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 Post subject: Re: Bill...a question
PostPosted: Sun Nov 07, 2010 5:14 pm 
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Van Canna wrote:

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There's ample midrange torque, along with a linear high-rpm surge that's a rarity in the world of turbocharging; moreover, the GTI-specific intake design sends a glorious sound through the firewall under full throttle.


I'm having trouble understanding what is meant by
Quote:
linear high-rpm surge that's a rarity in the world of turbocharging;


Can you help?

The best way to understand torque is to think about its units and apply it to a practical example. Torque is expressed in units of force times distance. Think of your foot on the pedal of a bicycle. Two ways to make that bike move from a dead stop or to change speeds are to push harder on the pedal (force) or to make the distance between the axis of rotation and the pedal greater (distance). Its the combination of those two factors that causes the pedal axis to change the rate of rotation (delta RPM), and that drives the gearing (transmission) which moves the rear bike wheel faster. In a car engine, torque is increased by changing the power of the explosion pushing down on the piston (force) and/or changing the dimensions of the crank shaft (distance).

Engine torque is what gives you that feel of being pushed back in your seat. While horsepower affords you the ability to be AT a particular speed given a particular load, torque gives you the ability to CHANGE the speed you are going.

Diesels are popular for trucks because they have maximum torque at very low RPM. Anyone who lived in the days of truly schitty engines and cars understands what it's like to push a car. You know that it's more difficult to get a car going from a dead stop than it is to keep it going once it's rolling. That is partly due to the difference between the coefficient of static friction (higher) vs. the coefficient of kinetic friction (lower). It's also based on preserving momentum (easy) vs. changing it (more difficult). So if you have an engine with a whopping amount of torque at low RPM, you will feel a massive load move from the stoplight when you put your foot on the accelerator. You may have to put 10 to 15 gears in a semi diesel to get that load moving even faster to achieve highway speeds. But that's just engineering on the truck side and training/licensing on the driver side.

Back to your question...

Turbocharging is one of many ways to ram the air-fuel mixture into the firing chamber and to increase the compression in that firing chamber before the spark ignites the air-fuel mixture to cause the rapid expansion of gases which drives the piston down. The stupid-simple way to do it is supercharging, where you have an electric blower which forces the air-fuel mixture in - regardless of how fast or slow the engine happens to be going. You get the electricity for a supercharger from the battery, and you maintain it with the alternator. With a turbocharger, you don't use electricity. Instead you drive the blower with the force of the engine exhaust leaving the combustion chamber. (You could also have a belt-driven blower.) The faster the engine goes, the faster the blower goes. The faster the blower goes, the greater the compression.

It's easy to see the problem with turbocharging. From a dead stop at the stoplight with the engine at idling RPM, there's very little "turbo boost." You need to rev the engine to get that pressure boost. The result? Something called "turbo lag." Your car is a mouse coming off the line until it gets to mid-RMP, at which time the mouse begins to roar. More turbocharging means more of that freakish differential from low to mid RPM. A side problem with turbocharged engines is the desire of the spirited driver to get that feel of the torque kicking in. That's achieved by operating the engine at high RPM, and that's a problem. The life of an engine isn't a function of how far you drive, but rather how many revolutions the engine goes. Diesels last forever because they achieve torque at low RPM. There is no need to beat the beast to get the feel. Poorly designed turbocharged gas engines don't last long because Mommy-o Andretti is constantly flogging her Vulv... I mean Volvo engine to get her mommy car with soccer kids on board to the game on time.

The best-designed turbocharged engines work by a combination of efficient blowers and modern control theory. How this is achieved takes years of engineering training and development. You need to be a master of things like fluid mechanics and odd mathematical tools like root locus analysis. Let's just say that the process is a combination of having raw power to spare, and then learning how to use it wisely.

Come to think of it... that's a lot like the Platonic ideal in martial arts. ;)

The goal is to get the feeling of being pushed back in the seat when you push down on the accelerator - NO MATTER WHAT SPEED YOUR VEHICLE IS GOING.

