How is a crankshaft design
High Power Engine Crankshaft Design and Analysis
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in 1860 the very first internal
combustion engine was produced at Baron
coal gas and had a four percent
efficiency displacing 18 litres to
produce two horsepower by 1876 Nikolaus
Otto and Eugene Langley began work on a
much more efficient combustion engine
1885 saw the first car but it wasn't
exactly fast the operating principles
however remain exactly the same as a
modern engine a piston constrained in a
bore moves up and down it is connected
to a rod with the rod connected to a
crankshaft generally placed overhead is
a pair of camshafts the camshafts
control the opening and closing of the
intake and exhaust valves on the engine
the system is timed using a synchronous
drive and a spark plug is placed in the
combustion chamber to ignite the fuel
air mixture this mixture is drawn into
the intake valve compressed by the
piston and ignited by the spark ball the
expanding gas mixture forces the piston
down the last stroke the exhaust stroke
is used to force the remaining gases out
of the combustion chamber it might
surprise you that a treasonable
production engine speeds the piston can
go up and down all hundred senses
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that's right the class you barely passed
was actually pretty important but we're
not going to do a detailed thermodynamic
analysis I just want to remind you that
as the compression ratio increases the
efficiency of the combustion process
increases too and the more air and fuel
you can pack into the combustion chamber
the more power you can get out of the
engine on the same note more cylinders
you stack next to each other the more
force you can push down through that
crankshaft you might hear some
old-timers say there's no replacement
for displacement well this is what
they're talking about because v8 usually
make more power than 4-cylinder here's
an example
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however with today's environmental
protection requirements engineers are
trying to squeeze as much power as they
can of small four-cylinder engines
because consumers want a fast car that's
going gas all the while environmental
regulation requires minimal hydrocarbon
emissions the key to making this work is
the turbocharger the turbocharger pre
compresses the intake air causing a huge
bump in cylinder pressure and an
increase in performance while retaining
a relatively small engine a small engine
pushing a lot of power means a lot of
stress
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all and all this stress gets
concentrated directly at the crankshaft
failure of the crankshaft would mean
catastrophic engine damage and put the
driver and other users of the road at
risk
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let's take a look at what an engineer
needs to do to make sure this doesn't
happen this study focuses on the
crankshaft of the engine the component
used in this example was designed using
SolidWorks 2016
like most engine components the
crankshaft is already very well
developed so what better starting place
than an already proven design this
crankshaft was laying in my garage so I
thought we might as well use it it
belongs to the 1.8 liter 20 valve turbo
Volkswagen engine the same one I was
taking apart just few seconds ago with
the initial design complete it's time to
find out how much force is going to be
pushing down on those crank arms to find
the maximum force we need to find the
maximum cylinder pressure using a
pressure transducer equipped to a
Renault engine the maximum in cylinder
pressure was found to be five thousand
three hundred and twenty kilo Pascal's
or about fifty three bar this was on a
fairly moderate two-liter engine
producing 145 horsepower estimates for
our 1.8 liter producing 180 horsepower
brings from 75 to 100 bar so great we
now know the maximum normal operating
pressures but there is one more thing we
need to talk about and that is pre
ignition it's the mother of all engine
killers when the fuel air mixture
detonates the four top assist
a massive pressure wave is released as
the engine experiences pressures
exceeding 200 bar pistons melt rods Bend
and crank shafts break so when designing
the crankshaft we applied a large factor
safety to do with things like
pre-ignition
this brings us to the static failure
portion of the project the maximum force
was found by multiplying the pressure
times the area of the piston we account
for cylinder pressures of up to 250 bar
this resulted in a force of 128
kilonewtons at 25 degrees to the
vertical this force is applied directly
and uniformly to the rod journal on the
crankshaft
lastly the crankshaft is supported by
bearing fixtures and fixed where the
flywheel mount this should give us a
good indication of the maximum stress
when the engine is operating at steady
state like going up a hill at full
throttle with a bit of pre-ignition this
basic design proves very strong with the
maximum stress concentrated at the main
bearings this maximum stress came out to
be 280 1.7 mega Pascal's this yields
effective design factors ranging from
one point six seven to two point five
two depending on material and surface
treatment choice I'm sorry what anyways
with such high factors of safety
downsizing comes with ease provided that
the material is strong enough the
crankshaft use in this example could be
shaved down substantially when talking
engines any rotating mass that you can
remove is a really big deal it means
less material used and less mass to
accelerate but comes at the cost of
decreased rigidity and increased
deflections
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it should be noted that too much
deflection can cause premature bearing
away
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the fatigue failure test will give us an
idea of how long our part will last
paycheck failure occurs when a micro
craft propagates through the metal even
though the yield strength hasn't often
read it ends with a small area
supporting the full structural load that
small area eventually experiences
brittle fracture in the part usually
fails rapidly
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fatigue failure can be avoided by
keeping the stress amplitude below the
endurance limit of the steel some
materials don't have an endurance limit
but we won't be focusing about earlier
we found that our maximum peak cylinder
pressure under normal operating
conditions was about 100 bar the goal is
to get the crankshaft to last the right
amount of time and not break prematurely
the following calculations require that
the crankshaft last 3.9 four times ten
to the ninth cycles this is a view of
the car for one hour every single day
for 30 years the peak cylinder pressure
was found previously to be 100 bar that
equates to 51.3 kilonewtons now we can
run this to our finite element analysis
and use the integrated SolidWorks
fatigue study to find the total lifetime
of the part the SN curves are included
with the SolidWorks Simulation package
and then as forged surface finish was
chosen this will put us on the
conservative side of an already accurate
simulation but how thin can you make
those crankshaft walls
finally we're going to check out
materials and post-processing perhaps
one of the most important design factors
is material choice and subsequent
processing the most commonly used
materials are forged steel and cast iron
the cast parts are usually cheaper to
make but not as strong as their forged
counterparts casting requires a void to
be filled by a molten metal whereas the
forging process involves forcing a
billet into shape one of the strongest
materials available would be a billet
alloy steel like 4340 billet Klink
shafts are usually very expensive and
reserved for specialty parts
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once machining is complete there's a
number of strengthening mechanisms
available these serve to increase the
durability of the crankshaft and push
its 2t device even further shot peening
WPC and cryogenic hardening are a few
common treatments cryogenic hardening
serves to increase the martensite and
can have a profound effect on overall
wear characteristics and strength here
CW SP describes the shot peening
processed the application of controlled
shot peening improves performance
reduces maintenance and provides a cost
effective solution to premature failure
in critical components shot peening is a
method of inducing residual compressive
stress into the component by bombarding
the surface with high-quality spherical
media in a controlled operation the
media can be steel ceramic or glass and
each piece acts like a tiny painting
hammer producing a small indentation in
the surface
the application locally yields the
material inducing beneficial residual
compressive stress the characteristics
of which are dependent on the base
material and component design at the
same time unwanted tensile stresses are
removed the proven results from
controlled shot peening show a dramatic
beneficial effect on both the life and
strength of components welds and
materials making the material resistant
to fatigue fretting and stress corrosion
cracking taking all design factors into
consideration a crankshaft of infinite
life should be very possible to produce
the vr6 engine I turbocharged and
swapped into my Volkswagen has living
proof of a seriously strong crankshaft
it was originally intended to output 177
brake horsepower with the turbo and
stand-alone EFI system I built the
numbers are over 300 and while that
might not seem like a high specific
output honda just released the new type
r which is pushing similar power through
a four cylinder
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ah
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