Aerodynamic Streamlining Mini-Course

[ERTW]

Originally Posted by ecomodded

Well that's no fun is it.

I think your forgetting that work or force is not done without energy , adding a wind load will not benefit fuel economy , removing wind load will. Driving at 1500rpm will save fuel over driving at 2500-3500 because it takes more fuel to drive at higher rpm's.

my point was that operating an engine far from its peak volumetric efficiency...is NOT as efficient. Even if it burns less fuel overall, but it's burning it LESS efficiently. rpm is not the determining factor (although there is slightly less engine friction to overcome). A car needs a certain amount of HP to cruise at a given speed and load. A downsized engine tuned to make just enough HP at cruise rpm, and peak VE, is the most efficient option. engine tech and driving style have more impact on consumption than a few counts of Cd.

My other point was, that OEMs know that aero doesn't make much difference at 60 mph...that's why they don't try. In city, it makes no difference at all, and hardly impact EPA mixed cycle ratings. Low Cd is more a marketing tool than anything.

[ecomodded]

You are convinced of your opinion even if it makes no logical sense from start to finish.

Your statements support each other only because they are both wrong.

I tried to explain it to you but your bent on peak performance not economy

[ecomodded]

I 'll paint a picture for you

Say you as in You are driving in your peak volume efficiency all day long.

But me I'll be cruising at 1000rpm all day, who do you think would consume the largest amount of fuel ?

[ERTW]

Originally Posted by ecomodded

I 'll paint a picture for you

Say you as in You are driving in your peak volume efficiency all day long.

But me I'll be cruising at 1000rpm all day, who do you think would consume the largest amount of fuel ?

How about we dispense with pictures and use numbers and actual logic?

Using the same car, you may use less fuel, and you'd be much slower. But that nowhere near what I'm saying. I say "efficiency", you understand "performance". ?! RPM does NOT automatically mean less fuel. It's about the load on a vehicle (drag and friction) and how efficiently the engine produces the power to balance the load. Newton's second law.

Haven't you ever compared highway consumption of a mid sized car with the four and six cylinder engines? The Malibu 2.4 gets 33 mpg highway vs the 3.6 26. That's 27% better. It cruises at a higher rpm. That is typical. Still feel I'm wrong?

There's an optimum engine size for a given car, at a given speed, which minimises consumption. Eg The 2014 Stingray is best at 5.7L per GM's SAE paper. The 6.2 was still marginally better than the 5.3 because it ran in V4 mode longer.

To take it a step further, design a smaller engine with the same power at part throttle, and lower pumping loss, and you get better economy.

This is the rationale to OEM downsizing engines. It's not "my logic". I'm done arguing.

[serialk11r]

I mean I guess you're technically correct, make an engine that runs slightly lean, 30:1 static compression ratio with a super long stroke, Atkinson cycle cam that closes 130 degrees after BDC, tuned headers and intake for VE below 2000rpm, and you'll have super high efficiency. If you need 15hp to cruise, then such an engine would only be capable of producing maybe like 25hp peak, and you wouldn't be able to accelerate.

An engine producing more power used at peak VE would be equally efficient possibly more, but it would also be sitting there wasting fuel more of the time.

VE and efficiency are very different anyhow, VE is a function of all the breathing on the engine, header, exhaust, valves, combustion chamber/piston, rod/stroke, and most importantly cams. Peak VE is very rarely where peak efficiency is, because peak VE on mild cams tends to be achieved where the intake and exhaust resonant frequency matches the engine speed, but that number is quite arbitrary.

The higher VE=higher efficiency argument only works when the whole system is designed for peak efficiency, because usually more VE doesn't increase efficiency downstream. For example past 60% load more VE means more energy wasted on the exhaust stroke. People who think higher VE means higher thermal efficiency are just practicing wishful thinking, BSFC is way harder to figure out and measure.

[ecomodded]

Originally Posted by ERTW

How about we dispense with pictures and use numbers and actual logic?

Using the same car, you may use less fuel, and you'd be much slower. .

Exactly !! you had MAJOR breakthrough just there , now hang on to the thought.

And go drive and save yourself some gas - instead of saving some time !

[ERTW]

So you're the wonderful guy who does 90 kph on the highway. It all makes sense now. Thank you for a thoughtful and insightful discussion.

[freebeard]

A clean car is a happy car!

[wmjinman]

Originally Posted by Xist

Okay, so:

[Google]

So, to simulate a car going 55 MPH, you need a model that is 761.207051 / 55 = 13.8401282:1

Otherwise, you would be creating a sonic boom indoors.

So simulate it at LESS than 55 mph. DUH!!!

[aerohead]

On page-1,in the 1st series of images I've added a 'golf ball',bowling ball,and Mair's boat tail.They're at the bottom.
If you compare the curvature on the balls leading up to the separation point with Mair's and the 'Template' you'll find a close correlation.
It is the gradual body convergence and pressure rise created by it, which allows the turbulent boundary layer to continue to receive kinetic energy,strafed in from the inviscid outer flow,critical for holding the TBL attached to the aft-body in spite of its 'desire' to flow backwards,against the current,towards the windshield headers low pressure region.