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Speed Torque characterstics of Crankshaft

  1. Jul 8, 2014 #1

    Raj

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    Hi all,

    I want to know reason behind the variation of torque with respect to the rpm of the engine. I understood that the torque would decrease as the engine rpm increases.But, When i look into the characterstics graph of speed vs torque of the crank shaft engine(Graph is attached),It increases to certain extent and then it starts decreasing.Could u please clarify me in this

    g2-3.jpg


    Thank you
     

    Attached Files:

  2. jcsd
  3. Jul 8, 2014 #2

    Simon Bridge

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    You need to look closely at what the document is calling "torque" here.
    Usually it is some sort of torque-delivered to the load - i.e. available to do work without stalling the engine ... so it can be somewhat more complicated than just force times moment arm.

    You are correct that as the engine speed increases the torque available to do work decreases - due to losses in the engine (friction etc) increasing with speed.

    At low RPM, the available torque will also be low - consider: zero rpm (the engine is off) you'd expect zero torque available right?

    The graphs are showing that there is more to the internals of the engine than you'd expect - especially the mysterious non-zero torque at 0rpm for some of the curves. You want to look at what they are calling "low rpm cam" and high rpm cam" - compared with the engine speed.

    Like I said - look to the details of the documentation.
     
  4. Jul 8, 2014 #3
    Ignoring the specifics of the figures posted, as its not entirely clear what the relative axis is. This can be answered by thinking of three key points.

    The amount of torque produced is based on how much fuel you have combust in a cycle.
    The amount of fuel you can combust is based on the amount of air available.
    An engine is basically an air pump.

    Large pumps, pump lots of stuff very effectively, but are poor at pumping at low speed.
    Small pumps, pump effectively at low speed but very quickly become choked.

    You may hear this referred to as breathing.

    Breathing is determined by the intake and exhaust geometry and cam profiles (overlap and lift) and is driven due to the relative pressure in the cylinder and the outside world.


    A given geometry will fill the cylinder most effectively at a single RPM (the peak). This was set up for the type of driving the engine was expected to do.

    Traditionally:
    Road cams traded high end power, for low end power, by favouring a low down torque curve. 1000-4000rpm is where people typically drive and the relatively wide power curve helps with drivability. GREEN CURVE

    If you were going racing you'd swap them out for 'fast-road' or 'full race cams'. Which would aggressively increase the valve overlap and lift (ie flow more). This trades the low end power, for high end power. BLUE CURVE


    More modern engines use variable valve technology. This has two discreet cam profiles that switch in given circumstances. The most well known of which is Honda's VTEC system.

    Very modern engines use a continuously variable valve system, that don't switch profiles discreetly, but in a smooth way. This means you get modern N//A engines producing very flat torque curves, which means a linear power delivery. So you don't have to compromise top end power for drivability.
     
  5. Jul 8, 2014 #4
    torque usually peaks where volumetric efficiency(VE) is greatest. That is, you get the most air in the cylinder compared to what it's actual displacement is. due to scavenging and the inertia of the airflow(ram effect), a good engine can often get more air into the cylinder than it's actual volume, even without any kind of forced induction.

    The RPM at which this peak occurs is determined, as xxChrisxx stated, by the breathing characteristics of the engine. The valve timing and port size and shape are going to have an optimum engine speed where below that rpm the air velocity is not high enough to take advantage of the scavenging and ram effect, and above that rpm the ports(or other part of the airflow) become a restriction to the flow. the cam timing and duration also affects how well the cylinders fill at various rpms differently.

    As revs go up, the events of opening and closing the valves happen quicker, as well as velocities of the airflow being higher. As everything is happening quicker, the slight delays in air movement for each event of the 4 cycle engine cause greater decreases in VE. by moving or extending the opening or closing of the valves, this is somewhat compensated for.

    Modern engines are good at varying the airflow with variable intake systems, and the valve events with variable timing and lift. this allows a much broader torque band. Turbocharging forces air in allowing almost flat torque curves, especially if the boost is regulated to keep the peaks from being as high.

    HP, being a function of torque and rpm, usually has the hp peak after the torque has started to fall off, since the rising rpms allow more work to be done even with less force.
     
  6. Jul 13, 2014 #5

    Raj

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    Thanks for your replies. I would like to know the explaination in terms of the phases that it undergoes for
    1. Increase 2. Max torque and 3. Decrease.
    May be the image that i attached you might have misguided you. Please explain the characterstics taking any one curve i.e., inverted 'U' shape.
     
