UNDERSTANDING FUEL INJECTION
The object of any induction system
is to put a combustible air-fuel mixture into the cylinders under all operating
conditions. Carburetors accomplish this by utilizing airflow to draw gasoline out
of a jet as the air passes through the carburetor. With fuel injection, fuel is
injected into the manifold using small valves called injectors. Fuel injection, as the name implies, is a
system of injecting fuel under high pressure as a spray or mist. This is a much
more efficient way of getting properly-mixed fuel and air where it needs to be.
Therefore less fuel is wasted than with carburetors due to the fuel being
There are two main types of fuel injection: port injection and throttle body injection. In throttle body injection, the injectors are mounted in a throttle body which is very similar in appearance to a carburetor. The air/fuel mixture is distributed to the individual cylinders via the intake manifold, just as if the throttle body were a carburetor. The drawbacks of this design are, first, that not all of the intake manifold runners are equal. This can result in uneven air/fuel mixtures in the different cylinders. Second, the fuel has a chance to condense or not be totally atomized as it moves from the throttle body to the combustion chamber. The longer the runners are, the greater the condensation problem. Port injection on the other hand, places an injector at each cylinder spraying on the back of the inlet valve so the fuel is sprayed or atomized very close to each cylinder. Jaguar has used the port injection on the V12 since late 1974 and on the 6 cylinder since 1978.
Early fuel injection systems were mechanical. The incoming air opened a door connected to a valve controlling the amount of fuel sprayed by the injectors. Jaguar used a Lucas mechanical system on the race cars of the late ‘50s and early ‘60s. Although more efficient, it was troublesome and not easy to fine tune.
Constant Injection System (also referred to as CIS or K-Jetronic) was a version system used on a lot of cars including the Aston Martin V8, Ferrari 308/328 series, BMW 320I, Mercedes, Lamborghini, and many others. This FI system metered out fuel under high pressure through nozzles based on incoming air flow and engine temperature. The later systems were modified to work with Lamba interfaces for finer mixture control. This is also referred to as oxygen sensor systems. CIS systems are more efficient than carburetors but do not give the accurate fuel metering needed for tighter emission controls.
Volkswagen introduced an electronic fuel injection system called “D-Jetronic” in the late ‘60s. The D-Jetronic was a very basic system using resistors and transistors without an IC chip in sight. Jaguar took this system and adapted it to work on the V12 with a few modifications. Acceleration enrichment was added, as was an amplifier to go from 4 cylinders to 12. In this system, the injectors were fired in groups of six timed by a set of two switches inside the distributor. Thus the injectors operated once per engine cycle. They were not timed to the cylinders but rather just sprayed fuel at the inlet valve at the same time regardless of whether the valve was open or not. Injector duration (which is how long the injector is open) was determined by engine RPM, engine temperature, air temperature, and engine load. A MAP (manifold absolute pressure) sensor was used which referenced engine load to vacuum rather than to air flow. This system worked a lot better than the carburetors of earlier engines but was not adaptable to the finer requirements of the HE engine.
The “P-Jetronic” injection system was installed on the HE cars around 1980. This was a digital system and utilized a program map for the injector duration based on the same inputs as the D-Jetronic. It did have the advantage of using Lamba feedback and also used smaller injectors that injected less fuel but fired twice per firing cycle for better atomization. This same system continued on the V12s through the end of the run in 1996 with few improvements needed.
The XJ6 in 1978 was fitted with “L-Jetronic after the German word “Luffa” for air. This system used an air box to sense the volume of incoming air and its temperature. Other inputs were RPM, which was picked up at the ignition coil, and engine temperature. One of the shortcomings with the L-Jetronic system is that it sensed air volume and not air mass. Higher altitudes required a connector in the trunk to be connected. Later, an altitude compensation device, mounted on the outside of the ECU, was added to address this issue. This system also had Lamba feedback and continued through the Series 3 run with minor modifications.
In 1988, the XJ40 brought about a newer system that used a Hot Wire sensor to actually measure air mass, which allowed the injection ECU to compensate for humidity and altitude, giving a more accurate control of fuel mixture.
One special Jaguar engine featured a sequential fuel injection system. In sequential injection systems, the injector spray pattern is timed to the opening of the inlet valves. This results in slightly better power and emissions. The 1993 XJRS used a sequential FI system made by Zytec.
As can readily be seen from the cars on the road today, power and drivability is on the increase along with fuel consumption decreasing. Tighter emission specifications require tighter control of the fuel. This has many benefits and will continue to improve as technology progresses.