In fact, would less fuel be required to produce the same or more power when swapping EFI for carburetion?
Other crucial questions posed by the Circle Track team concerned the overall efficiency of the fuel-metering systems. Would the carburetor require more fuel than the EFI system for a given power? How would fuel-injection affect the power curve and overall efficiency? Although the team was unable to answer all the questions first raised in the project due to time and testing limitations, some surprising answers came to light, as well as other questions we are eager to answer. Analysis of the issue regarding the fueling rate between the carburetor and the fuel injection system highlights the complexity of answering such engineering questions and provides guidance for future testing plans.
In Figure 9, a comparison of the A/F lambda values for the carbureted and fuel-injection metering system is shown for each of the seven test points (shown in Figures 2 and 5).
The first thing to notice is that the overall A/F control for carburetors is inconsistent. Lambda values for the seven test points vary from 0.63~0.85. Since the carburetor fuel flow rate depends largely upon the shape of the venturi(s) and airflow velocity, it may be seen that consistent A/F control is not achieved.
At lower intake air velocities, the fluid flow is dominated by viscosity effects resulting in inconsistent metering. For the fuel injection system, it becomes simply a matter of properly calibrating the fueling table, and under certain systems, utilizing closed loop O2 sensor feedback for consistency.
In analyzing the total fuel flow for each load point, it's counterintuitive that the fuel-injection system consumes more fuel. This is apparent in Figure 10, in which each steady-state test point fuel flow is compared. It may be seen that for all load points, the fuel-injection fuel flow rate is higher. Again, comparing Figure 9 with Figure 10, light is shed upon the reason. The fuel-injection system was calibrated for MBT timing and equivalence ratio.
In other words, spark timing was advanced to the knock limit for each load point, while fuel was simultaneously added until torque began to drop off. The discreet set point mapping of fuel-injection allows for each point to be optimized, rather than be a tradeoff for certain load points.
In the May issue, it was shown that there was a significant torque gain across the powerband for fuel-injection relative to the carbureted system, except at peak power due to tuning issues associated with the production fuel-injection intake manifold relative to the carburetor tuned manifold. This torque advantage was attributed to the discreet point tuning that is available with individual-port fuel-injection systems, which is not available with carburetors.
As the project moves to more advanced stages of engine testing, the team will examine more complex in-cylinder behavior to further differentiate the benefits of fuel metering technologies and will openly publish the results. Stay tuned.
Figure 8 - Full-load power...
Figure 8 - Full-load power pull, X-axis equals time (in seconds), Y-axis equals NOx emissions (g/kg-fuel).
Figure 9 - Individual steady-state...
Figure 9 - Individual steady-state test point lambda values, carburetor and fuel-injection comparison. Each number on the X-axis corresponds to the steady-state load points presented in Figures 2 and 5.
Figure 10 - Individual steady-state...
Figure 10 - Individual steady-state test point fuel flow rates, carburetor and fuel-injection comparison. Each number on the X-axis corresponds to the steady-state load points presented in Figures 2 and 5.