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Originally Posted by livi
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That would imply a direct and linear relationship between torque and horsepower in a car engine.
Now, what are the factors in an engine the affect the disruption of this linear relationship? Typically a big diesel truck engine makes a huge torque with a relatively moderate bhp output, while a small race engine develops a lot of horsepower but, conversely, a modest torque.
Is it the volume of the engine? Gear ratings? Density/weight of different engine components? Other things?
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hp = torque * rpm * constant
because rpm and torque are multiplied the net effect depends on the relative slope of the 2 curves.
rpm always has a + slope(ie goes up from left to right), torque slopes vary depending on whats going on inside the engine, they will be very + at points where the engine is working harmoniously inside, less + as things get out of sync and also go - when inertia catches up to gas flow volumes
what's happening in side all the cylinders collectively determines torque #s. The most torque comes from having the most massive fuel/air charge compress and burn most efficiently/completely in the right time window, this generally is easiest to accomplish at lower rpm. The diesel runs at low rpm and has minimal intake restriction and high compression ratio compared to a gas engine hence it's high torque.
when there is a wide rpm range it is harder to optimize torque at all points so w/o using variable geometry engines(to keep it simple) torque is optimized at a single point in the rpm range. If the point is down low you have a torquey engine, think truck or family sedan, if the point is up high you have a sportier engine used in a sports car, higher yet and you have a race car
w/i an individual cylinder there are many factors that determine the mass of the fuel/air mixture to be burned. The biggest is the available time during which the intake valve is open, the pressure differential between the intake manifold and the cylinder and the resonances/flow patterns that are occurring. At high rpm there is simply less time for the cyl. to fill so the torque curve tends to flatten then drop as rpm increases but the resonances in the intake and exhaust can be used to increase the mass of fuel/air charge at very specific and limited rpm points. If a variable geometry is used(vari ram & vari cam) the mass of fual/air charge ingested(volumetric efficiency) can be raised over a much wider rpm band
When both slopes are + hp goes up fastest, when one is positive and the other less so, hp increases at a slower rate, when one is more positive then the other( even if the other is -) hp can still go up but more slowly, when one is more - than the other is + hp goes down
small torques are amplified by gearing, gearing includes the transmission gear sets, cwp and tire dynamic loaded radius
Quote:
Originally Posted by livi
Also, where in an engine is torque measured?
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depends on the measuring tool
a chassis dyno measures at the rear wheels or hubs in some cases
an engine dyno measures at the flywheel