Precision Airmotive: RSA Fuel Injection Unpacked

In the intricate world of automotive and aviation mechanics, the fuel injection system stands as a cornerstone of engine performance, efficiency, and reliability. Moving far beyond the rudimentary carburettor, modern fuel injection ensures precise fuel delivery, optimising combustion and enhancing overall engine operation. For enthusiasts and professionals alike, understanding these complex systems is paramount to maintaining peak performance and diagnosing potential issues. This article will delve into the specifics of leading fuel injection systems, with a particular focus on the respected RSA® Fuel Injection System.

Who Manufactures RSA Fuel Injection Servos?

When it comes to the RSA® Fuel Injection System, the name you need to know is Precision Airmotive®. As the Original Equipment Manufacturer (OEM) of these highly regarded systems, Precision Airmotive has earned a reputation for unparalleled expertise and engineering excellence. Their products are not only respected but also widely trusted by engine manufacturers across the globe.

Precision Airmotive, as the original architect and producer of this sophisticated fuel injection system, offers a comprehensive range of components and services. This includes factory new RSA Fuel Injection Servos, meticulously rebuilt units, and thoroughly overhauled components. Beyond the servos themselves, they also provide essential parts such as Flow Dividers, precision-engineered nozzles, and a complete line of replacement parts and kits to ensure the longevity and optimal performance of your RSA system.

Understanding Fuel Injection Systems

At its core, a fuel injection system is designed to atomise and deliver fuel into an engine's intake manifold or directly into the combustion chamber. This precise delivery ensures the correct fuel-to-air ratio for efficient combustion across all engine operating conditions. Unlike carburettors, which rely on venturi effect to draw fuel, injection systems use pressure to spray fuel, leading to numerous advantages.

The Bendix/Precision RSA Fuel-Injection System

The Bendix inline stem-type regulator injection system, commonly known as the RSA series, is a continuous-flow system that precisely measures engine air consumption to control fuel flow. This system is comprised of three primary components: the fuel injector, the flow divider, and the fuel discharge nozzles.

The Fuel Injector Assembly

The fuel injector assembly is the brain of the RSA system, consisting of three crucial sections:

• Airflow Section: This section is responsible for measuring the engine's air consumption. It achieves this by sensing the impact pressure and the venturi throat pressure within the throttle body. These pressures are then channelled to opposing sides of an air diaphragm. As the throttle valve moves, altering engine air consumption, the air velocity in the venturi changes. An increase in airflow lowers the pressure on one side of the diaphragm (due to the venturi throat pressure drop), causing the diaphragm to move and open a ball valve. The impact pressure, picked up by impact tubes, contributes to this force, creating what is known as the "air metering force."
• Regulator Section: The regulator section features a fuel diaphragm that directly opposes the air metering force. Fuel inlet pressure is applied to one side of this diaphragm, while metered fuel pressure (fuel that has passed through the fuel strainer and manual mixture control) is applied to the other. The differential pressure across this fuel diaphragm is termed the "fuel metering force." A ball valve attached to the fuel diaphragm precisely controls the orifice opening and, consequently, the fuel flow. The distance this ball valve opens is determined by the pressure difference across the diaphragms, which is proportional to the airflow through the injector. This ingenious design ensures that the volume of airflow directly determines the rate of fuel flow. For low power settings, where the venturi pressure difference might be insufficient for consistent regulation, a constant-head idle spring is incorporated. This spring maintains a constant fuel differential pressure, ensuring adequate fuel flow in the idle range.
• Fuel Metering Section: Attached to the air metering section, this part houses several critical components: an inlet fuel strainer, a manual mixture control valve, an idle valve, and the main metering jet. In some models, a power enrichment jet may also be present. The primary purpose of this section is to meter and control the fuel flow before it reaches the flow divider. The manual mixture control valve allows for adjustment from a full rich condition to a progressively leaner mixture as the lever is moved towards idle cutoff. Both idle speed and idle mixture can be externally adjusted to meet specific engine requirements.

The Flow Divider

Once the fuel is metered by the fuel control unit, it is delivered to a pressurised flow divider. This unit serves several vital functions: it maintains the metered fuel under pressure, it meticulously divides the fuel to the various cylinders across all engine speeds, and it enables the shutting off of individual nozzle lines when the control is set to idle cutoff.

