Energy cost of air displacement? 1

[ChrisGreaves]

New here. Aeronautics because I am curious about the cost of airflow around an object.
In my case a Toyota Rav4 driving down the highway; calm weather, dry etc. Just the Rav 4.
I first proposed this discussion on Eileen's Lounge, but a user suggested I check out at this site.

The Rav4 is five feet wide and five feet high in cross-section. Assume that there is no space between the road and the underside of the SUV. It moves down the road like a block of wood, 25 square feet in cross-section, pushing air out of the way.
If I assume that it pushes air five feet vertically, I calculate 4,500 foot pounds per second of work; that is lifting 60 square inches of air vertically by five feet, assuming 15 psi. At 60mph=88 fps= about 1,000 inches/second, this has left me contemplating that the car engine must be doing 4,500,000 foot-pounds of work each second.

But one horsepower is defined as 550 ft/lbs/second, which suggests that the car in my simple model is an 8,182 HP Toyota Rav4.

The Toyota RAV4 is 203 HP.
Please and Thank you, where is my error in thinking?

I can see that air might be pushed to the side rather than vertically, but then that air must do work to push other air to the side.. For me the bottom line is that where there was air occupying a 5'x5' cross section, there is now no air in the cross section - just steel, plastic, rubber etc.

I am trying to determine how much work is done just to make a car occupy air-space on a 3.5 hour trip along the highway.
Thanks for any advice on the calculation, also any tips about a more appropriate forum.
Cheers, Chris

 

[EdStainless]

But your car isn't a block.
All shapes will have a drag coefficient, cv.
For simple geometric shapes (sphere or cylinder) you can look these up.
Most motor vehicles have cv's in the range of 0.3-0.4
The air drag is a function of the square of the speed.
So the drag at 85 is double that at 60

 

[GregLocock]

The air isn't pushed permanently out of the way, it flows around, under, and over the car and tumbles in behind it. So the air stays at roughly the same height overall. Otherwise there would be a vacuum behind it.

 

[WKTaylor]

A Wind tunnel [or analytical equivalent] is needed to separate-out aero drag from rolling drag, cooling-drag, etc.

Start with a few SAE books and documents...

SAE R-114 Fundamentals of Vehicle Dynamics
SAE R-366 Road Vehicle Dynamic
SAE R-392 Theory and Applications of Aerodynamics for Ground Vehicles
SAE R-430 Aerodynamics of Road Vehicles

SAE HS-1566 - Aerodynamic Flow Visualization Techniques and Procedures

SAE J1321 Fuel Consumption Test Procedure
SAE J1594 Vehicle Aerodynamics Terminology
SAE J2071 Aerodynamic Testing of Road Vehicles - Open Throat Wind Tunnel Adjustment, Information Report
SAE J2084 Aerodynamic Testing of Road Vehicles -Testing Methods and Procedures, Information Report

 

[3DDave]

It would take that (assuming calcs were correct) if a total vacuum was established behind the car as if the car was a piston in a tube. Instead the HP required is to have a breeze that blows air around the car as an obstacle.

Consider that 14.5 psi is pushing the rear of the vehicle forward and slightly more is pushing the vehicle back.

 

[MintJulep]

The force needed to move your car through the air is given by the drag equation.

Drag equation - Wikipedia

en.m.wikipedia.org

Work is force x distance.

 

[rb1957]

your assumptions are too simplistic and unreasonable, as is the answer they produce. At least you have the good sense to realise this (that a 200hp car can't generate 8000hp). Many references above will explain the problems with your assumptions. By guesswork I'd be surprised if it took more than 50hp to maintain a speed of 60mph, that most of your engine power is used to accelerate the car or to drive uphill. A commonsense view of this would be where is the accelerator pedal (to maintain 60mph) ? how does the engine sound ? Neither of this will be "balls to the wall", so the engine is producing much less than 200 hp

 

[XR250]

\By guesswork I'd be surprised if it took more than 50hp to maintain a speed of 60mph, that most of your engine power is used to accelerate the car or to drive uphill. A commonsense view of this would be where is the accelerator pedal (to maintain 60mph) ?

I am guessing it is much less than that.

]
The air drag is a function of the square of the speed.
So the drag at 85 is double that at 60

This makes sense but why is my fuel mileage not significantly worse at 85 mph versus 60? In my truck it goes from maybe 19 to 14 mpg.

 

[MintJulep]

In my truck it goes from maybe 19 to 14 mpg

(19 - 14) / 19 = .263

So, a 26% increase in fuel use. That seems significantly worse.

 

[MintJulep]

Horsepower & Aerodynamic Drag Calculator

www.stealthmotorsport.co.uk

 

[WKTaylor]

* MJ Your post, #10... RE: Cybersecurity risk. Please be careful about posting non-secure websites... http: VS https: .... my company is beating-us-up for following any non-secure link... especially with secondary internal data entry of any kind within that link. I almost had a 'Rut-Ro'... moment in my interest.

** ChrisGreaves -Student-. Long ago I learned to do some basic self-study research on a topic I needed help with... so I would ask 'smartly-framed' questions of the experts... gaining the expert's interest and enthusiasm and trust for talking with 'lowly-me', IE: my asking from a basis of 'basic knowledge' meant I was serious... not 'flirtatious'.

It seems that you may have done very little technical self-study on this subject for 'familiarization'... before asking this relatively complex question.

I've found a load of basic science/design related to 'automotive aerodynamics', by simple Google search. Most college engineering libraries should have one or more of these titles on the shelf... in addition to the Info I posted [see #4] earlier...

Vehicle Aerodynamics - Testing, Modification & Development: For road, racing and alternative transport
A Century of Car Aerodynamics: – the science and art of cars and airflow
Competition Car Aerodynamics
Race Car Aerodynamics: Designing for Speed