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The Science of Fuel Delivery


12/1/2002
By Larry Carley

 

One of the first things you have to learn abouot fuel delivery is the role of the air/fuel mixture on combustion, emissions and performance. To better understand what's going on, you need a short lesson in the chemistry of combustion. Chemistry is a pain - but if there's no pain, there's no gain, and you won't fully understand the rationale behind fuel delivery.

When fuel enters the engine through a fuel injector (or a carburetor on older vehicles), the fuel mixes with air to create a combustible mixture. It takes a lot more air than fuel to do this. Why, you ask? Because of the chemistry of air and gasoline.

Air is mostly nitrogen, which does not burn. Oxygen is the stuff that makes fire, but the amount of oxygen in the air is only about 16 to 20 percent. If air were pure oxygen, even a tiny spark would set off a blowtorch-like inferno because oxygen burns really HOT. If you mix it with acetylene gas, you can cut and weld steel.

As for gasoline, it's a hydrocarbon (HC) made up of long chains of hydrogen and carbon atoms. When hydrogen, carbon and oxygen get together in the right proportions and are ignited by a spark, the resulting explosion creates heat and energy. It takes a lot of air to do this because there's more hydrogen and carbon atoms in a molecule of gasoline than there are oxygen atoms in a molecule of air.

The ideal air/fuel ratio (called the stoichiometric ratio) for air and gasoline is 14.7 parts of air for every part of fuel. This ratio gives the best fuel economy and power, and produces the lowest emissions - just carbon dioxide (CO2) and water vapor (H2O).

If the air/fuel ratio contains too much fuel and not enough air (a "rich" mixture), there will be some unburned HC in the exhaust along with carbon monoxide (CO). HC is bad because it forms smog, and CO is poisonous. A rich fuel mixture can be caused by a dirty air cleaner, excessive fuel pressure, leaky injectors or a bad oxygen sensor.

If the air/fuel ratio has too much air and not enough fuel (a "lean" mixture), it might not ignite at all - causing a loss of power and a big increase in unburned HC in the exhaust. This is called "lean misfire" and may be caused by dirty or clogged fuel injectors, low fuel pressure or an air or vacuum leak in the intake manifold.

COLD STARTING
When a cold engine is first started, it needs a richer-than-normal fuel mixture until it warms up. But if the fuel mixture is way too rich (say 8:1 or less), there may not be enough oxygen to ignite the fuel. This is called "flooding" and it can prevent the engine from starting until the excess fuel evaporates.

By the same token, if the air/fuel mixture isn't rich enough when the engine is cranked it may not start either. And if the mixture isn't rich enough once it does start, the engine may idle poorly or hesitate and stall when accelerating.

Back in the days of carburetors, an automatic choke was used to provide cold start fuel enrichment. The choke closed a flap that blocked air entering the carburetor. The choke linkage was also connected to the throttle linkage so when the choke was closed the engine would idle at a higher speed. As soon as the engine started and began to warm up, the choke would gradually open allowing more and more air into the engine.

The problem with chokes was that they tended to stick and were tricky to adjust. If the choke was set too rich or too lean, the engine would be hard to start and might idle too fast or stall.

With fuel injection, there is no choke. Cold start fuel enrichment is provided one of two ways. On older vehicles (including many imports) an extra "cold start injector" was located on the intake manifold to give an extra dose of fuel during cranking and initial start-up. A timer was often used to control how long the cold start injector remained on. Failure of the timer or the injector would make the engine hard to start.

On newer fuel injection systems, the computer monitors air and coolant temperature with sensors and increases the 'on' time of the injectors to add extra fuel when the engine is cold. Fuel enrichment is programmed into the computer and depends on sensor inputs, so if the engine has a bad coolant sensor, it may cause a rich fuel mixture.

