general guide to engine management

[Retro-Man]

complete engine management explanation

I,ve already posted this in a different topic but thought people might find it useful or informative so have placed it here where it is most likely to be found by anybody interested

not written by me

The Basics of Power
Before we start upgrading a fuel-management system, it is very important to understand the dynamics of why an engine makes power. Simplified to a level that even Brauer-boy can understand, an engine requires three things to run: air, a certain percentage of fuel, and spark. An engine is most efficient when it burns every molecule of air and fuel that enters the cylinders. "Stoichiometry" is a chemical term that means the most complete combustion will take place. For gasoline, stoichiometry is 14.7 parts of air to one part of fuel by weight. Keep in mind this is only for gasoline. Other fuels like methanol will have different stoichiometric numbers. Car manufacturers usually like to have the air/fuel ratio at stoichiometric. Maintaining this number by examining the content of exhaust gasses is an oxygen sensor’s main purpose in life. If the oxygen sensor detects a lean ratio (meaning not enough fuel) at light throttle, then the computer will have the injectors add more fuel. If the sensor detects too rich of a mixture, the injectors will be asked to reduce fuel.

There are many cases where an engine will not have a 14.7:1 ratio. Start-up and warm-up are prime examples, as more fuel is needed in cold temperatures. Even when warm, an engine will only use a stoichiometric ratio in cruise or light-throttle conditions. At full throttle or for maximum power an engine will use a richer ratio. A turbo engine under boost will also use a richer mixture. A 14.7 mixture would cause a turbo motor to detonate.

OK, so we have air and fuel in the combustion chamber. Now what? It's time to light it off. The idea here is that expanding gasses and the pressure of the burning fuel and air will push the pistons down in the cylinder, turn the crank-shaft, and transmit power to the wheels. But when, during the combustion cycle, should the pressure occur? At some point in the history of the internal combustion engine, someone figured out that the best time for maximum pressure to occur is between 12 and 14 degrees of crankshaft rotation after top dead center (TDC). If something isn't set up right and maximum pressure occurs when the piston is at TDC, the rod journal of the crankshaft will be aligned with the centerline of the crank. The result is energy directed at the main bearings, rod bearings, the block, and the cylinder head instead of making the crank rotate. This means the engine will be trying to push its crank out of the bottom or lifting the head off the block. If maximum pressure occurs beyond 12-14 degrees after top dead center (ATDC), the piston will be too far away, the pressure will be lost, and the engine will not be efficient.

If this is difficult to understand, try picturing a stationary mountain bike that you are about to start pedaling. If the crank pedals are perpendicular to the ground (identical to an engine's TDC), the force from your legs is directed at the bike's crank assembly instead of making you go forward. But if you rotate a pedal slightly forward, your leg will be able to exert its maximum pressure on the pedal. And just like a car's engine, if the pedal is moved too far down, the force of your leg is lost.

So now we know the optimum time for maximum cylinder pressure to occur. The only problem is the time between the spark plug's firing and when maximum pressure is achieved. It takes time for the flame to travel through the combustion chamber and that time is constantly changing. It's dependent on a number of factors such as the density of the air, how much air and fuel are being ingested, engine load, and the air/fuel ratio. The more air and fuel in the chamber, the quicker it will burn. For example, at 3,000 rpm at light throttle a car might need as high as 40 degrees of timing. At 3,000 rpm and full throttle, a car might need as low as 30 degrees of timing.

What Is EFI?
An engine’s fuel injection system must manage three things: how much air an engine has, how much fuel is needed to mix with the air (dependent on conditions), and what the proper timing for the ignition of the mixture will be. All the basics of power that dictate how well an engine performs are controlled by a modern car’s EFI system. For example, let’s say your car has a turbocharged engine and at 4,000 rpm with full boost it will require 18-20 degrees of timing. The extra amount of air and fuel provided by the turbo result in a faster burn of the mixture. But with no boost and light throttle at the same rpm, 40 degrees of timing would be needed for the engine to operate properly. That’s quite a bit of timing range to cover and it’s up to the car’s EFI to figure it all out. That’s why today’s cars are so efficient. Old vacuum-advance distributors simply don’t have that type of range.

We've discussed timing a little, so now we'll blab about air. The computer must know how much air is entering an engine so it can tell the injectors how much fuel is needed. There are a few different ways for an engine to measure the amount of incoming air.

Mass-flow fuel-injection systems use a mass air sensor (MAS) to measure the mass of the air entering the engine. Most MAS devices measure the amount of air by directing air past a heated wire that is part of an electronic circuit. Air flowing across the wire draws away some of its heat and an increase in electrical current is required for it to maintain its fixed temperature. The current necessary to heat the wire is proportional to the mass of air flowing across the wire. Most mass-flow fuel-injection systems measure the air directly, so there is no need for the engine's computer to correct for air density. Once the computer knows the amount of air entering the engine, it looks at the other sensors to determine the engine's current state of operation (idle, acceleration, cruise, deceleration). It then refers to an electronic table or map to find the appropriate air/fuel ratio and selects the correct fuel-injector pulse width. A couple of drawbacks to a mass flow system include its price and overall design, which can restrict airflow in high-horsepower engines.

