aerodynamics question!

[deathegg]

Hi everyone! I need a lesson on aero. I know when they first design a car, they take aero into consideration along with engine size, gearing, etc.

However, when modding a car (let's say my '10 Yaris hatch)....how do any aero improvements I do help? That is, at a certain mph (let's say 65), it's in 5th gear, and therefore the engine is turning at a specific rpm at that speed. So If I do any aero work, the engine will still be turning that specific rpm @ a given speed, will it not? So how would any improvements help?

Does that make sense? Am I missing something?! (happens a lot!) lol

I CAN see how the improvements would help while coasting, but then again my Yaris has deceleration fuel cutoff, so if I'm coasting in gear I'm using no gas anyway, but I guess I can coast for a longer distance.

[blkfordsedan]

Aerodynamics has no connection to the engine RPM at all. Basicallly, all that aerodynamic improvements will do is decrease the power required to move through the air at any given speed. Less power = Less fuel consumption. In your case, lets say at 65mph your Yaris requires 12hp to maintain speed. If you add body mods, Yosemite Sam mudflaps, or whatever, and reduce the drag (or air resistance), it may only require 10hp to maintain a 65MPH cruise.

To expand a little, an engine is a variable output device. At any given throttle opening, it will produce a specific amount of power. Depending on the resistance the motor experiences, the RPM will vary. If the resistance is less than the power output then the motor speeds up. If the resistance is more than the power output, the motor slows down. A prime example is when you're driving on the highway and have your foot set in a fixed position on the gas pedal. When you start going up hill, the car will slow down (unless you give it more gas). When you go back down hill, the car will speed up. The correlation between engine RPM and vehicle speed is fixed...because there is a physical link between the crankshaft and the road. The transmission gears and differential all change the relationship (i.e. ratios), but for any specific cranshaft RPM there is a specific and exact RPM that the wheel will turn. What that wheel RPM is depends on what gear your in. ****unless you have an automatic with a torque converter to slip, but that's a little more complicated and not important at this point.

[Kraig]

It requires less horsepower to maintain that speed....less throttle...which means less gas! RPM and fuel consumption don't necessarily coincide.....

[deathegg]

Aerodynamics has no connection to the engine RPM at all. Basicallly, all that aerodynamic improvements will do is decrease the power required to move through the air at any given speed. Less power = Less fuel consumption. In your case, lets say at 65mph your Yaris requires 12hp to maintain speed. If you add body mods, Yosemite Sam mudflaps, or whatever, and reduce the drag (or air resistance), it may only require 10hp to maintain a 65MPH cruise.

To expand a little, an engine is a variable output device. At any given throttle opening, it will produce a specific amount of power. Depending on the resistance the motor experiences, the RPM will vary. If the resistance is less than the power output then the motor speeds up. If the resistance is more than the power output, the motor slows down. A prime example is when you're driving on the highway and have your foot set in a fixed position on the gas pedal. When you start going up hill, the car will slow down (unless you give it more gas). When you go back down hill, the car will speed up. The correlation between engine RPM and vehicle speed is fixed...because there is a physical link between the crankshaft and the road. The transmission gears and differential all change the relationship (i.e. ratios), but for any specific cranshaft RPM there is a specific and exact RPM that the wheel will turn. What that wheel RPM is depends on what gear your in. ****unless you have an automatic with a torque converter to slip, but that's a little more complicated and not important at this point.

First, I know aero aids DO help if done properly. The problem I'm having is how they do it.

What I'm not understanding is this: Since a specific speed in a specific gear = a specific rpm (lets pretend 65 m.p.h / 5th gear = 2500 RPM), how does aero improve MPG? I've always understood that at X-rpm, the engine makes a specific amount of power. For instance, my Yaris makes 106 HP @ 6000 rpm. HP does not (as far as I know), increase or decrease at 6000 rpm; if it's revved to 6000 rpm, it's ALWAYS 106 HP. Not 100. Not 115. It's 106.

So, for my 2500 rpm example, it makes X hp. Why would aero help m.p.g since the HP being created @2500 rpm has not decreased?

My brain is not happy right now. I feel stupid, because I think I should know this.

Of course, I may be over-thinking this and making it waaay more complex than needed. I tend to do that a lot too!

