Within the engineering community, there isn't a single design that is not a compromise. There is no perfect mechanical device. As an engineer, in the beginning stages of product development you identify the shortcomings you are willing to accept and those you cannot. Hopefully, the final product reflects those desires. Quite often, a theory may not be compromised in one area but, due to its method of operation, may create an undesirable effect in an unrelated region. The fuel and air mixing system of an engine has forced concessions to be made in desired mixture strength, volumetric efficiency, emissions from combustion, cost, underhood packaging and customer acceptance. This was always recognized, but when all things were taken into consideration, the carburetor had the least number of shortcomings and became the standard bearer for fuel delivery in Detroit and around the world.
The thought of injecting fuel into an engine instead of having it pulled though a carburetor booster venturi was not a new concept in 1957, when the General Motors mechanical fuel injection system was introduced on the Corvette and other GM vehicles. Many fuel-injected engines had already been produced, especially through aircraft innovations made as part of the war effort. Combat aircraft required the most precise and exacting carburetor performance and the military was the first to recognize this and work to eradicate the limitations on performance imposed by a carburetor.
Chevrolet became actively interested in the GM/Rochester fuel injection system for the Corvette in order to realize the full potential of the engine; it was a perfect sports car application. The engineers had been aware of the theoretical advantages of fuel injection, but were also intimidated by the high cost it brought along. The new GM system promised maximum advantage with a minimal cost. As the adaptation of the GM fuel injection was applied to the Chevrolet engine, it became obvious that a market for this fuel delivery method existed beyond the sports car line; it was eventually incorporated into the division's passenger vehicles.
The limitation of a carburetor
Even the most ardent supporter of carburetors would readily admit the device's difficulty metering fuel at varied engine operating conditions. If much care is exercised, it is possible to create a good calibration curve that delivers a proper air/fuel ratio for every engine load. As a wet flow fuel design, a carburetor has air and fuel travel through the intake manifold runners. Transporting the mixture to the cylinder head intake ports with a minimum of fuel fallout requires maintaining a high rate of velocity in the manifold. This is accomplished by using smaller-than-desired passages. In addition, heat needs to be applied during cold start and warm-up to aid in the vaporization of the fuel. At 60 degrees F, only 50 percent of the gasoline turns to vapor. Thus, even the best intake manifold design will often create a cylinder-to-cylinder mixture variation. To combat this, the overall mixture is made richer to assure an adequate amount of fuel reaches the leanest bores.
Other obstacles of employing a carburetor are that the engine may require up to 10 percent more fuel at wide-open throttle than it should in order to produce maximum power. Cold start and warm-up require substantial amounts of fuel to overcome initial wetting of the manifold runners. The intake manifold is almost always a compromise for volumetric efficiency and good distribution, and may not permit full utilization of the engine's potential. Decreased manifold vacuum during throttle opening transients increases the manifold wetting issue.
GM tests concluded that a fuel system capable of correcting even 50 percent of the average carburetor deficiencies could produce a 10 percent improvement in fuel economy that would ultimately result in substantial savings to the consumer.
It was with these goals and intentions that a development program was initiated by the General Motors Engineering Staff several years prior to the introduction of the Rochester Fuel Injection for the 1957 model year.
General Motors metering system
Two different fuel metering methods were considered. The method to meter the charge is linked to the load placed on the engine and can be considered a measurement of load instead of a metering method. The two theories explored were speed density and mass flow metering.
The speed-density system used on aircraft was controlled by means of an engine speed-air density responsive metering system. This choice may have been dictated by the wide air density range encountered in flight.
The speed-density metering system required a device that would sense absolute temperature and pressure accurately and correlate these absolute values to the control of the mixture strength. These requirements make the method very costly to execute. In addition, this design measures engine volumetric efficiency versus engine speed, which, on an engine equipped with a ram effect intake manifold, makes calibration complicated. This is due to the supercharging effect that takes places as the Hemholtz resonance theory is applied. A speed-density system usually requires a supply of fuel in equal amounts per cycle. The fuel pump must, therefore, be a uniform flow pump capable of maintaining an exacting flow and pressure throughout its life cycle.
In contrast, with mass flow metering, the amount of air consumed by the engine is measured and, in turn, used to determine load and the necessary amount of fuel. In this system, a high efficiency venturi is employed to measure flow rate. The temperature and pressure effect have to be corrected only as a function of the square root. A temperature swing that would require a metering change of 30 percent with a speed-density system would require only a correction of 14 percent with a mass flow design. And in actual process, fuel density variations with temperature cancel 4 percent of the change, and proper utilization of fuel vapor bubbles at high temperatures tends to cancel a large part of the remaining 10 percent.
