Talk about torque.

I guess it kind of depends on what torque you want to measure.

If what you care about at the end of the day is what torque the car can deliver to the wheels, why, measuring that torque at the wheels is a cinch.

If the answers you're trying to get are about various internal torques that are involved in making that happen, and those answers are important to you, then it could be a little more work making sure the questions are well-posed and planning ways to answer them.

 

Maybe I don't understand the way a dynomometer works. I though the math that produced a torque reading at various engine speeds begins with the wheels rotating a drum, then takes into account gearing (which on a conventional transmission is constant in any gear), and measures the force it takes to accelerate rotation of that drum.

It sounds much harder to constantly revise the gear ratio on the fly. I once wasted some time looking for a test on a Gen V Prius, but didn't run across anything. I did find instances of other cars with CVT that mimic geared transmissions being tested, but those were tested in a fixed gear mode.

A manufacturer can publish peak numbers measured at a stand, but the story for the Prius may be less transparent. The car has very soft launch characteristics, but seems almost peppy above 70 mph. My WAG is that the system is software limited at lower speeds.

 

Again, it comes down to what torque do you want to know?

If you want to know the torque delivered to the wheels, the dynamometer measures that directly. No gear ratios or math needed.

When people with conventional cars drive onto a dyno and want to learn the output torque of their engine, they do that by taking the torque at the wheels (what the dyno directly measures) and mathing it with the final drive ratio and the gear ratio.

So yeah, if the questions keeping you up at night are "what torques should I think my engine, or MG1, or MG2 produce?", then it'll take some work to make sure you're posing questions you've got a methodology to answer.

But getting back to basics, maybe it would help to know why those are the questions important to you. A car, however it's built, is a machine for delivering torque to the wheels. Torque at the wheels is easy to measure. You only need more work if you want to tease apart different torques that are internally involved in making that happen.

 

The primary reason for doing the maths to work backwards from the wheel torque to the engine torque is for the benefit (or at least bragging rights) of engine tuners who want to see numbers they can directly compare with others' engine tuning results, possibly in different cars.

Not really such a useful thing to determine for the Prius - higher engine torque in a conventional car will always directly translate to higher wheel torque, but that isn't necessarily the case with the hybrid synergy drive.

 

I'd suppose you couldn't even calculate IC engine HP from wheel torque because you'd never have the IC engine working by itself.

It isn't important but I am curious partly because I don't know a lot about the topic and at least in part because the experience of driving it lacks the references points to which I am accustomed -- gears, changes in engine speed directly correlated to vehicle speed, a tachometer and readily felt changes in gear. The published Toyota figure for the 2 liter engine is 150hp at 6krpm, but I couldn't tell you if I've ever reached 6krpm. It sounds quite a bit slower even when the meter on the dash shows full power delivery.

 

The car itself is measuring what's going on and will give you its own reports live over OBD.

You can see instantaneous power from the engine, and power from the battery, and the rpm of the engine.

If you trust those figures, you can use them to compute the torque from the engine, the torque from the MGs, and what the torque at the wheels should be.

You can then cross-check that with your wheel dynometer measurement.

 

BTW, I had made a thread here which had a few posts trying to figure out exactly when you would get peak power. Sounds like it's up your alley:

Some (deep) notes on the Gen 5 transaxles | PriusChat

Kind of stalled because we just haven't seen enough live data from the car - we're still not totally sure what the MG1 rev limit is, and hence at what point the engine can reach 6000rpm for max power.

 

So you'd be disaggregating by subtraction at each wheel speed?

It's an interesting thread. I couldn't add anything useful except the observation I've already made that the car feels a lot peppier above 70mph than below 50mph, not what I'd expect from an intuition formed in other cars.

My sense is that if this were most other cars, you'd have a lot of dyno videos and instructions on how to horse around with parts but that this topic just won't be interesting to a lot of Prius buyers.

 

You can do all the physics maths - engine torque can be computed from power and rpm.

MG1 and MG2 rpm can be computed from wheel rpm and engine rpm.

Torque and power on the 3 parts of the drivetrain can be computed from the mechanics of the power-split device.

It actually splits torque in a constant ratio, but that gives an arbitrary power split depending on rpm. Rpm relationship between the 3 parts is fixed, but the car gets to choose ratio between engine and wheels by freely choosing the speed of MG1 (subject to rpm limits). Here's an old thread doing maths on that:

Power split device and electrical/mechanical power | PriusChat

Exactly what "savagegeese" said in that video. I think it is because that's when you get maximum system power, when the engine can reach its maximum 6000rpm power, subject to the MG1 rev limit.

A somewhat new "turbo" effect in these new 2-litre cars. Older cars had a lower engine rev limit, which they could reach at lower speeds. But 6000rpm on the engine needs quite some wheel speed to reach.

I think there was far more of this sort of analysis here back in 2004 when the Prius was new - there's far less picking over details on this latest generation. Because there's nothing really that new in concept, just changes to the values.

Another old thread:

World's first 2010 Prius Dyno | PriusChat