How Does a Saturn Motor Control Fuel Delivery?

In the "Old Days," when enthusiasts tweaked on classic American V8s, motors used carburetors to combine fuel and air and to regulate the flow of the mixture into the motor. A carburetor is essentially a mechanical throttle control device that connects to the throttle pedal. It uses throttle plates to control airflow into the motor and it relies upon mechanical "jets" to control fuel flow. Larger jets deliver more fuel while smaller jets deliver less fuel. Although many older enthusiasts may tend to romanticize the effectiveness of the carburetor, in truth, by today’s standards, they were relatively inefficient and unreliable.

Modern motors, including the Saturn motor, use an engine management computer and electronic injectors to control fuel. As a result, the Saturn Powertrain Control Module – or PCM – is able to execute more precise fuel delivery and it is able to adapt (within limits) to varying conditions and fuel needs without making mechanical changes. Compared to the carburetor, modern fuel injection is much more efficient and reliable.

The process of Saturn fuel management becomes more complex, but the following explanation should provide a suitable basis of understanding:

One should first understand that the PCM is programmed to target specific fuel ratios, based upon the operating conditions of the car. (For full-throttle acceleration, it will seek richer mixtures. For highway cruising, it will seek leaner mixtures.)

The important tool for controlling these mixtures is the "pulse width," or the amount of time that the electronic injector is allowed to open. Since the size of the injector and the operating fuel pressure are known constants (or so the assumption goes), the time that the injector is open will determine the amount of fuel delivered. And thus, controlled time intervals will result in controlled amounts of fuel. Specifically, longer pulse widths will result in more fuel, while shorter pulse widths will result in less fuel.

The fuel pressure is governed by a "fuel pressure regulator." This device ensures that fuel pressure at the injector opening is regulated to a known value. If fuel pressure were to vary, then calculated pulse widths would no longer result in known amounts of fuel delivery, since higher fuel pressure results in more fuel for a given pulse width, and less fuel pressure results in less fuel for a given pulse width. (Note that the Saturn fuel pressure regulators are slightly variable as a function of manifold vacuum. More vacuum (less throttle) results in lower fuel pressure, while less vacuum (more throttle) results in greater fuel pressure.)

The PCM starts its fuel delivery attempts by considering sensor inputs that measure coolant temperature, incoming air temperature, throttle position, manifold pressure, and other vital operating information. It uses this information to formulate an "educated guess" for a mixture - which it executes by holding the injectors open for the "appropriate" corresponding pulse width.

The PCM will monitor the outcome of its attempts after the motor fires by evaluating oxygen content within the exhaust gases (through the use of O2 sensors.) Since "ideal" exhaust gas oxygen content can be mathematically calculated, the PCM can use O2 sensor input to make an evaluation as to whether current mixtures are "too rich" or "too lean."

To adjust fuel control to correct levels, the PCM can adjust the pulse width – or add or reduce fuel by increasing or reducing the amount of time that the fuel injectors are held open – during the next injection cycle.

So, in a nutshell, the PCM will make an estimation about fuel delivery prior to each engine cycle. It will execute by controlling the length of the injector pulse width. It will monitor its success through the O2 sensor, and then it will make a new calculation prior to the next cycle.

Open Loop and Closed Loop

The operation described above is known as "closed loop" operation. It determines the Saturn’s fuel management under almost all conditions – the notable exceptions being initial startup and full-throttle acceleration. The name "closed loop" is derived because it describes the complete-circle path of the decision-making process. During closed-loop operation, the PCM will work to maintain a near-stoichiometric (14.7:1) fuel ratio.

Under certain circumstances – such as wide-open throttle – it may not be appropriate or necessary to calculate and perform such exact adjustments. So instead, the PCM uses "open-loop" operation to control fuel delivery. During open-loop operation, the PCM does not consider the results of each cycle in the fuel decision for the next cycle. Instead, it resorts to a pre-programmed pulse-width that results in a pre-determined quantity of fuel. The PCM’s decision-making process during open-loop can be illustrated with this simplified example: if throttle position = 100%, then pulse width = "X."

Again, the name "open-loop" is derived from the graphical representation of the process. Since the results of the previous mixture are not factored into the calculations for the next pulse width, the loop remains "open."

