OutlawDon

I know the C5 coupes are rated at .29 and the Z06 are at .31,but I've always wondered how much more CD is added with the headlights up in the C5's. Anyone know?

0EG@EnglandGreen

They are airbrakes....

Glennm27

Plenty

MyVetteDream

You asked ... so here it is:

First, we will consider the air resistance of a moving vehicle. A parameter called the dynamic pressure is given by an application of the equation for kinetic energy: q = 0.5 * (rho) * V2, where (rho) is the density of the air and V is the vehicle's velocity to the air. This pressure must be multiplied by the frontal area (A) of the vehicle and by a factor called a shape coefficient (CD to get the actual total air-resistance force. Streamlined vehicles have a lower CD, as low as 0.2, while more boxy vehicles have coefficients nearly 1.0. These are all easily determined values. The result is a value (at 60 mph speed) of around 120 to 170 pounds of air resistance force, for a mid-sized sedan-style car.

There is also tire resistance, which, at 60 mph, is usually around 1.5% of the vehicle weight. For a modern 3,000 pound automobile, that's around 45 pounds of tire resistance force. The total vehicle drag is the sum of these two (air resistance and tire resistance), which ranges from about 150 to 230 pounds force (again, for standard-shaped cars, and not trucks or SUVs). Wheel bearings and other mechanical frictions also exist, but in a reasonably maintained vehicle, they are very small and will be neglected here. At 60 mph [ = 88 feet/second] this is the equivalent to around 30 horsepower. (200 pounds drag force * 88 feet/second = 17600 foot-pounds/second, divided by 550 is 32 horsepower).

That was for a mid-sized car. What about a sub-compact car? The frontal area is much smaller, so the air resistance drag is less, and the vehicle weight is less, so the tire resistance is less, too. Such a small car could therefore have a total drag (again, at 60 mph) of around 130 pounds, which is around 21 necessary horsepower. (This is why small cars can have less powerful engines! They generally need less horsepower to travel at a constant speed.)

But the headlights are just as bright on a small car, so they use just as much electricity, so they can still need engine power to generate it. In this case, the load on the engine might rise from 21 HP to 23, an increase of around 9%. So, for a sub-compact car, daytime driving with the lights on could increase gasoline usage (and cost) by around 9%.

Regarding the headlights, on low-beams, each headlight draws around 4 amperes of electricity, usually at around 14 volts (while the engine is running). That's 55 watts each (at 12 volts) or about 75 watts (at 14 volts) or about 150 watts between the two headlights. There are assorted other exterior vehicle lights that are on when the headlights are turned on, at least two taillights, two front parking lights, four side marker lights, etc. These lights add around 70 watts, so the entire exterior light electrical load is around 220 watts.

Since one horsepower is equivalent to 746 watts, that is actually around 1/3 horsepower equivalent of electricity. It turns out that there are significant losses in the production of the electricity, primarily in the fan belt drive and in the operation of the electric and magnetic fields inside the alternator itself. The net effect is that between 1.5 and 2 engine horsepower is used up in creating the electricity for the headlights. On a different subject regarding the production and distribution of electricity, did you realize that only around 13% of the energy in the coal or gas burned in an electric generating plant actually gets to our houses to be usable?

And that's for "low" beams. High beams are much brighter and take more electricity, so the total amount of electrical load is higher and so a little more horsepower must be taken from the engine.

On a somewhat similar subject, have you ever noticed that, when you turn on the air conditioning when the vehicle is idling, the engine revs up? The air conditioner compressor requires as much as 6 engine horsepower in order to create the air conditioning. If the engine stayed at normal idle, it does not create enough horsepower to provide this and the engine would immediately stall. So all vehicles have a provision for the engine to go to a fast idle, to provide the needed extra power. The automobile headlights do not require so much power to stall the engine, but the general concept is similar. Actually, in some cars, at idle, if the headlights are turned on, you can tell the engine speed slightly changes, as the engine speeds up to create the extra needed power.

I first noticed this effect because I generally drove from Chicago to Fort Lauderdale half a dozen times each year, the exact same trip. When I got a Corvette in 2003, that car made many such trips. It has a gasoline consumption readout, both for instantaneous and trip average. I often drive with the instantaneous value displayed. Since Georgia gasoline is always 10 cents lower than in other States (due to lower state gasoline taxes) I generally would fill up with gasoline at the same stations just inside the northern and southern borders. So I would drive that 350 miles with relatively few external variables each time. I quickly noticed that the average fuel mileage was slightly lower going north, which I attributed to a rise in altitude and the generally prevailing headwinds. But that was a minor effect, a small fraction of a MPG.

BiggieBoy

OutlawDon

MyVetteDream,

Thanks for the novel,but my question is still not answered I don't think. I am asking about air resistance increase with the headlights up...wasn't really concerned with electricity of the high beam,or low beam, or a mid-sized car, or a sub-compact car, or how many watts, nor my idle speed,etc. Thank you again for your knowledge of physics though.

-Don

MyVetteDream

The answer is that there are so many variables, one answer may not exist.

Besides, we all need to know more about "electricity of the high beam,or low beam, or a mid-sized car, or a sub-compact car, or how many watts, nor my idle speed,etc."

BTW, The whole long answer was meant to be a joke

I couldn't find an emoticon that said the difference was thiiiiis much

ls1vette97

I wonder how much it really effects top speed?!? If it's alot I'm going to have to look into some bright foglights!

jrose7004

Oh brother!

IdleCap

I thought my hood seal offset the coefficient drag of the headlights being up!

RJH20dog

I'm curious....you planning on driving over 170 MPH at night?

c5BlackBeauty

Right behind ya......

WolfeBros

If your running the 24 hours at Daytona you might notice a bit of drag.
99.98 % of the rest of us don't go fast enough to feel the difference.

FiberglassFan

Just use your instantanious MPG read out....Find a straight level hiway, put on cruze control, call up MPG display on the DIC and observe the reading, then raise the headlights and see what happens. True it won't give you a CX figure, but it will give you real world numbers of decreased MPG due to slightly higher drag.

gpotski

I was driving on the highway last month. Left home in the dark at 0430. I was getting about 27.5 mpg with the lights up (and on). When it got light enough, I turned off the headlights, and my mpg jumped to 27.7. In both cases, this was a value over a tank of gas.
So I saw a 0.2 mpg improvement with the lights down. Some may be the saved electricity, but I am sure some is in the reduced drag.

Ron C!

The answer is my MPG drops by 3 when I pop the lights up.

Cary

WERD!

I like spirited driving just as much as the next Vette owner, yet at night my vision is not as good as during the daylight. So, I could careless about the increased drag of the pop-ups.

DSOM Z06

Glad I ditched them!

0EG@EnglandGreen

They are barn doors...

ExRedRacer

Sometimes the simplest solutions are the best...