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Termyn8or -> RE: Would you drive 55 MPH if ? (5/14/2008 6:57:12 PM)
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SR, that is not necessarily true. I admit that it is a logical conclusion but this is another case of when one size does not fit all.
The first point I will make against what I am going to say is yes, lower RPMs are usually better because there is always the factor of friction. It is also true that the crank is reversing the direction of the piston all the time, which uses energy.
But, sometimes in the modern cars, when in a lower gear will reduce the injector duration to the point where the consumption would be the same if not for the friction. If you take a nicely broken in engine and take off the heads, remove the cam drive and everything else you can spin the crank quite easily. At low RPMs, especially as low as they are when you are hand turning it, the mass of the pistons means almost nothing. But at higher RPMs the force required increases exponentially.
That is why engines have a "red line", because the forces become too great if nothing else. Actually the valve train is usually what screws things up, unless it is a desmodramic system (sp).
Now more modern engines have roller lifters than most people are aware. They use exceedingly strong valve springs to eliminate valve float, which happens when the valve and the pushrod are not forced back to the resting (closed) position quickly enough. This is very bad for an engine because the valve might hit the piston which will necessitate a full head job, which is not cheap.
A desmodramic system which does not use springs is best for high RPMs, but is costly. It could only be used in the priceyest of cars, and I actually am not aware of anyone doing it right now, but then I do not know every little thing. Some company could be.
Now here is the problem, in the absence of variable valve timing they can only use so much duration (open valve time) and in the meantime they want the maximum performance, and these days, efficiency is a big part of performance.
So they go to roller lifters, and when they optimize the exact shape of the lobes on the cam, they know they can count on a bit more. And with modern injectors at each intake port they can make it idle smooth, something that was near impossible in the days of the carbeurator. They have ignitions now that will fire a plug even if the electrode is burned clean off, if you ever have a problem with lean misfire (high HC but NOT CO) you can be fairly sure you have an intake manifold leak. And it will set a code.
So the thing is the cam is optimized for a wider torque range, which is what makes a V6 seem to run like a V8 now. The high lift required because of the lack of being able to use more duration because they would be throwing emissions to the wind, requires that those valves close very quickly.
That is why they spent the money to put a roller cam in my car, and that is a 1989.
That is why on the shifter there is a position that disables the overdrive, because in the city you would really not rather have the tranny shift constantly. It saves very little money. Now when you get on the highway it is a different story.
If you look at the specs on an engine, you see torque and horsepower. Horsepower takes RPMs into account while torque does not. A pretty good picture of the torque curve can be derived mathematically from those numbers.
For example there was a year of Corvettes that had two options for the engine. One was a 350 HP V8 and the other was a 350 HP V8. But one was better. When you see those numbers you are looking at peak torque, and if it is an equal number there, a lower RPM is the better. When you see peak HP, the higher RPM is better.
The cheap engine had a very limited torque curve compared to the deluxe version. IIRC it was an LT1 and it was actually a Mercury Marine engine. But the point is, the ft. lbs. and hp. numbers were very close, but the better engine achieved peak torque at a lower RPM and peak HP at a higher RPM.
So there is a reason why auto manufacturers have not made the drive trains so tall that the engine does 1,000 RPM @ 60 MPH. There is what's called cam overlap and that is why. There is a time when both intake and exhaust valves are both open on each cylinder. The efficiency of doing it this way is dependent on velocity and the volumetric efficiency of the engine. If the gases move fast enough through the system, what seems to be inefficient is not.
The internal combustion engine, even with all of the refinements, is still quite inefficient.
We need something better that will perfom adequately. Somebody invent that and you might just be able to buy and sell these oil men who have a stranglehold on the world.
Any takers ?
T
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