Van Canna wrote:
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
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.
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.
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.
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.