Know-How Notes: Fuel Pump Basics
Deep in the bowels of your car, lives a much-maligned component. It lives its entire life to pump fuel from the gas tank to your engine, and most of us never give it any thought. That is, until it dies. Then you struggle to figure out what the problem is. When the fuel pump quits, you have no option but to replace it. Not all fuel pumps are created equal, however. There are three main types of OEM fuel pumps; diaphragm, rotary vane, and gerotor. Each has pros and cons to their design.
Diaphragm pumps use a membrane, usually a rubber composite, which moves in an up-down motion over a cavity. The cavity has an input and an output, each with a check valve in place to ensure a one-way motion of fluid. As the diaphragm moves up, it creates a vacuum, drawing fuel from the line to the cavity. As it moves down, the fuel is pushed out of the cavity under pressure. The check valves keep the fuel from being pushed out the wrong side. This is the design most commonly used in stock-style mechanical fuel pumps and cheap aftermarket low-volume electric pumps. This design is simple and they last quite a long time.
One of the biggest benefits of a diaphragm pump is the vacuum created on the feed line. This actually draws fuel from the tank to the pump. They are also very good for dirty fuel systems, since the fuel does not flow through the membrane, debris and other contaminants are less likely to damage the pump. Diaphragm pumps are not as efficient as other designs; therefore not suitable for high-performance applications.
The diaphragm pump often last many, many years. As a mechanical pump, they are also the easiest to change in most vehicles. The limit of the diaphragm pump is that they are not used on EFI (electronic fuel injection) engines as they can’t generate the required pressure and EFI requires fuel before the engine cranks.
Rotary Vane Pumps
Rotary vane pumps, such as the Holley Red and Blue fuel pump designs, operate with a paddle-wheel device inside a larger circular base. The wheel is offset to one side, creating a crescent shaped cavity. Paddles on the wheel slide in and out of the wheel as they spin inside the cavity, this draws fuel into the pump as the cavity opens up, then compresses it as it narrows again, finally pushing it out of the pump under pressure. The nature of the sliding vanes creates a lot of friction inside the pump. The slide must seal the pump, maintain pressure and slide in and out, resisting centrifugal force, all at the same time.
Sliding vane pumps are generally relegated to low pressure applications and are almost always T-style pumps (motor on top, inlet/outlet on the bottom), though there are a few inline sliding vane pumps. For high pressure application, the roller vane design is used. Where the sliding vane has a lot of friction on the flat edges, the roller vane uses the same basic principle, but instead of a paddle, a roller bar is used. The roller still moves in and out of the inner wheel, but much of the friction is reduced, increasing the sealing and efficiency of the pump. This style of vane pump is more suitable for high pressure than the sliding vane. Rotary vane pumps are efficient, but they are loud. Contaminates in the fuel can create problems in the pump, but they are more tolerant than gerotor pumps.
Gerotor pumps are the most common design for modern electric high-pressure, high-volume fuel pumps. A gerotor pump operates by spinning a spur gear that drives what is essentially an internal ring gear. This internal gear has teeth on the inside of the ring. As the spur gear spins, the ring gear rotates inside the cavity, creating suction on the inlet and produces pressure on the outlet. These pumps are very efficient, quiet and can build very high pressures. The drawback of a gerotor design is that they are highly susceptible to damage from contaminants and overheating. When the fuel feed is reduced to the pump, cavitation occurs, which destroys the pump in a matter of minutes.
A common misconception is that gerotor pumps do not produce vacuum on the inlet side, meaning they must be gravity fed by the fuel tank. In reality, gerotor pumps can generate significant vacuum. As the vacuum increases, the boiling rate of the fuel decreases, and the fuel to turns to vapor. This causes cavitation, which is an air bubble imploding. Cavitation is like setting off a bunch of tiny explosions inside the pump, it doesn’t take long, even a few minutes, to destroy the pump. By mounting the pump as close to the tank as possible, with a good gravity feed, you reduce the amount of vacuum generated by the pump, eliminating the vapor issues.
Direct-Injection and Diesel Fuel Pumps
The advent of direct-injected engines has brought the mechanical pump back into the limelight. These pumps use a piston to pressurize the fuel. Similar to the diaphragm pump, the piston design mounts the engine like a standard diaphragm pump, but it uses a piston drive similar to a master cylinder. These are often driven off of a lug on the camshaft. Like a diaphragm pump, the piston pump draws fuel into the pump and forces it out under pressure; the difference here is that there is no diaphragm to rupture and the piston action can create substantially more pressure and flow more volume.
Because the pump is mounted to the block and runs off the camshaft, the pump only produces what the engine needs; there is no need for a return line from the fuel rails. Instead, the piston pump is fed by an electric pump usually around 72 psi. This is the same for both direct-injection gasoline engines and diesel engines. The main difference is that gasoline engines are pressurized to 2,000-2,500 psi, whereas diesel engines use significantly higher pressures, between 10,000 and 30,000 psi range. This is necessary to atomize the fuel oil so that it combusts correctly. In gasoline engines, the 2,500 psi maintains a much better atomization level and increases fuel economy by a fair amount.
High-performance GDI (the term used for Gasoline Direct-injection) engines require some alteration when boosting power. Comp Cams now makes tuned cam lobes for the Gen V LT1 series GM engines, so you can boost your fuel output to match that new supercharger.
Choosing The Right Pump
For custom applications, regardless of the style of pump you choose, you need to know what size to get. While the old penchant of bigger is better is often true, there are situations where too much fuel pump can actually cause problems. A fuel pump is designed to move fluid and build pressure. A typical carburetor requires 7-14 psi of fuel pressure to ensure the bowls stay full. EFI systems require wildly different pressure, but typically range from about 26 to 60 psi, more with boost.
Recently we had a 400 horsepower small-block with EFI that required 26 psi of fuel pressure. Because of some fuel delivery issues, we ended up strapping a monster fuel pump that was capable of 100 psi and could feed a 1500 hp-carbureted engine. We noticed that the fuel pressure gauge would slowly creep up to almost 30 psi, even while the car was driving because the fuel pump was too big, causing the engine to run rich. It was simply pushing fuel past the built-in regulator on the TBI setup. A separate high-performance regulator could solve that problem, however the point is, too big can be a problem in certain situations. Eventually, the pump failed because the feed line from the tank was not large enough to support the needs of the pump, and it cavitated, which destroyed it. Setting up your fuel system requires more forethought than just “any pump will do.”
Keeping Your Fuel Pump Alive
When it comes to life span of a fuel pump, there are a few things you can do to keep your pump alive. Most OEM electric fuel pumps are located inside the fuel tank. This is to provide the pump with adequate fuel and to keep the pump cool and quiet. You can barely hear the pump if you turn your ignition switch to the “on” position and listen. A few seconds of whirring, and then it stops. This is pressurizing the fuel so that the engine can start.
The key here is keeping the pump cool. A hot pump is a dead pump, so the recommendation is to never let your gas tank get below 1/4-tank. Sure, the occasional dip below 1/4 is unavoidable, but consistently driving with less than 1/4-tank is very hard on in-tank fuel pumps.
The other trick is to replace your fuel filter. Most manufacturers advise changing the fuel filter every 20k-40k miles. When the filter gets plugged up, your engine struggles to get the fuel it needs, and the pump has to work harder pushing through the filter, shortening its life.
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