Figure 6 - Full-load power pull, X-axis equals time (in seconds), Y-axis equals HC emissio
How much power would be sacrificed when catalytic converters are installed? Was the use of such converters going to make any significant difference in emissions levels from a bona fide racing engine?
Important questions that the Project G.R.E.E.N team had concerning the use of catalytic converters involved the potential power loss and the type of conversion efficiency that would be achieved. In modern vehicles, great effort has gone into designing catalytic converters with high emission-conversion efficiency, while simultaneously minimizing engine power loss and fuel consumption. Would the higher power levels and flow rates associated with larger displacement race engines impact the loss? How effectively would HC, CO, and oxides of nitrogen (NOx) emissions be reduced? Would the catalyst endure strenuous, full-power dynamometer testing?
Comparing the full-power sweep results, we determined a good bit of information on the amount of power loss associated with catalytic convertor usage. For this test matrix, the engine was run utilizing E85 and the fuel injection manifold, along with two different catalyst substrates. One catalyst contained a 100-cell-per-inch (CPI) substrate and the second a 300-CPI substrate. The higher number of cells per substrate equates to an increased catalyst surface area, and, theoretically, would result in increased backpressure. This increase in backpressure should result in a loss of power, the results of which are shown in Figure 4.
In Figure 4, we see that a peak of 1 hp and 1 lb-ft of torque was lost using the 100-CPI catalyst, with almost 4 hp and 10 lb-ft of torque for the 300-CPI catalyst, compared with no catalyst use. Substantially more pressure drop is created with the smaller-celled substrate (300 CPI).
However, peak values do not always tell the complete story, so the entire power curve should be considered. For engine speeds less than 3,250 rpm, there is a slight increase in torque for the 100-CPI catalyst (relative to no catalyst) and almost no loss associated with the 300-CPI catalyst. Reversion and exhaust tuning effects are most likely the culprits, resulting in slight elevations of in-cylinder trapped mass and generating slightly higher torque values. However, as the engine flow (rpm) increases and we observe the area under the power curve that is important on the racetrack (3,500 to redline rpm), a consistent deficit is noted.
In the study of compressible fluid flow, for relatively low velocities (Mach numbers less than 1), as the cross-sectional areas decrease, the resulting pressure of the fluid drops and the flow rate increases. This pressure drop requires energy, and in the case of a reciprocating engine, some of the energy used to push the pistons drives this pressure drop (thus, less power at the crank).
As the exhaust flow enters the catalyst, the variation in flow diameter coupled with the small cells of the substrate causes this pressure drop across the catalyst. In theory, the more cells per substrate, the more restrictive the flow and the higher the loss. This is ultimately what was observed when comparing the 100-CPI, 300-CPI, and no catalyst engine configurations. However, it is telling to note how little power is lost using the higher flow, 100-CPI catalyst; a single lb-ft of torque; and approximate single horsepower across a portion of the powerband.
Final questions concerning the catalysts centered on their effectiveness in converting the criteria emissions of unburned HC, CO, and NOx. Since the engine was tuned for peak power at relatively rich A/F ratios, would there be sufficient oxygen (O2) in the exhaust stream to oxidize the HC and CO, while reducing the NOx? Would the catalysts be effective for such high loads and flow rates, and would the substrate remain durable during repeated full-load testing?
The results were very telling. To frame the discussion, we will review the relative A/F lambda values for which the engine was tested, as shown in Figure 5.