Mechanical Construction of High-Power Engines
(From The Design and Tuning of Competition Engines by Philip H. Smith, 1963)
Basic
materials
in
construction
The
first
requirement
for
ensuring
reliability
under
conditions
of
continuous
high-speed
operation,
is
that
the
materials
chosen
shall
be
of
the
correct
type,
and
adequate
for
the
duty,
of
any
particular
task.
The
advances
made
in
foundry
and
metallurgical
technique
of
recent
years
have
ensured
that
under
normal
conditions,
failure
of
a
component
is
comparatively
rare.
It
is
still
possible,
of
course,
to
encounter
cases
of
fracture
of
such
items
as
connecting
rods
or
crankshafts
under
abnormal
stresses,
as
in
racing,
but
these
can
arise
from
many
causes
other
than
faulty
material
or
dimensional
errors.
There
are
two
main
classes
of
materials
used
in
engine
construction-ferrous
and
non-ferrous.
The
former
comprise
those
with
an
iron
base,
such
as
cast-iron,
mild
steel,
and
alloy
steel.
The
last-named
can
be
case-hardened
or
otherwise
specially
treated
as
required
for
particular
duties.
A
fairly
recent
newcomer
to
the
ferrous
range
is
high-duty
or
hightensile
iron,
in
which
various
compounds
are
mixed
to
achieve
a
high
degree
of
toughness
and
strength.
This
material
is
coming
to
the
fore
as
an
alternative
to
steel
for
crankshafts,
Meehanite
being
a
typical
and
familiar
name
in
this
connection.
Present-day
foundry
technique
allows
castings
to
be
manufactured,
the
intricacy
of
which
was
unknown
thirty
years
ago.
This
allows
several
"
units
"
to
be
embodied
in
the
same
casting,
which
is
all
to
the
good
from
the
point
of
view
of
rigidity
and
strength,
though
perhaps
less
desirable
from
the
angle
of
accessibility
when
overhauls
are
contemplated.
For
instance,
it
is
the
custom
nowadays
to
make
the
cylinder
block,
crankcase,
and
main
bearing
scantlings
in
one
piece,
with
the
cylinder
head
detachable
above
the
top
of
the
swept
bore.
This
undoubtedly
makes
for
a
more
accurately
aligned
and
rigid
assembly
than
the
use
of
a
separate
crankcase
with
detachable
cylinder
block.
The
latter,
on
the
other
hand,
when
combined
with
its
head,
had
many
virtues,
notably
in
the
absence
of
a
complicated
gas-and
water
joint
with
its
attendant
studs,
bosses
and
sealing
gasket.
Non-ferrous
materials
Aluminium
alloy
is,
of
course,
the
most
used
of
the
nonferrous
range.
It
combines
adequate
strength
with
lightness,
and
is
on
the
whole
easier
to
cast
and
machine
than
iron.
Its
high
thermal
conductivity
is
also
a
very
desirable
feature
in
certain
applications.
For
large
castings
carrying
little
stress,
ordinary
cast
aluminium
is
excellent.
Thus
it
is
widely
used
for
oilcontainers,
valve
and
timing
gear
covers,
and
so
on.
As
far
as
oil-containers
are
concerned,
the
virtues
of
aluminium
from
the
point
of
view
of
heat
conductivity
in
comparison
with,
say,
a
pressed
steel
sump,
are
probably
over-rated,
since
oil
temperature
should
be
kept
within
bounds
by
other
aspects
of
design.
However,
the
aluminium
sump
does
combine
strength
with
lightness,
and
this
is
a
requirement
in
sumps
of
large
capacity
which
are
desirable
for
high-efficiency
engines.
The
thermal
conductivity
of
aluminium
really
shows
to
advantage
in
the
use
of
this
material
for
cylinder
heads
typified
by
Plates
59
and
62.
It
will
be
apparent
that
the
temperature
range
of
the
cycle
has
an
important
bearing
on
thermal
efficiency,
and
that,
whilst
rapid
heat
dissipation
at
certain
high-temperature
phases
in
the
cycle
is
essential
to
prevent
overheating,
retention
of
heat
at
other
phases
is
desirable.
Thus,
a
material
which
will
rapidly
transfer
the
heat
between
the
mixture
and
the
cooling
water,
in
whichever
direction
is
required,
will
make
for
high
thermal
efficiency
as
well
as
reliability
under
sustained
high
loading.
A
material
of
lower
thermal
conductivity
characteristics,
on
the
other
hand,
will
tend
to
retain
the
heat
within
itself,
leading
to
local
superheated
areas
in
conditions
of
high-temperature
operation.
For
very
high
pressures,
heads
of
aluminium-bronze
alloy
are
sometimes
used,
as
the
mechanical
strength
in
this
case
is
equal
to
that
of
cast-iron,
and
valve-seat
inserts
(required
with
aluminium
heads)
may
be
dispensed
with.
Barronia,
a
copper-tin
base
alloy,
is
another
successful
material
which
can
be
used
without
valve-seat
inserts.
