SPECIFIC FEATURES RELATED TO ENGINE MECHANICAL COMPONENTS
FEATURES DERIVING FROM THE FUNDAMENTAL CHOICE OF A 90° V6 ALUMINUM ENGINE
Dimensions of connecting rod bearings, length off-setting between V banks. - The interdependence between these two design parameters is well known to those skilled in the art; it derives from:
- the minimum diameter of the cylinder liner bore, which affects the dimensions of connecting rods, hence the diameter
- the specific pressure allowed on the aluminum tin alloy bearing material, which is based on the above-cited diameter and imposes a bearing width, hence the width of connecting rod big end.
From the two criteria above, a 17.5 mm. offset in length between the two cylinder-banks was derived.
It was essential to reach the minimum value since it affects both the overall length and the efficiency of transverse structure ribs.
Size of bearings. Definition of the crankshaft. - The dimensions of the crank-pins being defined, it remains to settle:
- the dimensions of journals;
- the thickness of arms (depending on the crankshaft material: modular graphite apheroidal cast-iron).
If we take into account the choice of aluminum tin alloy for the shells, and knowing the bearing forces, all the characteristics are fixed when the diameter of the journals are determined according to the following:
the front size of the main bearing-caps has to be chosen in consideration of two imperatives:
- the journal diameter must be large enough to give the desired stiffness of the crankshaft
- the overall size of the corresponding cap must be small enough to enable the accessories to be fitted close to the
The dimensions finally selected for the crankshaft are shown on Figure 3.
A 108 mm. centerline distance between cylinders of the same bank corresponds thereto.
Aluminum cylinder block (AS9U3) - All the foregoing concerning the dimensioning of some essential components, which finally conditioned the choice of crankshaft geometry, remains valid whichever material selected for the cylinder
Based on our experience in die-cast cylinder blocks for 4-cylinder engines, we have deliberately built the cylinder block around the crankshaft, assuming that the reinforcement of weak points experienced in the first tests would make it possible to reach exactly the same final overall dimensions as with a cast-iron block.
The analysis of stresses on the weakest points allowed distributing them by adding or sometimes removing material; this, in the end, led to a rigid, durable cylinder block.
Separate wet cant-iron liners have been chosen for the following reasons:
- to enable use of "low silicon" aluminum alloy in the cylinder block (simplification of the die casting and reduced machining cool wear);
- to be able to use conventional pistons.
Here are some interesting specifications concerning the cylinder block:
- Weight without caps and liners: 14.3 kg.
- Weight with cast-iron caps and liners: 27 kg.
- Overall length: 392.5 mm.
- Distance between crankshaft axis and cylinder head gasket joint face 221 mm.
- Distance between crankshaft and lower gasket joint face: 60 mm.
After leaving the die-casting molds, cast parts receive successively an impregnation and a blasting for sealing respective appearance purposes.
Attachment of bearing caps - The problem of attachment of the bearing caps is critical on a V-type engine. It is still more difficult with a die-cast light alloy cylinder block.
First of all, the main point is to determine the value and direction of forces applied to each bearing.
This is accurately calculated along the 720° cycle, by use of the gas pressure diagram with addition of the force from the reciprocating masses.
In addition, the position of counterweights that balance the 1st order rotating torque (see Balance) must be known:
By using six counterweights, the peak values of stress at the most heavily loaded bearings can be minimized.
Figure 4 shows a diagram of the instantaneous forces on bearing No. 3 at full load with an engine speed of 6100 RPM which represents the most severe bearing load. (Bearing No. 1 being the outermost bearing on the clutch side.)
It can be noted that the most severe condition occurs when we have a separating force of 2180 daN and a sliding force of 750 daN.
The size of these forces as well as the concern for reducing clearance between crankshaft and bearing (a reduction which was experimentally recognized as essential for controlling full-load noise) finally led to the arrangement shown in the cross-sectional view of Figures 5 and 6.
Main points to be noted are as follows:
The caps are made of cast-iron and separated; this requires that the bearing bore be finished with aluminum alloy (block side) and cast-iron (cap side), a process which has been used by the industry since 1965. This type of
construction permits both better resistance to stresses for a given geometry and a possibility of reducing clearances. Note that this arrangement allows avoiding the matching between crankshaft, bearings and block equipped with its caps
while still retaining the conventional machining accuracy in wide use for these components.
Each cap is positioned:
- laterally, by recessing its lateral faces between shoulders located on either side of conventional bearing zones;
- longitudinally, by one of the attaching studs thanks to adequate tolerances;
- vertically, each cap is enclosed and pressed by the main bearing bolts between cylinder block and lower housing (see Figure 6).
The advantage of this "enclosing" of the caps is to double the frictional force likely to resist the lateral sliding of the cap. It results in a considerable reduction of stresses in the areas where the bearing caps are tightened down
and the strength of these areas in the cylinder block is quite satisfactory.
The lower housing consists of a pressure die cast aluminum piece with the surface against the cylinder block and the caps machined in one milling operation; it is bolted to the crankcase without the use of a gasket.
Balance - The overall balance imperfection already mentioned being a torque of the 2nd order frequency, it cannot be corrected unless by resorting to complicated, costly devices, which might be prejudicial to the overall mechanical
However, it should be noted that the amplitude of this interfering torque is all the smaller as rod length is great (which is true for any 90° V-type engine; so as to avoid interference between moving components).
The 146.15 mm. center-to-center distance of a connecting rod corresponds to a connecting rod/crank ratio of 4.01 with a 73 mm. stroke. On the other hand, as concerns the balance of the first order, it amounts to balancing a rotating torque by means of counterweights. As already mentioned, those counterweights have been determined in position, mass and geometry
in order to balance the rotating torque and reduce stresses on bearings to a minimum.
As for a V8 engine, the overall balancing of the crankshaft must be done by attaching, manually or by using an appropriated cradled crankshaft holder, additional masses centered on each crank-pin and having a value of twice
the rotating mass of a connecting rod (2 x 0.557 kg.) plus one time the mass in reciprocating motion for one cylinder (0.796 kg.) -- (complete piston + connecting rod small end).
This technique is very well known and the stress will be laid only on a few particular points.
There is only one crankshaft type, one connecting rod type and one piston weight so as to enable full interchangeability without matching requirements.
With reference to Figure 7, this is achieved by bringing connecting rods to the same weight on big (T) and small (P) ends through automatic milling of weights arranged at both these ends.
Likewise on pistons, the calibration of the weight is carried out by removing material from weights (M) located under the piston pin.
Camshafts drive and location of oil pump - In order to minimize total length. the arrangement selected includes a separate chain to drive each of the camshafts; it is the only solution which takes advantage of the relative offset
between the two banks of cylinders and allows a reduction in length of 10 mm.
Each of the camshaft drive chains is kept under tension by a tensioner: on the slack side of the chain, there is a long steel blade padded with Nylatron (66-Nylon added with 30I Molybdenum disulfide), hinged at a point in the vicinity of the crankshaft and pressed at the other end by an oil-pressure activated pushrod including a non-return device.
The oil pump is of the straight gear type and its housing is integrated in the left-hand front face of the cylinder block (The left and right sides mentioned in the descriptions are valid for the engine viewed from the clutch side). The pump is driven through a chain without tensioner at l8/28ths of the crankshaft speed, which provides a fair compromise between size, pressure build- up at 1ow speed and safety against cavitation at high speed.
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