FUEL LOOP
Late-model engines also use a "fuel feedback control loop" to adjust the fuel mixture once the engine starts. The powertrain control module (PCM) uses input from the oxygen sensor(s) to determine if the fuel mixture is rich or lean. As soon as the O2 sensor gets hot enough to produce a signal (about 600 degrees F), the PCM goes into "closed loop" and starts using the O2 sensor signal to adjust the fuel mixture. Most newer vehicles have heated oxygen sensors so the sensor will reach operating temperature more quickly.

Oxygen sensors produce a voltage signal that bounces back and forth between 0.1 and 0.9 volts. Think of it as a rich/lean indicator switch. When the fuel mixture is rich (anything below 14.7:1), the O2 sensor signal shoots up to 0.8 volts or higher. When the PCM sees a rich indication, it compensates by making the mixture go lean. It does this by shortening the on time (duration) of the injectors to reduce fuel delivery. When the mixture goes lean, the O2 sensor signal drops to 0.2 volts or less. When the PCM sees a lean signal, it increases the duration (on time) of the injectors to add more fuel. This causes the fuel mixture to rapidly cycle back and forth between rich and lean, with the average mixture being close to 14.7 to 1.

The PCM also uses "short term" and "long term" fuel trim adjustments to make further refinements in the mixture. The end result of all of this is a closely balanced fuel mixture.

Several things can screw up the fuel control loop. One is a bad oxygen sensor. O2 sensors tend to slow down with age and can become sluggish or nonresponsive. If a technician hooks up an oscilloscope to an O2 sensor and sees a flat line waveform, the sensor needs to be replaced. A good O2 sensor should produce a signal that rapidly bounces back and forth between rich (0.8v) and lean (0.2v).

A bad coolant sensor can also prevent the PCM from going into closed-loop fuel control. If the PCM thinks the engine is always cold, it will keep adding extra fuel creating a fuel mixture that's too rich. The result can be a surging idle, poor fuel economy and elevated emissions. The same condition can also be caused by anything that causes the coolant sensor to read cold such as a thermostat that's stuck open or a low coolant level.

The PCM looks at inputs from the throttle position sensor, airflow sensor (if one is used) and manifold absolute pressure (MAP) sensor to determine throttle position, airflow and engine load. It then calculates the amount of fuel needed to balance the mixture. So any problems with these other sensors can affect fuel delivery, too.

FUEL PRESSURE
To provide the exact amount of fuel the engine needs under all conditions, the pressure inside the fuel lines that supply the injectors changes. When the driver steps on the gas and opens the throttle, intake vacuum drops. To deliver the same amount of fuel, pressure has to go up along with injector on time. Likewise, when the driver lets up on the gas and the vehicle decelerates, less fuel is needed to keep the engine running. Fuel pressure can drop along with injector on time.

The device that makes all of this happen magically is not the fuel pump (which runs constantly and provides steady pressure to the engine), but the fuel pressure regulator. On most engines, a fuel pressure regulator is mounted on the fuel rail that supplies the injectors. Inside is a diaphragm valve connected by a rubber hose to intake vacuum. When vacuum goes up, the valve opens and allows excess pressure to vent through a return line back to the fuel tank. On newer vehicles with "returnless" fuel injection systems, the pressure regulator is located back in the fuel tank just after the pump.

FUEL PUMPS
Some type of pump is required to push fuel from the fuel tank to the engine. Older carbureted engines mostly use an engine-mounted mechanical pump, though some may have an electric pump mounted in or near the gas tank.

Mechanical fuel pumps use a lever that rides on the camshaft to pump a rubber diaphragm inside the pump up and down. This creates suction that pulls fuel into the pump, and then pushes it along. A pair of one-way valves inside the pump only allow the gas to move in one direction (toward the engine). The output pressure is typically quite low with mechanical pumps, only 4 to 10 psi. But little pressure is needed to keep a carburetor supplied with fuel.

A leak in the pump diaphragm or one-way valves results in a loss of fuel pressure, causing the engine to run lean or stall. If the pump fails entirely, the engine won't start because there's no fuel.