[AlaskaFestivaGuy]

http://en.wikipedia.org/wiki/Parasitic_drag

Notice that parasitic drag (aka skin friction) is proportional to the SQUARE of velocity.

Induced drag is not really a factor until you add wings to your Festy.

[deathegg]

WTF?? Thanks for the link, but I glanced at that article and almost had a seizure!! Didn't even try to comprehend it.
My brain is not wired for that sort of complex math (it's math...or something, right??)

[Advancedynamix]

Horsepower is not always the same for a certain RPM. When a manufacturer lists the output of an engine, it is listed as the maximum HP that that engine makes at full throttle under a calibrated load. Your engine may make 106hp at 6000 rpm under a 95 ft lb torque load while at full throttle, but it wont make that same 106hp at 6000 rpm with 10 ft/lb torque load at 1/4 throttle. It may only be making approximately 11.5 HP. With modern fuel injection systems, fuel consumption can change drastically with less engine load.

All that said, you will be hard pressed to improve the fuel mileage of a Yaris with Aerodynamics. Toyota spent millions of dollars making that car aerodynamic and it has one of the lowest coefficient drag ratings in the modern automotive world. You would have to chop the top and lay down while driving it! Lol!

Hope that helped to clear things up.

[twopass]

more aero= less drag
less drag = less power used to do same work or maintain speed
less power used to maintain speed = less fuel used

[Movin]

One thing that does affect the Yari and other cars that require very little
power to maintain speed..Get the best rated low rolling resistance tires,
the tires themselves have less internal friction and are more aero shaped
than most popular 15's. Use gasoline with no alky in it clear gas is a little
hard to find but it contains more energy per gallon than the junk we are
told is best..These 2 things are more effective than aero mods on that particular
car. Next would be to keep the car empty of the 200lbs of junk most cars have
if you drive city a lot. 4th..aero.? nope ..quit loading the alternator so hard
with that huge stereo..5th..tape up rough edges and seams..? nope, driving
technique will outperform aero..study and use hyper-mileage techniques.
Aerodynamics may or may not out perform engine tune, that depends how
bad you are now.

Aero can be estimated by picking a section of road and marking the distance
it takes you to roll from the speed limit down to say 45mph. Average several
tries to get a base line. Do you mod for either rolling resistance or aero then
compare the results to your base line. Obviously if a change causes you to
roll further you have just reduced the power it takes to push your car and you
have just increased your gas mileage.

I live in a section of the country I can do this any time I want safely.
If you live in New York city, be careful and find a safe place to road
test!

Study what makes the highest BC of a bullet of the same weight
then look at your car, it might be one of today's best but it sure
isn't perfect! Very poor actually.

[deathegg]

All very interesting. Thanks!

Probably not doing anything to mod the Yaris, at least not for aero. Possibly a lip under the front bumper. But that's it. I get about 38 in around town driving, and I'm not even trying to get good mpg. I'm anxious to fill up and see what kind of milage it's capable of.

[Advancedynamix]

The front bumper lip will actually hurt gas mileage. The original intent with those was to improve effective down force. The increase in frontal surface area with the ever so slight (at most Yaris city speeds at least) addition of down force will not increase fuel mileage at all, but it may decrease it slightly (probably not noticeable, unless your putting a really big front lip on). But the cool factor is worth the mileage, so go for it! lol.

[blkfordsedan]

First, I know aero aids DO help if done properly. The problem I'm having is how they do it.

What I'm not understanding is this: Since a specific speed in a specific gear = a specific rpm (lets pretend 65 m.p.h / 5th gear = 2500 RPM), how does aero improve MPG? I've always understood that at X-rpm, the engine makes a specific amount of power. For instance, my Yaris makes 106 HP @ 6000 rpm. HP does not (as far as I know), increase or decrease at 6000 rpm; if it's revved to 6000 rpm, it's ALWAYS 106 HP. Not 100. Not 115. It's 106.

So, for my 2500 rpm example, it makes X hp. Why would aero help m.p.g since the HP being created @2500 rpm has not decreased?

My brain is not happy right now. I feel stupid, because I think I should know this.

Of course, I may be over-thinking this and making it waaay more complex than needed. I tend to do that a lot too!

