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The Rambler Rebel’s fuel injection – The Dream and the Legend

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[Editor’s Note: After the recent story on the Rambler Rebel’s 60th anniversary, we thought we’d include this section from Bill Lenharth‘s book, “The Amazing Rambler Rebel,” which explains the most misunderstood aspect of the Rebel: its never-produced electronic fuel injection system.]

The Rambler Rebel’s basic design program was innovative and set the tone for the future of a distinct class of automobile. The car itself also had very innovative components, among them electronic fuel injection. The fuel injection system was unique in the design concepts that it employed. These same concepts would continue to be used in the fuel injection system of vehicles into the 21st century.

The development of what was to become the Bendix Electrojector System started in 1953 at the Bendix Corporation. The Bendix company decided to explore the concept of an electronic fuel injection system as a replacement for the standard carburetor that would solve all of the carburetor’s very well known problems. However, during this time fuel injection systems were mechanical, costly, and fitted to only the most expensive vehicles. These systems worked extremely well, but the cost of the fuel injection system alone would equal the price of the AMC Rambler automobile.

In 1954 the only fuel injection offered by any manufacturer was by Mercedes Benz, which offered a mechanical fuel injection system built by Bosch. Mercedes first used the system in only the diesel engines in its cars and trucks. Mercedes then pioneered its use in a gasoline car, the famous 300SL gull wing sports car of the 1950s, which used a mechanical Bosch unit modified from the Mercedes’ diesel application.

Hot rodders were very familiar with fuel injection, employing the famous Hilborn fuel injection systems on their drag-racing cars. These systems were again mechanical, expensive, and required a lot of adjustments to work correctly. There was one injector per cylinder, and each one had its own adjustments. The injectors generally needed adjustments after every race. This approach was not practical for a production vehicle, even if one could ignore the cost.

Bendix started development of a fuel injection system in response to solving several problems experienced with the current carburetor and fuel injection systems. Carburetors had several well known problems that affected overall automobile performance, arising because of compromises made during the carburetor design process. These problems were cold starting, providing correct fuelair mixtures at all speeds and load conditions, flat spots in acceleration, and altitude effects. The design goals of the project were to offer a fuel injection system that would automatically recalibrate itself, self-compensate for different altitudes, be low in cost, have improved performance without sacrificing fuel economy, and provide improved cold weather starting/operation.

Several members in different Bendix divisions formed a design team to build the fuel injection system. The team was headed by Chief Engineer A. H. Winkler and his assistant, R. W. Sutton, of the Fuel Systems Engineering division. From the start the design approach was electronic instead of mechanical, the goal being to construct an adaptive system that would use inexpensive components, so that this system could be used in different applications and in low-cost cars.

The Bendix System employed the following components: port injectors that used electronic solenoids, a low-pressure electronic fuel pump, some type of timing device, and a computer. The system would use sensors to incorporate data on barometric pressure, engine vacuum, engine rpm, and engine temperature. The goal of the system was to have the proper fuel-air mixture at any engine speed and load combination. All the components used in the system would need to be newly manufactured with the exception of the fuel pump, which had already been developed for use in the aircraft industry.

Details of the Fuel Injection Design
A schematic of the basic fuel system structure is shown in the following diagram. Fuel is drawn from the tank by a nonmetered, electrically driven, low pressure pump that maintains a line pressure of 20 psi to each fuel injector valve, one per cylinder. Between the pump and the injectors is an in-line fuel filter, used in carburetors to filter fuel down to 20 microns. The Electrojector system could use this type of filter since the system did not have close-fitting parts. The proposed system design deviated substantially from any system in operation in 1957. All the mechanical systems required high fuel pressure (80 psi), and expensive, high-filtration type filters, both features adding substantially to the cost of the system.

Electrojector System
The fuel injector nozzle was aimed at the head of the intake valve. This reduced cylinder wall wetness and provided the best performance. The system also had a return fuel line circuit that returned unused fuel to the tank, purging any line vapor and air from the system, thus preventing vapor lock.

The solenoid fuel injector valve was a new design for this system. It operated mechanically, similar to mechanical fuel injection systems, but electricity instead of high fuel pressure was used to open the valve. The injector was designed so that the fuel enters down the center of the valve, flowing past the return spring to the discharge nozzle. The fuel discharges when the valve core is lifted off its seat. Fuel being expelled into the cylinder is quickly atomized. Bendix spent a lot of effort to test this part of the system and to refine it during development, attempting to insure that the valve would remain calibrated, require low power, and have a low manufacturing cost.

The next part of the design was the system components that told the injectors when to put fuel into each cylinder. The system was described by Bendix as a timed fuel injection system. Systems up to this point fed fuel into cylinders based on when a mechanical high-pressure pump forced a pulse of high-pressure fuel into each injector. This pump was driven directly off the camshaft and thus acquired the correct timing from the engine itself. Bendix needed the same type of control information, but in an electrical form.

The solution was relatively simple and straightforward. The Bendix team modified the stock Delco distributor with a second rotor and a second set of contacts. These components are inserted as a sandwich between the base of the distributor and the standard ignition distributor cap. Bendix referred to this unit as the “Triggering Selector Unit.” This unit consists of a set of breaker points and a distributing commutator. The distributing commutator has a contact section for each injector valve. The breaker points are operated by the same cam that operates the ignition breaker points. Therefore, of every two rotations of the engine, the distributor is rotated once, sending an electric pulse to the modulator box for each cylinder. The modified signal is then returned to the distributor’s fuel injector commutator for distribution to the cylinder. Through the triggering selector unit, engine speed and fuel injection timing are sensed and the electrical impulse is correctly distributed to the individual fuel injectors. Since this basic system used in the distributor modification employed the same basic approach as engine ignition, it would prove equally reliable. This modification of the distributor is the only modification that is required to the engine system to adapt it to the Bendix Electrojector system. Thus, the distributor fires the spark plugs as it injects the fuel.

