12 replies to this topic

[SideWaysBob]

Whislt sitting at my desk trying to summon up some interest for my work my mind wondered thus.

At what altitude does a F1 engine and car work best at ?

Am i right in thinking that there must be a point where the atmospheric density of the air going into the engine is less than that required to push the car through the air at max speed.
If a car is, in theory, stopped from getting to a "maximum" speed because of the air (pressure) infront of it, then reducing the amount of air infront will allow the car to go faster (i think), but reduced density of air means the engine is having to work harder to reach maximum output.

I'm thinking out loud here so don't shoot me down too quickly

so, and this is the point, would an F1 car go as fast at siverstone on a warm dry day and cold wet day, as it would at Mexico on a cold, wet day and dry day.

Where this comes from is Le Mans. as the race goes into the night and the air temp drop's and the moisture increases the cars go noticably quicker. -M Shuemacker set the fastest lap of the race the year he drove for Merc's at about 3 in the morning.

I'd be interested if anyone can enlighten me.

Cheers
SideWays Mr. Bob

[desmo]

As you pointed out the fastest laps at Le Mans are set when the air is coolest. The added density of the air benefits the engine during the vast majority of the lap when the throttle is open. Any benefit from reduced aero drag would be, due to drag being a square of velocity, most telling only in the brief time when the car is at or near max velocity. And of course the engine would be making less power as the car accelerated in thinner air.

Here is a link to a very cool calculator that corrects for ambient conditions. You can try various combinations on it and see the results:

http://rshelq.home.s.../calc_hp_dp.htm

[Powersteer]

That Le Mans time with could air would actually benefit from more power plus better air round aerofoils because of air density, ciao.

[Ursus]

If we are talking lap times we should not forget the tires.
If it's cool you can use softer tires and subsequently your lap times drop.
At Le Mans everybody swithes to softer compound tires when night falls and back again as it gets warm the next morning.

[Wolf]

Þhe question of pressure is quite interesting. I'd say cold and dry day- but at which altitude? Drag is reduced with altitude, but so is the power. The way I figure it is very simple. Take for example a car in Mexico, or somewhere, traveling at its max speed. Then reduce its altitude to gain a 10% increase in pressure. The car would need 10% more power to travel at that speed, due to pressure increase. The same increase would resulst in gained engine power in two ways- adding up to bigger increase than 10% turbo-charging. The benefits would be the same, but there is smaller (unwanted) increase in exhaust pressure than with turbo. That increase should be bigger than 10%, as I recall Volvo used turbo-charging with similar pressure increases, and power increase was bigger than above mentioned 10%. So, the car would travel faster at lower altitudes, and furthermore the engine pressure (because density should increase with pressure) should be more affected by the speed.
There's another aspect, regarding F1 cars, as well: the lower pessure means less aerodynamical grip, which should also increase lap times. Unless you gave it more wing (i.e. bigger angle of attack), resulting in further decrease of speed.

[Powersteer]

High attitude also means a severe dent on acceleration and medium speed plus the slight lack of grip in downforce. I believe even top speed is compromised because its how the engine react to air density compared to the body.

[SideWaysBob]

So follow up question. How much difference does Barometric pressure make to a performance car or in this case an F1 car in full flight ?

So i guess it's all relative. Low altitude gives more power (in an ideal model) but incresed drag,
High altitude gives less drag but also less power.

Does this mean that where ever you run an F1 car it's pretty much going to be running at it's potential maximum speed. If we ignore things like length of straight and cross winds.

Using that calculator desmo suggested i found that you could get 100% power at 50deg F at 1000ft. (more playing around than scientific deduction, that one) does that sound like any track used in F1 today.

Mr. Bob

[Yelnats]

The largest effect of increased air density is observed in snowmobile racing where the fastest times are achieved when the temps are the coldest. I racall a race in the mid seventies (Canada) where it was so cold that we had to carry our wineskins inside their snowmobile suits or have it freeze within 10 mins and beer drinkers were at a big disadvantage because a pint froze solid before it could be drunk!!!

Anyway the machines were superfast that day due to increased acceleration in spite of the increased drag.

Incidently the loss of HP with decreased preasure is dispropotionatly large because it is a gross loss. Net HP output is what we usually measure and ignores internal engine frictional loss.

A 10% loss in air preasure could perhaps produce a 10% loss in gross power at a given rpm. This perhaps equals a 13% loss in net output as internal engine frictional losses remain unchanged. But areodynamic drag would only decrease by about 10% and because chassis/tire related drag would remain the same we could expect total drag to decrease by maybe 9%. So considering that effective power would fall by 13% and drag by only 9% a reduced top speed could be expected with any lowering in atmospheric preasure. Of course these values are for discussion purposes only but whatever the correct values the conclusion would still be valid.[p][Edited by Yelnats on 09-23-2000]

[Billy Gunn]

Altitude:

The highest paved road in N.America is Mt Evans West of Denver. It runs up to a height of 14,250(ish) feet. At this altitude normal ambient air pressure is around 630Mb (9.13psi when std atmos = 14.7psi) a naturally aspirated engines performance, all other things being equal would be 61.5% of sea level. Unfortunately all things are not equal, the most notable being that at this altitude the oxygen/nitrogen balance is diminished from 20.9% to around 16.7% (and oxygen decrease of >20%) so power diminishes by another 20% i.e. 49.2%.

This weekends home work is "why does the O2 % drop?" Answers to teacher 9.00am CMT Monday 1st October.

[desmo]

I found this in an online text from NASA:

"2.3 Air Composition and Its Well-Mixed Nature

Air is primarily composed molecular nitrogen and molecular oxygen, with an assortment of minor or trace gases, such as argon, carbon dioxide, water vapor, and ozone, as well as many others, making up the rest. A parcel of air contains about 78% nitrogen molecules (N2, molecular weight of 28), 21% oxygen molecules (O2, molecular weight of 32 kg/kmol), and the remaining 1% are the trace gases. From this basic composition, the apparent molecular weight of air is about 28.964 kg/kmol. Both molecular nitrogen and oxygen decrease with altitude at exactly the same rate as overall air density. This means that the composition of air is approximately the same in both the troposphere and the stratosphere. The relative amounts of nitrogen and oxygen (78% and 21%) persist up to about 120 km, where atmospheric pressure is a tiny fraction of that of surface pressure."

Much to my surprise, I could find very little on the changes in the relative contents of N2/O2 at altitude. Is elemental (non molecular) N or O the reason? The differences in molecular weights don't seem to be a factor. I should call my father who was a professor of physics and taught some meteorology as well, but he now lives in Italy and I don't want to pay the bill just to ask him!

[palmas]

In fact the composition you are refering to in based on volume and I believe weight would be more apropriated.
In fact if we compare standart atmosfere at 0 altitude and 2,5 km altitude, we will find the following:

pressure : less 26,3% (1,013 bar to 0,747 bar)
Density :less 28% (1,225 kg/m3 to 0,957 bar)
Kinematic viscosity : plus 22%(14,61 cSt to 17,87 cSt)

Nevertheless I will not speculate on the practical results since moisture content and temperature will play a major role in this figures and in the car performance. Also the engine itself can make a big difference (whats the use of filling the cilinders with more O2 if the load is already 100%, that is, you cannot burn more gas).

[westendorf]

As a percentage there is as much oxygen at the top of Mt. Everest as at the bottom of Death Valley. Just less pressure therefore less "apparent" oxygen. Ciao, GFW

[Wolf]

OK, Billy Gunn, tell us why. I'd like to know!