Introduction:
Hastings Manufacturing Company has
published many previous reports on various factors influencing
oil control. This report's intent is to be comprehensive
of those reports, and identify all major factors in an engine
influencing the engines oil economy level.
Some of the points mentioned may
not be practical to implement with current production methods.
Others may be impossible, however, this reports intent is
to present all possible factors to the reader for consideration.
Another point to be considered is
at what point in good oil control do we encounter piston
, ring, and cylinder scuffing. This requires engine testing
as all engine designs react in different ways.
Conclusion:
Many factors in addition to piston
rings influence an engines oil economy characteristics.
Failure to control these other factors will result in an
engine that produces less than optimum oil control.
Discussion:
A reciprocating engines ability to
control the migration of lubricating oil to combustion chamber
where it is burned and thus lost depends on many factors
within the engine.
The assemblies and sub-assemblies
of the engine that affect oil control are listed below and
will be discussed separately as listed.
Positive
Crankcase Ventilation System (P.C.V.)
Engine Breathing System
Intake Manifold
Cylinder Heads
Rocker Arms - Push
Rods
Cylinder Blocks
Pistons and Connecting
Rods
Piston Rings
Crankshaft and Bearings
The P.C.V. system is designed to
reduce emissions by removing unburned gases from the crankcase
and reintroducing them in the intake system. The P.C.V.
valve itself is sometimes in a location of high degree oil
splash such as a valve cover. If the valve is not properly
baffled, oil along with crank case gases will be collected
and when returned through the intake system will be burned.
The engine's breathing system must also be properly baffled
as oil can be taken into the air cleaner and lost.
The major area of concern with the
intake manifold is gasket design and integrity. If even
minute leaks exist on the bottom side of a V-8 or V-6 intake
manifold, engine performance will suffer and oil will be
lost. There have been in recent years some warpage mainly
with aluminum intake manifolds.
Valve seals of good design should
be installed on both intake and exhaust valve stems of all
naturally aspirated engines. Intake valves have vacuum applied
when the valve is open and the oil can be drawn down the
guide from the top of the head. It is equally important
seals be used on exhaust guides as the rapidly exiting exhaust
gases past the bottom of the guide can create a vacuum at
the top of the guide moving oil down the exhaust guide and
into the combustion chamber. A turbo-charged application
does not normally exhibit this tendency as the exhaust is
normally pressurized.
Cylinder heads should have sufficient
drainage for oil that lubricates the rocker arms to return
to the crankcase rapidly. If drainage is not sufficient
it is possible for the base of the valve springs to be submerged
in oil that hampers the valve stem seals performance.
Any exhaust gases routed through
the cylinder head or intake manifold should be sufficiently
insulated or shielded to prevent choking of oil on the underside
of the intake or inside the valve cover in the top side
of the head.
In an overhead valve engine where
oil to lubricate the rocker arm is delivered through a hollow
push rod with holes in each end, the controlling orifice
is the meeting of the holes of the rocker arm and push rod.
During operation if wear occurs, which can increase the
hole size of the rocker arm or push rod, the oil allowed
out of the increased hole is increased by the third power.
If oil flow increases past the cylinder heads drainage capacity
(which in later periods of operation can be impaired by
sludge) the increased amount of oil can find its way down
the valve guides.
The cylinder bores are a very important
factor if an engine is to have good oil control. Cylinder
size, control, straightness, and finish are very critical
to pistons and rings performing properly. Cylinders must
not only be bored round and straight, this integrity must
be maintained through the honing process. Cylinders should
be round within .0005" or less and should have .0005"
taper or less. Cylinder diameter should be nominal + or
- .0005. When bore diameters are closely controlled, the
number of piston grade sizes needed can be reduced.
Bore finish roughness should be approximately
15 AA. It is recommended cylinders be plateau honed. This
is a two-stage honing process where after the cylinder is
honed to size, the stone pressure is reduced and a second
short pass of the hone removes the sharp edges of the pyramid.
The dark section in the sketch below is representative of
what is removed on the second pass.
The honing process can be more effective
if a stress plate is installed before the block is honed.
