Peugeot - Renault - Volvo 90° V6 Engine

(6x88x73 - 2.664 cm3)


Francois Gastinne
Societe Anonyme des Automobiles Peugeot

M.Laliere
Regie Nationale des Usines Renault

Stpehen Wallman
AB Volvo

Automotive Engineering Congress and Exposition
Detroit, Michigan
February 23-27, 1976


GENERAL CONSIDERATIONS BEFORE THE ACTUAL RESEARCH

(Factors considered in the choice of the 90° V6 engine)

In 1968, the design of an engine with a comparatively large cubic displacement to be fitted in prestige European cars was commenced. Already this consideration favors the 6-cylinder solution and a rapid analysis of the problem from the standpoint of perceptible qualities confirmed this orientation. The most convenient 6-cylinder engine was to be selected.

DETERMINING ADVANTASES OF THE 90° V6 ENGINE

Broad RPM range - The natural torsional frequency of the crankshaft of the 900 76 engine is very high in comparison to that of other 6-cylinder engines (thanks to its three crank-pin crankshaft instead of 6 for others); thus, it permits high RPM (see Figure 1 where torsional amplitudes through speed range are figured).

Figure 1: Crank-Shaft Torsional Amplitudes

As it will be seen however, the engine also has a good ability to operate properly at low speeds and full load. This broad RPH range enables quick passing which, even within the present context of speed limitations and fuel consumption reduction, is of importance for safety.

Length and weight savings - For the same displacement, the saving in length is about 250 mm. in relation to the in-line engine.

The length reduction is at the cost of a more complicated construction, which makes the engine itself heavier if cylinder blocks are made of the same material. However, the compactness of the le6 approach has two advantages, which permit weight reduction far more substantial than the weight increase resulting from the first remark.

These advantages are the following:

  • possible car length reduction by approximately the 250 mm., and
  • possible use of an aluminum cylinder block

In fact, the switch from the 13,000 daN/mm2 cast-iron modulus of elasticity to the 7500 daN/mm2 modulus of aluminum is consistent only with compact structures to avoid structural vibrations.

The following shows how the compactness of this engine can be used to advantage:

It is compact: It can be therefore be built with an aluminum alley block; It is therefore lighter.
The body is lighter.
It is shorter, so the vehicle can be shorter;

REVIEW OF FACTORS WHICH MAY BE CONSIDERED AS UNFAVORABLE

Overall balance imperfections - At first, the 90° V6 appears less satisfactory than the 6-cylinder in-line engine in terms of overall balance. As for us however, the comparison was made for A-cylinder engines (the only engine type in general used by a11 three manufacturers concerned).

A brief report on the overall resultants of all inertia forces, which are practically impossible to balance in both above-mentioned engine types, will follow.

For the sake of simplification, calculations were made through the evaluation of "free movement" for both engine types assuming that the weight of their mobile parts were the same, thus assuming the same displacement per cylinder.

Free movement of the engine subjected only to the influence of its own inertia force or torques, its weight supported by infinitely soft mounts.

The assumption above is obviously unfavorable to the V6 type but it is all the more significant.

A four cylinder engine is subject to a 2nd order sinusoidal force applied in the vicinity of the center bearing and parallel to the axis of cylinders, with an amplitude of:

Formula


*** M - reciprocating mass per cylinder
r - half stroke
ω - angular rotational speed (radian/second)
l - length of connecting rod

For the 90° V6, the resultant of inertia forces is a 2nd order sinusoidal torque, whose axis is perpendicular to the crankshaft and situated within the bisecting plane through both banks of the V. The amplitude ts as follows:

Formula

* M - reciprocating mass per cylinder
r - half stroke
ω - angular rotational speed (radian/second)
l - length of connecting rod
a - centerline distance of cylinders

In fact, this gain in weight certainly makes it possible to reduce the dimensioning of all components calculated as a function of the mass to be transported (suspension and braking components, structure, bumpers).

On account of the overall masses for each engine (which may be calculated a priori), the "free movement" of the 90° V6 measured at the outer main bearings, is six times smaller. This clearly shows that one obtains greater freedom in choice of engine mounts.

Experience in vehicle confirmed this to a great extent and the noise transmitted to the body because of the imperfection of overall balance is practically imperceptible.

Cyclical regularity - The 3 crank-pin crankshaft requires ignition at irregular angular intervals (900 - 1500 three times during a cycle).

Based on a gas pressure-angle diagram and taking into account instantaneous torques resulting from inertia, the instantaneous torque for 3 engine types having the same unit displacement (see Figure 2) was calculated:

  • 4-cylinder engine
  • 6-cylinder in-line engine (or 60° V)
  • 6-cylinder 90° V engine
Figure 2: Instantaneous Torque

Though these calculations do not take into account internal frictional forces, these results are accurate enough for comparison purposes.

To express more clearly the differences between the three engine types above, the following procedure was adopted:

One has calculated the areas enclosed between the straight line of the average torque and the instantaneous torque curve and related the sum of their absolute value to the energy supplied during one cycle. One obtains lower values as the engine runs more smoothly:

 2,000 RPM5,500 RPM
4 cylinder1,012,2
6 cylinder in-line (or 60° V)0,630,67
6 cylinder in 90° V0,730,71

Looking at the balance-sheet between the advantages and the drawbacks of the 90° V6 engine, we find that the former outnumber the latter, mainly due to the wide range of engine speeds and the substantial gains in compactness and total weight.

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