Figure 5 - Individual, steady-state points, lambda values. Each number on the X-axis corre
The team was able to collect engine rpm, throttle position, and time utilization data from a late-model stock car making laps at New Smyrna Speedway and apply that information to the CT 525 LS3 test engine. These data were then analyzed to determine the frequency at which the engine operated at specific speeds and throttle positions, and time-weighted histograms at each one of the points were determined. These throttle positions were then used on the dynamometer with the test engine, and the brake torque employed for each run was recorded to be used throughout the testing.
In this case, there were seven test points chosen, as shown in Figure 2. Given these conditions, one can see that for approximately 80 percent of the time, the engine experiences wide open throttle (WOT) conditions, with only a fraction of the time spent under part-load.
Why, the reader might ask, would a race engine-testing group focus on steady-state points rather than peak power? Typically, all the advertising we see for performance parts centers around one number-peak horsepower. The importance of the torque and power curves were discussed by Jim McFarland in multiple issues of Circle Track and the benefits of both fuel injection and E85 over race fuel were openly presented. Now, it is time to use the steady-state testing points to begin explaining their basis of use.
Was there a chance that combustion efficiency might be affected, where in the engine speed range could this occur, and could we link it to the differences between carburetion and EFI?
During the combustion process, it's inevitable that a portion of the fuel does not react to completion and release the full amount of energy present. This is the combined result of unfavorable local mixing consistency within the combustion chamber, cylinder wall wetting effects, and dynamic cycle-to-cycle variations.
At the onset of our engine testing, with regard to combustion efficiency, one question for which we wanted to gain some insight was whether we could make some generalizations between the race fuel and E85, as well as EFI versus carburetion.
Fortunately, by utilizing the Portable Emissions Measuring System (PEMS) unit for the series of seven steady-state operating dynamometer testing points, we could begin to answer some of these questions. For example, by measuring the unburned hydrocarbon (HC) and carbon monoxide (CO) emissions, we were able to gain some insight into the behavior of the various fuels within the engine. Taking the steady-state points presented in Figure 2, and weighting the emissions together, we generated the approximated on-track cycle-weighted emissions with the mass-specific units for each of the criteria emissions, measured in grams of emissions/kg fuel burned.
In Figure 3, we see that both the approximate cycle-weighted HC and CO emissions for the fuel-injected, 100-octane race fuel are lower than for the carbureted version. This indicates that the combustion efficiency for fuel-injection is greater than that for the carbureted version.
As mentioned earlier, mixing efficiency plays a significant role in this. The data show improved mixture preparation for the EFI system compared with carburetion. Since these results hold over a broad, steady-state operating range, it can be concluded that the fuel-injection metering system delivers better proportioned and/or mixed fuel (per cylinder) than the carburetor.
However, comparing the E85 EFI results, one sees higher CO and HC emissions relative to the gasoline-fueled counterpart. Since the energy density of ethanol fuels is lower than the petroleum counterparts, larger volumes must be injected per cycle (in this case, approximately 20 percent more fuel). This larger volume of fuel would require more mixing time, and/or better atomization by optimizing injector size. Something we intend to address during the on-track testing of the project. As the project progresses, the team will be looking further into these results to share with our readers.