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Old 07-05-2014, 03:59 PM   #1 (permalink)
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Engine and Aero Efficiency

My premise is that running a downsized engine at peak VE, producing just enough HP to maintain highway speed, is where you maximise fuel consumption.

The penalty of running a downsized engine at peak VE, is less available power. The penalty of having a bigger engine, is running off peak VE and BSFC, and more weight. The penalty of adding a turbo to regain power is the much higher BSFC under high rpm/load (with current technology). The penalty of a hybrid system (as a power adder) is the cost and weight of the motor/battery. Diesels have low rpm torque, lean burn/stratified charge, turbos...but expensive after treatment, to the point where it doesn't pay for itself vs a gas engine for several years.

If consumers wanted lowest fuel consumption, they would buy sub compact cars with 1L engines. 50% of new vehicle sales are full sized trucks. most people value style and status over fuel/energy consumption. And honestly, keeping a car for 10 years costs much less than buying a new car every three years...regardless of how much more efficient it is (plus manufacturing process).

diesels and gas ice will surpass 50 mpg very shortly. imo, EV are a real solution to lower life cycle energy usage. iirc GE recently announced a new battery chemistry that will undersell and outperform lithium by a significant margin. I believe they said a couple years to market. Even the projected cost efficiency of lithium batteries will reduce cost $2500 in the next few years (for the Volt's 16.5 kW battery).

The Spark EV, for instance, is currently a $2,600 premium over an equivalently equipped Spark gas car (with Ontario rebates). A cheaper battery and electronics will make the gas powertrain redundant in many locations. Same initial cost outlay, lower operating and maintenance costs. Instant torque, silent, smooth.

Power Japan has invented carbon/carbon batteries. cheap, power dense, stable.
Dual-Carbon Battery: Same Energy Density, Safer, Longer Life Than Lithium-Ion, Says Power Japan Plus
apparently they don't believe that "carbon containing" is the definition of "organic". Wait until they start using carbon nanotubes

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Old 07-05-2014, 04:27 PM   #2 (permalink)
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Quote:
Originally Posted by ERTW View Post
My premise is that running a downsized engine at peak VE, producing just enough HP to maintain highway speed, is where you maximise fuel consumption.

The penalty of running a downsized engine at peak VE, is less available power.
Yes, and that's why you don't see cars made that way. Acceleration to cruising speed would be VERY slow, and you can forget about hills.
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Old 07-05-2014, 04:33 PM   #3 (permalink)
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I'd heard the rule of thumb that rear wheel covers reduce Cd by 10%. I tested this in CFD last night. Keep in mind that rotating wheels affect Cd, and create really complex flows. I also tested wheel discs.

first picture shows the side view. Cd of 0.253. LaFerrari greenhouse. This sports car is 69.7" wide, and 46.8" tall. Frontal area is 1.61 m^2. Total drag is around 50 lb at 108 kph.

I wanted to show where induced drag comes from. Any pressure across a surface creates a force perpendicular to that surface (called normal force). If the normal force is at an angle from vertical, there is a component in the X direction, which causes drag. Downforce and lift both cause drag.

second picture is the flow in and around the wheel/tire. It moves around the tire, corkscrews within the wheel well, and through/around the wheel. it looks really nasty.

Wheel covers. Cd 0.252. ONE count :/ compared to some VERY open wheels (not even modeling brake rotors)

wheel cover to the wheel centreline - gives you most of the benefit of a full wheel cover. The tops of the tires are moving against wind direction, and stir it up. The bottom of the tire is moving with air direction, and so are not as important. wheel discs plus half cover = 0.246. seven counts over baseline. still not impressive.

full wheel cover. Cd = .242 eleven counts over baseline. four counts lower than half cover.
Total percent difference = 4.3%
Fuel economy savings ~2.2%
An average car getting 30 mpg will see ~0.66 mpg increase at 108 kph.

discussion: wheel covers and discs shield the airflow from nasty wheel vortex...so it may not be such a bad analysis. Just the cover-less model will be somewhat higher drag. The more shielded the tires are in front view, the less they benefit from wheel covers. The GM Precept achieves a Cd 0.17 with half wheel covers and multispoke wheels.

note: I put a cone on the tail which tapered at a 16 deg included angle. it improved Cd 20 counts, but more than doubled lift. At least on this vehicle, this isn't an optimal solution.
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Old 07-05-2014, 04:50 PM   #4 (permalink)
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Yes, and that's why you don't see cars made that way. Acceleration to cruising speed would be VERY slow, and you can forget about hills.
I'm gonna have to disagree with this statement. I rented a Corsa in europe this spring. It has a 1.4L engine iirc. The speed limits there are higher than Canada (130 kph on some highways). north america may have *hills*. Greece is mostly *mountain*. I didn't find myself wanting for more power. shifting gears ftw The only thing I'd change is low profile tires - you don't want to roll over on your tires when you're a km up on a 5m wide road with no guard rails, and an oncoming vehicle (which I did).