The next and most critical component of the system is the unit’s brain or electronic modulator unit. This box basically takes the voltage spike that it gets from the triggering sector unit and converts it into a pulse of specific and calculated width. The modulator also receives signals from sensing units on the operating condition of the engine. These sensors feed information into the modulator unit to modify the default pulse width continually so that the injectors always feed the engine the correct amount of fuel. Each injector valve is held open for the length of time necessary for the pulse to travel through the injector. This technology is referred to as pulsewidth modulation. These pulses are fed back to the triggering selector unit for subsequent distribution to the correct fuel injector. In a continuous process during the time of engine operation, the pulse width is constantly being modified by the modulator unit. The modulator uses 3.5 amps at battery voltage. In the 21st century the modulator would consume much less power using newer solid state technology. Besides controlling fuel, there is, of course, a need to control airflow so that the fuel mixture is kept in the proper ratio. The Bendix system used a two-barrel setup of throttle valves to control engine airflow. This setup looks like a carburetor, but the valves only control air and have no fuel delivery system. This carburetor-like device was called a throttle body. In the 21st century, throttle bodies also contain fuel injectors. The throttle body was fitted with a manifold intake pressure sensor that directly supplied data to the modulator. The information supplied provides insight into the relative density of the air charge entering the engine. The sensor increases the overall circuit resistance as the manifold pressure increases. All the other sensors perform similarly, adding resistance to the fundamental circuit in the modulator. This added resistance in the circuit causes the system to transmit a longer signal to the injector valve, holding the valve open longer to provide more fuel to a particular cylinder.

During engine acceleration, a small extra amount of fuel will smooth operation during the transition period. In carburetors, an acceleration pump provides this extra fuel. In the Electrojector system, however, an acceleration enrichment sensor measures manifold vacuum and temporarily increases the circuit resistance to allow the fuel injector valves to again remain temporarily open even longer than previously computed. This sensor employs a set of points that remain closed until there is a sudden change in manifold vacuum that opens them and introduces an additional resistance into the circuit until the vacuum bleeds across the divider and the sensor equalizes the pressure so that the points will close again. The period when the points are open allows longer pulses to be fed to the injectors and therefore more fuel to be put into the engine’s combustion chambers.

The Electrojector system also employed two other controls: idle enrichment and starting enrichment devices. The idle control allows the user to adjust the idle of the vehicle in a manner similar to a carburetor. An adjustment screw is provided to set idle speed. The starting enrichment device helps the system during starting. The Bendix system has a device that connects a solenoid to a variable resister and thermostat. Based on three conditions, this device sets the resistance at various positions. The first phase of its operation is during cranking; the resistor is set to the maximum resistance position. After the engine starts, the amount of resistance, and therefore the fuel added, is reduced as the engine warms up. The system also employs a conventional fast-idle cam and thermostat during the warm-up period.

Of the final two sensors one is for altitude compensation and the other deceleration. During engine deceleration, an engine can emit unwanted smog (unburned fuel). To solve this problem a sensor observes manifold vacuum, and when it detects an abnormally high reading, it cuts off the fuel when the reading reaches a certain point. With the injectors located close to the intake valves, there should be very little fuel carryover from the manifold, effecting a clean cutoff. The diagram in Figure 22 shows the entire system schematic, illustrating how the system functions and uses engine information.

However, there were bugs in the Bendix system that prevented the final sale of the Electrojector units for use in the Rebel. One of these bugs was cold weather starting that prevented the engine from starting at temperatures below 50 degrees F, and an owner of serial number 2 or 4 (1002,1004) had a Rebel fitted with the unit (a photo of the car appears in Chapter 7). It was reported that the fuel injection fitted cars could reach 60 within seven seconds. A photo of a fuel injection system installed in a test car appears in Winkler and Sutton’s SAE paper. The fuel injection system increased torque by only five foot-pounds, but it increased horsepower by 33. It was also reported that three Ramblers were fitted with the Electrojector unit for testing at the AMC proving grounds. This information came in conversations with former factory employees.

The production of the Rebel commenced in February of 1957. There has been discussion about how many Rebels received the Bendix fuel injection system. The number appears to be somewhere between six and none, although the official number is none. However, in several factory photo display models, Rebels are shown with a fuel injection equipped engine. One could assume that at least one show car must have received the fuel injection treatment. The list price for the fuel injection system was $395 retail, a real bargain for such a system.

One final note on the Electrojector fuel injection system for the Rambler: Bendix engineers commented that the horsepower and torque improvements were modest because the intake manifold and head/valve design were restrictive. It could therefore be expected that with a small amount of engine work the 327 could be made to pump out even more brute force power. Current drag race enthusiasts back this conclusion up by running Rambler 327 engines producing close to 400 horsepower.

The Bendix fuel injection as designed was revolutionary in 1957. Today fuel injection is commonly used in most production cars.

[For more on the Rambler Rebel, check out “The Amazing Rambler Rebel,” available directly from Bill Lenharth or from Pat Foster.]