FUEL INJECTION PART II

Let’s continue from where we left yesterday and delve a little deeper to understand how the ECU does its calculations.

The engine control unit uses a formula and a large number of lookup tables to determine the pulse width for given operating conditions. The equation will be a series of many factors multiplied by each other. Many of these factors will come from lookup tables. We'll go through a simplified calculation of the fuel injector pulse width. In this example, our equation will only have three factors, whereas a real control system might have a hundred or more.

Pulse width = (Base pulse width) x (Factor A) x (Factor B)

In order to calculate the pulse width, the ECU first looks up the base pulse width in a lookup table. Base pulse width is a function of engine speed (RPM) and load (which can be calculated from manifold absolute pressure). Let's say the engine speed is 2,000 RPM and load is 4. We find the number at the intersection of 2,000 and 4, which is 8 milliseconds.

In the next examples, A and B are parameters that come from sensors. Let's say that A is coolant temperature and B is oxygen level. If coolant temperature equals 100 and oxygen level equals 3, the lookup tables tell us that Factor A = 0.8 and Factor B = 1.0.

So, since we know that base pulse width is a function of load and RPM, and that pulse width = (base pulse width) x (factor A) x (factor B), the overall pulse width in our example equals:

8 x 0.8 x 1.0 = 6.4 milliseconds

From this example, you can see how the control system makes adjustments. With parameter B as the level of oxygen in the exhaust, the lookup table for B is the point at which there is (according to engine designers) too much oxygen in the exhaust; and accordingly, the ECU cuts back on the fuel. Real control systems may have more than 100 parameters, each with its own lookup table. Some of the parameters even change over time in order to compensate for changes in the performance of engine components like the catalytic converter. And depending on the engine speed, the ECU may have to do these calculations over a hundred times per second.

This leads us to discussion of performance chips. Now that we understand a little bit about how the control algorithms in the ECU work, we can understand what performance-chip makers do to get more power out of the engine. Performance chips are made by aftermarket companies, and are used to boost engine power. There is a chip in the ECU that holds all of the lookup tables; the performance chip replaces this chip. The tables in the performance chip will contain values that result in higher fuel rates during certain driving conditions. For instance, they may supply more fuel at full throttle at every engine speed. They may also change the spark timing (there are lookup tables for that, too). Since the performance-chip makers are not as concerned with issues like reliability, mileage and emissions controls as the carmakers are, they use more aggressive settings in the fuel maps of their performance chips.

Various Injection Schemes

Multi-Port Fuel Injection (PFI or EFI or SEFI):
The goal of all fuel injection systems is to carefully meter the amount of fuel to each cylinder. On most gasoline applications, the system uses a single injector per cylinder and injects fuel immediately ahead of the intake valves.

Direct Injection:
Recently many diesel engines feature direct injection (DI). The injection nozzle is placed inside the combustion chamber and the piston incorporates a depression (often toroidal) where initial combustion takes place. Direct injection diesel engines are generally more efficient and cleaner than indirect injection engines, but tend to be noisier, which is being addressed in newer common rail designs.

Some recently designed hi-tech petrol engines utilize direct injection as well. This is the next step in evolution from multi port fuel injection and offers another magnitude of emission control by eliminating the "wet" portion of the induction system.

That’s it. I think after this you should not feel uncomfortable when the salesman talks to you about fuel injection and emmission control (infact you could teach him a thing or two now). Anyways if you want to know more or have any queries about the info above please feel free to mail me. Till next time them.

TECH STUFF: FUEL INJECTION

In this two part series i'll try and explain the modern fuel injection systems which have replaced carburetors (in India only in cars till date). I'll try and keep it as simple as possible so that everyone can appreciate it despite his/her engineering info level. Hope you find it interesting and informative, but as always pls do write back your suggestion/queries to me.