With a mass flow system, the volumetric efficiency of the engine is sensed by the venturi and no additional correcting signal is required. The fuel pump delivery does not have to be constant as long as it is in excess of the desired flow at the injectors.
With the preliminary testing done on a single-cylinder engine with an 8.8:1 compression ratio, GM engineers came to the following conclusions:
Direct injection into the combustion chamber had no appreciable advantage over injection into the intake ports.
Timed injection in the intake ports produced slightly less power and used more fuel than continuous injection.
Injection directed toward the intake valve gave the most power, the best economy, the fastest warm-up and the best acceleration response.
With these facts established, the metering system of choice for the GM fuel injection was to be mass flow.
The fuel was supplied by a conventional six p.s.i. diaphragm pump and passed through a 10-micron primary filter to the fuel meter. The fuel was admitted through a float valve, similar to its counterpart in a carburetor, into a float chamber where any vapors that had formed due to a temperature rise were vented. A small gear pump located in the bottom of the float chamber delivered the fuel through a second inlet filter and a fuel valve to the metering cavity. The fuel valve contained an anti-percolation ball check to keep the fuel between the valve and the pump at a sufficiently high pressure to eliminate vapor pockets.
Some of the fuel delivered to the metering cavity flowed directly to the engine's continuous spray nozzles, with the remainder going through a spill port back to the float chamber. The amount of fuel returned was regulated by the spill plunger.
The air entered the air flow meter through the annular venturi section and flowed past the throttle blade to the manifold header and then to the individual intake ports. The air velocity through the venturi caused a depression signal related to the mass flow. This venturi signal acted on the control diaphragm to create a force, which was transmitted to the spill plunger (fuel return) by means of the diaphragm link and control arm, the latter being pivoted on the ratio lever.
An increase in air flow through the venturi caused a relative increase in venturi signal, which, acting on the control diaphragm, resulted in an increase in the force acting on the top of the spill plunger. The spill plunger then moved to a new balanced position to create a fuel pressure increase proportional to the venturi signal increase. Since the increased fuel pressure resulted in a fuel flow proportional to the increase in air flow indicated by the venturi signal, a constant air/fuel ratio would be maintained as long as the linkage ratio was not changed.
The mixture strength could be varied by changing the linkage leverage between the control diaphragm and the spill plunger. This was accomplished by shifting the pivot point on the control arm. When the ratio lever rested against the Power (P) stop, the air/fuel ratio would be correct for maximum horsepower. When the lever was in the Economy (E) position, part-throttle fuel requirements were fulfilled. For automatic operation, the ratio lever was controlled by the spring-loaded enrichment diaphragm, which was subjected to manifold vacuum. At light load (high vacuum) the lever was held at the "E" position; at full load (low vacuum) it was at the "P" position.
The stops were set to obtain a best economy air/fuel ratio of approximately 15.5:1 at part throttle (carburetors were tuned to a richer 14.7:1) and a maximum power mixture strength of around 12.5:1. The enrichment diaphragm spring was set to hold the ratio lever in the economy position above seven inches of mercury.
The nozzle (injector) had an accurately calibrated fuel orifice of less than 1/64-inch diameter. The nozzles were mounted at the entrance of each intake port in plastic holders. Just below the fuel orifice there was a small air chamber, which received filtered air through four 0.100-inch diameter air holes from the air cleaner. This air supply assured that the nozzle discharge was at all times near atmospheric pressure, regardless of manifold vacuum fluctuations. Consequently, the amount of fuel injected depended solely on the operating fuel pressure. In line with the fuel orifice across the air chamber was the opening to the intake port. This was a 0.040-inch diameter air orifice that enhanced the atomization of fuel by the mixing of air under all manifold vacuum conditions. The air volume passing through the air orifice represented about one-fourth of the air used at closed throttle for idle.
The tubular skirt below the air orifice was cooled by evaporation of the fuel, which in turn cooled the fuel line by conduction, thus preventing metering disturbances caused by fuel vapor bubbles. The fuel at the nozzle and in most of the line was at a lower temperature than when it entered the system from the diaphragm pump. By cooling the fuel through latent heat, the open orifice type of nozzle became suitable for handling high vapor pressure fuel, even under summer conditions. The nozzle operating temperature range, due to its location and insulation, was much lower than that of nozzles used in other fuel injection systems. The lower temperature greatly reduced vapor problems.
The fuel orifice size was selected to provide a minimum pressure at idle and engine crankover of about eight p.s.i. and a maximum pressure of 200 p.s.i. at full throttle. Under normal driving conditions, the fuel pressure seldom went over 20 p.s.i. and was easily maintained by the gear pump.
