Automatic Boost Control

[wuzak]

They reduced manifold pressure to shift because the gears had to mesh and its both easier and less stressful to mesh gears at lower speeds than it is at higher speeds. Anyone who has ever shifted a manual gearcase will recall that. It's in all the two-stage pilot's manuals to reduce power to shift supercharger gears, and turbocharger gears operate the same way. It would be different, of course, if the coupling were hydraulic.

Like regular gearboxes, the gears are always in mesh. The clutches are used to engage or disengage the gears. In regular car manual gearboxes this role is done by the syncromesh.

Still, even when shifting something like a Formula 1 transmission, they momentarily remove power from the geartrain by interrupting the ignition to ease the gear engagement. Every single button-operated motorcycle gear changer I have used (several) has had ignition interruption, too.

Current F1 transmissions have zero power interruption.

Sequential gear systems used on racing touring cars, for example, do have ignition cut for upshifts, and can be operated without using the clutch. Some Supercars drivers only use the clutch pedal for the start of the race and in the pits.

These transmissions tend to not have synchromesh, but rather have dog rings for engaging gears.

 

[GregP]

Well, Wayne, if they are always "meshed," why is it that if you slightly miss a clutchless shift, or if you engage the clutch rather too quickly before the sift is fully completed, the gears will grind? EVERY manual transmission I have ever driven will grind, making it rather plain that the gears are not always "meshed." "Synchromesh" doesn't mean they are always meshed, it means the "consumable" synchos try to get the gears that are going to be meshed moving at the same surface speeds before they actually mesh. Worn synchors are sure sign of a bad driver.

The ignition cutout is VERY plain in an Indy car. Less so in an F1. I don't think either of us has any insider knowledge about an F1 transmission, but it surely is NOT a CVT. There are a mandated number of forward and reverse gear ratios, so there ARE gears that mesh and unmesh.

 

[Joe Broady]

They reduced manifold pressure to shift because the gears had to mesh and its both easier and less stressful to mesh gears at lower speeds than it is at higher speeds. Anyone who has ever shifted a manual gearcase will recall that. It's in all the two-stage pilot's manuals to reduce power to shift supercharger gears, and turbocharger gears operate the same way. It would be different, of course, if the coupling were hydraulic.

That first sentence makes no sense. Reducing manifold pressure does not change the speed of the supercharger drive gears because the Hellcat has a constant speed prop. The real reason to throttle back is the manifold pressure surge when you shift up from neutral to low blower, or low to high. If you're already at max allowable pressure, you'll briefly overboost the engine until the regulator reacts. "Retard throttle as necessary to avoid exceeding desired manifold pressure after shift," says the operating manual.

It also says, "reduce RPM if practicable" when upshifting, so an RPM reduction is not as important as a boost reduction. No MP or RPM reduction is necessary before a downshift, however.

The induction and supercharger diagram in the Hellcat operating manual shows a countershaft driven by the crankshaft through step-up gears at a fixed ratio. On the countershaft are two gears which mesh with corresponding gears on the blower drive shaft. Each countershaft gear has a clutch (actuated by oil under pressure) which can lock it to the shaft. As someone has already noted, the two speed drive is a constant mesh arrangement. The two gear pairs are always in mesh. Low or high blower is selected by actuating one clutch while the other lets its gear freewheel on the countershaft.

The clutches work by friction. They are not dog clutches as in an automotive manual tranny, so no grinding of gears is possible. According to the Hellcat operating manual, "Do not shift the supercharger control more often than at five minute intervals, while in flight except in an emergency, to allow the dissipation of heat from the clutches. The control must be at the extremity of its travel in either ratio to prevent clutch slippage."

 

[GregP]

Reducing power past a very easy-to-find point DOES affect rpm. The point is where you aren't feeding enough power to the engine to maintain the constant rpm.

It happens every time I land, say, a Cessna 182 or a Piper Dakota. They aren't turbocharged or supercharged, but they have constant-speed propellers that operate the same as the big iron. The P-51D and P-40N I rode in operated exactly the same when we came back to the field, and I have read the manuals for most US fighters.