In most cases, open-loop operation will deliver unusually large amounts of fuel – often resulting in mixtures richer than 12.0:1 on a stock motor. This not only ensures adequate fuel supplies, but it keeps piston and cylinder temperatures lower which will lessen the likelihood of detonation and engine damage.

Throttle-Body vs. Multi-Port vs. Sequential Fuel Injection

Since 1991, Saturn has used three types of electronic fuel injection systems. The 91-94 SOHC (single overhead camshaft 8-valve) motors used a throttle body injection – or TBI. The 91-95 DOHC (double overhead camshaft 16v) and 95 SOHC motors used the multi-point injection – or MPI. And the 96-99 SOHC and DOHC motors used the sequential fuel injection – or SFI.

Each of these three systems works in the way that this article has previously described – altering the pulse width of the electronic injector to deliver the proper amount of fuel. However, the three systems vary in terms of where and when the fuel is added to the mixture.

The TBI system is characterized by one single injector that is used to deliver fuel to all four cylinders. This single injector is mounted just above the throttle body. This makes the TBI system unique since both air and fuel travel through the throttle body and intake manifold while en-route to the cylinder head. (This is similar to the carburetor – except for the fact that the carburetor introduces fuel through mechanical jets, while the TBI system uses an electronic injector.) Of the three systems, the TBI is the least precise. Since fuel is mixed with air before entering the intake manifold, there is time for the fuel and air to separate before it reaches the motor. This problem is known as poor "atomization," or the tendency for the mixture to settle from its atomized – or vaporized state – into separate pockets of fuel and air. And it is one of the reasons that the 91-94 SOHC motors are rated at only 85HP.

The MPI system advances beyond the ability of the TBI system by employing four separate injectors – one for each cylinder. And, unlike the TBI system - which adds fuel at the throttle body - the MPI system mounts each injector directly into the intake port of the cylinder head. This ensures that only air travels through the throttle body and intake manifold - delaying the introduction of fuel until it actually reaches the cylinder head – and avoids many of the TBI system’s problems with atomization. However, even though atomization is improved as a result of the injector positioning, potential problems still exist with regard to injector timing – since, like the TBI, all four injectors are controlled simultaneously. This allows time for the mixture to settle while waiting for certain intake valves to open.

The SFI system is yet another evolution beyond the MPI system. Like the MPI system, four separate injectors are positioned directly into the cylinder head. However, unlike the MPI, in which all injectors are controlled simultaneously, the sequential system fires each injector at its own optimized instant – allowing more precise injector timing and reducing the chance of poor atomization as a result of settled mixtures.

Ways to Increase Open-Loop Fuel Delivery in the Saturn Motor

There are only three factors that determine the amount of fuel delivered during open-loop operation, so getting more fuel is as "simple" as adjusting one or more of these three factors.

  1. Adjust the Pulse Width. The pulse width is, of course, the length of time that the injector is held open. And it is the PCM’s primary tool for controlling fuel delivery during both closed-loop and open-loop operation. As long as fuel pressure and the size of the injector opening are constants, then a longer pulse width will obviously result in more fuel being delivered. The big advantage of adjusting the pulse width is that very precise amounts of adjustment are possible. However, since the Saturn PCM is not re-programmable, the only way to make these changes is to purchase an aftermarket engine controller and start calibrating!
  2. Use Larger Injectors. If the fuel pressure at the injector and the pulse width are constant, then the size of the injector will determine fuel flow. Obviously, a larger injector orifice will result in greater fuel flow, while a smaller injector orifice will result in reduced fuel flow. Finding the appropriate orifice size and the labor involved with replacing injectors is relatively difficult. And the cost of purchasing electronic injectors is steep, so we do not necessarily expect many enthusiasts to be quick to replace injectors. Nevertheless, SPS is currently working with OE suppliers to develop bolt-on performance injectors with an enlarged orifice.
  3. Increase Fuel Pressure. Increasing the fuel pressure is the obvious solution for most enthusiasts. If the size of the injector is constant, then adding more pressure to the injector will result in more flow for each and every pulse width. (And vice-versa, reducing pressure will reduce flow for each and every pulse width.) Best of all, manipulating fuel pressure is easy and inexpensive, as parts are readily available from reputable tuners such as SPS.