During engine idle, the throttle plate was closed and about one-quarter of the air requirement was supplied through the individual nozzle air chambers. The rest was provided via a passage around the throttle blade and was regulated by the idle air bypass screw in the throttle body. The correct idle mixture was set with the idle mixture screw. It controlled the amount of venturi signal boost caused by bleeding in manifold vacuum from the idle orifice.
To yield the best economy for steady-state operation, the system provided a leaner mixture at low air flow volume, but due to the plunger weight and the nozzle placement below the spill level in the fuel meter, there was some inherent enrichment near idle. The remainder of the required idle mixture enrichment was supplied through the idle orifice, and the balance of the required fuel in the off-idle range was provided by the off-idle orifice.
As the throttle was opened from the idle position, the off-idle orifice was subjected to manifold vacuum and resulted in a stronger venturi signal. The amount of off-idle bleed was regulated by a restriction in the bleed passage.
Steady state engine operation, whether at idle, part-throttle or full load, was relatively easy to calibrate for, but as with a carburetor, transient fueling required more effort in order to achieve the proper mixture strength.
The Rochester fuel injection system did not require an accelerator pump to supply extra fuel during throttle opening. Two factors were responsible for this. Manifold wetting with fuel injection was kept to a minimum, and the inherent instant fuel metering response of the system also contributed. Upon throttle opening, the manifold is filled by an immediate rush of air greater than the steady state air requirement, thus causing an increase in venturi signal strength. Since the fuel metering linkage parts were very light and the travel limited, their inertia was negligible, so that the incoming air would instantly cause a rise in fuel pressure; this resulted in a shot of fuel proportional to the volume of air.
Cold starting and warm-up did not require the same amount of richness over normal operating temperature that was required with a carburetor. This was due to the elimination of most of the intake manifold wall wetting with the injector being close to the valve. The starting enrichment was achieved by means of a solenoid connected to the starter circuit. The solenoid, when activated, completely cut off the spill by covering the spill ports, and also uncovered a passage from the float chamber inlet to supply extra fuel directly from the diaphragm pump.
During engine warm-up, the cold enrichment mechanism kept the ratio lever at the power stop position by cutting off manifold vacuum to the enrichment diaphragm. This enrichment assured proper engine operation with low coolant temperature.
By directing the fuel stream on the intake valve, the incoming mixture was warmed up considerably faster than by means of the heat riser used in a carburetor application. A fast idle cam kept the engine idle speed high enough to eliminate stalling by slightly opening the throttle blade during warm-up.
The new fuel injection system operated with a dry manifold and, thus, the amount of fuel vaporization in the runners during deceleration at closed throttle was negligible. A further step to eliminate hydrocarbon exhaust emissions during coast down was provided by a "coasting shut off" (CSO), located directly above the gear pump outlet. This device consisted of a valve that, under normal operating conditions, was closed by a spring. A link connected the valve to the coasting shut-off diaphragm, which was subjected to manifold vacuum at the throttle body.
During coasting, high manifold vacuum exerted force on the diaphragm in excess of the spring load, and the pull rod lifted the valve off its seat, thereby allowing the pump to discharge directly back into the float chamber and cut off fuel flow to the nozzles. The vacuum valve was so arranged that vacuum can be applied to the diaphragm only when the throttle was closed, to eliminate fuel shut-off as the engine was accelerated with no load, such as with the transmission in neutral.
A good theory
When fuel injection is employed on an engine, there is substantially more freedom in intake manifold design--fuel drop-out due to runner length is no longer a concern. The GM engineers recognized this and used the opportunity to create a "tuned" induction system. The phenomenon is called inertia supercharging. By adjusting the runner length and diameter, a ram effect takes place that causes an increase in pressure in the cylinder from increased volumetric efficiency. GM engineers used pressure transducers placed near the intake valve to measure and adjust the effect.
When installed in a vehicle, the Rochester fuel injection system, along with the companion ram-tuned intake manifold, provided superior performance, reduced fuel consumption and instantaneous throttle response at all loads and engine speeds in comparison to the production carburetor used at the time. It also allowed a performance car such as the Corvette to fully utilize the potential of the engine while providing docile low-speed manners--a trait that was previously unattainable.
As with many new technologies, the added up-front cost and the service industry's lack of familiarity with the design meant that the Rochester fuel injection was not greeted by the public with the enthusiasm the engineers enjoyed. The system was labeled as being complicated and fussy about weather conditions, but in reality it was a lack of understanding and not a design or calibration flaw. General Motors is in the business of selling cars, and only advanced the technological envelope when there was a financial reward. The fuel injection system passed on quietly a few model years after its introduction, a mechanical marvel that was never recognized for its achievement.
This article originally appeared in the October, 2007 issue of Hemmings Classic Car.
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