For most of the existence of the internal combustion engine, the carburetor has been the device that supplied fuel to the engine. On many other machines, such as lawnmowers and chainsaws, it still is. But as the automobile evolved, the carburetor got more and more complicated trying to handle all of the operating requirements. For instance, to handle some of these tasks, carburetors had five different circuits:

1. Main circuit - Provides just enough fuel for fuel-efficient cruising.
2. Idle circuit - Provides just enough fuel to keep the engine idling.
3. Accelerator pump - Provides an extra burst of fuel when the accelerator pedal is first depressed, reducing hesitation before the engine speeds up.
4. Power enrichment circuit - Provides extra fuel when the car is going up a hill or towing a trailer.
5. Choke - Provides extra fuel when the engine is cold so that it will start.

In order to meet stricter emissions requirements, catalytic converters were introduced. Very careful control of the air-to-fuel ratio was required for the catalytic converter to be effective. Oxygen sensors monitor the amount of oxygen in the exhaust, and the engine control unit (ECU) uses this information to adjust the air-to-fuel ratio in real-time. This is called closed loop control - it was not feasible to achieve this control with carburetors. There was a brief period of electrically controlled carburetors before fuel injection systems took over, but these electrical carbs were even more complicated than the purely mechanical ones.

At first, carburetors were replaced with throttle body fuel injection systems (also known as single point or central fuel injection systems) that incorporated electrically controlled fuel-injector valves into the throttle body. These were almost a bolt-in replacement for the carburetor, so the automakers didn't have to make any drastic changes to their engine designs.

Gradually, as new engines were designed, throttle body fuel injection was replaced by multi-port fuel injection (also known as port, multi-point or sequential fuel injection). These systems have a fuel injector for each cylinder, usually located so that they spray right at the intake valve. These systems provide more accurate fuel metering and quicker response.
The gas pedal in your car is connected to the throttle valve - this is the valve that regulates how much air enters the engine. So the gas pedal is really the air pedal. When you step on the gas pedal, the throttle valve opens up more, letting in more air. The engine control unit (ECU, the computer that controls all of the electronic components on your engine) "sees" the throttle valve open and increases the fuel rate in anticipation of more air entering the engine. It is important to increase the fuel rate as soon as the throttle valve opens; otherwise, when the gas pedal is first pressed, there may be a hesitation as some air reaches the cylinders without enough fuel in it. Sensors monitor the mass of air entering the engine, as well as the amount of oxygen in the exhaust. The ECU uses this information to fine-tune the fuel delivery so that the air-to-fuel ratio is just right.

A fuel injector is nothing but an electronically controlled valve. It is supplied with pressurized fuel by the fuel pump in your car, and it is capable of opening and closing many times per second. When the injector is energized, an electromagnet moves a plunger that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle. The nozzle is designed to atomize the fuel - to make as fine a mist as possible so that it can burn easily. The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU. The injectors are mounted in the intake manifold so that they spray fuel directly at the intake valves. A pipe called the fuel rail supplies pressurized fuel to all of the injectors.

In order to provide the correct amount of fuel for every operating condition, the engine control unit (ECU) has to monitor a huge number of input sensors. Here are just a few:

1. Mass airflow sensor - Tells the ECU the mass of air entering the engine.
2. Oxygen sensor(s) - Monitors the amount of oxygen in the exhaust so the ECU can determine how rich or lean the fuel mixture is and make adjustments accordingly.
3. Throttle position sensor - Monitors the throttle valve position (which determines how much air goes into the engine) so the ECU can respond quickly to changes, increasing or decreasing the fuel rate as necessary.
4. Coolant temperature sensor - Allows the ECU to determine when the engine has reached its proper operating temperature.
5. Voltage sensor - Monitors the system voltage in the car so the ECU can raise the idle speed if voltage is dropping (which would indicate a high electrical load).
6. Manifold absolute pressure sensor - Monitors the pressure of the air in the intake manifoldThe amount of air being drawn into the engine is a good indication of how much power it is producing; and the more air that goes into the engine, the lower the manifold pressure, so this reading is used to gauge how much power is being produced.
7. Engine speed sensor - Monitors engine speed, which is one of the factors used to calculate the pulse width.