Stress plate honing is advantageous because a thick plate
is bolted to the block's deck and torqued. This stresses
the cylinder as if the cylinder head were in place. The
cylinder can now be honed round in the stressed condition.
Thermal distortion will occur when
the engine is run, however, total cylinder distortion will
be less if the cylinders have been stress plate honed.
Cleaning of the cylinders after the
honing process is important and cannot be over emphasized.
During the honing operation, two abrasives are produced,
cast iron dust and honing stone residue. If these materials
are not completely removed from the cylinder the abrasives
will damage the rings, bearings, and all other moving parts
in the engine.
The entrance chamber at the top to
the cylinder must be smooth, free of tool marks or burrs,
and blend smoothly with the cylinder wall. If any of the
above adverse conditions do exist, serious damage can be
inflicted on the rings when they are installed in the cylinder.
The pistons role in blow-by and oil
control is critical. It should be noted that not only must
new piston designs be engine tested, but modifications to
already existing and proven pistons must be thoroughly tested.
The features of pistons which affect oil control are numerous.
Listed below are the major points. These points will be
briefly touched on.
Piston Skirt Fit in Bore
Skirt Design
Oil Drainage
Piston Stability - Land and Groove Width
Groove Squareness
Rod Alignment
The piston fit in the bore is important
because on the downward stoke it is the pistons skirt which
first shears the oil film prior to the piston ring passing.
If the oil film is too thick the piston ring will "hydroplane"
over the oil and it will be scraped to the combustion chamber
on the upward stroke where it will be burned.
Skirt design determines how effective
the pistons oil shearing ability is. For example the old
style full skirt pistons which are bore size around their
entire circumference are more efficient oil scrappers than
the current open slipper type pistons of today. The open,
or modified slipper pistons are necessary to clear crankshaft
counter balances although full skirt pistons are still used
in some engines.
Oil drain back in the oil ring area
of the piston is important to good oil control. If the oil
scraped by the oil ring cannot be removed or drained rapidly
behind or under the oil ring, it will cause the oil ring
to ride over the oil film leaving too thick a film on the
cylinder wall. Since the oil ring is responsible for scraping
the largest part of the oil film from the cylinder wall,
drainage of this oil down the inside of the piston must
be efficient.
In today's engine, with friction
and weight reduction being a prime consideration, pistons
have become much shorter. Both ring grooves and piston lands
have been reduced in width substantially. This causes two
problems that must be evaluated. Piston stability is affected
by today's shorter piston when the piston is protruding
from the bottom of the bore at B.D.C. At the start of the
upstroke the piston tends to "cock" 'in the bore.
This interferes with the rings ability to stay in contact
with the bore. It increases piston groove and ring side
wear. In some instances piston skirt wear and skirt collapse
is accelerated. As stable a piston as is possible is desirable.
With the reduced piston height and
reduced groove and land width it is important that the second
land, under the top ring, be sufficiently strong to resist
deflection during the engines power stroke. Land deflection
caused top edge bearing of the piston ring and can in time
fatigue and break the land.
The ring grooves of the piston must
have no downward tilt.
If the groove is tilted down as shown,
top edge bearing of the piston ring can occur. The upward
stroke of the piston will cause oil to be scraped up to
be burned during the power stroke.
Piston grooves must be perpendicular
to the center line of the piston and not as shown below.
If the ring grooves are as shown,
the rings will spin rapidly as the piston moves up and down
in the bore. Accelerated ring side wear and breakage will
be the result.
The sketch below depicts a misaligned
rod.
When a rod is misaligned two things
happen which will affect oil economy. The piston rings are
not perpendicular to the center line of the cylinder bore
and will rotate rapidly. The rod bearings will be tilted
on the crank journal and excessive oil will be spun onto
the cylinder wall. Abnormal bearing wear will also occur.
There have been many changes made
with respect to piston rings in the last few years. These
changes have stemmed from two main factors, engine downsizing
and vehicle weight reduction. Bore sizes have been getting
smaller, pistons have been made shorter, and block sections
have been reduced. Shorter pistons have required narrow
rings. The thinner block cross sections and resulting thermal
distortion of cylinders has resulted in reduced radial wall
dimensions of both compression rings and oil rings. The
axial and radial reduction of piston rings has served to
make the ring pack much more conformable to the cylinder
bores.
