-0.622));
// D = P/(T*R)
AirDensity = Baro_pa / (TempVirtual_k*GasConstant);
% Std Density = AirDensity/StdAirDensity*100
Additional Conversion Factors
1 Centimeter - 0.0328084 foot; 0.393701 inch
 
1 Circular Mil - 7.853982 x 10 to the negative seventh square inches; 5.067075 x 10 to the negative sixth square centimeters
 
1 Cubic Centimeter - 0.061024 cubic inch; 0.270512 dram (U.S. fluid); 16.230664 minims (U.S.); 0.999972 milliliter
 
1 Cubic Foot - 0.803564 bushel (U.S.); 7.480520 gallons (U.S. liquid); 0.028317 cubic meter; 28.31605 liters
 
1 Cubic Inch - 16.387064 cubic centimeters
 
1 Cubic Meter - 35.314667 cubic feet; 264.17205 gallons (U.S. liquid)
 
1 Foot - 0.3048 meter
 
1 Gallon (U.S. liquid) - 0.1336816 cubic foot; 0.832675 gallon (British); 231 cubic inches; 0.0037854 cubic meter; 3.785306 liters
 
1 Grain - 0.06479891 gram
 
1 Gram - 0.00220462 pound (avoirdupois); 0.035274 ounce (avoirdupois); 15.432358 grains
 
1 Hectare- 2.471054 acres; 1.07639 x 10 to the fifth square feet
 
1 Inch - 2.54 centimeters
 
1 Kilogram - 2.204623 pounds (avoirdupois)
 
1 Kilometer - 0.621371 mile (statute)
 
1 Liter - 0.264179 gallon (U.S. liquid);0.0353157 cubic foot; 1.056718 quarts (U.S. liquid)
 
1 Meter - 1.093613 yards; 3.280840 feet; 39.37008 inches
 
1 Mile (statute) - 1.609344 kilometers
 
1 Ounce (U.S. fluid) - 1.804688 cubic inches; 29.573730 cubic centimeters
 
1 Ounce (avoirdupois) - 28.349523 grams
 
1 Ounce (apothecary or troy) - 31.103486 grams
 
1 Pint (U.S. liquid) - 0.473163 liter; 473.17647 cubic centimeters
 
1 Pound (avoirdupois) - 0.453592 kilogram; 453.59237 grams
 
1 Pound (apothecary or troy) - 0.3732417 kilogram, 373.24172 grams
 
1 Quart (U.S. dry) - 1.10119 liters
 
1 Quart (liquid) - 0.946326 liter
 
1 Radian - 57.295779 degrees
 
1 Rod - 5.0292 meters
 
1 Square Centimeter - 0.155000 square inch
 
1 Square Foot - 0.09290304 square meter
 
1 Square Inch - 645.16 square millimeters
 
1 Square Meter - 10.763910 square feet
 
1 Square Yard - 0.836127 square meter
 
1 Ton (short) - 907.18474 kilograms
 
1 Yard - 0.9144 meter
POWER - AC CIRCUITS
Efficiency = 746 x Output HP / Input Watts
3ø KW = Volts x Amps x PF x 1.732 / 1000
3ø Amps = 746 x HP / 1.732 x Eff. x PF
3ø Eff. = 746 x HP / 1.732 x Volts x Amps x PF
3ø PF = Input Watts / Volts x Amps x 1.732
1ø KW = Volts x Amps x PF / 1000
1ø Amps = 746 x HP / Volts x Eff. x PF
1ø Eff. = 746 x HP / Volts x Amps x PF
1ø PF = Input Watts / Volts x Amps
HP (3ø) = Volts x Amps x 1.732 x Eff. x PF / 746
HP (1ø) = Volts x Amps x Eff. x PF / 746
1 KW = 1000 Watts
 
Eff. = Efficiency, PF = Power Factor, KW = Kilowatts, HP = Horsepower
POWER - DC CIRCUITS
 
Torque = HP x 5280 / RPM
HP = Torque X RPM / 5280
1 HP = 36 lb.in. @ 1750 RPM
1 HP = 3 lb. ft. @ 1750 RPM
 
Eff. = Efficiency, HP = Horsepower
OHMS LAW
 
Volts (E) = Amps (I) x Ohms (R)
Amps (I) = Volts (E) / Ohms (R)
Ohms (R) = Volts (E) / Amps (I)
 
R=Ohms, E=Volts, I=Amperes

To figure miles per hour, multiply the engine RPM by the Wheel Diameter in inches and divide this by the Gear Ratio times 336
or
MPH = RPM * wheel diameter (in inches) / gear ratio * 336

To figure engine speed (RPM), multiply by the Speed in MPH, by the rear axle gear ratio times 336. Divide this by the tire diameter in inches.

or

RPM = MPH * gear ratio * 336 / tire diameter







 

MacDizzy Home | Blog Page | MacDizzy M.U.L.E. Engine Builds | MacDizzy Update 2011! | MacDizzy's Two Stroke Technology Exchange | MacDizzy Tee Shirts | Compression Ratio Chart | YFS200/DT200 Yummie Yami | YFSYZ Hybrid | Two Stroke Engines | TRX Specs | TRX Dyno | TRX 270cc Engine | TRX Intake & Shifter | Two-Stroke Software Review | Blaster Rebuild | 2 Stroke Cylinder Mapping | Basic Porting | Banshee 370 cc Long Rod | Engine Building Formulas | LT250R Melt Down | RZ500 - Old Friend | TRX Internal Discoveries | More TRX Internal Discoveries | Little L Tease | GP760 Observations | KTM


MacDizzy's Two Stroke Technology Exchange is the Two Stroke Engine Capital of the Internet.


Date Last Modified: 11/29/2012
Mystery Achievements, Ink™© 1991-1999 - MacDizzy© 1993-1999

Copyright 2010, 2011