Fuel Injection

There is a reason this in on a black background. To many mechanics who were raised on carburetors, fuel injection is a black art. And to some it still is. Reading this will not enable you to go repair injection systems, but it will give you a little idea of how they operate which can lead you to a point that you have a chance to isolate an injection problem.

I worked for years for Beck Arnley / World Parts Corp. as a hot line answer man. Part of my job was to travel around the country conducting seminars on Import car repair. Our objective was to attempt to get the traditional domestic car mechanic to work on Import cars. Other than a lot of over head cams, the engine was just an engine. So that part was not too much trouble. 

However, when it came to fuel injection, that was another case. Each tech designed his own seminar based on his experience  and on his brands of cars that he was most familiar with. I tried several different methods of training and found one that worked well. 

I used an old single carburetor from an old domestic car that everyone knew well and broke it down into each section of the carburetor. I then took a early version of the Bosch "L" jetronic injection system and showed that each section of the carburetor had an comparable section in the injection system.  I got a good response using this method and was able to cover several of the different systems this way.

I will do the same thing here so I might at least give you some indication how injection systems work. I can't cover all injection systems and all of each system details but knowing how a system works enables one to have a good chance of correcting one when it fails.

All SI (spark ignited) piston engines require the same thing to run so all of the different injection systems must supply those needs in some manner. 

An old carburetor has an idle circuit to meter small amounts of fuel into the small amount of air allowed past the throttle plate. An idle stop screw controls the amount of air that can enter past the throttle plate. This sets the idle RPM. An idle mixture screw sets the quantity of fuel allowed into the air stream.  An injection system has the same thing except most don't use a stop screw to hold the throttle plate open a little,  instead they close the throttle plate and have a small port that bypasses the throttle plate and they  use a adjusting screw to control the amount of air that can bypass the throttle. Some control that small bypass electrically. That now controls idle RPM. Control of idle mixture is done by a bypass of the "L" shaped door in the air flow meter. This door is spring loaded to the closed position and the air entering the intake forces the door open which sends an signal to the ECU or ECM as some like to call it. The ECU then grounds the injectors (usually in groups) for a short time measured in milliseconds.  This controls the mixture at idle. The second bypass screw controls the amount of air allowed to bypass the door, thus controlling idle mixture.

Most carburetors have an accelerator pump to supply extra fuel on a quick throttle opening. The "L" Jetronic injection system also needs extra fuel on a quick throttle opening. To achieve this, they add a vacuum port to the  diaphragm of the fuel pressure regulator. When the throttle is opened quickly, the manifold vacuum drops and the regulator increases the fuel pressure. Supplying extra fuel just like an accelerator pump does.

As the air flow  increases in a carburetor another jet system is used in a carburetor and because the air flow is not very high yet, the fuel that comes out of the jet is not well atomized so an emulsion tube / jet pre mixes air with the fuel so there is a better atomizing of the fuel.  An injection system usually does not need to add an extra system for this because the fuel is sprayed at a high pressure through the injector thus atomizing the fuel very well.

An engine requires extra fuel per air when cold starting and a carburetor uses a choke to supply this extra fuel. An injected engine also needs this extra fuel for cold starting so the "L" Jetronic system adds a "Cold Start" injector to spray extra fuel into the intake manifold on a cold start. This injector is not controlled by the ECU like the other injectors. It is controlled by a thermotime switch in the coolant system.  The carburetor has a fast idle cam on the throttle linkage to open the throttle plate to set the idle speed higher then a normal idle. Some carburetors control the choke automatically by a bimetal coil heated by a small tube from the exhaust manifold and some use engine coolant to control the choke. The injection system uses a separate item to control fast idle during warm up. It is called an "Aux Air Valve". The Aux Air Valve controls the amount of air allowed to bypass the throttle plate with a pipe and by use of a rotary valve that opens and closes this pipe. The rotary valve is operated by a bimetal plate the is heated by an electric heater around the bimetal plate.

The carburetor uses a main jet to control the mixture at high RPM. The L-Jetronic system controls the mixture by several items. The "L" shaped door in the air flow meter sends a signal to the ECU that there is a large amount of air entering the engine. (Later injection systems use a hot wire in place of the door to send air flow information to the ECU) (There were injection systems that relied on manifold vacuum to determine how much fuel to inject) and the ECU lengthens the pulse duration to allow more fuel to enter through the injectors. In addition to this the throttle is open more so the manifold vacuum has fallen and this raises the fuel pressure.

In a carburetor when the throttle gets close to full throttle a "Power Valve" allows more fuel into the intake. In the injection system a switch attached to the throttle shaft makes a contact so the ECU will again lengthen the pulse duration to allow more fuel in.

Some basic items that need to be tested on most injection systems when you suspect a problem are, fuel pressure ( different on each different type of system) (a low of 14 PSI on some on up to a high of 80 PSI on CIS injection) Injector pulse needs to be tested. (a inexpensive "Noid" light can detect power and grounding pulse) Most multi port injectors are either powered by 12 volts or by a resistor pack and grounded by the ECU in short pulses to control the mixture. Most of the various components of the injection system can be tested by a volt/ohm meter. Most injection systems rely heavily on a coolant temperature sensor.

The injection system has added a few extra items that earlier carburetors didn't have until much later. These include, a coolant temp sensor, altitude compensation, feed back info from the exhaust and better load compensation info. The injection system has greatly improved efficiency and in a few cases improved power a little.

Even though the fuel delivery system has improved a lot, the engine itself has changed little in basic design. Subtle changes and advancement in metals has enabled the piston engine to produce much more power than a few decades ago. However the piston engine is still held back by the quality of the fuel.

The quality of the fuel has a lot to do with it's BTU or Caloric value and other properties. For example gasoline has more BTUs than alcohol so one might think that a gas engine should out power an alcohol engine. This is true if both fuels were used in an engine designed for gasoline. If you take the best gas engine and put alcohol in it, then you will loose power and mileage. However, if you modify the engine to take advantage of another property of alcohol (octane rating and oxygen in the fuel) , the story reverses and the alcohol engine will by far out power the gas engine.

Injection systems have evolved since then but still need to address the needs of the engine. No matter how well we meter and atomize and vaporize the fuel you still are held back by what power is available from the fuel.

It will be a race as to weather we run out of gasoline before we squeeze the last little bit of power available in the gasoline.

Tech 12

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