question about aero
 

does a car feel slower or faster with better aero, given the same speed?

Faster... why?

Perception can vary based on several factors, including how quickly the vehicle accelerated to the test speed.

If the individual were introduced to a vehicle which was already at speed, the perception shouldn't vary from test subject to test subject, regardless of aero.

It could seem faster if it is more responsive to acceleration as basjoos claims.
But then it could seem slower because I've noticed that the quieter a vehicle is, the slower it seems to be going, and bellypans and such can quiet a vehicle down.

That's why I said that it could vary.

In a true test where one would hope to gather any useful information, most of the perceptive cues would have to be muted or averaged out over the sample because something as simple as a change in sound can change ones perception of speed, direction, proximity, etc.

Quick question, if the aerodynamic drag component is reduced how does this change the amount of power the engine is producing at a given vehicle speed? For example lets say the car has a cd of 0.33 and a gear ratio spread such that the car is turning at 3000 rpm at 68 mph an N/V ratio of 44.12 rpm/mph. The engine produced maybe 50 horse power at this point, if the aerodynamic loading used to require 40 hp at this point and the aerodynamics is improved such that it now takes less power how can this effect the engine speed at a certain vehicle speed? I can see it allowing for increased coasting, however the gear ratio is fixed and therefore for a fixed vehicle speed the engine produces a certain power output at a given throttle input, say around 50% load (ie its a 105 hp engine)

Come on Hubert, think about that one a little bit.

Is this one of those "the answer is given in the question" type questions?

Depending on the car, it may be somewhat trivial to change the gear ratio. It is about $20 and a trip to the u-pull-it for my saturn to get a taller 5th gear.
http://ecomodder.com/forum/showthrea...swap-2779.html

My rabbit was pretty easy too, but needed more careful adjustment than the saturn when reinstalling the 5th shift fork (still could do it in the car).

*maybe* you have a C transmission, and can get a .725 5th gear out of a corolla or a tercel or a yaris? It might take some research to be certain: Toyota C transmission - Wikipedia, the free encyclopedia

Taller drive tires are another option, which may or may not affect drag, as well as swapping in a used tranny with better ratios (and a new clutch disk and throwout bearing while you are there)

There is no inherent scope to modding :)

P.S. if you need 50hp to cruise, you are driving a barn ;)

Ok the 50hp was pulled from thin air, even the Taurus I first learned to drive in only required 13-15 hp at around 50 mph or was it 60 I forget, but someplace I have the dyno slip that stated this. Also the 50 hp was derived from a 50% load based on the wot peak hp of 105 bhp and 100lb/ft or was that the other way around, I need to see which gear ratios/gearbox is in the car, I know its a 5 speed, but the overdrive isn't tall enough, I can engage 5th gear as low as 30 mph and then get it down to 25 around slowing down for corners, but it also turns 3000 somewhere near 70mph per the speedometer and the gps (closer to 68 at 3000 exactly by the tach). I'd like either a taller 6th gear on top of the five now as a way of reducing the revs further, but unless it can be done reasonably by some one with some mechanical inclination but no prior experience and for not too much money vs saving and replacing it when it did wear out.

I'm being serious, it doesn't seem logical that the engine would spin any slower even if the load is reduced due to the aerodynamics being improved, vehicle speed is a function of available tractive effort power - road load power- mass effective*acceleration (ie the inertial forces) reducing the aerodynamic component is incorporated inside the road load force, as an example I studied in a hybrid and electric vehicle course we took a theoretical extended range electric conversion of a Ford Fusion Hybrid and the peak power usage was 40kw or about 53hp for both the city and highway epa cycles, however with the net energy usage much more severe in the city cycle without regen, which was cut in half theoretically with regen, however due to the way the highway cycle works there is virtually no gain to regenerative breaking in the epa highway cycle test

The engine doesn't spin any slower when you reduce load. An engine, at any point where it is not accelerating or decelerating, is idle. The way we tend to use the term idle is quite a misnomer. Idle means at rest, or not laboring. When you load the engine, once it stabilizes, it's effectively at idle again, even though it's at a higher speed than normal "idle".

Anyway, the point is that an engine only produces exactly enough power to satisfy it's current operating status. If you demand 30 mph, the engine will produce enough power to maintain 30 mph in addition to overcoming static and parasitic loads. Just because the RPM that coincides with 30mph happens to be capable of producing twice the required power, doesn't mean that it is producing that much.

It takes less power to move at a given speed with better aerodynamics. The engine is still spinning, so to make less power, you give it less throttle, which gives it less air (and thus less fuel), so it will make less power at the same engine speed. Try going from a flat road to a slight downhill. In order to not speed up, you have to lift up on the throttle a bit. Its the same with better aerodynamics.

Again this is where the part open throttle engine curves get really messy when talking about bsfc, its easiest to represent them as contour maps or islands of efficiency, speaking of which no one has located one for the 4a-fe either, but that's not relevant to this topic, however at reduced loads the engine makes less power but may not be operating at its most efficient state, other optimizations non withstanding. However does lower power output lead to lower fuel consumption if the combustion events are still occurring as frequently as ever? I guess as a mechanical engineering student who has only just begun to get into the exciting powertrain classes this is still a bit fresh theoretically vs physical experience.

