Returnless systems are found on many late model cars and trucks. "Returnless" refers to the fact that these systems have the fuel pressure regulator inside the fuel tank. This eliminates the need for a fuel return line from the fuel injector rail on the engine to reroute excess fuel back to the fuel tank.
The first returnless fuel injection systems appeared back in 1993 on certain Chrysler V6 and V8 truck engines. By 1998, all Chrysler cars and light trucks had them. In 1996, Toyota introduced its first returnless system, followed by General Motors and Ford in 1999. Honda went "returnless" in 2001, and today you'll find returnless fuel injection systems on almost all new vehicles.
In older return-type systems, the fuel pump delivers more fuel to the engine than it actually needs. The excess fuel is then routed back to the fuel tank through a pressure regulator and return line. This can increase the temperature of the fuel because of the heat it picks up while circulating through the fuel rail in the engine compartment. Eliminating the return line keeps fuel temperatures lower and more consistent for better fuel economy and emissions.
FUEL SYSTEM OPERATION A returnless fuel injection system manages fuel pressure a little differently. Instead of using a spring-loaded vacuum diaphragm in the regulator to change fuel delivery when throttle opening and intake vacuum change, the regulator in a returnless system operates at a constant pressure. The older return-type systems need to vary fuel pressure to maintain the same pressure differential across the injectors when intake vacuum drops. When vacuum drops, the regulator increases pressure to compensate. But in a returnless system, this isn't necessary because the line pressure is always the same. So how does the system compensate for changes in engine load and vacuum? A returnless system uses the Powertrain Control Module (PCM) to regulate fuel delivery. A fuel pressure sensor mounted on the supply rail allows the PCM to monitor fuel pressure. When pressure in the supply rail drops as engine load or speed increase, the PCM compensates by increasing injector duration (on time) and/or the operating speed of the fuel pump. Some systems (Ford, for example), vary the fuel pump's output by changing the voltage supply to the fuel pump module. When more fuel is needed, pump speed is increased by increasing the pulse-width (on-time) of the pump's voltage signal (pulse-width modulation). WHY RETURNLESS FUEL INJECTION? The older return-type fuel injection systems circulate a lot of fuel between the engine and tank. This keeps the fuel from getting too hot and boiling as it passes through the fuel rail on the engine (which can cause vapor lock and hard starting or stalling on hot days), but it also carries a lot of heat back to the fuel tank. Heat increases fuel vaporization inside the fuel tank, and puts more of a strain on the Evaporative Emissions (EVAP) control system. The EVAP system's job is to contain fuel vapors so they do not escape from the fuel tank and pollute the atmosphere. Fuel vapors pass through a vent hose to a charcoal-filled canister which temporarily traps and stores the vapors. Later, the vapors are purged from the canister via a control valve and routed into the engine while the vehicle is being driven. The trouble is, EVAP systems have limited capacity and can only store so much fuel vapor. If the fuel is getting hot and vapor pressure is building up inside the tank, it can saturate the charcoal canister and overload the EVAP system's ability to contain the vapors creating a potential emissions problem. On newer vehicles with OBD II, the onboard diagnostic system is required to monitor the fuel system for vapor leaks. If the fuel in the tank gets too hot and builds up excessive pressure, it may cause a leak that will turn on the Malfunction Indicator Lamp (MIL) and set a diagnostic trouble code (DTC). What's more, the U.S. Environmental Protection Agency tightened the limits for evaporative emissions, making it even more important to control fuel vapor pressure inside the tank. With returnless systems, there is no return line and no circulation of fuel back to the fuel tank from the engine. Consequently, there is no heating of the fuel in the tank and no increase in fuel vapor pressure from driving the vehicle. This reduces the risk of excessive pressure build up inside the fuel tank, vapor leaks, and triggering an OBD II EVAP monitor DTC.
OTHER RETURNLESS FUEL INJECTION DIFFERENCES Another difference is that returnless systems typically operate at a higher pressure than return-type systems. This is necessary to reduce the risk of fuel boiling and vapor lock in the injector supply rail during hot weather (since there is no recirculation of fuel from the engine back to the tank to keep the supply rail cool). Returnless systems are very sensitive to fuel pressure, and if pressure is more than a few pounds out of specifications, it may be enough to cause a driveability or emissions problem. Fuel pressure checks on returnless systems can be done in the usual way by attaching a gauge to the service valve fitting on the fuel supply rail, or you can hook up a scan tool and read the pressure value via the pressure sensor. Using a fuel pressure gauge to cross-check the accuracy of the electronic reading is a good way to check for a fuel pressure sensor that is out of calibration. Remember, returnless systems are designed to operate at a constant pressure. A simple pressure check with a gauge or scan tool will tell you if the system is within specifications. Pressure should also be monitored with a scan tool while driving the vehicle to check for pressure loss under load. If the operating pressure is out of range, the PCM will compensate by increasing or decreasing the short term fuel trim (STFT) and long term fuel trim (LTFT) values. As a rule, these numbers should usually be within plus or minus 10 points. If you see a higher or lower value on your scan tool, it may indicate a fuel mixture problem due to incorrect fuel pressure (bad fuel pressure sensor, bad fuel pressure regulator, a weak pump or low pump voltage), or an air leak, or dirty fuel injectors. VOLUME JUST AS IMPORTANT AS PRESSURE Fuel volume is just as important as pressure in all fuel injection systems. The pump has to push enough volume of fuel to keep up with the engine's demands when it is under load, accelerating hard or running at wide open throttle. A weak pump may still produce enough pressure to be within specifications at idle, but may not deliver enough fuel at high rpm and load, causing fuel starvation, lean misfire and a loss of power. As a rule, a "good" pump will deliver at least 750 ml (3/4 quart) of fuel in 30 seconds. Sometime a low pressure or volume problem isn't the fuel pump but a clogged filter, plugged fuel inlet filter sock, restricted fuel line or a faulty fuel pressure regulator. A low voltage supply to the fuel pump due to a wiring problem, low charging voltage or bad relay may also prevent the pump from operating at normal speed.
Aucun commentaire:
Enregistrer un commentaire