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"Everything You Always Wanted to Know About E.F.I. But"
Posted by Uptight Motorsports Nerd on June 17, 2010
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Authors note: The complete title of this article was to be "Everything NASCAR Fans wanted to know about Fuel Injection (But Were Afraid to Ask)"; however, I am limited to 60 characters.
Carburetors are fickle things. They deliver fuel to an intake manifold much in the way that a toilet delivers water to (you know). A hinged float will raise and lower a metering rod in a jet, controlling the rate at which fuel (under about 5psi) enters the bowl. Vacuum from the engine will draw fuel from the float through a variety of hydraulic circuits that provide fuel as vacuum is applied. That is an oversimplified way of describing carburetors in 67 words.
Fuel injection (electric, multi-point fuel injection in this case) is somewhat simpler. Fuel (at about 60 psi) is squirted into the cylinder head intake port by a computer controlled injector. The computer fires the injector when sensors on the crank and camshafts locate a piston?s intake stroke. The time spent firing is determined by throttle position, manifold pressure, and temperature. That is an oversimplified way of describing fuel injection in 48 words. Such relative simplicity has let me to be amazed by the proliferation of carburetors.
The first big difference you noticed (assuming you are already familiar with such terms as "manifold" and "cylinder head") is the difference in fuel pressure. The carbureted engine only needs its fuel pump to move fuel from the tank to the carburetor. This fuel pump can be mechanically driven, meaning that it will start and stop with the crankshaft. The fuel injection pump plays a role in how fast fuel can travel through an injector; however, to make 60 psi the pump must be driven by an electric motor. This has been raised as a safety concern as an electric motor does not necessarily shut off with the engine. This issue is easily addressed by installing inertia switches that would shut off the fuel pump in a hard crash. Frankly, I have never heard of this being a problem with the millions of fuel injected cars on the road.
The second big difference is where the fuel is delivered. When fuel enters the manifold from the carburetor, it must pass through the entire manifold. This means the manifold is charged with the task of keeping fuel atomized. If the inside of the manifold is smooth, fuel forms liquid drops and air flows freely; inversely, an abrasive manifold slows the movement of air but keeps fuel atomized. Since the fuel injected manifold only moves air, fuel atomization is not an issue. Without millions of dollars being spent on finding the perfect intake manifold for each rpm range, fuel injection proves itself to be less expensive. Again, this is oversimplified as it ignores the effects of the shape and length of the intake manifold?s runners as well as the speed of the air in the manifold.
The third big difference is the presence of a computer. In order to fire each fuel injector at the proper moment, the computer must know the location of each piston in relation to its cylinder and what stroke it is on. This information is gained through sensors (either analog permanent magnet sensors or digital Hall effect sensors) located on the crankshaft and camshaft (take that, Heisenberg). The amount of fuel to be delivered is based on the density of the air being drawn into the engine and the driver?s demand, and this data may be monitored in real time to maximize performance in different atmospheric conditions. Carburetors are only adjusted as fine as they were when they were last jetted. In high performance applications, unexpected changes in weather can alter the air/fuel ratio in ways that a carburetor cannot compensate for leading to engine failures. This makes fuel injection the safe and inexpensive choice.
Finally we come to the issue of cheating. There is a lot of fear that fuel injection is a gateway to traction control. I would like to remind everyone that NASCAR intends to impose a lifetime ban on anyone attempting to use traction control in a Sprint Cup race. I am in no way advocating that anyone cheats, but here is how one would cheat.
A traction control system needs the following information: the speed of the vehicle, the speed of the driven tires, throttle position, and brake pedal position. Vehicle speed can be determined by wheel speed sensors located at each of the front tires. Speed of the driven tires can be determined by sensors at each wheel or on the driveshaft. Throttle position is already known for fuel delivery purposes, and brake pedal position would only confirm that the front tires are not locked under breaking. The idea of traction control is to use the computer to shut off fuel to the next injector set to fire in the event that one (or both) driven wheels has lost traction from excessive power in a turn. As long as the computer has no information on wheel speed or is a sealed unit provided by the sanctioning body (which could also prevent unauthorized testing), no one will have traction control.
With all the differences in function covered, let's address the matter of form. The on-track product will be mostly unchanged. The increased precision of fuel injection may lead to better fuel mileage (particularly in caution situations). Widening the power band may make cars harder to drive on short tracks and road courses, but there won?t be much change at faster speedways. Restrictor plates would be replaced by very narrow throttle bodies so the problem of pack racing will not be resolved.Rumor has it NASCAR will have fuel injection as early as next year; we can only pray the sky doesn?t fall.
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