FACTORS INFLUENCING OIL CONTROL IN A SPARK IGNITED GASOLINE ENGINE Back

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

Positive Crankcase Ventilation (P.C.V.)

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.

Intake Manifold

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.

Cylinder Heads

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.

Rocker Arms / Push Rods

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.

Cylinder Block

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.

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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.

Pistons and Connecting Rod

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.

Piston Stability

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.

Piston Groove Squareness

The ring grooves of the piston must have no downward tilt.

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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.

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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.

Rod Alignment

The sketch below depicts a misaligned rod.

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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.

Piston Rings

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.

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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.

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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.

Crankshaft and Bearings

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)

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