Also my car probably has the C50 5-Speed Manual Transmission
Gear ratios for this transmission.
1st 2nd 3rd 4th 5th Reverse Final
3.545 1.904 1.310 0.969 0.815 3.250 3.722
At least it was used on the equivalent corollas and some of my research indicates that it may be the same as in my car. that 0.82 overdrive is why its still spinning quicker than is desired, peak torque is closer to 2-2.5k iirc.

Even though aero with no gear change may move you lower on the load scale and farther from the bsfc island, you are still using less power, so that usually eclipses the decrease in gm/kwh.

aero + gear change can provide more than the sum of their independent changes.

aero + smaller engine is the next logical step (and just deal with slower acceleration) ;)

And the engine should not spin any more slowly at a given speed, if you improve the vehicle's aerodynamics.

However, the throttle controls the amount of engine power being produced, right? If you give the engine less throttle, it'll make less power. That means the engine requires less airflow through it, and less gasoline that must be mixed with that airflow.

I suppose that indirectly the throttle controls the power output by regulating the air intake in a gasoline powered vehicle with a throttle plate, however in a modern common rail diesel or some direct injection gasoline engines with no throttle plate can control the fuel directly. If the reduced loading would allow for a lighter throttle application then yes the power output should be reduced to compensate at the same revs, meaning that on a throttled engine less air/fuel mix should be inducted, however with direct injection I'm not as certain how it is programmed.

Diesels are throttled by fuel, and only fuel. Purple who say they don't have a throttle are incorrect; They don't have an air restrictor, with a few exceptions. When you let off the pedal slightly, your changing the amount of fuel being injected. This in turn, throttles the engine down, asking for less pro out power at a given output speed.

Controlling the fuel intake effectively throttles the power output of the diesel engine yes, but I was referring to the throttle plate on the intake air charge, which modern diesels typically don't require and are effectively at wot all the time on the air side ie the volumetric efficiency goes up, and the precise amount of fuel required can be injected just prior to the top of the compression stroke. This increase in volumetric efficiency coupled with the higher energy density of the diesel fuel helps lead to higher fuel efficiency/lower fuel consumption

Are you trolling?

If you already understand how this works, what is the point of your questions?

What is the end point of this thread?

No I was not trolling, and therefore have no need to continue this conversation at present, however on an aerodynamically related question which would be more important, blocking the upper grille which is already narrow, and adding an air dam versus a belly pan and closing up the wheel arches.

^^ You're asking for imperical data of which is better than what when we have no idea what you drive or what your car specifically looks like.

all those modes u listed can have different impact from one car to another...

From the Front, Same car mentioned on the left hand side of my posts as well as in the fuel log in the signature.
Although I was asking in general which is more important aerodynamically, sealing off the undercarriage, or redirecting more air around it by effectively lowering the front end with an air dam and possibly blocking the grille opening. It didn't have to be empirical data, just a rough estimate would suffice for the theory, as my current knowledge is more in tune with the powetrain side of things.

I nominate this for one of the weirdest threads. :confused:

Hi All,

At speed, with a quiet engine, the car will seem to be slower. As the aero vibration in the chasis is not sensable, and the noise is very low.

Down a hill, however, the car will seem very quick. That is not the same as fast, but many people confuse the two. The car will take off like a bandit, with no additional accelerator pressure.

And accellerating, in a Prius, the car will seem slower, as the engine will run slower for a given speed and/or accelleration.

feel
 

'Feel' is a subjective evaluation and can't be quantified.
Actual changes to aerodynamics will produce quantifiable changes in pitch,yaw,roll,lift,top speed,wind noise,and mpg.

Hucho/Sovran
 

Hubert,I think the aero/BSFC info you need was covered in Hucho's book and earlier by Gino Sovran at GM in his SAE Paper.
Both works deal with maximizing mpg after drag reduction by gear-matching to maintain the engine on the island of highest BSFC.
As early as 1934,Chrysler's Carl Breer realized that to get the biggest bang for the streamlining buck,a car would need to have taller gears to get the 'load' back up and prevent over-revving of the engine.
The best BSFC is generally said to occur at the torque-peak rpm.If so,then,after aero mods you'd want your 'cruise' rpm to reflect that engine parameter.

high-speed fall-off
 

Historically,a Diesel engines mpg advantage diminishes with speed.As an Otto cycle engine car gets into higher load situations ( faster ) the throttle is moving closer to WOT all the time.Honda 3-barrel carburetors were designed to have both the idle and the #2 barrel throttle plates at WOT at highway cruise speeds for nice volumetric efficiency,something modern EFI fails to do.
The Diesel will still have the chemical energy density advantage over gasoline.You can argue the cost/benefit ratio.
Presently,my gasoline pickup is returning better mpg than the VW Diesel Rabbit at higher speeds.

Prizm
 

You might block the upper opening with cardboard and tape.The lower grille is at a higher pressure.See if anything shows at the pump.Watch your coolant temp. If you can make the inlet airtight,even better.
You'll notice,in late model cars,that getting air around,rather than under the car is preferable,so the airdam isn't a bad idea.