Yes, you're overthinking. Re-read my second paragraph again and think about it....
The throttle controls the amount of air the motor ingests and thus the potential power output. The rpm of the engine changes based on the load or resistance it is experiencing. If you apply 1/4 throttle and hold it there, the engine will have the potential to put out "X" amount of power. If the load or resistance is LESS than "X" horsepower, the engine will increase RPM and accelerate the load (thus increasing the power requirement) until the load requirement reaches "X" horsepower. The engine RPM will change until the power requirement of the load and the power output of the engine are the same.

What's messing you up is thinking that an engine puts out a specific amount of power at a specific RPM. The power output is determined by the throttle position or the amount of air & fuel being ingested. You can put your car in neutral and free rev the engine up to 5000 RPM and hold it there. It will only take a very slight amount of throttle to do so. Why? Because the only load you're trying to overcome is the internal friction of the engine itself...maybe a couple HP. But, drive the car on the highway in 5th gear and try to cruise at 5000 RPM. You'll probably have to go almost full throttle, because the power required to move the car at that speed requires way more power and thus more air and fuel.

[Movin]

I think I got this figured out..
If I make 20 HP at 2000 RPM
and I make 60 HP at 6000 RPM
I will make 120 HP at 12,000 RPM!!

[Safety Guy]

Charlie answered your question really well:

"Horsepower is not always the same for a certain RPM. When a manufacturer lists the output of an engine, it is listed as the maximum HP that that engine makes at full throttle under a calibrated load. Your engine may make 106hp at 6000 rpm under a 95 ft lb torque load while at full throttle, but it wont make that same 106hp at 6000 rpm with 10 ft/lb torque load at 1/4 throttle. It may only be making approximately 11.5 HP. With modern fuel injection systems, fuel consumption can change drastically with less engine load."

He also wrote:

"All that said, you will be hard pressed to improve the fuel mileage of a Yaris with Aerodynamics. Toyota spent millions of dollars making that car aerodynamic and it has one of the lowest coefficient drag ratings in the modern automotive world."

I'll disagree just a little bit here. Almost all "off the shelf" products are going to be compromises, due to the manufacturer's inability to satisfy all customer usage requirements and desires. That means that though the Yaris probably is one of the most aerodynamic cars out there, there may still be ways to mod it for better CD numbers. I'd look into ride height, since the Yaris will be made to go on all sorts of roads, including rougher ones, without too much chance of breakage under "normal" use. You might put on some lowering springs and add a deeper front air dam, which may improve your numbers a bit. Then again, maybe not. Air dams will keep air from going under your car, but they also add frontal area. Whether or not your mileage decreases will depend on how the whole deal acts in the real world. Maybe the Yaris has a very slick underside and the air dam will not be as effective.

Getting rid of all the gaps on the body surface may be a safer option. Again, the manufacturer will have their specs, which are probably not quite as good as you could do with some careful thought and the right materials to keep air out of these gaps. As was already suggested, low rolling resistance tires could also help. The question is, will they help save more gas than the extra money they will cost?

Hypermiling driving techniques, more air in your tires, reducing extra weight, and keeping your engine in proper tune are all low or no cost ways to improve your mileage. After you investigate those, then maybe look into better aero mods to see what you can do.

Karl

[deathegg]

What's messing you up is thinking that an engine puts out a specific amount of power at a specific RPM. The power output is determined by the throttle position or the amount of air & fuel being ingested. You can put your car in neutral and free rev the engine up to 5000 RPM and hold it there. It will only take a very slight amount of throttle to do so. Why? Because the only load you're trying to overcome is the internal friction of the engine itself...maybe a couple HP. But, drive the car on the highway in 5th gear and try to cruise at 5000 RPM. You'll probably have to go almost full throttle, because the power required to move the car at that speed requires way more power and thus more air and fuel.

OK, this paragraph makes it clear(er) for me, thanks!

Probably not doing much in the way of modding it; the Yaris is pretty aerodynamic as it is. I'd tend to rely on hypermiling than almost anything else; what I want to do to the Yaris would tend to increase drag (lower chin spoiler, rear roof spoiler), or add rolling resistance (wider 16" wheels and tires instead of the 15")