Driving style is different. where a european driver uses the on ramp to accelerate, an american driver punches it right before merging (or after, as is sometimes the case). power makes lazy drivers. I found that out in autocross.
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Old 07-05-2014, 04:56 PM   #5 (permalink)
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I'm gonna have to disagree with this statement. I rented a Corsa in europe this spring. It has a 1.4L engine iirc. The speed limits there are higher than Canada (130 kph on some highways). north america may have *hills*. Greece is mostly *mountain*. I didn't find myself wanting for more power. shifting gears ftw The only thing I'd change is low profile tires - you don't want to roll over on your tires when you're a km up on a 5m wide road with no guard rails, and an oncoming vehicle (which I did).

Driving style is different. where a european driver uses the on ramp to accelerate, an american driver punches it right before merging (or after, as is sometimes the case). power makes lazy drivers. I found that out in autocross.
I'm gonna have to disagree with your disagreement. That car had way more power than necessary to keep it at cruising speed.
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Old 07-05-2014, 05:22 PM   #6 (permalink)
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So...are you saying that 120 peak hp is too little or too much?

. If you're comparing hp necessary vs peak hp, then yes. Some cars more than others.

Engine hp is dependent on load. The hp charts you see are at wot. Part throttle hp is much less.

If you look at hp needed vs actual hp at part load cruise rpm, not so much. Typical american cars have much more reserve than necessary. My 169 hp malibu has more han enough to merge safely. Are you arguing that cars "need" multiples of that?
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Old 07-05-2014, 05:33 PM   #7 (permalink)
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So...are you saying that 120 peak hp is too little or too much?
No, I'm saying that your original statement in Post 1 is erroneous:

Quote:
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My premise is that running a downsized engine at peak VE, producing just enough HP to maintain highway speed, is where you maximise fuel consumption.
It only takes 10-20 hp to cruise at 60. The car you drove in Europe had way more than that.
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Old 07-12-2014, 03:04 PM   #8 (permalink)
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consumers

I feel like the automakers are certainly capable of creating remarkably high-mpg vehicles,however have not realized a market which insists such vehicles exist.
When CRX's and Metro's,and such were on the market,they constituted less than 2% of total vehicle sales.That's not a 'driver' for product development.
Hucho mentioned in 1985 that when fuel economy was a paramount specification in vehicle design,that we'd see vehicles based upon streamline bodies and wing sections with Cd 0.15 and below.
Ferdinand Piech,in the 1990s,insisted that 'his' cars would be at Cd 0.10 within a decade.Cd 0.10 was the Cd of a penguin,and certainly,his company would be able to mimic the efficiency of a penguin (paraphrasing).
In the early 1980s,automotive engineers were discussing Cd 0.10 as a reasonable lower limit for automobiles.
Hucho believes we might get to Cd 0.08.
Only university Eco Challenge and World Solar Challenge cars are utilizing this 92-year-old technology.
Automakers are joint-stock corporations and have a legal,fiduciary responsibility to pay the largest dividend to shareholders.They don't like to take risks.
The upshot is,that anyone interested enough in the technology,will go to the trouble to mimic the technology,and by default,serve as a technology demonstrator to other consumers,perhaps educating by default.Kinda like EcoModder's.
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Old 07-12-2014, 04:32 PM   #9 (permalink)
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If 4.5 hp can propell my 200 pounds at 40 MPH and 150 MPG, why would you need more than the 25 HP I will have in my 3 wheeler at 1100 pounds versus the 50 cc super cub at 350 pounds, me included in both even including another passenger (not on the super cub).

If I don't need to go over 40 MPH, I'll take the scooter. Over that. I'll take the trike. 4 cents a mile on the GZ250. A little over 2 cents a mile on the scooter. A little less than 7 in the Fiesta and 11 in the truck.

I'ver put 300 miles on the truck in two months, last fill May 16th.

If you need the same power, then dowsize the engine and use some form of forced indiction for temporary bursts of power. Right now your grossly oversized engines are costing us say 100 billion a year in wasted energy.

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Old 07-12-2014, 06:42 PM   #10 (permalink)
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Both.

This was in response to a post that was deleted that implied that you have to choose between ample power and insuffecient power. I believe and evidence supports this belief that you can have ample power and high efficiency in the same vehicle and there are several ways to accomplish this.

regards
Mech


Last edited by user removed; 07-13-2014 at 10:00 AM..
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