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ECU Inputs
In this chapter we will cover some of the basic working components of the EFI system and the theory behind how they work independently and as part of the entire system. We will begin with a look at the inputs the ECU uses to collect information, and then in the next chapter we will look at the outputs, which it uses to carry out its commands.
There are many different sensors and actuators that make up the components of a fuel-injection system. In this chapter we will take a look at some of the more common ones used in most aftermarket EFI systems.
MAP Sensor
The MAP (manifold absolute pressure) sensor is most commonly used as the primary input for determining the engine’s load. The term load in this instance describes how hard the engine is working at any given engine speed. In other words, the amount of air that flows through the engine per cycle does not necessarily increase simply because of an increase in engine speed. The overall amount of air moving through the engine is greater for a given amount of time, but the amount per cycle doesn’t vary greatly unless there is a change in engine load.
As an example, picture a car sitting at idle in neutral. In order to get the engine to climb up to say, 5,000 rpm, the operator need only push down very lightly on the accelerator pedal. Now picture the same car in overdrive going 20 miles per hour and going up a steep hill. In order to get the engine speed up to 5,000 rpm the accelerator pedal will need to be pushed to the floor and the engine will be working much harder because the load on the engine is greater for the same engine speed.
The term MAP represents manifold absolute pressure — the amount of vacuum or boost inside the manifold at any given time. The MAP sensor uses absolute pressure because this type of reading will always be constant and will not be affected by changing atmospheric conditions like a normal pressure gauge would.
The MAP, manifold absolute pressure, is the amount of vacuum or boost inside the manifold at any given time. The sensor uses absolute pressure because this type of reading will always be constant and will not be affected by changing atmospheric conditions like normal gauge pressure would.
Before we go any farther it will be useful to briefly touch on what absolute pressure is and what the units of measurements are that represent it. Normal atmospheric pressure is rated in units of pounds per square inch, or psi. The pressure at sea level is 14.7 psi. As the altitude changes, so does the density of the air. An increase in altitude produces a decrease in air density and a corresponding decrease in the measured pressure. It is also worth mentioning that as the altitude increases, the temperature of the air decreases in direct proportion. The air’s altitude, pressure, density, and temperature are all related, and a change in any one of these factors affects all the others.
The MAP sensor consists of a flexible diaphragm that conducts electricity. The resistance of the diaphragm changes when it flexes or bends. When pressure is applied to the MAP sensor, it causes the diaphragm to flex. The resistance acting on the electricity passing through the sensor changes in proportion to the actual pressure acting on it, and the computer is able to read the changes in the signal that returns to the ECU.
If we have a tire pressure gauge and we connect it to a tire that reads 32 psi, we can say that the tire is filled to a pressure of 32 psi. This number is referred to as the gauge pressure. The absolute pressure that actually exists inside the tire would be the atmospheric pressure plus the gauge pressure. So, if we measure the tire’s absolute pressure at sea level the tire would have an absolute pressure of 14.7 + 32 = 46.7 psi absolute.
It is important that we only use absolute pressure when dealing with calibrating our ECU. That way, when the vehicle is operated in different geographic areas, at different altitudes and air densities, the calibration for any given manifold absolute pressure will remain the same.
Again, it is important to remember here that the density of any quantity of air is directly related to its absolute pressure. The higher the pressure, the denser the air will be for that given volume. This is easily recognizable in engines that are supercharged or turbocharged. The more boost you have to pressurize the engine with, the more total airflow it uses, and thus the more power it makes.
It will help here to also understand that nearly all automotive computers measure absolute pressure in metric units of bars, or kPa, meaning kilopascals. The reason for doing this is because when the absolute pressure inside the engine is less that 0 psi, it is operating in a vacuum. When discussing amounts of vacuum, we typically use terms like inches of mercury, or InHg, and when we talk about positive pressure or boost in an engine, we use pounds per square inch, or psi. It can be very tricky and time consuming to constantly convert these units to make them something easy to understand.
Instead, we use metric units of pascals. A pressure of 14.7 psi is considered to be one atmosphere, or 100 kPa. One kPa represents 1,000 pascals. Also a pressure of 14.5 psi is equal to one bar, so the two terms are very closely related in actual measurements, and are often used in place of each other. A bar of boost, which indicates a manifold pressure of roughly 14.5 psi gauge pressure or 25.2 psi absolute (at sea level). These measurements are also the same as having 200 kPa of manifold pressure.