There are two main types of control for multi-port systems: The fuel injectors can all open at the same time, or each one can open just before the intake valve for its cylinder opens (this is called sequential multi-port fuel injection). The advantage of sequential fuel injection is that if the driver makes a sudden change, the system can respond more quickly because from the time the change is made, it only has to wait only until the next intake valve opens, instead of for the next complete revolution of the engine. The algorithms that control the engine are quite complicated. The software has to allow the car to satisfy emissions requirements for 100,000 miles, meet EPA fuel economy requirements and protect engines against abuse. And there are dozens of other requirements to meet as well.

In the next part I'll we'll see how is this accurate fuel metering achieved, types of injection schemes and much more.

TECH STUFF: CARBURETOR

The carburetor was invented by the Hungarian engineer Donat Banki in 1893. Fredrick William Lanchester of Birmingham, England experimented early on with the wick carburetor in cars. In 1896 Frederick and his brother built the first petrol driven car in England, a single cylinder 5 hp (4 kW) internal combustion engine with chain drive. Unhappy with the performance and power, they re-built the engine the next year into a two cylinder horizontally opposed version using his new wick carburetor design. This version completed a 1,000 mile (1600 km) tour in 1900 successfully incorporating the carburetor as an important step forward in automotive engineering. The word carburetor comes from the French carburet, meaning ‘carbide’. To carburet means to combine with carbin. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fuel by mixing it with a volatile hydrocarbon.

The carbureter is a device which mixes air and fuel for an internal-combustion engine. Carburetors are still found in small engines and in older or specialized automobiles. However, most cars built since the early 1980s use computerized electronic fuel injection instead of carburetion. The majority of motorcycles still are carburated due to lower weight and cost. Most carbureted (as opposed to fuel-injected) engines have a single carburetor, though some, primarily with greater than 4 cylinders or higher performance engines, use multiple carburetors or multi-choke carburetors (the latter being a number of intakes in a single body). Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. This had the advantage of never "flooding" the engine, as any liquid fuel droplets would fall out of the carburetor instead of into the intake manifold; it also lent itself to use of an oil bath air cleaner, where a pool of oil below a mesh element below the carburetor is sucked up into the mesh and the air is drawn through the oil covered mesh; this was an effective system in a time when paper air filters did not exist. Today, most automotive carburetors are either downdraft (flow of air is downwards) or side-draft (flow of air is sideways).

The carburetor works on Bernoulli’s principle: the fact that moving air has lower pressure than still air, and that the faster the movement of the air, the lower the pressure. Generally speaking, the throottle or accelerator does not control the flow of liquid fuel. Instead, it controls the amount of air that enters the carburetor. Faster flows of air and more air entering the carburetor draws more fuel into the carburetor due to the partial vaccum that is created. The goal of a carburettor is to mix just the right amount of gasoline with air so that the engine runs properly. If there is not enough fuel mixed with the air, the engine "runs lean" and either will not run or potentially damages the engine. If there is too much fuel mixed with the air, the engine "runs rich" and either will not run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel. The carb is in charge of getting the mixture just right.

- A carburettor is essentially a tube.
- There is an adjustable plate across the tube called the throttle plate that controls how much air can flow through the tube. You can see this circular brass plate in pic 1.
- At some point in the tube there is a narrowing, called the venturi, and in this narrowing a vacuum is created. The venturi is visible in pic 2
- In this narrowing there is a hole, called a jet, that lets the vacuum draw in fuel. You can see the jet on the left side of the venturi in pic 2.

