How much power to maintain speed?

[retepsnikrep]

How much power is reqd to maintain an Insight at 30mph on the flat in good calm conditions? BHP? KW?

Assume tyres at say 50psi.

Any offers?

[Willie Williford]

Assuming you are in 5th gear, the engine is produceing app. 20 ft. pounds of torque and probably 15BHP.

HTH
Willie

This is based on the HP torque graph in the "Technicians Information Guide" booklet I have.

[b1shmu63]

I did a test on a windless summer day. I chose a very long straight hill with if memory serves a 3 degree slope and zero traffic, started at the top at about 55 MPH and was doing 70+ and still accellerating at the bottom. In any case I calculated from this rather crude test that 50 MPH should be attainable with less than 12 HP. Testing for 30 MPH should be much safer if you can just find a very long hill with perhaps a one degree slope, no wind and no traffic. I posted the actual test results on one of the threads. You need a good transit or GPS and a little spare time. For accuracy you need to find a speed you can start down the hill and maintain in neutral with just the slope of the hill. Calculate the force due to gravity using a log table or scientific calculator. Multiply force X distance X time, then convert to HP using an Internet online converter. The toughest part is finding the right hill.

[retepsnikrep]

Interesting I thought 15bhp was bit high, my 2200lb ev with the aerodynamics of a brick and a 12kw motor could do 50mph on the flat. I would suggest 3-4kw might be nearer the mark.

[b1shmu63]

There was a corner at the top of the hill that I wasn't comfortable taking at more than 55. The hill at 3 degrees was just too steep for a real test without feeling like I was going for the sound barrier, and anything other than a steady speed versus constant slope requires too much rocket science (calculus).

[IamIan]

Originally Posted by retepsnikrep

How much power is reqd to maintain an Insight at 30mph on the flat in good calm conditions? BHP? KW? Assume tyres at say 50psi. Any offers?

30 mph = ~48.28 kph = ~13.411 m/s

The formula for Instantaneous Rolling resistance force in ( Newtons ) N is:
F = -Cr * m * g (rolling resistance)
F = Force of rolling resistance ( Newton )
Cr = Coefficient of rolling resistance of LRR Tires on a smooth road 0.006-0.01
m * g = Total Weight
g =9.8 m / s^2
m = 834 kg for a 5 speed Insight without AC.

So under good conditions the Instantaneous Rolling resistance = 834*9.8*0.006 = ~49.0392 N
1 Watt = 1 Nm/s
49.0392 N * 13.411 m/s = ~657.6647112 Watts

-------------------

Aerodynamic Drag :
Instantaneous force of drag is:
P = -1/2 * p * V * V * A * Cd
where :
F is the force of drag ( Newton )
p is the density of the Air ( kg/m³ )
V is the velocity of the car ( meters per unit time )
A is the frontal Area ( Insight = 1.9 Square Meters )
Cd is the drag coefficient ( Insight = 0.25 )
The air density will change with humidity and temperature as will Cr, but at about...
At +30 degrees C the Density of the air is about 1.164 kg/m³

Instantaneous Aerodynamic drag force:
1/2*1.164*13.411*13.411*1.9*0.25 = ~49.721 N
1 Watt = 1 Nm/s
49.721 * 13.411 = ~666.807 Watts

-----------------

~657.6647112 Watts Rolling resistance + ~666.807 Watts Aerodynamic Resistance = ~1,324.47161 Watts
= ~ 1.3245 kW
1kW = ~1.34 HP

So it takes a minimum of ~0.9884 HP ( just about 1 HP ) for an insight to maintain about ~30 MPH on a flat level ground under good conditions and ~30 degrees C.

Of course real world variable will cause variation.

[Guillermo]

Ian is right that it takes at LEAST 1 HP to maintain 30 mph, but his calculations are missing other important factors such as mechanical frictional losses etc..

A better way to determine the power required to drive at a specific speed is to do a coast down test on a level road with no wind.
Since finding a completely level road and no wind (therefore no traffic as well) is extremely difficult, just find an empty stretch of road that comes close to that description and do coast down testing in both directions multiple times to average out the error.