At idle speeds, the fuel pressure from the regulator must build sufficiently to overcome the spring force acting on the diaphragm and valve assembly within the flow divider. This action lifts the valve, allowing fuel to pass through to the fuel nozzles. Since the regulator delivers a fixed amount of fuel at idle, the valve opens only as far as necessary to pass this specific amount. As fuel flow from the regulator increases beyond idle requirements, pressure builds in the nozzle lines, fully opening the flow divider valve. At this point, fuel distribution to the engine becomes primarily a function of the discharge nozzles.

A notable feature of the Bendix RSA system is the ability to use a fuel pressure gauge, calibrated in pounds per hour, as a fuel flow meter. This gauge connects to the flow divider, sensing the pressure applied to the discharge nozzles. This pressure is directly proportional to the fuel flow, providing valuable insight into engine power output and fuel consumption.

Fuel Discharge Nozzles

The RSA system utilises air bleed fuel discharge nozzles, with one nozzle located in the cylinder head for each cylinder, directing fuel into the intake port. Each nozzle incorporates a calibrated jet, sized according to the available fuel inlet pressure and the engine's maximum fuel flow requirements. Fuel is discharged through this jet into an ambient air pressure chamber within the nozzle assembly. Before entering the individual intake valve chambers, the fuel is mixed with air to aid in atomising the fuel, ensuring a finer spray and better combustion. Fuel pressure before the individual nozzles is directly proportional to fuel flow, allowing a simple pressure gauge calibrated in gallons per hour to serve as a flowmeter. Engines equipped with turbosuperchargers must use shrouded nozzles, which are vented to the injector air inlet pressure via an air manifold.

The Continental/TCM Fuel-Injection System

The Continental fuel-injection system, another prominent design, injects fuel directly into the intake valve port of each cylinder head. This is a continuous-flow type system, designed to precisely match fuel flow to engine airflow. A key characteristic is its use of a rotary vane pump, which does not require timing to the engine, simplifying its operation.

Fuel-Injection Pump

The fuel pump in the Continental system is a positive-displacement, rotary-vane type, driven by the engine's accessory drive system. It includes a spring-loaded, diaphragm-type relief valve, with its diaphragm chamber vented to atmospheric pressure.

Fuel enters the pump at the swirl well of the vapor separator. Here, a swirling motion separates fuel vapour, ensuring that only liquid fuel is delivered to the pump. The separated vapour is drawn from the top centre of the swirl well by a small pressure jet of fuel and directed into a vapour return line, which carries it back to the fuel tank. This effectively prevents vapour lock and ensures a consistent fuel supply.

Given that it's an engine-driven, positive-displacement pump, changes in engine speed directly affect total pump flow. Since the pump's capacity exceeds engine requirements, a recirculation path is incorporated. By placing a calibrated orifice and relief valve in this path, the pump delivery pressure is maintained proportionally to engine speed, ensuring proper pressure and fuel delivery across all operating speeds.

A check valve is also integrated, allowing boost pump pressure to bypass the engine-driven pump for easier starting. This feature also helps suppress vapour formation in high ambient temperatures and allows the auxiliary pump to act as a backup fuel pressure source should the engine-driven pump fail.

Fuel/Air Control Unit

The fuel/air control assembly is responsible for regulating engine air intake and setting the metered fuel pressure to achieve the correct fuel/air ratio. The air throttle, mounted at the manifold inlet, features a butterfly valve controlled by the aircraft's throttle, which governs air flow to the engine. This aluminium casting is precisely sized for the engine and does not utilise a venturi or other restrictions.

Fuel Control Assembly

The fuel control body, typically made of bronze for optimal bearing action with stainless steel valves, contains a metering valve at one end and a mixture control valve at the other. Each stainless steel rotary valve features a groove forming a fuel chamber.

Fuel enters the control unit through a strainer and proceeds to the metering valve. This rotary valve has a cam-shaped edge that controls fuel delivery to the manifold valve and nozzles. The fuel return port connects to the return passage of the centre metering plug, and the alignment of the mixture control valve with this passage determines the amount of fuel returned to the fuel pump. By connecting the metering valve to the air throttle, the fuel flow is precisely proportioned to airflow, maintaining the correct fuel/air ratio. A control lever on the mixture control valve shaft connects to the cockpit mixture control.

