Drag

NiMH · 03-11-2005, 01:55 AM

I've been curious how significant the car's drag is at different speeds, so I calculated the power lost to drag at different speeds and also at different altitudes and temperatures. I've attached the plots in case anyone else is interested. The constants I used are:
Coefficient of drag, published by Toyota = 0.26
Frontal area (published for similar sized cars) = 1.9 sq. meters
Density of air varies with temperature and altitude ~ 1 kg/cubic m

Since the drag force is proportional to the square of velocity, and the work done to counteract that force is the force times the velocity, the power loss ends up proportional to the cube of the velocity. Now of course at a higher velocity the car covers proportionally more distance per unit time, so you can think of the effect on mileage as being down to a square-law relationship again.

I plotted the power in horsepower since this is the most familiar unit for the engine power. I understand there are still a few nations that have not yet adopted the metric system, so for those readers from Liberia, Burma or the United States of America you can multiply the velocity scale by 5/8 to get mph.

This agrees with some comments I saw from other posters:
- drag really isn't too significant until you start to go above residential speed limits (but notice how brutal it becomes on the highway)
- density change due to temperature really is significant (at high speeds)

I also learned that humid air is less dense, so a hot humid day is a low drag day.

DanMan32 · 03-11-2005, 11:24 AM

I thought I was getting better mileage on humid or foggy days.

jtmhog · 03-11-2005, 11:45 AM

Vehicle drag is just one of many variables that effects mileage. For instance, how does the humid air effect combustion?

rick57 · 03-11-2005, 12:05 PM

I always thought that a humid day would cause more drag,not less with the water vapor in the air. Just think if you could get the graph to slope upwards in a more straight line,the Bonneville Prius would have turned in a higher top speed

rick57 · 03-11-2005, 12:06 PM

Quote:

Vehicle drag is just one of many variables that effects mileage. For instance, how does the humid air effect combustion?

All I know is in NHRA Drag Racing,the Pro Stocks slow down in high humidity.

drash · 03-11-2005, 05:41 PM

I'm guessing here and maybe rick can correct me, but I think the Atkinson engine might actually benefit from higher humidity as it adds a little more compression. Of course for the Drag Racers, don't they use more alcohol based fuels? They might separate more under high humidity causing incomplete burns. I really reaching here as this is way outside my line of work.

NiMH · 03-11-2005, 11:33 PM

For sure the humidity and temperature of the air have effects on the efficiency of the combustion engine, which is a whole separate topic from drag. Nonetheless, it's interesting and a bit counter-intuitive but water vapour is actually less dense than air. Fog, on the other hand is actually condensed water vapour and I would expect it to have the opposite effect.

xevious · 03-12-2005, 12:37 AM

Wait... what's this... "metric" system?

rick57 · 03-12-2005, 12:36 PM

Quote:

I'm guessing here and maybe rick can correct me, but I think the Atkinson engine might actually benefit from higher humidity as it adds a little more compression. Of course for the Drag Racers, don't they use more alcohol based fuels? They might separate more under high humidity causing incomplete burns. I really reaching here as this is way outside my line of work.

I don't think any engine would like high humidity. As for the Atkinson cycle,by a reference in the 1st gen NCF it meantions that it does not generate high output but is highly Thermal Efficient. As for the racers,the Pro Stock class is based on production cars with a max of 500CID and run on high octane racing gas. There is no alcohol in it. But in the fuel classes,dragsters and funny cars,you will find some.

DaveinOlyWA · 03-12-2005, 01:23 PM

actually high humidity would create a loss of power. water simply does not heat up very well. that is good for us because the abundance of water on this planet helps to regulate and moderate the extremes in climate this planet has to offer.

the amount of energy required to heat water is one of the highest of any substance so it would be a detriment.

03-11-2005, 01:55 AM	#1
NiMH Join Date: Dec 2004 Location: Calgary Posts: 15 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	Drag I've been curious how significant the car's drag is at different speeds, so I calculated the power lost to drag at different speeds and also at different altitudes and temperatures. I've attached the plots in case anyone else is interested. The constants I used are: Coefficient of drag, published by Toyota = 0.26 Frontal area (published for similar sized cars) = 1.9 sq. meters Density of air varies with temperature and altitude ~ 1 kg/cubic m Since the drag force is proportional to the square of velocity, and the work done to counteract that force is the force times the velocity, the power loss ends up proportional to the cube of the velocity. Now of course at a higher velocity the car covers proportionally more distance per unit time, so you can think of the effect on mileage as being down to a square-law relationship again. I plotted the power in horsepower since this is the most familiar unit for the engine power. I understand there are still a few nations that have not yet adopted the metric system, so for those readers from Liberia, Burma or the United States of America you can multiply the velocity scale by 5/8 to get mph. This agrees with some comments I saw from other posters: - drag really isn't too significant until you start to go above residential speed limits (but notice how brutal it becomes on the highway) - density change due to temperature really is significant (at high speeds) I also learned that humid air is less dense, so a hot humid day is a low drag day.

03-11-2005, 12:05 PM	#4
rick57 Senior Member Join Date: Feb 2005 Location: Circleville, Ohio Posts: 565 My Car: Other Non-Hybrid Model: Package: N/A Thanks: 0 Thanked 0 Times in 0 Posts Friends: 1	Re: Drag I always thought that a humid day would cause more drag,not less with the water vapor in the air. Just think if you could get the graph to slope upwards in a more straight line,the Bonneville Prius would have turned in a higher top speed

03-11-2005, 05:41 PM	#6
drash Member Join Date: Jan 2005 Location: Upstate NY Posts: 72 My Car: 2005 Prius Model: Package: #3 Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	Re: Drag I'm guessing here and maybe rick can correct me, but I think the Atkinson engine might actually benefit from higher humidity as it adds a little more compression. Of course for the Drag Racers, don't they use more alcohol based fuels? They might separate more under high humidity causing incomplete burns. I really reaching here as this is way outside my line of work.

03-11-2005, 11:33 PM	#7
NiMH Join Date: Dec 2004 Location: Calgary Posts: 15 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	Re: Drag For sure the humidity and temperature of the air have effects on the efficiency of the combustion engine, which is a whole separate topic from drag. Nonetheless, it's interesting and a bit counter-intuitive but water vapour is actually less dense than air. Fog, on the other hand is actually condensed water vapour and I would expect it to have the opposite effect.

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03-11-2005, 11:24 AM	#2
DanMan32 Join Date: Aug 2004 Location: Tampa Bay, FL Posts: 3,775 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	I thought I was getting better mileage on humid or foggy days.

03-11-2005, 11:45 AM	#3
jtmhog Senior Member Join Date: Jan 2005 Posts: 151 My Car: 2006 Prius Model: Package: Base Thanks: 0 Thanked 1 Time in 1 Post Friends: 0	Vehicle drag is just one of many variables that effects mileage. For instance, how does the humid air effect combustion?

03-12-2005, 12:37 AM	#8
xevious Join Date: Jan 2004 Location: Oregon Posts: 187 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	Wait... what's this... "metric" system?

03-12-2005, 01:23 PM	#10
DaveinOlyWA 3rd Time was Solariffic!! Join Date: Apr 2004 Location: South Puget Sound, WA Posts: 12,378 My Car: 2010 Prius Model: IV Package: Solar Roof Thanks: 24 Thanked 121 Times in 103 Posts Friends: 10	actually high humidity would create a loss of power. water simply does not heat up very well. that is good for us because the abundance of water on this planet helps to regulate and moderate the extremes in climate this planet has to offer. the amount of energy required to heat water is one of the highest of any substance so it would be a detriment.