With fuel injection, it's a different situation. The injectors need high pressure to spray fuel into the engine. A carburetor uses intake vacuum to pull fuel through jets and venturi, but injectors have to spray it directly into the intake ports. This requires anywhere from 35 up to 85 or more psi of fuel pressure depending on the type of injection system. This requires a high-pressure pump, which is usually mounted inside the fuel tank.

Electric fuel pumps come in a variety of designs: a single or double-vane, roller vane, turbine or gerotor-style pump. Most run constantly and use the fuel for lubrication and cooling. Running the tank empty may damage a high-pressure fuel pump.

Fuel pressure must be within specifications for the vehicle application, otherwise the engine won't run right. So one of the first things a technician will often check if he suspects a fuel delivery problem or lean fuel condition is fuel pressure. If pressure is low, it may be a bad pump - or it might be an electrical fault in the pump wiring circuit (bad ground, high resistance in connectors, etc.) Low voltage can cause a pump to run slower than normal.

The volume of fuel delivered is also important. A weak pump may produce adequate pressure at idle, but it may not keep up with the engine's fuel requirements when the engine is under load or at high rpm. A technician can check this by measuring fuel flow. If fuel delivery volume doesn't meet specs, the pump needs replaced.

If a fuel-injected vehicle won't start because there's no fuel, the cause may be a bad pump - or a bad pump relay or anything in the pump or relay's wiring circuit that controls pump operation. On most vehicles, the PCM oversees the operation of the pump. The pump circuit may also be wired through the oil pressure sending unit or an impact sensor to kill the pump in the event of an accident. So if there's a fault in any of these other components, it may prevent the pump from running.

Replacement fuel pumps for fuel-injected applications must have the same pressure rating as the original. The pump's inlet filter strainer should also be replaced when a new pump is installed. Also, the inside of the fuel tank should be inspected and cleaned if it is found to contain rust, dirt or other contaminants. A new pump can be quickly ruined by a dirty fuel tank.

CLEAN FUEL
Annual fuel filter replacements used to be part of the annual tune-up ritual. Replacing the fuel filter every year or two is still a good idea for preventive maintenance, but on most late-model vehicles, fuel filters are designed for 50,000-mile or higher service intervals.

The filter's job is to trap dirt, rust and other contaminants before they reach the injectors. If the filter plugs up, it can restrict the flow of fuel and cause a lean fuel condition.

One way technicians used to test fuel filters was to remove the filter and blow through it. If they couldn't blow through the filter, the filter was assumed to be plugged. But that's not the case anymore. The filter media inside many fuel filters today is so fine that it's almost impossible to blow through a new filter once it becomes wet.

Fuel filters should be replaced at the recommended interval in the owner's manual, or more often as needed if the fuel system has been contaminated with dirty gas.

DIRTY INJECTORS
Because the fuel filter stops dirt before it reaches the injectors, the main cause of injector clogging is not dirt but fuel varnish. When tiny drops of fuel vaporize around the injector nozzle, they leave behind trace compounds that can buildup over time and restrict the nozzle opening. Gasoline is supposed to contain detergents to wash away these deposits and keep the injectors clean. But recent reports indicate that many cut-rate brands of gasoline don't contain adequate levels of detergent for this.

Dirty injectors restrict fuel delivery and cause a lean fuel condition. The result can be rough idle, hesitation and stumbling when accelerating - and even lean misfire. What's the cure? Remove the deposits by cleaning the injectors with a dose of fuel system cleaner. If that doesn't work, a technician can clean the injectors on or off the vehicle using a more powerful solvent. And if that doesn't work, a new set of injectors must be installed.

Another reason for replacing an injector is if the solenoid inside it has failed. The solenoid opens the injector valve when energized by the PCM. The cure for a dead injector (assuming the power and ground connections to it are okay) is a new injector.

Replacement injectors must have the same flow characteristics as the original. Many injectors look the same on the outside but are calibrated differently for different engine applications.















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