In today’s automotive landscape, many of the headlines are being made by the steady progress of the electrification and automation of the automobile. This column has often featured some of this high technology as applied to Porsches, but it is sometimes instructive to look back at prior innovations and their reason for being to better understand how the modern systems came to be. A majority of Porsches on the road today feature an electronic fuel injection (EFI) system, and some of these are over 40 years old!
Regardless of the fuel induction method, there are some basics that apply to all automotive gasoline engines. Gasoline does not combust in liquid form as stored in a car’s fuel tank—it must be in vapor form and mixed with air in order to ignite. Furthermore, because the boiling point of any liquid is directly related to air pressure, fuel vaporization is aided by the relatively low pressure downstream of the throttle plate(s). However, to maximize the potential of the vaporization of all admitted fuel, it must first be atomized, or broken up into very small droplets, before entering the combustion chambers of the engine.
While not terribly accurate at metering fuel, a carburetor is actually quite efficient at fuel atomization due to careful placement of a venturi in the intake throat(s). The venturi is a calibrated restriction that serves to speed up the throughput of air, which also lowers the pressure in this area. The fuel is mixed with the air at the center of the venturi to aid atomization of the fuel and to improve intermixing of fuel and air.
A fuel injection system uses pressurized fuel forced through an injector valve/nozzle to achieve atomization. Porsche used Bosch’s diesel-based mechanical fuel injection (MFI) system in racing engines for decades and in the 1969-1973 911. While a full description of the operation of Bosch MFI is beyond the scope of this article, an important feature of this system is that it operated at about 250 psi, which allowed fine atomization of each precisely measured and timed shot of fuel into the intake ports. This served to promote the evaporative cooling effect of the injected fuel and was quite conducive to the high-revving Porsche racing engines with individual throttle bodies.
Porsche successfully used Bosch’s more emissions-friendly K-Jetronic (or CIS for Continuous Injection System) mechanical injection system for many years on the 911, which continually injected fuel into each of the engine intake ports. While the system pressure of 80 psi still allowed for adequate atomization, some of the fuel would always remain in the intake port until the next intake valve opening event. While this would seem less than ideal, there was always some atomized fuel going into the cylinder, and CIS-equipped engines had special piston crown designs that were designed to generate a tumbling effect to promote intermixing of the air and fuel.
Bosch’s electronic port fuel injection systems use fuel injectors that each contain an electromagnetic solenoid. The injectors are fed a constant supply of pressurized fuel (at about 2.0-2.5 bar, or 29-37 psi in earlier systems) via a common supply tube (often called a fuel rail), for each bank of cylinders, with fuel pressure maintained by a regulating device that restricted the return line to the fuel tank. While the fuel atomization capabilities of the early EFI systems were not as good as their mechanical predecessors, they made up for this deficiency with precise fuel metering and flexibility.
Fuel injection quantity is controlled by varying the amount of injector open time via the length of the pulse-width of the ground signal to the injector negative terminal. Typical fuel injector pulsewidths can be as little as 1.5-2.0 milliseconds (ms, or thousandths of a second) to over 20 ms higher engine speeds and loads. The positive fuel injector terminals are always supplied with voltage while the engine is cranking or running; the switchable ground path needed to complete the circuit and open the injector is controlled by a transistor inside the electronic control unit (ECU).