The best way to see this with your eyes is to look at plots of torque vs. RPM. A "flat" torque curve is the ideal. A "peaky" torque curve means you'll be driving a temperamental vehicle that needs to be flogged (kept in the ideal range with a combination of engine speed and gearing) in order to respond.

To show the ideal, I often go to Wards Auto and look at their yearly review of "Ten best engines." They do their homework, and they display it for all of us to see.

Here's a GM 2.4 liter DOHC, naturally aspirated engine. This is as good as you're going to get with this simple architecture, and an inline 4-banger. They get this with high displacement in an ideal architecture. It's worth mentioning that an inline design means a long travel for the piston (because of the geometry you are afforded vs. a "V" design) which means a long "U" in the crank shaft. So you achieve better torque through the geometry of getting a better "distance" on that force times distance equation.

Image

Note the torque peak at 4900 RPM, and the general "peaky" shape of the torque by RPM curve. The spirited driver is going to want to feel the not-so impressive 172 lb-ft of boost. And that happens by flogging this engine at a higher RPM. Four-bangers "whine" because they're operating in this high RPM (= high tone) range. They whine more if you can't get appreciable torque unless you're operating in this narrow, high-RPM band.

Oh and note how precipitously the torque drops off after the peak. That's common.

Here is a supercharged engine - the Audi 3.0L DOHC V-6.

Image

Note the very broad band of flat (and high) torque. You get the maximum 325 lb-ft of torque at 2900 RPM. And that torque keeps going, and going, and going all the way up to 5300 RPM. The engine responds so well for so long that there's no feeling like you constantly need to change gears to hit "the sweet spot." That sweet spot is instead a sweet acre of performance.

However supercharging increases compression, and higher compression does shorten engine lives. A little...

And if it looks like that curve is "flattened", well... it is. This is control theory at work. The goal is smooth and constant response across a wider range of conditions. The blowers are working just as hard as they need to work - and no more - across a broad RPM band. There are many other "variable" components in play here such as valve timing. This isn't a simple engine, but it does produce a simply wonderful result.

Here is an example of the true genius of Audi-VW engineering. They're pulling out all the stops here. You have the economy of a 4-banger, the geometrically ideal design of an inline engine, the power boost of superbly designed blowers in a turbo- (as opposed to super-) charged design, and the added genius of control theory to tame the beast.

And they didn't have to use diesel fuel to do this. Diesel is capable of massive torque at low RPM because long-chain hydrocarbons have more chemical bonds to break per unit volume of fuel. If I didn't know better, I'd swear this was a turbocharged diesel engine.

Here is the Audi AG 2.0 L turbocharged I-4 engine.

Image

The maximum 258 lb-ft of torque is achieved at 1500 RPM. That's barely above idle. And that torque keeps coming from the deep, throaty 1500 RPM all the way to 4200 RPM. The engine is done before the GM engine above is just getting started. This engine will not whine like a weasel; it will roar.like a lion. It will push harder with less shifting, sound better. and last longer. Oh, and fuel economy ain't bad either.

- Bill


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 Post subject:
PostPosted: Mon Nov 08, 2010 3:47 am 
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Bill,

Thanks _ brilliant explanation of how things work...

So as to this
Quote:
linear high-rpm surge that's a rarity in the world of turbocharging


How does this translate into my driving feel of the GTI
_ as opposed to another vehicle without the 'linear surge'?
Quote:
"Drivability is the other main advantage the 2011 Volkswagen GTI has over other sport hatches. Output from the 2.0-liter turbo engine is smooth and linear, and that power is more manageable in the real world than that of more feisty rivals." -- Edmunds


What does 'linear' feel like to my driving of the car, Bill?

Image

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 Post subject:
PostPosted: Mon Nov 08, 2010 1:22 pm 
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Van

The language is perhaps unfortunate, but I know exactly what he's talking about.

Linear would mean that the torque curve is "flat" in the power band as per the two Audi/VW engine examples above. What you feel is a consistent push back onto your seat when you push on the accelerator pedal - across a fairly broad RPM band. You won't feel a need to be shifting up and down a lot, because your engine gives the exact same "kick" across a broad range.