One of Newtons laws says that an object in motion will remain in motion until an external force is acted on it. (something like that)
What this means is that there are forces acted on the Insight that prevent it from maintaining a constant speed on level ground unless it is outputting some power to overcome air resistance, rolling resistance and mechanical friction in the bearings etc...

The main equation to use is
Force = Mass x Acceleration

Mass can be measured at truck weigh station or at a dump weigh station or just use the published car weight plus driver and passenger/cargo weight.
Acceleration can be calculated by plotting the data from the coast down test.

Coast down test:
Use a passenger to help to improve accuracy and for safety reasons
Use a stopwatch with slip times or lap time memory
Use a level straight stretch of road with little or no wind and no traffic (because you will be slowing down a lot)
Start at a high speed for example 60 mph.
put the car in neutral and have the passenger stare at the speedometer.
Have the passenger start the stopwatch as the car speed passes exactly 50 mph and measure "lap time" at every 10 mph interval.

Acceleration = Change in Velocity / Time

Rolling resistance and mechanical friction changes very little with speed.
But wind resistance increases exponentially compared to the increase in speed.
Therefore at higher speeds such as highway driving wind resistance is significantly more then rolling resistance/friction.
But at low speeds wind resistance can be less significant then rolling resistance/friction.
So during the coast down test the time measured increases between the 10 mph drop intervals.
So the calculated Acceleration will the be the average acceleration between the speed interval range.
Since this is a coast down test when the vehicle is in neutral, the calculated power is the engine power applied to the clutch to maintain a constant speed.
So the calculated power is closer to crank HP then wheel HP.

I haven't done this before so I don't have numbers to work with, but hopefully I explained in a way that anyone that remembers high school level physics can continue where i left off.

Let me know what you find out.

[gpsman1]

I dont know the accuracy, but the ScanGauge and other scan tools will give you a real-time number for Horsepower at any given instant. I assume it is ICE horsepower only, so run your tests without Regen or Assist.

My 4,000 pound shaped like a brick with huge tires Escape Hybrid with eCVT takes the following:

22 HP to maintain 55 MPH
33 HP to maintain 65 MPH
44 HP to maintain 75 MPH

50 HP will maintain 55 MPH up a 6-7% grade.

I've only had a scangauge in the Insight for a few days and have not run the same tests yet.
If there are interested parties, I would be happy to repeat the above.
Often, I see numbers in the single digits when drving the Insight.
-John

[IamIan]

Originally Posted by Guillermo

Ian is right that it takes at LEAST 1 HP to maintain 30 mph, but his calculations are missing other important factors such as mechanical frictional losses etc..

100% correct... the 1 HP is the amount of power needed to be applied to over come the aerodynamic and rolling resistance under those limited conditions.... the Transmission is not 100% efficient so losses.... etc... this means you have to generate more than 1 HP to get 1 HP worth of applied power.....

Also going up even a 1% incline will change the results as well...
rolling resistance will be reduced by 1% as the normal force against the road is no longer the whole weight...
so ~6.57 Watts less rolling resistance... but you traded it for gravitational force acting against you... which means ~1% of the gravitation force of the car becomes a counter force... or about 834*9.8*0.01= ~81.732 N * 13.411 = ~1,096 Watts = ~1.1kW of gravitational resistance to even a 1% incline.

and it assumes a wind speed of zero... faster winds speeds will increase the aerodynamic resistance exponentially.

Like I said in the calculation post ... "real world variables will cause variation."

Unfortunately it is nearly impossible to achieve the limited "Good" conditions in the real world... Wind speed will only be near zero in a large warehouse with all the doors closed... you would also need such a wear house with a perfectly level floor as even small variations in angle will throw off the results.... etc...etc... the real world is tooo complicated.

[Mike Dabrowski 2000]

Not a very exact test, but should give you some guidelines.
My e-wheel has taken me 40+ miles at 24-30MPH on a full charge 100% to 40% SOC of my AGM31 110AH 48V pack. The motor current is limited to 190A. Cruising at 30 mph, flat ground, it sits at about 20-30A. The road I traveled during this test run included the stretch of road displayed in this topo graph, http://www.99mpg.com/Projectcars/TheBes ... file.jpg,0

I would guess that no more than 2-3HP would be required. The acceleration with that small amount of power will be painfully slow though.