Gear ratio question. ugh i'm confused.

[xxChrisxx]

so...at this point ur not gaining any hp or torque yea? when does this usually happen in most vehicles?

This is a very difficult answer to try and convey. Exhuast gas itsself has nothing to do with gains in torque and power. However it's pressure pulses that allow exhuasts and inlets to be tuned, which is why you get such elaborate exhaust shapes.

You've got to remember the ONLY way you gain torque (and therefore power at a given rpm) in an engine is by burning more fuel, and to do that you need to cram more air in the cylinder. Everything tuning wise has that goal in mind.

 

[jakksincorpse]

This is a very difficult answer to try and convey. Exhuast gas itsself has nothing to do with gains in torque and power. However it's pressure pulses that allow exhuasts and inlets to be tuned, which is why you get such elaborate exhaust shapes.

You've got to remember the ONLY way you gain torque (and therefore power at a given rpm) in an engine is by burning more fuel, and to do that you need to cram more air in the cylinder. Everything tuning wise has that goal in mind.

well i ment as in, if no exhaust is leaving the cylinder, then obviously an air/fuel mixture isn't going to combust right? which would result in crappy performance.

 

[xxChrisxx]

well i ment as in, if no exhaust is leaving the cylinder, then obviously an air/fuel mixture isn't going to combust right? which would result in crappy performance.

Pretty much all the exhaust gas does leave the cylinder. Once it's gone its gone. I don't see what it has to do with making more power.

The only way to get more exhaust out at very high rpm is to have bigger exhaust ports. In all engines, they are made as large as possible. If you look exhuast valves are always larger than intake valves to ensure a clean cylinder.

Also 2 stokes have dreadful emissions, as they burn oil and throw fresh charge straight out the exhuast due to them having no valves. 4 strokes for production engines don't have this problem.

 

[PaulS1950]

Jakksincorpse,
I am no physicist but I have been an engine builder and mechanic for 35+ years so take this for what it is worth;
Placing a partial vacuum on the exhaust is likely to make less torque and horse power due to the lean condition that will result by drawing part of the air fuel mixture out during the overlap period (that is when the exhaust valve is closing and the intake valve is opening). Not only will that increase HC emissions and reduce fuel mileage but it will also lower the pressure in the cylinder.
The reason compressors are put on the intake side is so more fuel is drawn in with the fuel raising the cylinder pressure and thus making more torque and more horse power.
What you are proposing has been done before - even without a mechanical pump. Headers with very good extraction can cause an engine to go lean enough that the mixture has to be made richer to compensate for it. Properly tuned exhaust and intake systems can provide for volumetric efficiencies of over 1.2 - essentially super-charging the engine but this only works at a very narrow rpm range when the effects gets to the exhaust valve before the intake valve opens.
I suggest you take a class on engines in a community college to get the basics of how and why they work.

Paul, the 60 year old student

 

[xxChrisxx]

Paul is so right, it is a good idea to get the basics down. Many of the ideas you are coming up with are creative, but with a more in depth understanding you will see why they will not work.

It would also make sense to get a course specifically about engines and transmission systems, as the best we can do here is give you some reference material which doesn't fully convey the fiddly pita nature of designing and optimising an engine.

 

[russ_watters]

Something missed before:

...instead of relying on the cylinders vacuum pressure alone, if you have 14psi of air/fuel being shoved in, why not 14psi of exhaust being sucked out. [emphasis added]

You didn't fully grasp what Chris was telling you here and though he said it, it wasn't completely made clear to you:

Exhaust valves are usually sized to be as big as possible, and inevitably flow is choked at higher rpm across the orifice so sucking any harder won't make any difference anyway.

At low rpm there is more than enough pressure differential and time for all the exhuast gas to be evacuated.

Since the exhaust is the burned fuel and air that just powered the stroke of the engine, you already have vastly more than "14psi of exhaust being sucked out." I'm not sure how much, but "choked flow" requires a minimum of something like 75psi, but I'd expect the initital pressure of the exhaust gases is far above that.

 

[xxChrisxx]

This acutally reminds me that I need to take my reference books to work! I really miss working on problems with things that acutally move. Static stuff is really boring.There is a formula to work out the critial pressure drop for choked flow, but I can't remember it at present.EDIT: Before we go any further. OP I suggest you get the Haynes "4 stroke performance tuning". by A Graham bell. It's a great book that's aimed at the enthusiastic amateur. So it only assumes a minimal amount of technical knowledge. You'll prbably be able to pick one up dirt cheap 2nd hand off amazon.

There was another book I was going to reccoment that outlined the basics of engine and transmission operation too, but I can't remember that. (memory like a seive these days)

 

[jakksincorpse]

Jakksincorpse,
I am no physicist but I have been an engine builder and mechanic for 35+ years so take this for what it is worth;
Placing a partial vacuum on the exhaust is likely to make less torque and horse power due to the lean condition that will result by drawing part of the air fuel mixture out during the overlap period (that is when the exhaust valve is closing and the intake valve is opening). Not only will that increase HC emissions and reduce fuel mileage but it will also lower the pressure in the cylinder.
The reason compressors are put on the intake side is so more fuel is drawn in with the fuel raising the cylinder pressure and thus making more torque and more horse power.
What you are proposing has been done before - even without a mechanical pump. Headers with very good extraction can cause an engine to go lean enough that the mixture has to be made richer to compensate for it. Properly tuned exhaust and intake systems can provide for volumetric efficiencies of over 1.2 - essentially super-charging the engine but this only works at a very narrow rpm range when the effects gets to the exhaust valve before the intake valve opens.
I suggest you take a class on engines in a community college to get the basics of how and why they work.

Paul, the 60 year old student

oh, well thanks for clearing that up, its fine, i took an autotech class, didnt really care for it. speed is more my thing.

 

[jakksincorpse]

like i said above guys, I'm not really into the mechanics of it, I am just ready to make a motor that's wicked fast and gets awesome gas mileage. like the cars in the SEMA competition. i would do anything to figure out how that mustang guy got 400hp out of a v8 and still got over 100mpg cruising the highway.

 

[PaulS1950]

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.

 

[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