Nonliner would be like the less-sophisticated (simple architecture) engine above. Push on the accelerator in the power band and you get a variable feel of push back onto the seat. It feels like the engine is surging and then it wanes past a very narrow sweet spot. When trying to get the most out of the engine, you'll find yourself constantly shifting up and down to stay in that narrow sweet spot.

- Bill


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 Post subject:
PostPosted: Mon Nov 08, 2010 4:51 pm 
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Thanks Bill...got it :)

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 Post subject: Hi Bill
PostPosted: Mon Nov 08, 2010 9:22 pm 
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You wrote
Quote:
The life of an engine isn't a function of how far you drive, but rather how many revolutions the engine goes.


What's your opinion then on whether or not to shut off my PC when the day's done?

I hear all kinds of opinions...restarting it over and over will wear it down faster then if left running 24/7?

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PostPosted: Wed Nov 10, 2010 1:30 pm 
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Hey, Van!

This is a good question with a not-so-simple answer.

The most important thing to remember is that - like cars - not all computers are built alike. The Dell that some of us have at work is built a bit more robustly than the appliance you get at Best Buy (uugghh!!!) or some other local shop.

Actually it reminds me of lawn and garden equipment. Because I lift weights and throw bodies around in my spare time AND I have 1.4 acres of property with hills and burms and lots of trees to go around, I can break an average push mower in a year. And I only use the push mower to do the parts I can't get to with the riding mower. The engine will be fine, and I'll donate it to someone who wants to make a go cart. But the frame will have cracked, or a wheel will come off, or a handle will break. Meanwhile the lawn services that come next door operate a level of equipment that's designed to go 5 days a week for 5 or more hours a day.

At work, I usually leave my computer on when I leave because I've unleashed some program up against 3 years of medical claims for 6 million people. It takes a while for all that computation and all the I/0 against the databases on the server hard drives. But when I'm not doing that, my company wants me to put my computer in a standby mode which basically reboots it and then sets it in a ready-to-log-on mode. So when IT headquarters wants to do a Windows update on all the machines, their processes can find machines in that mode, check up on all the updates, and bring software plus virus definitions up to date.

Your home machine's biggest weakness is the hard drive. I generally leave my everyday machine on 24/7 for one reason - to allow Windows updates (automatically at night) and to allow Norton to seek out new virus definition files. I may have to replace a hard drive every 2 to 3 years, but it's worth it. I DO find a need to reboot my machine every once in a while for several reasons. Some software can behave badly and refuse to give memory back when it ends in a not-so-graceful fashion. Before you know it, your machine is running out of available RAM, and your programs are using your hard drive rather than your RAM for available memory. That slows things down and it's hard on the hard drive. An occasional reboot brings everything back to square one. Another problem I've noted is the way Norton treats certain virus invasions. My Norton will shut my machine off the wireless before it kills the virus. Then it won't let it get back on again. I guess that's not so bad if the only price to pay is an occasional reboot after Norton has saved my machine from a virus invasion.

To give you an example of why you want a machine on... My PC in Virginia now is only on when I am there - about once a month. And when I turn it on, it takes half a day for it to go find all the updates of Windows, Adobe, Skype, ooVoo, Firefox, Norton, etc., etc. I wish I could leave it on and let that happen automatically. But too many bad things can happen if you leave a machine unmonitored for a month.

So...
  • Do an occasional (at least once a week) reboot of your machine.
  • Make sure your machine does automatic Windows updates, and know when it is scheduled. And make sure your machine is on when those updates are scheduled to happen. Doing this manually is just too much of a PIA.
  • Make sure your machine is on long enough to let your anti-virus software update its programs and virus definition files, and know when this happens. Again... too much of a PIA to do this manually.

I hope that helps.

- Bill


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PostPosted: Wed Nov 10, 2010 4:46 pm 
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Thanks, Bill. You confirm what I have read elsewhere. It is indicated that constant rebooting wears out the motor.

I do leave my pc on 24/7...I find that Norton does the rebooting automatically every few days anyway. I have a Dell.

But now and then I also reboot manually when some stupid things seem to hang up...like my outlook express.

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