Quote:
Originally Posted by SPL
H2O Doctor — Without instrumentation more sophisticated than the car's MFD or a ScanGauge, I don't think it's easy to determine exactly when the TCH is in "heretical" mode. In addition to the ICE's speed, one needs to be able to monitor MG1's speed, and also its direction of rotation. I believe that heretical mode is commonly employed when travelling at steady high speeds on relatively flat highways. It should not be confused with "fuel-cut" mode which can occur at speeds above ~64 km/h (~40 mph) when the car is coasting (or, at least, when the accelerator is only very lightly pressed). In fuel-cut mode, all fuel flow to the ICE is stopped, but it is being spun at ~1000 rpm by MG1 (powered by electricity from MG2), in order to prevent it from the over-revving that would occur were the ICE allowed to stop completely. Fuel-cut mode can be detected unambiguously from the fact that ScanGauge shows "open-loop" ICE operation, and the dash's FE gauge simultaneously reads precisely zero fuel usage (0 L/100 km or 60 mpgUS). In both cases, electrical power is flowing from MG2 to MG1. The open-loop indication will clearly distinguish fuel-cut mode from heretical mode.
Stan
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You can tell if the ICE is on by a slight (very slight) accelerator demand. It will bring the FE indication off of the 60
MPH indication and move it to some decreasing value, typically in the 55-40 MPG range. This fuel flow indication will not occur in fuel cut mode and if the accelerator demand caused the ICE to restart it will move initially to above 40MPG. From my posts you probably know my advocation of heretical mode. As mentioned in several of them, I often cruise in the 50-60
MPH speed range using what I beleive to be heretical mode. My indication for this is a fuel flow in the 40-59 MPG range, but that occasionally backs down to 60 MPG.
You could be correct that the ICE fuel flow is cutting out for these 60 MPG intervals. I think it unlikely at that speed on undulating hills with a average very small grade (in either direction). However, it matters not even if it is, because a certain amount of enery will be required over the average grade for the distance. The basic condition is the ICE spinning (under power or not) with a very efficient "gear ratio" (irregardless of which way MG1 is spinning) and an average FE of well above 40 MPG. A coast, whether in neutral or not, simply trades off a loss of inertia against the enegy demanded for the next small hill or the inertial loss that will eventually result in a drop in cruise speed. In these conditions, the expenditure of the energy to maintain a constant power demand will be more efficient.