Calculating HP from speedometer video

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In summary, the 2016 Toyota Prius has a confirmed 0-60 time of approximately 9.8 seconds, similar to the current model. However, the initial acceleration horsepower is lower than expected, possibly due to a limitation of the drivetrain and the use of an electronic, constant velocity transmission. Further testing is needed to determine the cause of this limitation. It is also suspected that the gas motor is not used at low speeds, and the power is limited by the batteries' discharge rate.
  • #1
bwilson4web
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Recently, Toyota held a 'press drive' day for their 2016 Prius and Jason of Jalopnick had the good sense to make a video of the speedometer during a maximum acceleration. I downloaded the video and coded the timestamps of each speed change to generate this curve:
2016_metrics_200.jpg

This confirmed the 0-60 is ~9.8 seconds like the current model. But we also knew the curb weight, gasoline level, and Jason's weight.

So three of us independently calculated the power required with similar results and this is mine:
2016_metrics_210.jpg

  • HP (KE) - the horsepower calculated by the kinetic energy change of the accelerating car.
  • HP (drag) - the previous model roll-down coefficients with the velocity squared term (aerodynamic drag) scaled by the improved coefficient of drag change, .24 / .25.
What has me puzzled is the low, initial acceleration horsepower:
  • 40 hp @ 1 second
  • 65 hp @ 2 seconds
  • 77 hp @ 3 seconds
  • 85 hp @ 4 seconds
  • levels off at 90-100 hp
Now the Prius uses an electronic, constant velocity transmission, effectively an infinite ratio eCVT. My first thought was the low, early horsepower was to avoid breaking traction and uselessly spinning the tires. Someone else speculated it was a limitation of the Prius engine and eCVT. So my question is how to resolve this?

I am planning to do a pull-test with our current model, Prius and measure how much traction force it asserts. My thinking is the control laws should limit the zero speed, torque to a finite limit. This is probably in the same order of magnitude as the new Prius since both are rated at the same 0-60 times.
The only other approach would be to set a torque limit and see what it does for the early acceleration curve.

Thoughts?

Bob Wilson, Huntsville AL

ps. Toyota changed the control laws so the engine speed is proportional to the speed. It would make sense that this avoids a useless engine wind-up if so little power is actually needed to the wheels.
 
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  • #2
Breaking traction would only be an issue for a second or less.

My suspicion is that if you add torque to the graph, you'll find the torque is constant. That would be a limitation of the electric motor (high torque+low rpm = overheating) and/or a choice by the engineers to improve fuel economy.
 
  • #3
The low early horsepower is likely a limitation of the drivetrain. Even with a CVT, the gear ratios are not infinitely variable, and at close to zero speed, the available power will be limited by the fact that the drive motor is also spinning at near zero speed (where it cannot make peak power). Once the car is moving fast enough that the drive motor is able to spin at the correct speed to make maximum power, the CVT changes ratios continuously to hold the drive motor at this speed, and thus the power levels off. If you looked at the tachometer, I would bet that from 0-5 seconds in your graph, the drive motor is accelerating, and from 5 seconds onwards, the motor RPM would be held constant while the gear ratio changed. This isn't really a problem for the car because at low speed, you do not need much power for adequate acceleration, and by the time you reach a speed where having the additional power is important (30 or 40 mph, in this case), the motor is spinning fast enough to provide that power.
 
  • #4
I know very little about the prius but I heard that the gas motor isn't used at low speeds - so at what point does it cut in? Is it used at low speed when max acceleration is called for?
 
  • #5
If the gas motor is not used at low speeds, then the power is probably limited by the batteries maximum discharge rate, not the electric motor's maximum rating.
 

Related to Calculating HP from speedometer video

What is "Calculating HP from speedometer video"?

"Calculating HP from speedometer video" is a method used by scientists to determine the horsepower (HP) of a vehicle by analyzing the speedometer reading in a video recording of the vehicle's acceleration.

How accurate is this method?

The accuracy of this method depends on the quality of the video recording and the consistency of the speedometer calibration. With precise measurements and a well-maintained vehicle, this method can provide accurate results.

What are the steps involved in calculating HP from speedometer video?

The steps involved in this method include recording the vehicle's acceleration on video, measuring the distance traveled, and using a formula to calculate the vehicle's horsepower based on the speedometer reading and acceleration time. A more detailed process may involve factoring in other variables such as weight and air resistance.

Can this method be applied to all types of vehicles?

While this method can be applied to most vehicles, it may not be accurate for vehicles with continuously variable transmissions (CVT) or electric vehicles, as they do not have a traditional speedometer. Additionally, the accuracy may vary for different types of engines.

Are there any limitations to this method?

One limitation of this method is that it only provides an estimate of the vehicle's horsepower and is not as accurate as other methods such as dynamometer testing. Additionally, external factors such as road conditions and driver behavior can affect the results.

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