  7. Jul 14, 2014 #6
    It is mainly related to volumetric efficiency. Torque is simply the amount of force being applied, so the only reason it goes up from idle to the peak torque rpm is that on each ignition cycle more air and fuel are burned due to better filling of the working chamber due to the inertia of the incoming air coming in, and the timing of the opening of the valves, and also the ability of the exhaust gas to escape, and even help pull more intake air in during overlap.
    If the airflow into the engine were perfect, and the cylinder was filled with the same amount of air each time, the torque would be the same at all rpms. Horsepower would increase at a linear rate with regard to rpm, and would peak at max rpm.

    But, since no engine is perfectly tuned for all rpms that it will be used at, the phases are basically:

    1. rpms increase, port velocities increase, and valve events happen more quickly. the air going in and coming out of the engine has momentum increasing, as well as the pulses getting closer together. more and more air is getting into the chamber each time it fires as the velocity of the air allows the inertia to push/pull it's way into the chamber. It also gets more into the range where the pulses are lining up with the opening of the valves, so there is a pressure wave coming down the intake at the same time as the valve opens, and this also increases the airflow. similarly the exhaust pipe will have a negative pressure when the valve is opening, helping to pull the spent charge out, and even pull some of the intake air in.
    2: at peak torque rpm these events are all lined up so that the velocities are right at the most efficient level, where the increased inertia and the pulses are timed just right to correspond with the valve opening and closing. the working chamber is getting as much air and fuel as possible for each firing event.
    3: after the peak rpm, flow is still increasing, and the velocities are such that there is more and more resistance to flow, and there is lower pressure behind the valve when it opens, and the pulses are starting to lag behind the valve opening and closing, resulting in the cylinders not being as completely filled. Scavenging becomes less effective in the exhaust as the negative pressure wave is making it to the valve at the wrong time. Power continues to increase for a while after the torque peak, since the torque is not falling off as fast as the rpms increase, but after a certain point the resistance to flow will choke out the engine more and more, and the power will come down as well.
     
  8. Jul 21, 2014 #7

    Raj

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    Thanks.....Your answer really convincing and clarified my doubt. :-)
     
  9. Aug 6, 2014 #8
    See also the prior post on similar torque vs lower RPMs-- here https://www.physicsforums.com/showthread.php?t=244434

    I've worked with engines for 50 years (tractors,cars,trucks,heavy eqpt), off & on &have a BSci Physics/Math degree. I agree with MOST of what is said in re volumetric efficiency, air flow,valve timing, etc, BUT.. it appears a bit neglected as to fuel characteristics & burn time (combustion curve). The key to torque is prime lever arm angle of the crank vs burn peak (beside heat losses to walls as in thread above.
    That's why diesels THAT HAVE WARMED UP do so much better at low RPM, as in road graders, and of course their long stroke-high leverage arms. The slow burn of diesel with hot combustion walls shows how important those factors are- more time with burn without wall losses while longer time at prime long stroke arm leverage. That is all near missing in small high RPM cars.

    Engine designs, even for gasoline, like the Old John Deere Model A - Poppin Johnny proved that very slow gasoline engines could function with extreme torque at about 300-400 RPM, a very long stroke, but were inefficient and ran rich mixture. (I owned one of those & it weighed Tons but would pull a wheelie at idle if not careful. )

    Typical cars with short strokes for high RPM already have a disadvantage of not much time burning in the prime torque crank angles. When you look at the torque arm of the crank throw, it's obviously tiny and briefly at a useful angle, so the only thing that helps is for cylinders to fire more often-up to the point that there remains never enough lever arm for much torque.

    My points are that working with real and varied engines appears to verify much of what is suggested, but that prime crank angle (20 to 80 degrees past TDC), stroke and burn time/intensity create torque more than any other factor. If at slow RPM the fuel was delivered near TDC, it's probably burnt by 12-15 degrees after TDC at low RPM (700-1200) in a small car, & heat rapidly absorbed by walls. Remember the small engines are designed to run a bit hot at mid-higher RPM, which cooling capacity is way high for low RPM.
    Fuel injection also varies tremendously on how pulses are delivered, some with variable multiple pulses for lower rpm, increasing burn at key angle.
    I'd appreciate a couple of comments. I'm new here. And I'm designing a new type of engine.
     
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