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Thread: Fantasy engine

  1. #131
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    You are right when you say that a high torque at low / mid range rpm is important for a good driveability and a fast acceleration out of small corners, but thats because it means that you will have a lot of power in the lower rpm range.
    It does not matter how much absolute torque you have. If the power output is the same (again i am not talking about max power, but power over the rpm range), there will be no difference in torque at the wheel after appropriate gearing.

    A completely flat power line is not desirable. Simplified your tire can always transfer a fixed amount of torque. The transferable power increases with the revolutions. So for your lowest gear you will want a flat torque curve and a gearing ratio, so that your wheel torque is slightly higher then your transferable torque. In the next gears you will want a flat power curve, so you have the maximum power the engine can deliver, all the time.

    As i said before you can of course also increase the Volumetric Efficiency (which is what a turbo does), but still, you wont be able to deliver more power/torque. It only helps to reduce the rev range to increase your engines efficiency.
    You can not directly transfer some street tuning stuff to a restricted engine like in FSAE.

    I don't really get your last sentence about the 600cc. A 600cc 4 cylinder engine can easily choke the restrictor, have more torque then needed (you can get almost 70 Nm) and have a large usable rev range from 5000 to 11.000 rpm.

  2. #132
    I'm not sure what the fuss is about with choking the restrictor. Surely the aim is to make enough power to be traction limited for most (define most according to team goals) of the lap. That shouldn't require that much power - we never had any problem achieving it with a basically stock-internals 600.

    I'd say the main challenge would be to deliver that power in such a way that the driver can easily keep the tyres on the edge of grip, which means a fairly linear torque curve and fast throttle response at all RPM. Things like turbos and large, slow revving engines detract from both of those to my mind. An engine with multiple small cylinders, like a 600RR, fulfills the requirement for adequate power and sharp throttle response, leaving only the powerband problem. I'd argue that a 1000 is likely to have too much torque - one inch of throttle travel corresponds to 50Nm rather than 20, so a normal human would have to drive 10Nm from the limit rather than 5 as a tiny movement is more likely to break the tyres loose. Functionally, the 600 is delivering more torque.
    MUR Suspension & Steering '11
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  3. #133
    I think everyone is taking the liter bike engine too literally. I agree it is overkill. My point is that the greater displacement means you will reach the maximum amount of power and therefore torque allowed by the restrictor sooner. To what extent one needs that power is up for debate.

    RenM power doesn't do work, that's torque. Power is the measure of how fast you can do that work and is one way of looking at how an powertrain package will perform. I like to look at rear wheel torque because it is directly translatable to force, and F=MA. With a conventional transmission you are stuck with one ratio for a part of the rev range and you want to maximize torque in that range.

    I agree with you that max power through an RPM range can be geared to do the maximum amount work given the restrictor. That is what I'm saying about the larger engine. It will choke the restrictor and make max power at a realistic RPM.

    That brings me to what I was saying about the 600cc engine. A 600cc engine can choke the restrictor for about 2,000 rpm right at the end of its rev range. Without expensive internal modification you can't get one to spin fast enough to choke the restrictor through a whole gear. On top of that you would need a huge final reduction to make that useable, like 4-4.5:1, and that doesn't fit.

    I don't get what you are saying about the power and torque curves though. In a perfect world you would have a huge flat torque curve and the driver can use a higher gear or the right pedal less if they want less torque at a speed. We can't have that flat torque curve because of the restrictor, so the next best thing is to have a flat power curve that maximizes torque. Are you suggesting running different maps for different gears to change power output?

  4. #134
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    The conversation has drifted toward the limitations of real engines.
    More make believe please.

    For a fantasy engine I would want an OEM set up with:
    A 600cc parallel twin
    Air cooling
    A dry sump
    An intake already with a 20mm restrictor
    Two speed transaxle

    And the whole thing packaged to be as narrow as possible.

    For the first year you could use the OEM intake and tune.
    Later you could redo the intake to better fit your needs.
    Or if you screw up your design you could go back to the OEM intake and tune.

    I feel that for a first year team this would be perfect.
    Ordered directly from the OEM and it is ready for plug and play.

    -William

  5. #135
    Okay:

    600cc SOHC 12V triple
    Transverse crankshaft
    Liquid cooling
    Low height wet sump with accumulator
    Integral two speed transaxle (spool diff)
    Cylinders angled rearward and reversed (intake goes straight up from the front of the motor)
    ~11k redline

    Why narrow? I'd would have thought short, for mass centralisation.

    Liquid cooling means you can bury it under the seat, three cylinders gives you the balance of throttle response and linear power I was talking about, and SOHC is fine as you won't make much more power revving above ~10k RPM anyway because restrictor. Wet sump for simplicity and packaging. Two speeds is enough, as has been said quite a lot on this thread.

    It's heavier and not as fuel efficient as a 'perfect' engine, but it should be more reliable and driveable, which means heapo testing, which means you go faster at the end of the day anyway.
    MUR Suspension & Steering '11
    MUR Suspension advisory/annoyance '12
    BJR data engineer '13
    Maker of things

  6. #136
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    Originally posted by Racer-X:
    RenM power doesn't do work, that's torque. Power is the measure of how fast you can do that work and is one way of looking at how an powertrain package will perform. I like to look at rear wheel torque because it is directly translatable to force, and F=MA. With a conventional transmission you are stuck with one ratio for a part of the rev range and you want to maximize torque in that range.

