Mechanical Design > Part Design
I'm showing how to model an automotive engine in CATIA and to measure some of it's properties. For this entry, I'm writing about all the basic parameters required to model a working automotive engine. Later I'll write about modeling other automotive engine parts and show how to assemble them and make a kinematic simulation.
To make a valid and functional assembly, all parts need to have correct dimensions that will fit to each others. Therefore every dimension has to be measured, calculated and recorded. I don't have any actual model with me therefore I'm going to make all other parts based on the engine displacement. All dimensions for the parts are just made up based on calculations from the engine capacity.
Measuring the Bore and Stroke:
I'm going to make a short-stroke, high-rev V8 engine with oversquare piston just like a racing engine. The engine capacity is 2.4 liter like the F1 engine. Some calculations is required:
Assuming an F1 engine with 19000 rpm maximum revolution would have Stroke length of 39.77 mm.
Engine capacity = 2.4 liter = 2400 cm^3
V = N*(π/4)*(B^2)*S
V = Total engine capacity
N = Number of cylinders
B = Bore size
S = Stroke length
B = [(4/π)*(V/N*S)]^(1/2) = [(4/π)*(2400cm^3/8*3.977cm)]^(1/2) = 98.0 cm
Therefore the Bore size is 98.0 mm
Measuring the Crank Offset and Connecting Rod Length:
These parameters are required at early stage because the length of the pistons depend on the crank offset (crank circulating radius) and the connecting rod length. The engine should be compact to reduce space and to have low center of gravity. Piston size will influence the overall engine dimension.
Distance between crank axis and piston pin axis is:
s = a cos θ + (r^2 - a^2 sin^2 θ)^(1/2)
a = crankshaft offset
r = connecting rod length
θ = crank angle measured from cylinder centerline.
When piston at TDC, θ = 0°
s = a + r
Stroke = 2 * crank offset
S = 2a
a = S/2 = 39.77 mm/2 = 19.89 mm
Assuming the engine to have connecting rod length to crank offset ratio, R = 5.13
(R ranges from 2 for small engines to 10 for large engines)
R = r/a
R = Ratio of connecting rod length to crank offset
r = connecting rod length
a = crank offset
r = aR = 19.89 mm * 5.13 = 102.04 mm
s = a + r = 19.89 mm + 102.04 mm = 121.93 mm
*Note the use of capital S and small s
V-Block Configuration Angle and Bore Spacing
A complete 4-stroke cycle requires 720° revolution. For a V8 engine, the best angle is:
720°/8 = 90°
For this angle, spark firing occurs at every 90° rotation of the crankshaft. This angle provide the best firing sequence that help to reduce vibration. Other angles are possible but mostly requires adjustment to the firing order and timing in order to balance the engine. There is also a need for balancing shaft to reduce vibration.
Common V8 engines use crossplane crankshaft where the first and last of the four crank pins are at 180° with respect to each other as are the second and third, with each pair at 90° to the other, so that viewed from the end the crankshaft forms a cross. The cross-plane can achieve very good balance but requires heavy counterweights on the crankshaft. This makes the cross-plane V8 a slow-revving engine that cannot speed up or slow down very quickly compared to other designs, because of the greater rotating mass. However, racing engines usually use flatplane crankshaft where the crank pins are at 180°. Flatplane crankshaft does not require counterweights therefore having less mass and inertia and allow higher speed and acceleration. Flatplane crankshaft are imperfectly balanced and thus produce vibrations unless balance shafts are used, with a counter rotating pair flanking the crankshaft to counter second order vibration transverse to the crankshaft centerline. I'm still looking if this engine we're going to build will require a balancing shaft or not.
*This post will be updated when further calculations are required.
Automotive Engine In CATIA 1 - Calculations
Wednesday, March 11, 2009