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How to calculate the torque of an engine?

  1. Mar 6, 2017 #1
    Actually I have 2 questions.

    1-) How do we calculate the torque that the engine generates? t=F.d so F= The force piston exerts on crank arm. and d= length of crank arm. Can we calculate like this or is it completely different?

    2-)Torque values of a car in the technical information is the torque engine generates or the wheels exerts on ground? or these two are the same? In otherwords, is the torque of the engine directly transfered to the wheels?
  2. jcsd
  3. Mar 6, 2017 #2


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    The power is transferred from the engine through the gearbox and differential gear to the wheels with only minor losses along the way.
    Remember that; Power = Torque * RPM.
    So gearing will increase the torque as the RPM is reduced through the gears in the drive train.
  4. Mar 6, 2017 #3
    Conceptually T = F.d is correct.
    The question is: How do you determine what F and d to use?
  5. Mar 6, 2017 #4
    Determination is easy for the engineers I think. Crank arm's lenght is already known, F can be determined by calculating some thermodynamics calculations.
  6. Mar 6, 2017 #5


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    The problem with crank throw and cylinder area is integrating the cycles of all cylinders. The torque from an engine can be estimated by volumetric capacity and your experience. Other than that there are too many unknown variables. You must know things like dynamics of inlet airflow, cam profiles and exhaust temperatures.
    If you want to know the true torque, you must measure it on a dynamometer.
  7. Mar 6, 2017 #6
    I got it. Then if we know the power, we can calculate the torque roughly by dividing RPM can't we?

    Thanks for your answer by the way.
  8. Mar 6, 2017 #7


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    Yes. Power is the flow of energy which is conserved.
    RPM = Power/Torque. Torque = Power/RPM.
    Use ratiometric values or appropriate units.
  9. Mar 7, 2017 #8
    Remember there is Brake Horsepower (BHP) and Wheel Horsepower (WHP). BHP is measured at the flywheel and WHP is measured at the wheel(s). WHP averages about 15% less than BHP. The difference is due to the rotational mass of the drivetrain. Rotational force is what?
  10. Mar 7, 2017 #9

    jack action

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    ##F## and ##d## are both vectors that varies according to the crank angle and that is over one cycle, not one revolution. So the torque output also varies with respect to the crank angle. Below are examples for a V8, L4 and single cylinder. Note that the torque can even be negative at some point in the cycle when considering one cylinder only (and even 4):


    When we talk about the torque of an engine, we talk about the average torque at the crank over one cycle.

    The rotational inertia of the rotating components will attenuate the RPM variations that could be caused by those torque variations. That is why flywheels are often used on reciprocating engine.

    If we know the average power, we can calculate the average torque exactly by dividing it by the RPM.
  11. Mar 7, 2017 #10
    That graph looks familiar.
  12. Mar 7, 2017 #11
    Both d and F vary as a function of crank angle.

    Its best doing this for a single cylinder engine at first to understand it better. Start with d as its relatively easy to work out.
  13. Mar 7, 2017 #12

    jack action

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    That is because it originally comes from this post. Even though it is not available on PF anymore, Google (:cough: Big Brother :cough:) made a copy of it and still serves it in searches. I guess we only loose some definition.

    Once on the Internet, it is there forever. :nb)
  14. Mar 7, 2017 #13
    I suspected it was one of mine, nice to see it still floating round. I lost all that work about 5 years ago when my external HDD died.

    It was a crude first look into engine forcing on a flat frank V8 based on two inline 4 engines. Ahhhh memories.
  15. Mar 7, 2017 #14
    Very impressive
  16. Mar 8, 2017 #15
    Torque is function of crank angle and numerous other parameters
  17. Mar 8, 2017 #16
    http://r.search.yahoo.com/_ylt=A0LEVwl0TcBYZVsAGfFXNyoA;_ylu=X3oDMTBzdWd2cWI5BGNvbG8DYmYxBHBvcwMxMAR2dGlkAwRzZWMDc3I-/RV=2/RE=1489026549/RO=10/RU=http%3a%2f%2fwww.seas.upenn.edu%2f~cse400%2fCSE400_2005_2006%2fPhillipsSaris%2fFinalPaper.pdf/RK=0/RS=9H190RVZ8RpLATbnMAifNFD3REQ- [Broken]