Manifold absolute pressure directly relates to the amount of air that exists inside an intake manifold. A higher MAP value indicates greater airflow, and thus greater engine load, while a smaller MAP number indicates less load and airflow in the engine.
When referring to automotive ECUs, we typically only use measurements of kPa. Thus, an engine operating in vacuum would have a manifold absolute pressure of something less than 100 kPa, and when the engine is at wide open throttle in a state where it has no vacuum or boost, it would have a MAP value of about 100 kPa. When the engine sees positive pressure, or boost, the MAP signal would be higher than 100 kPa. This means that a gauge pressure in the manifold of 14.5 psi of boost would be equal to 200 kPa, and a gauge pressure of 29 psi would net a MAP value of 300 kPa and so on.
All this may sound complicated, but it helps to just remember that less than 100 kPa is vacuum, and more than 100 kPa is boost, and every 100 kPa is equal to about 14.5 psi of positive gauge pressure.
Now we can begin to discuss the actual method the MAP sensor uses to relay information to the ECU. The actual MAP sensor signal is an electrical value sent from the sensor to the ECU. The ECU determines the amount of pressure inside the engine from that signal.
The MAP sensor consists of a flexible diaphragm that conducts electricity. The resistance of the diaphragm changes when it flexes or bends. So, one side of this diaphragm is exposed to a closed circuit leading to the manifold pressure signal coming from the engine. When the pressure is applied to the MAP sensor, it causes the diaphragm to flex an amount that is proportional to the actual pressure acting on it.
The ECU sends a small amount of voltage, usually 5 volts, to the sensor and then measures the amount of voltage that returns to it through the resistance of the MAP sensor. The more pressure that is applied to the sensor, the more its resistance value changes. This now affects the amount of voltage that is sent back to the ECU. The sensor’s resistance is carefully calibrated to be exactly the same value for any given amount of absolute pressure so that the ECU can mathematically calculate the pressure against it for any voltage value.
Once the ECU receives the electrical signal from the MAP sensor, it uses that value, along with values from several other sensors, such as the engine speed, water temperature, and air temperature sensors, to determine the actual density and mass of air inside the engine. It does this using a property of mechanical physics called PV=NrT.
This equation explains how the computer knows how much air is in the engine and is the very essence of our EFI system.
P: represents the absolute pressure in the engine
V: represents the volume we are measuring
N: represents the mass of air (which is what we are trying to find)
R: represents a real gas constant for air, which is used in all air calculations (287.05 for dry air)
T: represents the absolute temperature of the volume being measured
If we can produce all but one of these variables, we can then apply them to this formula to find the missing one. In our case, we have pressure from the MAP sensor (P), we have volume (V), which is the size of our manifold, and we have temperature from our intake air temperature sensor (T), which we will discuss shortly. The R is a constant number, called the real gas constant, which is always the same for air. So the only thing we don’t know is the mass (N).
Once we figure out the mass, we will have a quantity, which can be easily related to other quantities like the amount of fuel. Then we can obtain an air/fuel ratio. It is this method that the ECU uses to determine how long to turn on our injectors to obtain the correct mixture of air and fuel for the engine to operate properly.
Using the PV=NrT formula, and ECU can calculate how much air there is inside the manifold. Although you won’t actually need to know how to use this formula, it is fun to at least understand how it all works! Things happen fast inside a manifold like this one, so if you use the formula to solve for one of the variables, the answer you get will only be true until the throttle plate opens or some of the intake charge gets sucked into one of the cylinders.
Take heart my friends in knowing that you will never need to actually do any of these calculations in the process of building and tuning your own EFI system, as this is all being done behind the scenes in the software for us by the lovely engineers who manufacture these systems. I just felt it necessary for you to understand the basic concepts behind how the system works in order to really appreciate the value of each component!
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Table of Contents |
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Chap. 1 - Carburetors vs. Fuel Injection |
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Chap. 2 - The Basics of Electronics |
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Chap. 3 - Tools and Equipment |
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Chap. 4 - ECU Inputs |
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Chap. 5 - ECU Outputs |
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Chap. 6 - Tuning Maps and Basic Engine Calibration |
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Chap. 7 - ACCEL/DFI |
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Chap. 8 - AEM Plug & Play |
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Chap 9 - Autronic |
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Chap. 10 - Edelbrock Pro-Flo and Advanced Programmable Fuel-Injection Systems |
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Chap. 11 - EFI Technology |
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Chap. 14 - Haltech |
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Chap. 15 - Holley Commander 950 |
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Chap. 16 - MoTeC |
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Chap. 17 - Simple Digital Systems (SDS) |
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