PISTONS: Mahle high strength, forged,
aluminum pistons with
anodized top ring land were selected due to the high
engine combustion pressures.
The outer piston skirt is
anti-friction coated.
Quote:
CONNECTING RODS: Finite element stress analysi
s was used to optimize the
design and material of the connecting rod assemblies
during the Ford GT engine design program. A forged
steel “H-beam” connecting rod was chosen early in the
project due to the success of a similar design for the
4.6L supercharged engine application. During the
connecting rod analysis study, the H-beam connecting
rod design was optimized for this engine application.
Special attention was paid to the small end of the rod to decrease mass as much as possible while ensuring the
strength and durability of the connecting rod.
Many different connecting rod design versions were
developed with the help of the designers and design
engineers. To analyze many design iterations in a short
period of time, second order tetrahedral solid elements
were used to represent the connecting rod assemblies.
Finite element analysis was of the inertia relief type, with
loads applied at the small end and big end of the
connecting rod assembly simultaneously. Loads were
calculated with a multi-body
dynamic model of one of the
piston and connecting rod assemblies; see Figure 8.
The input to the multi-body dynamic model was the
estimated Ford GT cylinder pressure. Forces at the small end and big end of the connecting
rod were calculated using a multi-body dynamic model
with unique mass and inertia properties for each design
iteration under investigation.
The multi-body dynamic
models were simulated at several steady-state engine
speeds in order to determine the engine speeds that
produced the highest forces on
the connecting rods. At
very high engine speeds the gas load was assumed to
be an insignificant contributor to the forces on the
connecting rods and was ignored. See Figure 9 for a
graph that shows example out
put from one of the multi-
body dynamic models at 7,000rpm, with no gas load. In the 7,000 RPM, no gas load steady-state engine
speed simulations, the point in time in the crankshaft
cycle selected for loads transfer to finite element
analysis was at piston TDC in the model. This is where
the connecting rod was in pur
e tension; that is, the
connecting rod was being “pulled apart” by the wrist pin
and crankshaft pin at this point in time in the crankshaft
cycle. Figure 10 shows the st
ress contours in one of the
connecting rod assemblies that was analyzed at 7,000
RPM no gas load, piston TDC. The finite element analysis helped to define the tapered
shape of the small end of the connecting rod, and it
helped in the selection of 4340 steel as the connecting
rod material. Other materials were considered for the
connecting rod assembly, including grade 5 titanium
alloy. In the case of t
he titanium alloy, stresses decreased in some components of the connecting rod
assembly, and increased in other components of the
assembly, mainly due to significant differences in
material properties compared to steel such as static
stiffness. Finite element
analysis was used to show that
a significantly more expensive material such as a
titanium alloy would not necessarily increase the
expected life of the connecting rod assembly as a whole
in the Ford GT engine application.
