How much engine braking a gen 3 is capable of

When a gen 3 Prius is headed downhill using engine braking alone (battery having reached 80% so no more regen is accepted), how much braking can it do?

TL;DR: at least 12.6 kW, possibly a bit more (engine reached 3936 RPM in my test, but the car can allow engine revs up to 5300). So, it can hold your speed downhill as long as the loaded weight and speed of the car and the rate of elevation loss work out to an energy gain rate from gravity of that many kW or less.

The rest of this post just describes how that was measured, back in February 2023; it's a lightly-edited repost of the original 2023 post. Trouble with that old post is it combined a couple different topics, and the measure-engine-braking part was the later part. Linking to that older post from discussions about engine braking isn't ideal, because someone following the link sees the first half of a post that seems off the topic. So only the measure-engine-braking part is the focus of this post.

Preliminaries

I'm in Indiana (and not even southern Indiana where there's terrain), so it's a little hard to find adequate hills, but on 3 February 2023 there was a power outage after lunch that left me with little else to do and it was a beautiful day for a drive, so I went and found an adequate hill I remembered not too far away. And by the time I returned, the power was back on.

Even this hill was so short I had to start with a force-charge to 70% at the top in order to observe the car's transition from regen to engine braking on the way down. Indiana....

I did not touch the brake pedal, but had the cruise control at its lowest setting. The shift mode selected was D, as a gen 3 will not allow using cruise control in B. Because the set speed was a dozen MPH below the road speed, the cruise control was hard at work to get the speed down.

That work proceeded all by regeneration into the battery until the state of charge reached 77.2%. At that point, light engine braking began, and increased while regeneration decreased, until at state of charge 80.0% it was all engine braking and no regen.

At 77.2% SoC just before that transition began, road speed was 37 MPH and increasing; the power being regenerated was not quite equal to the input from gravity. Speed increased to 47 MPH during the transition and was back down to 36 MPH by the time it was all engine braking at 80.0% SoC. (The hill had already begun to flatten out by that point. Indiana....)

The PIDs that were datalogged during the run are referred to below using the names Techstream shows for them.

A couple incidental notes:

• The power management control ECU has PIDs for "Regenerative Brake Torq" and "Rqst Regenerative Brake Torq". Both remained zero throughout, both at first when regenerative braking was employed, and later when engine braking was employed. It seems these PIDs are used only when regen is requested by a message from the brake ECU, and not when employed by the cruise control (which is implemented within the power management control ECU itself).
• The power management control ECU also has a "Request Power" PID, giving the kilowatts the ECU has requested from the ECM when it wants (positive) engine power. But this PID remains zero during engine braking; it does not report the kilowatts the engine is being asked to soak up.* But we do have enough other PIDs to complete the picture anyway.

Measuring the regen braking (before transition starts at 77.2% SoC)

Just before the transition, the braking power is easy to calculate. MG2 was doing 4606 RPM at 27.88 Nm, so it was subtracting 13.45 kW from gravity's input. Of those 13.45 kW, what arrived at the battery was 45.06 amps at 262 volts, or 11.81 kW.

1.64 kilowatts have gone missing. The car uses around half a kW just being awake, and the remaining 1.1 kW or so, around 10% of the power being handled, would not be too astonishing as losses along the motor to inverter to battery path. Also, we have to expect a certain fuzz in the calculations involving different PIDs because Techstream is polling for them, and not getting answers that all come from the same instant.

13.45 kW does not represent an upper limit on the car's regen braking: much stronger regen is available (if the battery isn't full) when requested by the brake pedal. What was measured here was the amount of regen braking applied by the cruise control to try holding speed, but with no use of the brake pedal.

Measuring the engine braking (after transition completes at 80.0% SoC)

After the transition, at the point of strongest engine braking I saw, we can directly compute the portion of the power that is taking the MG2 to MG1 path. MG2 is doing 4331 RPM at -20.50 Nm, so it is subtracting 9.30 kW from the gravity input. None of that is going to the battery, and what's arriving at MG1 is 9974 RPM at 7.37 Nm, or 7.70 kW. Again there's a ~ 1.6 kW gap to explain with a combination of the car's usage, electrical losses, and PID polling fuzz.

That 7.70 kW is not all of the power the engine is twirling off, but only the portion that came over the MG2 to MG1 path. But we also have enough data to compute what followed the mechanical path.

The MG1 torque was 7.37 Nm and is always 30/108 of the torque on the engine, so that was was 26.5 Nm at 3936 engine RPM, or 10.94 kW being soaked up.

MG2's RPM was 4331 and a Gen 3 has a 58:22 reduction between MG2 and the PSD ring, so the PSD ring RPM was 1643. The PSD ring sees 78/30 of MG1's torque, so that was 19.16 Nm, and at 1643 RPM that's 3.30 kW, which is what was left of the gravity input power after MG2 had subtracted 9.30 kW from it.

So the gravity power input being handled was 12.60 kW, of which MG2 removed 9.30 kW to follow the electrical path, leaving 3.30 kW to follow the mechanical path.

The engine was soaking up 10.94 of those 12.60 total kW, leaving 1.66 kW to explain with the car's power usage, incidental losses, and PID polling fuzz.

The engine was turning 3936 RPM and Gen 3 will allow up to 5300, so there was surely more engine braking capacity available if I'd had the hill to show it. Indiana....

What gear in a familiar stick-shift car would have given roughly the same engine braking?

Because the PSD ring was doing 1643 RPM and the engine turning 3936, the CVT had an effective gear ratio of 2.396 at that moment.

In my last stick-shift passenger car, 1st gear was 3.416 and 2nd was 1.842. That car also had a higher final drive ratio, 4.105, than Gen 3 Prius 3.267. So that car's 2nd gear was 7.561 to the wheels, and the Prius in this example was effectively 7.828. Not a bad match.

In that other car, I probably would have downshifted to 2nd on that hill at that speed, and the engine braking would have sounded and felt pretty much the same. While our transmission is built differently inside, it isn't behaving any differently in this example than a conventional transmission would.

* I think member RGeB has presented at least one graph showing an Engine Torque PID that did show negative values during engine braking, a PID available from the ECM. But I was logging only PIDs from the power management control ECU, and its Request Power PID doesn't reflect that, and just sits at zero instead.

 

That's an interesting behavior to report, and I've also seen such reports before (though it's not something I've ever experienced in my own car). It might deserve its own thread, as it appears to be another interesting behavior that's not the same as engine braking.

In all the cases where a Prius with me in it reached 80% on a descent, the car just dropped the charge current to zero and left the battery alone for the rest of the descent. The engine braking that took place was simply a matter of power from the wheels being twirled off in the engine, as #1 above shows, while the battery current remained negligible. Once the descent flattened out and the car needed forward propulsion again, it would eagerly use the 80%-charged battery for that—an obvious way of bringing the charge state back down from 80% without having to throw that energy away.

Having the car use power from the battery for no purpose but to twirl the engine seems like a fairly desperate measure, and it would be interesting to know more about the circumstances when that's been observed. Must be something making the car decide it can't just wait for the next flat or uphill and use the battery charge then in a worthwhile way.

It's most fun to have enough data to complete the picture of how much power was going where, as was done in #1 above. That would require having the battery voltage and current data you supplied here, and also at least two of (engine RPM, MG1 RPM, MG2 RPM), and the MG1 and MG2 torques from the same moment. (Also, which generation/model Prius it is, so gear ratios can be looked up.)