Thread: EFI basics
07-26-2007, 03:45 PM #1
This is a basic article explaning (3) types of 4 stroke fuel injection systems out there. Pay close attention to the highlighted section.
Once upon a time an engine needed three things to run: fuel, air, and fire. Thatís what carbs, coils, and distributors are for. Modern EFI engines still need these three elements, but they use different hardware to provide them, and a computer to run the whole process.
Todayís electronic engine management systems can process millions of instructions per second to continuously adjust spark and fuel for optimum performance. The computer regulates the electronic fuel-injector pulse width (the time that the fuel injector is open) and ignition lead with input from various sensors. One of the key things the computer needs to know is how much air the engine consumes under a given set of conditions. Three different measurement strategies have evolved to supply the computer with this basic information; in order of sophistication they are: N Alpha, Speed Density, and Mass Flow metering.
A relatively simple design, N Alpha systems use only engine speed and throttle angle to calculate the amount of fuel needed by the engine. This system doesnít measure airflow directly; instead, engine load is assumed based on throttle-angle versus engine rpm. The various load-rpm points make up the computerís lookup table, with the amount of fuel needed at each point manually programmed by the tuner. N Alpha systems work well on engines that operate primarily at wide-open throttleósuch as race carsóbut are much less accurate at part-throttle than more sophisticated systems because of their relatively simple fuel map. They generally do not have a closed-loop mode for air/fuel correction, resulting in part-throttle calibration that is crude at best when compared to other systems. This also makes them incompatible with modern catalytic converters. Any significant engine change requires remapping.
Speed Density systems accept input from sensors that measure engine speed (in rpm) and load (manifold vacuum in kPa), then the computer calculates airflow requirements by referring to a much larger (in comparison to an N Alpha system) preprogrammed lookup table, a map of thousands of values that equates to the engineís volumetric efficiency (VE) under varying conditions of throttle position and engine speed. Engine rpm is provided via a tach signal, while vacuum is transmitted via an intake manifold-mounted Manifold Air Pressure (MAP) sensor. Since air density changes with air temperature, an intake manifold-mounted sensor is also used.
Production-based Speed Density computers also utilize an oxygen (O2) sensor mounted in the exhaust tract. The computer looks at the air/fuel ratio from the O2 sensor and corrects the fuel delivery for any errors. This helps compensate for wear and tear and production variables. Other sensors on a typical Speed Density system usually include an idle-air control motor to help regulate idle speed, a throttle-position sensor that transmits the percentage of throttle opening, a coolant-temperature sensor, and a knock sensor as a final fail-safe that hears detonation so the computer can retard timing as needed.
Because a Speed Density system still has no sensors that directly measure engine airflow, all the fuel mapping points must be preprogrammed, so any significant change to the engine that alters its VE requires reprogramming the computer.
By contrast, Mass Air Flow (MAF) systems use a sensor mounted in front of the throttle body that directly measures the amount of air inducted into the engine. The most common type of mass-flow sensor is the hot wire design: Air flows past a heated wire thatís part of a circuit that measures electrical current. Current flowing through the wire heats it to a temperature that is always held above the inlet air temperature by a fixed amount. Air flowing across the wire draws away some of the heat, so an increase in current flow is required to maintain its fixed temperature. The amount of current needed to heat the wire is proportional to the mass of air flowing across the wire. The mass-air meter also includes a temperature sensor that provides a correction for intake air temperature so the output signal is not affected by it.
The MAF sensorís circuitry converts the current reading into a voltage signal for the computer, which in turn equates the voltage value to mass flow. Typical MAF systems also use additional sensors similar to those found in Speed Density systems. Once the electronic control module (ECM) knows the amount of air entering the engine, it looks at these other sensors to determine the engineís current state of operation (idle, acceleration, cruise, deceleration, operating temperature, and so on), then refers to an electronic map to find the appropriate air/fuel ratio and select the fuel-injector pulse width required to match the input signals.
07-26-2007, 04:00 PM #2
So then the TPS only affects fuel curve not timming?
07-26-2007, 04:05 PM #3
07-26-2007, 04:07 PM #4
SO the TPS should have really little to no affect on the top RPM's (assuming you retune your EFI)??
07-26-2007, 04:14 PM #5
07-26-2007, 04:17 PM #6
07-26-2007, 04:48 PM #7
07-26-2007, 05:35 PM #8
The big question here is what does the controller do to give us the ability to control fuel ? Obviously its manipulating input signals to the ECU. Maybe it uses the TPS signal already ? I dont know enough about this system and dont write software or fuel maps, so I cant really explain why, just that I tried it and it must be on the factory spec to run correctly.
07-26-2007, 06:07 PM #9
to bad macboost or advent did not do as promised. it would change everything. good post Rich my brain is hurting now
07-26-2007, 06:09 PM #10
Ok good enough
just trying to understand...
I bet adjusting the TPS also plays with timming
I bet the EFI just opens the pulse width on the injectors....
Many conpanys make univeral injector controlers...
Last edited by r33pwrd; 07-26-2007 at 06:13 PM.
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