DANGER: Cruise Control and mountains

On the Fuelly website, the average MPG I see for all 6 model years of the Gen 4 Prius (excluding the Prime) is between 51 and 52. It may be possible through tire over-inflation, hypermiling, etc. to squeeze out a few more MPG, but I am skeptical about any claims of much more than that on a consistent year-round basis. Fuelly is showing 54.6 for the average on my 2017 Touring model over its 23k mile lifetime, but the car has been driven exclusively in warm (not hot) weather and on long trips between the East and West coasts and between Florida and NY. The MPG per tankful has been as high as 59 (presumably with wind behind me) and as low as 49 on one other occasion. I don't stress about the 49 and didn't get too excited about the 59.

The car has an Engine Control Module that monitors all aspects of the engine's performance and will not allow the engine to operate outside a safe RPM range. It does not consume gasoline when the hybrid ECU instructs the ECM to rev the engine to reduce the amount of charge in the hybrid battery. The system is a lot smarter than we are.

 

how many miles on her?

 

The engine revving up, especially when the battery is getting full, or during longer downhill roads, is perfectly normal when slowing down or trying to maintain a constant speed downhill.

It's basically engine braking, where the ICE converts energy from the wheels into heat via the compression of air (adiabatic heating) and internal friction in the engine. This is the same on nearly all vehicles with combustion engines.

The Prius will prioritise converting excess energy at the wheels into electrical energy to store in the battery first, that's what a modern hybrid does. However, if the battery can't accept all of that energy (e.g. because it's full, too hot, too cold, too much too fast, etc) then the remainder of that energy has to go somewhere else or you will continue to accelerate downhill. You've told the car what speed you want to maintain via the cruise control, so the car will try to prevent you going over that speed downhill. The energy can be burned off to the atmosphere as heat via engine braking like you're experiencing (the engine will rev up), or burned off to the atmosphere as heat via the friction of the brakes. The former is preferable as it provides less wear, even if it sounds scary. The latter will wear out parts of your brake system and if it heats up too much it will fail.

Putting the car in B mode makes the car favour the use of engine (B)raking more than D. The idea being that it reduces strain on the battery and brakes on long downhill stretches, at the expense of reduced fuel economy. That's the driver's call.

Whenever engine braking is needed or used, the dash will show the ICE as on, and it will sound like it's on (especially if it's revving up) but no fuel is used while engine (or any other type of) braking.

SM-T813 ?

 

That sounds like one of the questions you're really worried about, so it should have a direct answer:

Yes, the car completely protects itself from over revving.

What's happening as you go down that grade is pretty much the same as if you were in a manual-transmission car and you had downshifted to help control the speed. And in the manual car, you would be right to worry, because it would be all on you to mind the RPMs and do something different (upshift or use the brakes) to keep the engine at a safe speed.

In the Prius, though, all of that is under a computer's control, and it knows the designed max RPM for the engine, and it has a continuously-variable transmission to play with, not just a few fixed gearings.

That means if you plug in a scan tool and start heading down that grade, if it is steep enough, you will see the RPMs go right up to the designed limit (might be around 5200, don't quote me) and hold steady right there, even as your road speed increases or decreases. The computer will just use the CVT to convert whatever road speed to that same engine RPM, and it could do that all day and be perfectly happy.

That limit on the engine RPM means there's a limit on how much engine braking you've got. If the grade isn't too steep, that braking will be able to hold your chosen cruise speed constant. Steeper than that, still nothing bad happens to the engine, but its braking effect won't be enough to hold your set speed, and the car will pick up speed and you'll have to snub it back down from time to time with the brakes.

In that situation, you also have the option of setting the cruise to a lower speed that the cruise will be able to hold given the available engine braking. If you've been setting for 10 above the limit and it can't hold that, you might try setting at the posted limit for that descent and see how that goes. If there's no impatient traffic behind you, you could even try lower than that.

 

I am not the best to FYI on this given my location but I believe engine braking on cruise control with ICE on DOES use gas. I had a scan gauge on my car for a while and I noticed when the engine revved up when braking on cruise control or on long downhill jaunts it does show an increased amount of fuel consumed. I believes it showed about .6-.7 gph consumed with an engine RPM consistent at 5000 RPM. I am in a non-hilly area flat florida so maybe someone else can chime in and give more details. In Zardoz defense the very first time I heard my engine rev up like this I thought it might be possible that it was over-revving the engine but confirmed with the scan gauge it is not.

 

That's interesting. I don't remember offhand whether GPH on the ScanGauge is an actual raw PID that it reads from the car, or a number it calculates from other raw PIDs like RPM and MAF. I would be tempted to double-check with Techstream in the same conditions, watching the Data List to see the "fuel cut" status values. (In Gen 3 at least, it is "values" plural; I think I've seen more than one such flag on the engine data list. Not sure why or what the difference is between them.)

My prediction would be that would show fuel cut, at least after the first few seconds of descent. No point burning fuel in those circumstances, and the engineers didn't make a habit of leaving efficiency on the table.

 

The title of the thread is misleading. I thought a dangerous condition was being reported.

I don't experience any of the problems that the OP reported. BUT.... I drive through the Siskiyou mountains once a month as well as the mountains around Mt Shasta. I've found that using dynamic cruise control on a mountain road will cause conditions where the car you are following will go around a sweeping left curve and leave the radar picture. If this happens when there is a truck ahead in the slow lane the radar locks onto it and may suddenly slow your car from 55 mph to whatever speed the truck is going.

This is frightening the first time it happens, to say the least. It's not as scary the second time.

It's a simple problem to overcome. Just turn off cruise while in twisty mountain roads. It's not meant to be used under those conditions.

Dan