SPECIAL FEATURES OF FUEL SUPPLY SYSTEMS
Currently, the P.R.V. V6 engine is available either with a carburetor fuel supply system or a fuel injection system.
Each of the main versions of both these types of fuel supply systems will now be described.
CARBURETOR FUEL SUPPLY SYSTEM
In the process of development, it soon became obvious that, as only manifold water preheating was used, it was necessary:
- to use a double barrel carburetor with simultaneous opening, whose throttle plate axis are parallel with the crankshaft;
- to position the carburetor right in the middle of the V at a well determined point in relation to engine length.
The fuel mixture distribution results obtained with this single double barrel carburetor were already relatively satisfactory and we noted that two options were available.
The first option consisted in separating almost completely the manifold ducts between the two cylinder banks so as to have each of them supplied by one barrel. A connection hole 20 mm. in diameter would allow correct balancing at idle
and very low loads, whereby a rather high specific torque (related to engine displacement.) would be achieved (abouc 8 m.daN/ - liter).
The second option consisted in connecting the six ducts of the manifold under the carburetor.
In this way, the specific torque was slightly lower (7.75 m.daN/liter) but the specific power was increased.
Introduction of an additional small nozzle carburetor upstream of the distribution center further improved the air/fuel preparation at low loads.
This arrangement enables good distribution at all load conditions and a slight increase in maximum power output.
Figure 12 shows the manifold installed in production. It illustrates the principle of this fuel supply system.
The corollary of this principle was to develop a "compound" fuel supply system. The double barrel carburetor is automatically opened by use of a diaphragm.
The development has been performed in close cooperation with the Solex Carburetor Company.
The following documents represent results from this development:
- chart showing maximum differences of air/ fuel ratio (CO level %, which is a valid fuel expression because of the high enough medium CO level) at 1/4, 1/2, 3/4 and full load. (Figure 13)
- Chart showing actual CO levels obtained with the production carburetor described above, along the complete operation range of torque/RPM (so-called ISO-CO curves). (Figure 14)
- Chart showing ISO consumption curves in g/hp hour on the range of torque/RPM. (Figure 15)
FUEL INJECTION VERSION
Choice of system - The adoption of the continuous fuel injection system (Bosch K-Jetronic) was decided upon on the basis of the following major advantages:
- Direct load sensing by the air metering device which continuously measures air flow through the engine.
- Simple injector design since the fuel is continuously injected.
- Fewer and simpler service requirements. Most components within the system can be checked by a fuel pressure gauge.
- Partial altitude compensation by the air metering device.
- Less influence on actual air/fuel ratio by the EGR rate since actual air flow is measured.
Fuel injection system description - The fuel injection system was invented by the Bosch Company and is now in production; its description was already widely diffused; however, we describe it in Appendix II.
Air intake system of the injection version engine - One of the major design constraints in adapting a fuel injection system to a new engine is the requirement for good cylinder-to-cylinder air distribution. The air distribution is more crucial in any feed system where the air and fuel supplies are separated, as a variation in cylinder-to-cylinder air distribution solely results in a variation in cylinder-to-cylinder richness. When the fuel and air are well mixed in advance, a variation in flow results mainly in a variation in respective charge to each cylinder.
Two different intake manifold systems have been designed.
For markets where emission requirements are moderate, top priority has been given to simplicity and serviceability. The major components of this air intake system is shown in Figure 16. The intake manifold resembles a six-legged crab which straddles the air/fuel metering unit, enabling the latter to be placed low in the V between the cylinder banks. Intake air passes upward through the metering unit and through the single throttle into the plenum chamber, which forms the body of the "crab". The air then travels downward through the six "legs" to each cylinder.
In order to permit a lean overall mixture without having one or more cylinders running so lean that the lean firing limit is reached, a dual manifold has been designed for the US fuel injection version with the objective of optimizing air distribution (Figure 17).
The total amount of air is measured, as in the other injection version, in the air metering device, and the air flow is then divided into two and passes a dual throat throttle, each throttle feeding one of the manifolds. Each manifold supplies one of the cylinder banks and is located at the opposite side of the V, thus facilitating comparatively long intake ducts.
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