Fuel Manifold Valve

The fuel manifold valve serves as a central point for dividing fuel flow to individual cylinders. It contains a fuel inlet, a diaphragm chamber, and outlet ports leading to the individual nozzle lines. A spring-loaded diaphragm operates a valve in the central bore. Fuel pressure provides the force to move this diaphragm, which is enclosed by a cover retaining its loading spring. When the valve is down against its seat, the fuel lines to the cylinders are closed. The valve is drilled for fuel passage from the diaphragm chamber to its base, and a ball valve is installed within it. All incoming fuel must pass through a fine screen in the diaphragm chamber.

From the fuel-injection control valve, fuel is delivered to this manifold valve. Here, a diaphragm raises or lowers a plunger valve, simultaneously opening or closing the fuel supply ports to the individual cylinders.

Fuel Discharge Nozzle

Similar to the RSA system, the Continental fuel discharge nozzle is located in the cylinder head, directing its outlet into the intake port. The nozzle body features a drilled central passage with counterbores at each end. The lower end acts as a chamber for fuel/air mixing before the spray exits the nozzle. The upper bore contains a removable orifice for calibrating the nozzles. Nozzles are calibrated in various ranges, and all nozzles for a single engine are of the same range, identified by a letter stamped on the nozzle body's hex.

Drilled radial holes connect the upper counterbore to the outside of the nozzle body, drawing air through a cylindrical screen fitted over the nozzle. A shield is press-fitted over most of the filter screen, leaving an opening near the bottom. This design provides both mechanical protection and an abrupt change in airflow direction, helping to keep dirt and foreign material out of the nozzle interior.

Advantages of Fuel Injection Systems

Both the Bendix/Precision RSA and Continental/TCM fuel injection systems offer significant advantages over conventional carburettor systems:

• Reduced Induction System Icing: With fuel vaporisation occurring in or near the cylinder, the temperature drop associated with fuel evaporation is localised, significantly reducing the danger of induction system icing, a common issue with carburettors.
• Improved Acceleration: The positive and precise action of the injection system provides immediate and consistent fuel delivery, leading to much improved engine acceleration characteristics.
• Enhanced Fuel Distribution: Fuel injection systems ensure a more uniform distribution of fuel to each cylinder. This eliminates the variations in mixture often seen with carburettors, which can lead to uneven performance and overheating of individual cylinders. By providing an optimal mixture to all cylinders, engine life and efficiency are improved.
• Better Fuel Economy: Because fuel is distributed more evenly and precisely, the engine can operate with a leaner overall mixture without risking a too-lean condition in any single cylinder. This often results in better fuel economy compared to carburettor systems, where the mixture to most cylinders might need to be richer than necessary to ensure the leanest cylinder operates properly.

Comparative Overview: RSA vs. Continental/TCM

While both systems achieve the goal of precise fuel delivery, their approaches and components differ. Here's a brief comparison:

Frequently Asked Questions (FAQs)

Q1: What is the primary role of a fuel injection servo?

A fuel injection servo, particularly in the RSA system, acts as the central control unit that meters and regulates the flow of fuel based on the engine's air consumption. It ensures the correct fuel-to-air ratio is maintained across various engine operating conditions, from idle to full power.

Q2: How does a flow divider ensure even fuel distribution?

The flow divider, present in systems like the RSA, keeps metered fuel under pressure and precisely divides it to the individual cylinders at all engine speeds. At lower engine speeds, it precisely opens to deliver a fixed amount of fuel, while at higher speeds, it fully opens, allowing the discharge nozzles to primarily control distribution. This ensures that each cylinder receives an equitable amount of fuel for balanced power output and reduced hot spots.

Q3: Why is a vapour separator important in some fuel injection systems?

A vapour separator, as seen in the Continental/TCM system, is crucial for preventing vapour lock. It separates fuel vapour from liquid fuel, ensuring that only liquid fuel reaches the pump and ultimately the engine. This is particularly important under high ambient temperatures where fuel can more easily vaporise, which would otherwise disrupt consistent fuel delivery and engine performance.

Conclusion

Whether it's the robust RSA system from Precision Airmotive or the innovative Continental/TCM design, modern fuel injection systems are vital for the efficient and reliable operation of engines. Their ability to precisely meter and distribute fuel offers significant advantages over older carburettor technologies, including improved fuel economy, better performance, and enhanced safety. Understanding the components and operating principles of these systems is crucial for anyone involved in the maintenance or operation of such machinery, ensuring they continue to perform at their best for years to come.

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