Light-alloy
crankcases,
at
one
time
common
when
separate
cylinder
blocks
were
the
rule,
are
found
occasionally,
in
which
case
the
casting
also
incorporates
the
water-jacket,
special
iron
cylinder
liners
being
used.
These
are
frequently
of
the
wet
type,
seating
on
suitable
sealing
rings
at
top
and
bottom
to
form
a
water
joint
with
the
aluminiurn
casting
shown
on
Fig-
4
:
I.
A
form
of
construction
used
with
dry
liners
is
to
cast
the
aluminium
around
the
iron
liners
in
the
mould;
the
liners
in
this
case
have
a
specially
finished
exterior
to
form
a
mechanical
interlock,
such
as
threading
or
roughened
"
sandpaper
"
pockmarking.
Both
forms
of
construction
make
for
commendably
light
power
units
and,
although
the
manufacturing
operations
are
to
some
extent
increased
in
complexity,
large
scale
production
has
a
habit
of
overcoming
such
drawbacks.
Aluminium
alloy
is
used
without
exception
for
pistons,
usually
in
die-cast
form,
though
forged
pistons
may
be
preferable
for
very
high-speed
work.
The
metal
is
chosen
as
much
for
its
heat-conducting
properties
as
for
its
lightness,
since
for
adequate
strength,
the
pistons
are
sufficiently
robust
in
section
to
have
considerable
weight.
No
other
metal
would,
however,
be
suitable,
as
ultra-rapid
heat
conduction
from
the
piston
crown
to
the
cylinder
walls
is
of
outstanding
importance.
Connecting
rods
have
at
times
been
made
of
duralumin
or
similar
alloys,
but
forged
steel
is
generally
favoured,
while
as
far
as
crankshafts
are
concerned,
steel
is
still
mostly
used,
though
high-duty
iron,
notably
Meehanite,
has
recently
come
to
the
fore,
with
cast
construction
instead
of
the
usual
forging.
Fatigue
Failure
Failure
of
highly
stressed
parts
was
at
one
time
frequently
caused
by
an
actual
fault
in
the
metal.
Nowadays,
such
failures
are
very
exceptional,
modern
methods
of
production
and
inspection
being
almost
foolproof.
Breakages
in
modern
engines
almost
invariably
arise
from
fatigue,
or
tiring
of
the
metal
under
abnormal
stresses,
resulting
in
a
crack
developing.
Once
such
a
fracture
has
started,
it
will,
of
course,
rapidly
spread
until
complete
breakage
occurs.
The
greater
the
factor
of
safety
in
the
component,
the
less
liability
there
is
to
fatigue,
but
where
reciprocating
parts
are
concerned
superfluous
weight
of
metal
is
undesirable.
Unfortunately,
these
parts
are
the
very
ones
in
which
fractures
are
most
serious
and
frequent.
Old
age
also
leads
to
changes
in
the
metal
structure
which
lessen
its
resistance
to
fatigue,
a
point
to
watch
where
"vintage"
engines
are
under
consideration.
Elimination
of
places
which
are
liable
to
encourage
the
start
of
cracks
is
an
important
part
of
correct
design.
Such
places
may
be
found
at
sudden
changes
of
section,
at
the
bottoms
of
screw-threads,
junctions
of
bolt-heads
with
their
shanks,
and
so
on.
Accidental
scratches
or
file-marks
can
lead
to
early
fracture,
and,
conversely,
a
high
polish
is
a
distinct
discouragement
to
breakage.
In
this
connection,
it
has
been
established
by
test
that
an
accidental
scratch
on
a
polished
surface
causes
a
reduction
in
fatigue
resistance
of
15
per
cent.,
while
the
finishing
of
a
normal-smooth
surface
with
fine
carborundum
to
a
high
polish
will
increase
it
by
2
per
cent.
Examination
of
a
fracture
can
often
provide
useful
information.
The
final
breakage
point
is
usually
discernible
by
the
rough
portion
at
the
break,
the
remaining
part
being
almost
polished
in
appearance,
with
curved
lines
back
to
their
starting
point.
This
semi-polished
surface
is
caused
by
the
working
together
of
the
surfaces
before
the
final
parting,
and
the
start
of
the
trouble
is
sometimes
traceable
to
the
commencement
of
the
curved
lines
aforementioned.
Investigation
by
an
expert
metallurgist
can
often
give
a
clue
to
the
direction
of
the
force
causing
the
breakage,
and
thus
help
in
determining
whether
an
abnormal
load
in
the
normal
direction
was
responsible
or
whether
some
unexpected
additional
stress
made
its
presence
felt.
In
very
many
respects
the
design
of
the
competition
car
power
unit
follows
closely
its
more
sober
counterpart.
In
fact,
many
of
the
components
used
on
the
latter
types
can
be
employed
with
equal
success
in
engines
of
greater
power
output,
providing
they
can
cope
with
the
extra
stresses
involved.
A
study
of
the
chapters
dealing
with
specifications
of
typical
engines
will
help
to
indicate
how
various
manufacturers
deal
with
particular
aspects
of
design.
Here,
we
will
consider
briefly
some
general
points
of
particular
interest.