Gear ratio question. ugh i'm confused.

[russ_watters]

There is a formula to work out the critial pressure drop for choked flow, but I can't remember it at present.

Bernoulli's compressible flow equation will get you close.

 

[jakksincorpse]

He got the power with boost and got the mileage by not using the horse power.
The amount of power used is proportional to the amount of fuel burned.
Build a light and aerodynamic car with a relatively small engine with a turbo-charger on it and then cruise at 60 mph using a light throttle, no boostand an electronic engine management system and you can easily duplicate or even exceed those results.
The biggest factor in mileage is the nut behind the wheel (steering wheel that is). Just by changing your driving habits you can get an increase of up to 50% in your mileage. Add to that skinny, hard tires running on low friction bearings (loose but not sloppy) and a completely locked up drive train (rotating as a single member) and you begin to understand that we as drivers waste a lot more fuel than our cars do.

locked up drivetrain as in like what they use in dirt track races with modifieds? just a forward and reverse hooked up to a planetary gear?

how exactly do you not use the horsepower? like give your motor lots of low end torque to cruise on?

 

[russ_watters]

how exactly do you not use the horsepower? like give your motor lots of low end torque to cruise on?

Just lift your foot halfway off the gas when you get to the speed you want to cruise at.

And a small engine with a turbocharger can get much better fuel economy than a big engine without one (if they have the same peak horsepower). Turn off the turbo and the engine is exactly the same as a small engine without a turbo, running at a better spot on its rpm/efficiency curve.

 

[jakksincorpse]

http://www.youtube.com/watch?v=9dKlOCde4Vk&feature=related

we ain't in kansas anymore, this is how that mustangs motor ticks. there's a lot of neat crap under that hood.

 

[xxChrisxx]

As we said before, the motor may be capable of a peak output of 400hp but that's not is what is being used when cruising. Also as has been pointed out, power is directly reflected in the amount of fuel that is burnt. At 400hp it's going to be burning a LOAD of fuel. Asmuch or more than a normal 5.7L V8.

From that video, it's got active cyliner management. So what it does when it only need the 5-10hp to cruise at 60mph it will shut off some (nearly all) of the cylinders, the firing pattern will cycle, but maybe only 2 of the cylinders will be firing in a full cycle. This kind of thing gives you fantasic fuel efficiecny, but will give unacceptable driving quality in a normal car.

Also it really does depend how they are measuring the mpg, as funny maths has been used in the past to get figures like that. I have a feeling that the claim of 100mpg is rubbish, but it's promising enough that I'm certainly not going to write him off as a crackpot yet.
EDIT: For some reason they've dropped out of the x prize and not turned up at a motor show with the new car... interesting.

 

[Lsos]

i don't think there would be a huge loss. you can freely spin a turbocharger with ur hand. i figured running the hot side of a charger through the auxilery belt to drag out the exhaust would be pretty nifty. maybe if there were some changes in design of the motor this could be useful at high rpms.

When you "freely spin a turbocharger with ur hand" you are hardly doing anything. A turbocharger does work when it's plumbed into an engine that provides a resistance. That, and when it's spinning at something like 100,000rpm...which is much more than you can hope to accomplish with your hand.

Again, you are underestimating how much brute force it can take to move air around.

 

[jakksincorpse]

so in the end, god i swear I've re-written this 4 times. so i'll keep it simple

more low end torque will increase mpg with smarter driving because...the engine is working more efficiently and using less fuel?

 

[jakksincorpse]

When you "freely spin a turbocharger with ur hand" you are hardly doing anything. A turbocharger does work when it's plumbed into an engine that provides a resistance. That, and when it's spinning at something like 100,000rpm...which is much more than you can hope to accomplish with your hand.

Again, you are underestimating how much brute force it can take to move air around.

i wasnt underestimating, i was making an observation, there are "fake" superchargers called prochargers that use the same design i stated only shove air into the motor,

i've been told the suction idea won't work so the idea is irrelevent.

 

[xxChrisxx]

so in the end, god i swear I've re-written this 4 times. so i'll keep it simple

more low end torque will increase mpg with smarter driving because...the engine is working more efficiently and using less fuel?

Yes, that's correct. But you are suggesting methods that require lots of power to achieve very little (like the 'reverse supercharger'). Spending a pound to save a penny so to speak.

Frankly smarter driving makes a bigger difference to MPG than technology. If you drive with a lead right foot, then you are going to have low mpg no matter what technology you use. If you drive with MPG in mind, you'll be slower (not necessarily by that much though) but have fantastic mpg.

i wasnt underestimating, i was making an observation, there are "fake" superchargers called prochargers that use the same design i stated only shove air into the motor,

i've been told the suction idea won't work so the idea is irrelevent.

What on Earth is a procharger?

 

[jakksincorpse]

http://www.britishv8.org/Articles/Images-V15-2/Supercharger-A.jpg

normal supercharger

http://image.hotrod.com/f/9642210/hrdp_0612_more_power_on_pump_gas_03_z+procharger_blower.jpg

procharger

 

[jakksincorpse]

i'm lost on something though.

for a 20" wheel to travel one mile per minute it needs to rotate 2933 times right?

with all the gear reduction of the rpm it doesn't make sense.

like an engine at 2000rpms in a transmission overdrive gear of .83:1 and a differential of 3.75:1 the wheel is only rotating 642rpm

what am i not getting..

 

[Redbelly98]

A 20" diameter wheel has a circumference of 63" or 5.2 feet. So it's about 1000 revolutions per mile:

1000 x 5.2' = 5200' = 1 mile​

At 642 rpm, it takes about 1.5 minutes to get to 1000 revs or 1 mile.

 

[jakksincorpse]

A 20" diameter wheel has a circumference of 63" or 5.2 feet. So it's about 1000 revolutions per mile:

1000 x 5.2' = 5200' = 1 mile​

At 642 rpm, it takes about 1.5 minutes to get to 1000 revs or 1 mile.

did u get the circumfrence by multiplying 20" by pi?

 

[jakksincorpse]

how much torque is required to keep a car at 60mph?

like 3000lb weight, 0.30 coefficient of drag, 63" wheel circumfrence

 

[xxChrisxx]

http://www.britishv8.org/Articles/Images-V15-2/Supercharger-A.jpg

normal supercharger

http://image.hotrod.com/f/9642210/hrdp_0612_more_power_on_pump_gas_03_z+procharger_blower.jpg

procharger

All it is is a centrifugal supercharger, fundamentally no different to any other type of supercharger. Why did you describe it as a 'fake' supercharger?

 

[Redbelly98]

did u get the circumfrence by multiplying 20" by pi?

Yes.

 

[jack action]

how much torque is required to keep a car at 60mph?

like 3000lb weight, 0.30 coefficient of drag, 63" wheel circumfrence

$$T_{eng}=\left(\frac{1}{2}\rho C_d A_f V^{2}+ fW \right)\frac{r_{tire}}{\eta GR_{trans} GR_{axle}}$$

Where:

$$T_{eng}$$ = torque of engine (N.m)
$$\rho$$ = air density = 1.23 kg/m³
$$C_d$$ = drag coefficient
$$A_f$$ = frontal area of vehicle (m²)
$$V$$ = vehicle speed (m/s)
$$f$$ = rolling resistance coefficient = 0.015
$$W$$ = weight of the vehicle (N)
$$r_{tire}$$ = tire radius (m)
$$\eta$$ = efficiency of drivetrain = 0.85-0.90
$$GR_{trans}$$ = gear ratio of transmission
$$GR_{axle}$$ = gear ratio of axle