[brian_]

units of measure

... Now the kinetic energy of motion at 70 mph is (.5mv^2) = 510682 lb-ft...

kinetic energy is measured in joules. lb-ft is a measure or torque, not energy.

In any system of units, the product of force and distance can be torque (moment of force), or energy, depending on context. For instance, in SI the units of torque are N⋅m, while the unit of energy is kg⋅m2/s2, or joule... which is also N⋅m. To keep these straight, the usual convention in the old units is to put the force unit first for torque (lb-ft), and the distance unit first for energy (ft-lb), so arguably kennybobby should have used ft-lb... but I don't think there's a consistent rule. Certainly in this context it is clear that the quantity is energy.

For a first order power estimate take the kinetic energy divided by your acceleration time, let's assume 0 to 70 in 13 seconds.

This gives a power of 39283 lb-ft/sec, a horse can pull 550 pounds over a distance of one foot in one second, so divide by 550 to get about 72 horsepower required for acceleration.

The fact that ft-lb is a unit of energy is demonstrated as kennybobby continued to show that the rate of change of energy is power... including that a "horsepower" is defined as 550 lb-ft/sec (just as a watt is a J/s or N⋅m/s or kg⋅m2/s3

Learn SI units, it makes the calculations a lot simpler.

Yes, that avoids the unit conversion constants, but the physics is the same regardless of the system of units.

 

[brian_]

Re: An Intuitive Approach to Fissiks and Mechanics

Every mechanic knows what a 5 lb sledge hammer feels like in the hand, but who has an intuitive feel for 15 Newtons?

15 newtons is the weight of 1.5 kg. That's far more familiar than "5 lb" for most people in the world outside of the U.S. Neither system is inherently easier; for each person, one system is more familiar.

How many mechanics have a 1 meter long ratchet in their toolbox?

Probably not many, unless they work on heavy equipment. On the other hand, doesn't everyone have a breaker bar about a metre long? I do.

 

[brian_]

You have quite modest battery requirements.

You only want 30 mile range. For your light car, perhaps 250 watt-hours/mile, so, 7500 watt-hours. That's incredibly small, you won't find anything that small in an OEM EV.

True, but you will find that in the lower-capacity plug-in hybrids. The Volt is a higher-capacity plug-in hybrid... like the Chrysler Pacifica Hybrid and Mitsubishi Outlander PHEV.

Perhaps more importantly, all modern EVs and hybrids use higher voltage than will likely be used with a brushed DC motor. If you use, for instance, one-third of a 360 V EV pack to get 120 V, you have one-third of the energy capacity... that's only 8 kWh for an early Leaf.

 

[brian_]

I see a few hybrid battery packs from Prius and Camry available. So, those would be suitable? And I'm looking for around 8kWh?
Looks like Prius Gen 1 is only 1.6kWh and Gen 2 is 4.4Kwh.
And are controllers and BMS hard to find/match for those?

P.S. Please let me know if I should start a thread in the battery section instead of here.

Yes, this is a battery question, not a motor question, unless you are determined to use the motor, controller, and battery all from the same donor.

And the answer to that battery question is that non-plug-in hybrid batteries are too small, but plug-in hybrid (Prius Prime, Chevrolet Volt, Chrysler Pacifica, Mitsubishi Outlander PHEV, etc) batteries are around the target energy capacity... as long as you want 360 volts.