Mechanical engineers: help me calculate stuff.
 

Such as how much HP does it take to move an NA down the highway at a constant 50 MPH on level ground, assuming ordinary all-season tires in the ~195 width range.

With the top up / down?

With the headlights up / down?

What about 65 MPH?

Etc.

Is this what you are looking for ?
Calculate HP For Speed
and
Part 6: Speed and Horsepower

It all depends on the coefficient of drag on the car, which can be then converted into a force, which can then be converted into power/torque

so unless someone has the experimental C_d values for what your looking for from full scale model testing, then your guess is as good as mine. Even then its going to differ from real-world results

I mean, how accurate of an answer are you looking for?

Doesn't MS have a HP readout given the required sensor inputs? I feel like dataloging that once its calibrated would give the best answer for your brake drag, bearing condition, headlight gaps, etc etc

Whoever wrote Virtual Dyno assembled all that. His documentation and possibly the program has it in it.

this calculator has the basic necessities included on an accurate estimation

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

The problem with this is that yes you theoretically should be able to estimate the power the car MAKES given the sensor inputs/outputs, but this doesnt tell you the power the car NEEDS to go that speed.

Drag Force = .5 * Drag Coefficient * Velocity^2 * Frontal Area * Air Density

^this can be converted to power fairly easily if all the variables are known

Estimated Engine HP based off MS = some complex function of the sensors given calibration is accurate. The uncertainty in this I'd imagine would be pretty high, but then again it should give you a ballpark

Vehicle Coefficient of Drag List - EcoModder

Calculate HP For Speed

Top down drag coefficient is .46, which means that 50mph takes 10.9 hp
Top up drag coefficient of .38 means 50mph takes 9.69 hp

That algorithm relies on acceleration. You can get the drag force empirically by doing a cost down test from 80 to 60mph and then get the rolling friction force by doing a coast down test from below 40 to like 10 mph. And thats just some math. And once you have that you can do your calculations on the horsepower required.

I love this thread already.

lol.

Attachment 234593
EDIT: For the record, I did not write the paragraph seen there using the word "ration"

the mpgs in my WRX drop significantly when cruising above 60mph compared to cruising at 55mph behind a big truck.

hope that helps.

“( ͡° ͜ʖ ͡°)”

https://www.miataturbo.net/attachmen...1&d=1418393510

Note Frontal Area and Cd. VD includes drag.

Anyway, there is the info for a NB. I don't know where to find lights up / lights down. CC gave you top up and top down.

It would be FAR easier and better to do some logs then hire a dyno for an hour.

Dann

I remember that class!

^This. What Leafy describes is basically a SAE test to determine the rolling resistance as well as the drag force, but with a slight deviation.

The TL;DR process is in red.

For the rolling resistance, hook a force gauge up to a strap that is connected to the front tie down hooks. Pull the car from a stop using the force gauge, as close to parallel to the ground as possible. This has to be done on a smooth surface that is level. At a steady 1mph or so, record the steady state force reading, just make sure that you are not accelerating or slowing down, this will greatly affect the readings. This is your rolling resistance in Lbs.

Perform a coast down test at a relatively high speed, such as 70-40mph. Knowing the weight of the car as well as the frontal area and other parameters, subtract the rolling resistance from the resistance calculated from the high speed tests, this will tell you the aero drag force, and from this you can find the coefficient of drag.

Using the information above with the calculations you guys posted, you can find out approximately how much HP is needed to go a certain speed.

Keep in mind, when you find this HP rating, this is the raw HP needed to accomplish this speed, it does not take any driveline or other losses into account. So if you are doing some project like trying to do a EV conversion and want to know how powerful the motor needs to be to achieve a certain speed, take a conservative estimate of the losses and go from there.

Long winded, but hope this helps.

why can't we just get a REALLY LONG inclined road of known angle and let the car coast to terminal velocity?

with frictionless rollers on it in a perfect vacuum at absolute zero?

Simple. Build a road at Mt. Everest Peak down to the bottom with a run off road and all in a sealed tube to pull a vacuum.

:pitlab:

Going back to why dont youy just do a log and then use a dyno?

Because the VD software, just like dynojet software, doesnt work if you're not accelerating.

has anyone asked the most important question- why do you (Joe P) care?

you making an electric miata commuter?

No no no.

Drive the desired speed in the desired conditions. Log manifold pressure and rpm or use tps or whatever you prefer. Simulate on dyno. Read HP output.

Easy.

No. Joe never asked for theoretical frictionless coefficient of drag. He wants to know steady state. Which means I have to revise my suggestion to using several ramps of varying angle until you find one that sustains 50 mph terminal velocity.


I think nitrodann has the easiest solution. do it empirically. except for the dyno part. I hear those aren't so great at determining drag. maybe an inside out dyno. One where the car is stationary and the wind moves. I'm sure there's one at Ames:

http://upload.wikimedia.org/wikipedi..._NASA_Ames.JPG

I'm merely curious as to the practicality of constructing a series-hybrid Miata, utilizing a small Li-ion battery back capable of providing the vehicle with a very short pure-electric range (say, 20 miles or so), while affording the capacity to travel unlimited distances on an occasional basis by utilizing a gasoline-powered generator.

Such vehicles have been constructed in the past which accommodate an external generator mounted on a small trailer, however I find this to be both grossly inelegant and also quite inconvenient. Sadly, gasoline-powered generators, at least those available at the consumer-level at a reasonable cost, are not especially compact.