    The link is to a PDF of some really impressive work done by two University of Pennsylvania students for their final paper. Some might appear familiar. The title is "Computer Simulated Engine Performance." They took four engines and using MATLAB mapped a complete cycle of the engines. There goal was to compare actual engine performance to simulated performance. Unfortunately they ran into a common problem, no manufacturers would share their performance specs so actual and simulation could be compared. Regardless, it's some ambitious and impressive work from some undergrads.
    Last edited by a moderator: May 8, 2017
  18. Mar 8, 2017 #17
    Yeah, I'm sorry for them and I also surprised why none of manufacturers didn't share their results. Of course, we wouldn't expect Mclaren or Porsche to share their test results but some local manufacturers could help them and their work wouldn't be wasted. Thanks for your share btw.
    Last edited by a moderator: May 8, 2017
  19. Mar 9, 2017 #18
    No problem. I thought it was a good paper for showing the extent of the interactive relationships going on in an engine. The extent of the answers to your question here a good example of the relationships. You can discuss the topic of torque with 5 different but knowledgeable people and become confused. It depends on the interests, experience and focus of the person your talking with. An engineer whose job is related to air flow through an engine has a comprehensive job involving multiple systems of an engine when compared to engineer whose job focus is materials and stresses on components have different mindsets with their approaches of an engine. I assume you are a mechanic and student but as a mechanic, if you are, you know the best mechanics are those that best understand the relationships of systems. A modern engine that won't start can have a fault that has no relationship with what could be called traditional solutions but that doesn't mean the traditional solutions have no value or should be ignored.
    Any student pursuing an education related to the automotive industry would serve their education well to spend a couple of days in the archive of The Henry Ford Museum. It's where Ford sends their "program" information when they determine there is nothing proprietary about the program and has no more commercial value. I'm sure there are other, similar places. I just don't know of them. Anyway, the papers of Don Sullivan, "Sully" are there. He was in on the Model A design, he designed the Ford flat head V8 and his last job was the SVO V6. By writing, "he designed the flat head V8," what I mean is he was tasked with taking his idea and leading the team of engineers that designed, developed and proved the engine. Taken in total it's what Ford refers to as a "program." In his papers the entire process of a program can be seen. From the early conceptual meetings to developing conceptual ideas with slide rule math to early drawings and all along the process what is accepted and what is rejected along with why, it's all there. The process of prototyping parts to the team machinists and mechanics building the first engines to finally proving a design out. Most interesting and important though was his ability to see all the systems and components of an engine at once, put it all down on paper and then organize and lead a large team of people to make it all happen. There are other programs and leaders to review but none I've reviewed had his abilities.
    I took the time to write this because it might help explain why a question on torque can be about crank angle in one answer to typical equations in another. To make practical use of it all it takes someone who can bring the different approaches and focuses people bring to a task and make it work to complete the task. It's not a simple process and it's often skewed with biases and personalities.

    If you're interested in reasons why manufacturers are tight with information I can share some. It has even less to do with torque then this though.
  20. Mar 9, 2017 #19
    "1-) How do we calculate the torque that the engine generates? t=F.d so F= The force piston exerts on crank arm. and d= length of crank arm. Can we calculate like this or is it completely different?"
    During the design phase, that is essentially what is done.

    2-)Torque values of a car in the technical information is the torque engine generates or the wheels exerts on ground? or these two are the same? In otherwords, is the torque of the engine directly transfered to the wheels?

    Torque values of a car display (approximately) how much power the engine generates on an engine dyno. The engine, as it sits in the car, will generally make within 15% of that advertised torque value (net power). The amount of torque that makes it from the engine's crankshaft to the rear wheels to be transmitted to the ground will be about 85% of that net value. Manufacturers very rarely will ever advertise rear wheel horsepower numbers. They vary between different models speced with different transmissions or wheels or other basic modifications. 2 cars with the same advertised power output will have different rear wheel horsepower measurements if they have different transmissions or wheels or tires etc etc. For example, I had a vehicle advertised by ford to have 320 horsepower. That is not what the car made on a chasis dyno. It made about 265hp on a chasis dyno at the rear wheels.
    Gross HP > Net HP > RWHP.

    Torque is not complicated and anybody who diverges from this definition is wrong: "Twisting force". The only other version of torque is twisting force.

    Horsepower is a function of the amount of twisting force being generated, and how many revolutions per minute the crank is spinning while generating that amount of torque.

    Think of torque as the measure of the amount of force that the crankshaft is turning with. Torque is twisting force. Torque tends to cause rotation unless there is a greater force preventing the crank from rotating.
    Think of horsepower as the work rate of the engine. X amount of work being done in X amount of time. The easiest way to calculate HP is (RPM x Torque) / 5252

    You could also think of it in an analogy. Say there are 2 people that need to each unload their own car full of groceries. Exact same car. Exact same groceries. Exact same house/kitchen/fridge. All factors are equal, except the way that they carry groceries.
    - Joe carries huge loads of groceries, but can only do one load every half hour. It takes him 2 hours to unload his car full of groceries
    - Bob carries small loads of groceries, but does the trips very quickly. It also takes him 2 hours to unload his car full of groceries.
    - Joe has more strength at one time, but has a slower cyclic rate than Bob.
    - Bob has less strength at one time, but has a faster cyclic rate than Joe.
    - Joe has more "torque" because he applies more force during each trip. They both showed the same amount of horsepower, because they did the same amount of work in the same amount of time as each other.
    Last edited: Mar 9, 2017
  21. Mar 9, 2017 #20

    Was going to say this is en engine dynamics thread. But you were responding to Q2.

    Q1 is far more interesting, as it attracts less power vs torque nonsense.
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