    I agree with you that max power through an RPM range can be geared to do the maximum amount work given the restrictor. That is what I'm saying about the larger engine. It will choke the restrictor and make max power at a realistic RPM.

    That brings me to what I was saying about the 600cc engine. A 600cc engine can choke the restrictor for about 2,000 rpm right at the end of its rev range. Without expensive internal modification you can't get one to spin fast enough to choke the restrictor through a whole gear. On top of that you would need a huge final reduction to make that useable, like 4-4.5:1, and that doesn't fit.

    I don't get what you are saying about the power and torque curves though. In a perfect world you would have a huge flat torque curve and the driver can use a higher gear or the right pedal less if they want less torque at a speed. We can't have that flat torque curve because of the restrictor, so the next best thing is to have a flat power curve that maximizes torque. Are you suggesting running different maps for different gears to change power output?
    oh come on. Talking about power simply takes out the gear ratios out of the debate. Torque can be altered by gear ratios, power cant, so its much easier and much more feasible to talk about power and not about engine torque. If you insist so much on using torque, the torque at the wheel multiplied with its speed gives you the power at the wheel, which is the power the engine can deliver.

    you can have a 600c engine choke at 9.000 rpm and have a nice power plateau for 2,500 rpm without expensive modifications. All it takes is some thorough engineering and knowledge about engine tuning.

    We can have a flat torque curve until the restrictor chokes. In the lowest gear used for driving you can start at a low rev and at the end of that gear you reach the higher revs and your power plateau and stay there with your next gears.

  7. #137
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    @Tom Wettenhall

    I would want it to be narrowed for latter aero implementation.
    I always struggled to get a decent diffuser to fit around our F4i.
    Not that it can't be don't; a narrow engine would just make it easier.

    -William

  8. #138
    Power only takes the ratios out if you run some sort of cvt or other non conventional transmission. Power does not do work, I think you will agree that a car geared to do 200mph with 2 gears is going to have poor acceleration times even if it has the same power as an appropriately geared car.

    Even at that my argument for a larger motor is that you can reach choke flow and peak power at a reasonable RPM. On our car we run the f4i and we are maxed out on rear sprocket size and we still have issues with falling out of the power in slow corners. Without increasing the half shaft angles or lengthening the wheelbase we can't go any lower. I calculate that in a 25mph corner we are at about 5,500 rpm in first.

    If you are choking at 9,000 rpm you have poor restrictor design or you are using forced induction. The flow bench data off our intake showed that we choke just before 11,000 RPM, I'd believe 10,000 if you guys did something very right, but not 9k. Even then you would need to be able to choke the restrictor from 7,000 to 12,000 rpm if you want to maximize rear wheel torque. I say the only ways to do that are with a larger engine or forced induction.

    Maybe you guys have a better time packaging your engine or something and can run more friendly ratios. With our car we are always looking for ways to maximize low end torque. Its a great safety net for the drivers and helps with acceleration from a dead stop.

  9. #139
    Originally posted by murpia:
    V's package well (longitudinally).

    What's a boxer if not a 180deg V? Doesn't that result in too high a crank height, cramped exhaust routing and no space for accessories along the sump?

    A turbo V works quite nicely: Put the plenum in the V and the turbo behind the engine above the gear cluster. Oil pumps down one side of the sump, alternator down the other. Shallow out the V as far as you can without cramping the exhaust exits (120deg?).

    Regards, Ian
    Well in the same light, what is an inline engine other than a zero degree V?

    As for a V, it might be beneficial to keep the heat in the exhaust and reduced weight of your exhaust manifold by keep it in the valley and keep your light manifolds outside of the V.

    I imagine any arrangement would really depend on layout/packaging.

    Originally posted by murpia:
    OK, yes, I can see this setup packaging nicely, but I have a hard time imagining (fantasising?) the camshaft drive(s).



    Assuming that image link works, it highlights my CoG & crank height concerns ref a flat engine vs. a V...

    Regards, Ian
    Well considering that is with the factories deep oil sump, of course it seems tall. Now layer the image of a 209mm deck height V8 with an equally deep oil pan on top of that.

    Keep it apples to apples...
    Micah McMahan
    Red Bull Powertrains Performance Design Team Leader
    3MI Racing LLC Owner/Engineer
    Former MSI Defense Solutions - Sr. Design Engineer/Project Manager
    Former Roush Yates Engines - Sr. Design & Analysis Engineer
    ODU FSAE 04 member, 05 controls leader, 06 control/ergo/brakes leader, 07 brakes/MC22 turbo engine/Asst Team Leader

  10. #140
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    we are running in circles. As i said i am not talking about the maximum power, but about the power over the rev range.
    Once again: you will not gain a torque advantage at the rear wheels with a higher displacement, because you will have to change your gear ratio, because of the restrictor choking. You can of course decide to run in high rev ranges, but you will get a lot of throttle and mechanical losses which lead to a bad efficiency.

    We neither have a poor restrictor design nor use forced induction. We have a very good designed gas exchanged cycle, with properly tuned intake and exhaust runner lengths and adjusted cam timings and profiles. You do realize that you can reduce the rpm at which you get the maximum possible flow by increasing your VE?

    It is not a good target to run your engine choked all the time. As i said, you need a flat torque curve at the lower rpms where you are traction limited and you need a power plateau of not more then 2,500 rpm for your higher gears.

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