Sled Puller said:
Now lets review, quote from Jetdude. :
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"But honestly, what difference does it make? Torque is not HP. HP is what makes things happen.
You can have 1000lb/ft of torque on something that doesn't go anywhere. If nothing is moving, no work is being done, hence nothing to brag about. Give me a long enough lever, and I'll make 1000lb/ft of torque with one hand. Big deal!
It's HP that's hard to get out of these engines, not torque. "
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That statement, is a line in the sand, Hohn, we didn't put it there. It shows this man does not understand what a Diesel is all about.
We will be glad to teach him.
Quite true. Torque is work, and work is what makes things happen. HP is work through time, and is what keeps things going. My engine might make 200HP these days, and it accelerates my truck respectably. But put a 200HP motorcycle engine in my truck, and it won't get going very quickly.
To put it reasonably simply, torque is a measure of a device's ability to accelerate; the more torque a device has, the faster it can accelerate. On the other hand, HP is a measure of a device's reluctance to slow down; the more HP something has, the less likely it is to slow down.
If you've has ever paid attention to a gas race car going down the strip, you'd've noticed that the driver has the engine revved up moderately high before launch, then runs it close to the limit at launch. This puts the engine in its best HP position; when the transmission shifts, the engine is quite reluctant to slow down at such a high RPM. Gas race cars also depend on fairly loose torque converters that have pretty good torque multiplication.
Diesels, on the other hand, don't much need HP or torque multiplication to get down the track. They have an abundance of torque, and their owners are always trying to find ways to better couple that torque to the wheels, via 93% torque converters, multi-disk TC clutches, and even multi-disk manual clutches.
I see you still don't believe me when I say torque it what accelerates. Hmmm. How about this analogy: take a 1000 pound wheel set up on a nice axle so it can turn freely, and supported on only one side. Let's make the wheel 5 foot in diameter. Place a 1/2" hole near the outer edge of the wheel and a 1/2" hole near the center of the wheel. Now stop the wheel. Place a 1/2" hand-grip peg in the inner hole and apply 50 pounds of force for0. 2 second to start the wheel turning. Measure the wheel's RPM at that 0. 2 second mark. Stop the wheel. Place the hand-grip peg in the outer hole and apply 50 pounds of force for 0. 2 second to start the wheel turning. Measure the wheel's RPM at that 0. 2 second mark.
Remember, the wheel was not turning when you initially applied the force each time, thus you were generating no HP at all. I can guarantee that the wheel's RPM was higher when you used the outer peg hole. But that's not the true point of this exercise. The true point is that, at the start of each exercise, the wheel is not turning at all, thus absolutely no HP can be imparted to it. Remember, HP=TQ*RPM/5252; if RPM equals 0, HP must equal 0. I think this fairly proves that HP does not and cannot get things going. It is torque (work) that gets things going, that gets things turning faster. Once a state of equlibrium has been reached, it is HP that will maintain that state.
So why do race cars accelerate as well as they do? For that, you have to understand the basic principle of a torque converter. It, in two words, converts torque. HP in *must* equal HP out. (For the purposes of this example, we'll ignore power lost to heating the fluid. ) If you are putting 200 HP into a TC, you must get 200 HP out. "But," you say, "what about when the vehicle isn't yet moving? Didn't you just say that no HP can be applied to something that isn't turning?" Yup. I shore did; this is where we are ignoring the power lost due to heating the fluid.
So, let's suppose that engine is putting 200 HP at 4000 RPM into the TC; it is thus generating around 260 ft-lb of torque (200*5252/4000). Now, let's allow the output of the TC to turn ever so slightly, say, 1 RPM. The TC is putting out 200 HP, since HP in must equal HP out. The output torque of the TC will be in the vicinity of 1. 05 million ft-lb of torque (200*5252/1); since the real output is likely closer to 600 ft-lb, there's about 1. 05 million ft-lb of torque being converted to heat.
Now let's take a look when the output of the TC has reached 1000 RPM, which will be significantly closer to the TC's efficiency range. Still with the 200HP in and out, the TC is now generating around 1050 ft-lb of torque. At 2000 RPM, the torque has dropped to about 525 ft-lb, and at 3000 RPM, it has dropped to around 350 ft-lbs. Remember, the input of the TC has been constant at 262 ft-lb all this time. Do you now see where the 'multiplication' happens?
Now compare that 200HP/262TQ engine with a 200HP (@1800 RPM) diesel. The diesel engine is generating more than twice the torque at less than half the RPM (about 580 ft-lb). With a 93% efficient TC, 186 HP is making it out of the TC. When the TC output reaches 450 RPM, TC is putting out around 2170 ft-lb. At 900 RPM, the TC is putting out about 1085 ft-lb of torque. At 1350 RPM, torque output is around 720 ft-lb.
So, comparing the gasser with the diesel (and losing 7% in the diesel's TC), the diesel still puts out over twice the torque at each of the 75%, 50% and 25% output/input RPM ratios.
Now put that diesel in a car next to a gasser with the same HP and weight. Which one will reach the traps first? I say the diesel will, because it has far more torque than the gasser and will accelerate significantly faster.
Now take two IROCs side-by-side at the dragstrip. They both weigh the same. One has a 500 HP (@5000 RPM) gas engine and the other has a 500 HP (@3500 RPM) Duramax. I'd say the Duramax will reach the traps first, even though the HP is the same, because it has more than twice the torque, and torque is what accelerates a vehicle.
Need another example? Take a turbocharge where the compressor output has been connected to the turbine input, and propane is injected near the turbine input past a spark plug (for ignition). Let's suppose the turbo, in this configuration, can produce 17. 5 ft-lb of torque on its compressor shaft. At 60,000 RPM, this little turbo would thus be capable of generating 200 HP. But I really don't think it's possible to couple that 200HP to a transmission.
"Hey, Bill! I need a TC with a stall speed of 50,000 RPM, and 93% efficient from 50,000 RPM and up!"
Geez! I need to pack for SEMA! Night gents!
N
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