Friction reduction in engines has
been possible in large part due to reductions in tangential
tension reductions of the oil ring. Smaller cross section
oil rings that are also more conformable aided the tangential
tension reduction.
An illustration of the most popular
ring pack used by today's engine manufacturers is shown
below.
The typical top compression ring
is barrel faced, molybdenum filled, and frequently high
strength iron. It is important that the porosity of the
moly is low as high porosity creates many oil carrying pockets
reducing oil economy of the engine. Molybdenum has a very
high melting point and offers excellent scuff resistance.
The barrel face ring is advantageous because It offers fine
line contact with the cylinder for rapid break-in. The specification
on the barrel contour should be as low as possible and still
assure no top edge contact. A lower contour barrel leaves
a thinner oil film on the cylinder wall than a high barrel
contour. A possible specification would be .0002" /
.0007" barrel drop.
The intermediate compression ring
is a reverse torsional taper face ring. The intermediate
compression ring has a dual purpose. It must seal compression
and combustion gases and must also assist the oil ring in
scraping oil down the cylinder wall. For this reason this
ring is a very good choice for oil control as its attitude
in the groove offers side sealing as shown in the sketch.
This prevents oil from getting around
behind the ring to be left on the cylinder wall. The rings
taper face also offers line contact with the cylinder wall
and rapid break-in. It is best for higher early oil economy
if the ring has high taper (2° / 2° 30'). This higher taper
facilitates rapid break-in for the intermediate compression
ring.
The Hastings Flex-Ventâ oil ring is capable of excellent oil control with today's
reduced tangential tension. It is regularly designed to
operate at minimum tension levels of 6.0 pounds (26.7N)
and is being tested at even lower levels.
The Flex-Ventâ performs well for several important reasons. Its open design
offers excellent drainage of oil from the cylinder walls,through
the oil ring groove, and back into the engines oil pan.
The pad which the rail side rests on is broad and flat which
offers excellent outboard support for the rails. The Flex-Ventâ design also makes It very conformable to the cylinder walls.
In general, the following criteria
should be considered for piston ring design. The radial
wall of compression rings should be SAE. regular. This is
the lowest diameter to wall ratio and offers the maximum
of conformability. Ring width should be 1.5mm this reduces
the mass of the ring and helps to prevent groove pound out
of the piston.
Oil ring axial and radial dimensions
should be as low as practical as again the conformability
must be of prime concern to run at reduced tension levels
for engine friction reduction.
Specifications and tolerances for
piston rings such as width, wall, tension, light tightness,
finishes, etc. are well established. It is of the utmost
importance when considering maximum oil control that the
specifications are held as piston rings are the primary
component in good oil control.
There are some areas that must be
considered with respect to the crankshaft and bearings if
an engine is to have maximum oil control.
Bearing clearances determine how
much oil is "spun off" by the rotating crankshaft
some of that is deposited on the bottom end of the cylinders.
If the oil film thickness from the "spun off' oil becomes
too thick the piston rings will hydroplane on the oil film.
This oil will migrate to the combustion chamber to be burned.
With vehicle motion, oil is constantly
moving in the oil pan. Even with baffles in the oil pan,
oil waves can be splashed onto the crankshaft to be "wind
whipped" by the crankshaft. A windage tray attached
to the bottom of the engine block, between the oil in the
pan and the crankshaft is generally effective in preventing
the above conditions.
Crankshaft end play also can affect
oil control. If end play is excessive the back and forth
movement also moves the pistons in the cylinders causing
rings to "spin". This affects their ability to
control oil and causes excessive side wear.
It should be noted camshaft bearing
fit and hydraulic tappet fit is important because the oil
that flows out drops on the crankshaft and is spun on the
cylinders.
It has been mentioned earlier the
items recommended in this report should be engine tested
if they are to be implemented. All engines respond differently
to changes in specifications. It is also difficult to predict
when an engine will reach the point of scuffing cylinder
bores, pistons, and pistons rings. It would be well to implement
changes during engine testing one at a time to evaluate
their worthiness.
(P79-87)
Back To Top
|
