Power meter for car?

[Nicko]

Hi, first post
My main hobby is racing bicycle time trials. I'm very involved in the field testing "community" when it come to bicycles, aero drag and rolling resistance. The tool that has made CdA and Crr measurements available to "anyone" is the power meter. With the PM, there is no need for perfectly flat roads, known constant grades, coastdowns etc. Using the PM, a scale, a thermometer, air pressure data and a suitable stretch of "quiet" road you can measure you Crr and CdA with surpricing accuracy, often within 2%.
I have looked around here for any references to car power meters but found nothing. Can anyone point me in the right direction?

I have a ´96 1.6L Corolla and have an idea to put a load cell under one of the two longitudal engine mounts. That way I should be able to measure the "driving torque". Add a speed sensor and a data acquisition system hooked up to a laptop and I got myself a power meter

Anyone done something similar?

[RobertSmalls]

I would love to do exactly this. It's probably been done at various carmakers' proving grounds, but I've never seen a car power meter "in the wild".

Ideally, you would have strain gauges in each CV shaft to get driving torque, and read either the vehicle (i.e. transmission output shaft) speed sensor, or ABS wheel speed sensors (for higher precision).

You'd need an altimeter and knowledge of your car's mass if you wanted to account for gravitational potential energy.

Count fuel injector pulsewidth and divide by output shaft torque, and you'd have an instant engine+transmission efficiency readout.

If you can move this from "would love to do" to "have a proposal for a prototype, with bill of materials", then I'm very, very interested.

P.S. Second best is the ScanGauge. It's an excellent tool to coach yourself on driving technique, and to monitor your car's various sensors in real-time, but it's not a power meter.

[Mustang Dave]

The ScanGauge II with X-gauge capability can display engine BHP. It displays the gross value, including the power to drive all the accessories and overcome internal friction.
I'm not sure how accurate it is, though..

[Nicko]

To use the fuel consumption meter to calculate power to calculate coefficients of drag and RR wouldn't add any new information, I don't think. A proper power meter would enable measuring the "demand" side of the equation directly, regardless of engine "state".

I propose a force sensor that measures the reaction force to the "propulsion" force. I only see two mounts on my car that counteract the torque from the drive shafts (the other two are on a "left-right" axis) and both of them should see a force that is proportional to the propulsion force.
I'd like to know what practical problems there are with a sensor in that environment. How bad are the vibrations in comparison to the measured force? Are there high sideways forces too?

[Nicko]

Originally Posted by RobertSmalls

...
If you can move this from "would love to do" to "have a proposal for a prototype, with bill of materials", then I'm very, very interested.

I have a plan, (four) surplus 1000kg load cells and a logger that writes directly to a spreadsheet. I need to make a replacement engine mount that incorporates one load cell, some signal conditioning and a suitable wheel speed sensor.
I need a reason not to go forward...

[RobertSmalls]

A power meter on a car would immediately quantify the benefit of a new grille block geometry, or of front wheel skirts. It would be sensitive enough that you measure the energy consumption of the 10kg of tools you left in the car, and it can help you decide whether to open the window or turn on the fan. Do it. The feedback is way more precise and immediate than that of a ScanGauge.

The biggest reason against it is: you need a lot of strain gauges.

I don't know exactly where the Corolla's motor mounts are, but my Honda has three motor mounts, and all three are six-axis connections, and they all resist driving torque. You should get out a crowbar and see how stiff your mounts are in the relevant directions, and from there decide whether you're comfortable with the assumptions you want to make.

Honestly, the best argument against direct measurement of engine torque is that it's easier to estimate engine torque from engine operating parameters (RPM, MAF, fuel:air ratio, fuel injector pulsewidth, etc). I imagine that an eight-channel DAQ attached to sensors already on your car would be plenty to create beautiful logs and real-time bar graphs of energy consumption, broken down by aero, rolling, and inertial forces.

I'm an inexperienced mechanical engineer. Take my input for what it's worth.

BTW, welcome to the site!

[Nicko]

Originally Posted by RobertSmalls

A power meter on a car would immediately quantify the benefit of a new grille block geometry, or of front wheel skirts. It would be sensitive enough that you measure the energy consumption of the 10kg of tools you left in the car, and it can help you decide whether to open the window or turn on the fan. Do it. The feedback is way more precise and immediate than that of a ScanGauge.

The biggest reason against it is: you need a lot of strain gauges.

I don't know exactly where the Corolla's motor mounts are, but my Honda has three motor mounts, and all three are six-axis connections, and they all resist driving torque. You should get out a crowbar and see how stiff your mounts are in the relevant directions, and from there decide whether you're comfortable with the assumptions you want to make.

Honestly, the best argument against direct measurement of engine torque is that it's easier to estimate engine torque from engine operating parameters (RPM, MAF, fuel:air ratio, fuel injector pulsewidth, etc). I imagine that an eight-channel DAQ attached to sensors already on your car would be plenty to create beautiful logs and real-time bar graphs of energy consumption, broken down by aero, rolling, and inertial forces.

I'm an inexperienced mechanical engineer. Take my input for what it's worth.

BTW, welcome to the site!

Thanks!

I'm not sure you need a lot of strain gauges to get a signal that can be calibrated as an alias for propulsion force, that's why I need the opinion of others. The crowbar will come out soon

I doubt it's easier to estimate engine torque from a plethora of "indirect" parameters, than to put a suitable sensor "within the force flow"
I started a new thread in the proper forum, see you there.

[NiHaoMike]

Not helpful for a regular car, but a hybrid that uses HSD has capability to measure power. Not sure if a Scangauge can read that data, however.

[Daox]

A g-tech uses accelerometers to measure torque and therefore horsepower. I'm not sure if that is exactly what you are looking for, but its a start.

[MetroMPG]

Originally Posted by RobertSmalls

A power meter on a car would immediately quantify the benefit of a new grille block geometry, or of front wheel skirts. It would be sensitive enough that you measure the energy consumption of the 10kg of tools you left in the car, and it can help you decide whether to open the window or turn on the fan. Do it. The feedback is way more precise and immediate than that of a ScanGauge.

Perhaps, but it is still subject to all the other confounding factors that would invalidate the data that the device is providing.

I realize you know this, but some people reading this might think that a super accurate meter automatically equals valid data, but it doesn't.

If the user doesn't attempt to control for external variables as much as possible (eg. speed, grade, road surface, vehicle weight, wind, engine/drivetrain temperature, traffic, other vehicles' effects on your aero, cruise control vs. driver's right foot, etc. etc.) then comparisons between state A & B are suspect, regardless of the measuring tool.

Just saying.