The carb is operating "normally" at full throttle. In this case the throttle plate is parallel to the length of the tube, allowing maximum air to flow through the carb. The air flow creates a nice vacuum in the venturi and this vacuum draws in a metered amount of fuel through the jet. You can see a pair of screws on the right top of the carb in photo 1. One of these screws (labelled "Hi" on the case of the chain saw) controls how much fuel flows into the venturi at full throttle. When the engine is idling, the throttle plate is nearly closed (the position of the throttle plate in the photos is the idle position). There is not really enough air flowing through the venturi to create a vacuum. However, on the back side of the throttle plate there is a lot of vacuum (because the throttle plate is restricting the airflow). If a tiny hole is drilled into the side of the carb's tube just behind the throttle plate, fuel can be drawn into the tube by the throttle vacuum. This tiny hole is called the idle jet. The other screw of the pair seen in photo 1 is labelled "Lo" and it controls the amount of fuel that flows through the idle jet.

Both the Hi and Lo screws are simply needle valves. By turning them you allow more or less fuel to flow past the needle. When you adjust them you are directly controlling how much fuel flows through the idle jet and the main jet. When the engine is cold and you try to start it with the pull cord (choke in layman terms), the engine is running at an extremely low RPM. It is also cold, so it needs a very rich mixture to start. This is where the choke plate comes in. When activated, the choke plate completely covers the venturi. If the throttle is wide open and the venturi is covered, the engine's vacuum draws a lot of fuel through the main jet and the idle jet (since the end of the carb's tube is completely covered, all of the engine's vacuum goes into pulling fuel through the jets). Usually this very rich mixture will allow the engine to fire once or twice, or to run very slowly. If you then open the choke plate the engine will start running normally.

This is just the basics. If you want to know more please feel free to mail me. Along with data, I have couple of videos showing the working of a carb and choke plate (with audio explanation), which I can mail to anyone who is interested. This will make it easier for you to visualise the process. Hope this article aided in your understanding of an automobile engine, that would be mission accomplished for me. For more technical articles watch this space (maybe some of you could mail me any particular topic that you want to know about, I’ll be happy to help out).

SCIENTISTS have built the smallest petrol engine - tiny enough to power a WATCH. The mini-motor, which runs for two years on a single squirt of lighter fuel, is set to revolutionise world technology. It produces 700 times more energy than a conventional battery despite being less than a centimeter long - not even half an inch. It could be used to operate laptops and mobile phones for months on end - doing away with the need for recharging. Experts believe it could be phasing out batteries in such items within just six years.

The engine, minute enough to be balanced on a fingertip, has been produced by engineers at the University of Birmingham. Dr Kyle Jiang, lead investigator from the Department of Mechanical Engineering, said: "We are looking at an industrial revolution happening in peoples' pockets. "The breakthrough is an enormous step forward. Devices which need re- charging or new batteries are a problem but in six years will be a thing of the past."

Other applications for the engine could include medical and military uses, such as running heart pacemakers or mini reconnaissance robots. At present, charging an ordinary battery to deliver one unit of energy involves putting 2,000 units into it. The little engine, because energy is produced locally, is far more effective. One of the main problems faced by engineers who have tried to produce micro motors in the past has been the levels of heat produced. The engines got so hot they burned themselves out and could not be re-used. The Birmingham team overcame this by using heat-resistant materials such as ceramic and silicon carbide.

Professor Graham Davies, head of the university's engineering school, said: "We've brought together all the engineering disciplines, both materials, chemical engineering, civil engineering, and mechanical engineering.”What better place to have the second industrial revolution - in nano-technology - than where the first took place, in the heart of the West Midlands."

But it makes you think where are we heading to. I mean tomorrow we just might be driving cars that run from a battery (saying fuel is expensive and it pollutes), and use gadgets that are powered by an internal combustion engine (because batteries are inefficient and have less life)!!! Guess this is what they mean when they say “One man’s dream is other’s nightmare”.

p.s.: sorry for the quality of photograph, couldn't get a better one.