The effect of horsepower on gas mileage

[omgwtfbyobbq]

Run your numbers yourself, sum up the rolling and fluid friction @48mph for each vehicle. There is ~27% difference in energy required to move the Vette @48mph EPA highway, but the difference in mpg is ~136%! Where does this extra inefficiency come from? Primarily, the fact that the Vette has an engine six times the size of the Insight's, and pumping losses at 50mph are huge!

Hybrids aside, since we're talking about pumping losses, so any tech like electric motors, idle shut-off, mds, that minimizes those, and masks the impact of pumping losses should probably not be looked at...
The Mustang you mentioned (mt) has a difference in displacement of 4.6L/4L=~15%, and a difference in highway efficiency of 28mpg/25mpg=~12%. The Camry you mentioned, not the hybrid version, get's 28mpg for the the 3.3L 4sp auto, and 34mpg for the 2.4L 4sp auto. 3.3L/2.4L=~38% difference in displacement, and 34mpg/28mpg=~21% difference in mpg. So, the drop in efficiency is not linear, probably because friction losses are greater in smaller displacement engines. But, the point I'm making is that pumping losses are a big part of gasoline FE! And the Vette would still get ~48~50mpg (maybe more, maybe less, ballpark) if it had the Insight's engine, since there is only a ~27% difference in glider energy requirements. The only way to get the vette to 50mpg would be to drop the force needed at 48mph by ~67%, LRR tires only give about 8%, so the remaining must be done via aero, and like you said, if we improve that by 50%, we'll be there.
Here I go off topic again!
Anyway, pumping losses>friction losses. *EV motors>ICE engines!

*I've been thinking about a li-ion powered velomobile.

[The Toecutter]

You also have to account for the fact that the Corvette's alignment isn't tuned for efficiency but for traction. Same with its brakes, which are likely the cause of much drag. For that matter, there's also its transmission.

When it comes down to it, the difference between a V8 and a L3/electric drive would be more closer to 50% than 100%. Lets run some numbers, a set for the Corvette, and a set for the Hybrid.

2007 Chevrolet Corvette Coupe:

Mass(W): 1,442 kilograms
Drag Coefficient(Cd): .28
Frontal Area(A): 2.08 square meters
Rolling Resistance Coefficient(Cr): .012
Transmission Efficiency(TE): .90

Velocity(V): expressed in meters per second
Force Drag(FD): expressed in newtons
Force Rolling(FR): expressed in newtons
Stray Force(SF): 40 newtons
Wheel Power(WP): expressed in watts
Engine Power(EP): expressed in watts

Air Density(Rho): 1.25 kg/m^3
Gravitational Constant(G): 9.8 N/kg


Equations used:

FD = .5 * Rho * Cd * A * V^2
FR = Cr * W * G

WP = (FD + FR + FS) * V
MP = WP / TE

Results:

At 21.6 m/s(48 mph):

FD = 170
FR = 170
SF = 40
WP = 8,208
MP = 9,120

At 26.8 m/s(60 mph):

FD = 261
FR = 170
SF = 40
WP = 12,623
MP = 14,025

2006 Honda Insight:

Mass(W): 840 kilograms
Drag Coefficient(Cd): .25
Frontal Area(A): 1.9 square meters
Rolling Resistance Coefficient(Cr): .006
Transmission Efficiency(TE): .94

Velocity(V): expressed in meters per second
Force Drag(FD): expressed in newtons
Force Rolling(FR): expressed in newtons
Stray Force(SF): 20 newtons
Wheel Power(WP): expressed in watts
Engine Power(EP): expressed in watts

Results:

At 21.6 m/s(48 mph):

FD = 139
FR = 49
SF = 20
WP = 4,493
MP = 4,780

At 26.8 m/s(60 mph):

FD = 213
FR = 49
SF = 20
WP = 7,558
MP = 8,040



At 60 mph, the power at the motor of the Honda Insight would be about 57% that of the Vette. At 48 mph, the power required by the Insight is about 55% that of the Vette!


That's where the bulk of the mileage differences are at. Put the V8 in the Insight, and fuel economy would probably drop around 40% or so. And it would still embarass most anything else on the road when it comes to fuel economy.

I chose a higher amount of stray friction for the Vette over the Insight due to the Insight being designed with efficiency in mind, as opposed to maximizing traction and other performance aspects. The Insight also has a more efficient transmission and the numbers were chosen accordingly.