Y8s, with the Dann Plan, the dyno needs no drag. On the street, run all test conditions, logging the MAP and RPM, with maybe pulse width as well. Then on the dyno, run those same settings and read the HP. Now you have the HP required for each of the street-logged conditions.

Top down, 60 mph, MAP was 62kPa. On dyno, run same gear at 60 mph, and 62 kPa. Record the HP. This is the requirement for 60 mph with top down. And so on for each condition. Need no wind and flat road, or average multiple runs on the street for each condition; then a single, multipoint, load controlled dyno session.

That seems like more work than just expecting someone to magically spoon-feed me the exact information I want.

My guess would be something in the 28 to 32 Hp for the conditions you are describing.
Maybe a bit less even.

I've found some sources within the DIY community which suggest that a generator capable of supplying 15kWe, powered by an engine capable of producing 20HP, is sufficient to sustain 60-65 MPH continuously in an average passenger car.

Of course, commercially-available generators so rated, while extremely inexpensive, are hilariously large and heavy.





Now, I'm going to branch off here for a moment to point out a few select pieces of the 2012 California Air Resources Board regulations on the sale of range-extended EVs.


First, the vehicle must have a rated all-electric range of at least 75 miles, which is higher than the 50 miles required of a zero-emission vehicle.

Second, the auxiliary power unit must provide range less than, or at most equal to, that battery range.

Third, and I quote directly: "The objective of the BEVx is not to develop a PHEV with universal appeal."


Dafuq?

I have no idea what CARB is trying to do here. On the one hand, they want to address range-axiety and broaden the appeal of battery-powered EVs. So to do that, they're first going to drive up and weight and cost of these vehicles by loading them with unnecessarily large battery packs (50% larger than a conventional, battery-only EV), and then they're going to deliberately cripple them by not allowing the added gasoline engine to give them a range which is sufficiently useful to cover those 10% of use-cases which rational, ordinary people will cite as the reason why owning an EV as their only car is not a practical proposition.

Well, I guess they achieved the objective of not developing a car with universal appeal.


https://www.miataturbo.net/attachmen...ine=1418924590

Joe if the Volt is sold in california then sum ting wong with what you just posted since the volt doesnt meat any of those requirements.

The Volt is classified as PHEV (Plug-in Hybrid Electric Vehicle), not a BEVx (Battery Electric Vehicle, extended.)

The distinction between these two classes is a point of great controversy with the greenies, but what it really boils down to is that California has pretty much eliminated any possibility of buying a small, lightweight, inexpensive vehicle which has just enough battery capacity to deal with your everyday commute, yet is still able to be driven on long trips from time to time as the need arises. Instead, you must either own two cars, or you must own a single car which is larger, heavier, more expensive and less efficient than is necessary.

They might as well outlaw four-door Sedans and require that everyone in CA instead purchase both a Miata and a Chevy Surburban.

So... make your car meet the PEHV standards? Whats the difference between a PEHV and a BEVx besides the electrical cord that plugs into the wall and not being forced to meet stupid rules?

Well, the electric cord itself isn't even a difference, since both PHEVs and BEVxs are able to plug in and charge their battery from grid power.

The honest answer is that the difference between a PHEV and a BEVx is whatever the regulatory agency says it is, and I have utterly no idea why its necessary to have a distinction between the two in the first place.




https://www.miataturbo.net/attachmen...1&d=1418926568

My palm and forehead are very sore right now.

I totally understand. I was just trying to clarify the concept. Seems like, on occasion, Dann has plausible ideas but people don't understand what he is trying to convey. ;) ;)

Joe, it sort of sounds like the BEHX is supposed to be a small, lightweight vehicle for around-town stuff, while the PEHV is more open to be an EV with an unlimited-range extension. I mean, depending on what you do with it, the name itself may not exactly describe the vehicle, but who cares?

If I missed something in there, I hope somebody will correct me.

I have the chance to buy a citicar (actually a Comuta-Car) for fairly cheap. Need batteries & some basic TLC. should I do it?

How cheap? Lithium batteries are cheap now a days, even buying your own LiFePO4 cells from alibaba is cheapish now.

Not as cheap as I was lead to believe. I was talking a grand, 1200 tops. Turns out he's talking 1700-1800. For a cheap plastic box with unknown electronics, that's a lot.

One thought I've always had, it would be "neat" to build an AC setup and have a clutch to connect/disconnect a fuel powered engine directly to the AC "motor" to make it a generator AND to directly power the car at typical cruising speeds. Advantages are: still no need for transmission, now only 1 motor that's now also a generator, lighter, less components, and more efficient when cruising with fuel engine as it's direct drive to wheels. Typical small diesel engine could be sized to do the job and allow you to drive at normal cruising speeds while charging too.

or just go get a prius?

Homebuilt=cool. Prius=fag.

Handwaving over some added complexity, you've described the Synergy drive used by Toyota in their hybrids.

Well then it's a conceivably reasonable approach. I wonder how DIY friendly/not friendly this would be? Or another thought, what cost is a used wrecked prius to salvage the system from? Knowing what batteries/motors/controllers cost, a wrecked old prius might be a bargain if it's not impossible to swap over the hybrid system.

rigging up a fag powertrain in a fag car = full circle? straight?

some of these "ideas" crack me up.

meanwhile, our prius is the most practical and convenient car we own.

I find that sick green NB with electric powertrain running 9's at like 142 really cool though

prius drivetrain in a CRX would be winning. With the HF motor thats a 50mpg chassis. With a hybrid setup it would be pretty interesting to see how much better it could get. With the HF motor and aero mods its a 136mpg chassis.