The other popular method of determining airflow is a speed density system. Unlike mass flow systems, there isn't an airflow meter that can cause airflow restriction. Speed density fuel injection systems use the speed of the engine, a measurement of manifold vacuum, and the density of the air to calculate engine air flow. This is accomplished by using a manifold absolute pressure (MAP) sensor and a pre-determined table of how efficient an engine is at flowing air in all conditions. The inherent problem with the table is that it's created at the factory and is based on a new, stock engine. The table of volumetric efficiency does not take into account wear-and-tear of an engine or if an intake or exhaust manifold was changed. To compensate for this, a speed density system uses an oxygen sensor to measure the air/fuel ratio. If the sensor is reading any errors, then the computer will correct fuel delivery of the injectors.

An injector is the engine component that allows fuel to be applied into the combustion chamber. Most import cars have one fuel injector for each cylinder. An injector works by having an internal plunger that is activated by the application of voltage. When the plunger is activated, an opening is created, allowing pressurized fuel to flow past it. An injector's primary concern is fuel delivery in all types of conditions. Fuel flow is controlled by varying the pulse width or duty cycle of the injector. Pulse width is the time in milliseconds that the injector is open, while duty cycle is the injector's overall percentage of open time.

That's a basic overview of the components in a fuel-injection system. It's important to have a background on how it works, but it's also pretty boring. What can be done with fuel injection to give your car more power is much more interesting.

Let us preface this by saying that the fuel-injection systems on modern cars are very efficient. Honda, Mazda, Mitsubishi, and everybody else have spent a lot of money and time perfecting their fuel delivery systems. A stock car and a stock EFI system work very well together. There's no real reason to modify the combo. But when you start adding aftermarket parts, things change. A stock computer should be able to handle the first steps to increased engine performance like an air intake or header. But what about a turbo or nitrous? Did the engineers who created the EFI system on a Civic foresee an owner bolting on such a thing? Don't think so. At this point an aftermarket EFI system must seriously be considered.

Chips
An aftermarket chip works by optimizing the timing and air/fuel ratio of an engine. Some car models will benefit more from a chip because they have more conservative EFI programming. German cars like VWs and BMWs are stereotypically conservative and will respond better to a chip. But if a car already has an optimized EFI setup, it will be difficult for a chip manufacturer to improve the stock EFI programming. Chips are also somewhat limited since they are only as good as the car they were programmed on. Every engine is different. If you change the setup of your car, such as adding camshafts, then the chip will no longer be calibrated properly.

Injectors
An aftermarket air intake or header will require the computer to add more fuel. Stock injectors can add more fuel but only up to a certain point. Increasing fuel pressure is an option, but increasing the fuel pressure changes the calibration of the injector. An EFI system bases its calculations on the known calibration of the injector. If the injector calibration is changed, the computer won’t know this and will create a fuel curve most likely detrimental to performance. The only way around this is modifying the stock computer or adding larger injectors.

Choosing the correct-size injector is always difficult. A balance must be struck between having enough fuel for full-throttle acceleration runs and being able to cut fuel output for part-throttle puttering around town. More fuel is not always better. If there is too much fuel, the engine will idle poorly or possibly even refuse to start. This is because many larger injectors will not operate with an adequately short duty cycle to lean the air/fuel mixture enough at idle.

Lucas injectors are a popular aftermarket choice for upgrading injectors. Instead of having a pintle-type design that factory injectors have, Lucas injectors have a plate-type design. The plate design allows the injector to operate with a much shorter duty cycle while still being able to provide enough fuel for maximum power. Lucas injectors also work much better with turbocharged engines because they are more resistant to the heat that a turbo creates.

Even if your engine is stock, increased performance can come from balancing and calibrating the injectors. Injectors can be defective right from the factory or become clogged after a few years of use. A clogged or improperly adjusted injector will create an uneven distribution of fuel mixture between the cylinders. If one cylinder is lean, the computer will retard the timing for the entire engine, meaning the engine will lose a lot more power than it should.

Aftermarket EFIs
Stock computers do have limitations. The engineers of a Honda Civic’s computer probably never thought that a turbo would be bolted on. Consequently, the computer doesn’t recognize the changes that a turbo creates and problems will arise if the engine is boosted too much.

This is when an aftermarket EFI system should be used. Aftermarket systems allow the complete customization of an engine's timing, air measuring, and fuel delivery. For example, consider you have added a 90hp nitrous system. As we said earlier, maximum cylinder pressure should occur at 10-14 degrees after top dead center. With nitrous, more fuel and oxygen will be added and cause the flame front to travel faster, meaning the timing must be retarded, but only when the nitrous is flowing. Driving around town without nitrous and with retarded timing will translate into a pig of a car. Now consider the same car but with ACCEL's DFI system installed. The DFI system can be setup to progressively retard the timing as nitrous and fuel are applied.

Some of the more popular aftermarket systems are ACCEL's DFI system, Electromotive's TEC system, or a Motec system. With any of these systems, changing the air/fuel ratio and timing can be done with just a few computer keystrokes. Wink
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