[omgwtfbyobbq]

I think your FR's are way, way high because the manufacturer curb weight already includes gravity, since they weight them on scales. If the vette weighs 1.442kg than we're already including gravity, and in this context weight is in the N=kg as opposed to the actual mass kg. If you divide your FR's by 9.8, than I think you're at the force needed to overcome rolling resistance. As per the rolling resistance expression, Crr is dimensionless, so the W must be in N already.

[The Toecutter]

Kg is a unit of mass, not weight. At the Earth's level of gravity, there are ~2.2035 pounds of force for every kilogram of mass. In space, mass and weight would basically have no relation, as mass would stay the same as it was on Earth and weight would basically be 0 pounds.

To find the total force gravity in Newtons, you take the mass in kilograms and multiply by the gravitational constant.

With pounds as opposed to kilograms, the gravitational constant is already included and it's a simple conversion. Newtons and pounds measure the same thing: weight. Kilograms is not a unit of weight.

It is not unusual for a car to have 30 pounds of rolling force. That equates to a 3,000 pound car with a .010 Cr. 1 pound is equal to 4.45 Newtons. Convert accordingly.

[omgwtfbyobbq]

Originally Posted by The Toecutter

Kg is a unit of mass, not weight. At the Earth's level of gravity, there are ~2.2035 pounds of force for every kilogram of mass. In space, mass and weight would basically have no relation, as mass would stay the same as it was on Earth and weight would basically be 0 pounds.

To find the total force gravity in Newtons, you take the mass in kilograms and multiply by the gravitational constant.

With pounds as opposed to kilograms, the gravitational constant is already included and it's a simple conversion. Newtons and pounds measure the same thing: weight. Kilograms is not a unit of weight.

It is not unusual for a car to have 30 pounds of rolling force. That equates to a 3,000 pound car with a .010 Cr. 1 pound is equal to 4.45 Newtons. Convert accordingly.

I think I see where you and I both went wrong! If like you stated, with pounds, the gravitational constant (~32ft/s^2) is already included, to convert to the mass in kilograms, we need to first divide the weight by the acceleration due to gravity to arrive at the mass in pounds, then we convert that mass in pounds to mass in kilograms, then multiply that by 9.8m/s^2, and finally multiply by the dimensionless Cr.

In other words, if the weight in pounds is 3174pdl, then we divide by 32ft/s^2 to arrive at a mass of 99lbs. Converting to kg we get 45kg, and the normal force is 45kg multiplied by 9.8m/s^2, which is 441N, so the rolling friction is the product of this and .012, which is roughly 5N, which converts to ~38pdl, which is what you get if you multiply 3174pdl by .012.

So, it must be that rolling friction is much small than either one of us though, or, in fact, the weight in pounds is actually the mass in lbs and we must multiply by 32ft/s^2, then multiply by the Cr to get the rolling friction, and the same goes for the mass in kg. So, the question is, provided that the units convert correctly in both cases, is weight generally a measurement of mass, or of force?

[omgwtfbyobbq]

I hate standard units.
This sucks beause I was seriously underestimating rolling friction, but it's very very cool because if my tires have a Crr=.012, getting LRR tires Crr=.006 and aligning everything 0,0,0 should bring my mileage up to almost 70mpg@50mph! Anyway, as we've seen before, I hate units!
Not that pumping losses aren't significant, there just not as significant as my ignorance of the imperial units, crazy *** things.

Engineering Challenges: Adding another technology to a vehicle does not necessarily improve fuel efficiency. For example, variable valve timing and cylinder deactivation both target the energy ?pumping loss? that occurs as the engine ?breathes? (that is, operates at a lower intensity). Once the first technology is applied, that pumping loss has been largely reduced so the second technology will provide less benefit.

[The Toecutter]

If I had my way, the auto industry would be bringing on the 40+ mpg musclecars, 80+ mpg biodiesel-powered family sedans, and 250 mile range pure electric cars.

My design philosophy is that performance and fuel economy need not be mutually exclusive. Both can be had in a properly designed car that retains much of the amenities of the cars seen today.

[white90crxhf]

i wouldn't say a force is essentially weight. the drag on the car is a force, same with the wind pushing it forward or against it.

incase anyone is wondering, a slug is the measurement for mass in the engilsh system.

[JanGeo]

you measure mass with a balance scale using weight against weight so gravity doesn't matter - weight measured with a spring scale is essentially the same here on earth as mass, just scientifically speaking, mass is used so the units are correct.

[omgwtfbyobbq]

But we can't convert a force to a mass without an acceleration. If the pounds is actually pound-force, then to convert we need to go to another force. In either case, the common use of SI kilogram must actually be an acceleration, meaning kilogram force instead of kilogram, just like pound force instead of pound.