By on April 15, 2011

What keeps powertrain engineers up at night? C’mon, get your mind out of the gutter. The move towards downsized, turbocharged engines is creating a number of new engineering challenges, and “torsional excitations” grabbed the spotlight at this year’s Society of Automotive Engineers Congress. Steven Thomas, manager of Ford’s global transmission and driveline, research and advanced engineering, illuminated the issue [via Wards].

As we reduce the engine torque, particularly just off idle prior to the boost coming on, we’re going to adversely impact the ability to accelerate the vehicle. I would challenge you all to think about new ways of dealing with this. We could really use new designs to deal with these challenges to optimize the fuel economy, but at the same time deal with (noise, vibration and harshness) and performance issues presented by these new engines.

The problem: the increased inertia of forced-induction engines. The practical example: a turbocharged Fiesta. A worthy adversary, a worthy cause. Let’s do this.

Inertia is already a challenge for the Fiesta, as Thomas reveals that

Ford’s new DCT, which also appears this year in the ’12 Focus, is “great for CO2 reductions and fuel economy, but I have to tell you one of its challenges is the amount of inertia in a dual dry-clutch assembly.”

Add a downsized, forced induction engine, which the Fiesta was not designed for, and the potential for “torsional excitation” rises. One possible solution, the use of dual-mass flywheels, is being tested by Ford for use in a possible turbocharged Fiesta, but initial results show it could actually increase engine friction by as much as 15%. If “DMF”s don’t work, the options become somewhat more limited:

Pendulum dampers are being considered to address the problem. And with automatic transmissions, torque converters incorporating improved dampers can quell some of the vibrations, but more work is necessary.

And, says Thomas, three-cylinders are even tougher to keep smooth, as their uneven firing pattern works with torsional excitations to create severe NVH conditions. The future of engines may be downsized and turbocharged, but it’s still got a few bad vibrations to work out.

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27 Comments on “Phrase Of The Day: “Torsional Excitations”...”

  • avatar
    Educator(of teachers)Dan

    How bout honest to god manual transmissions and expecting that a powerful car is going to… oh I don’t know… not drive like a f#$%ing appliance?  That’s my humble submission for a solution.  Or keep using regular automatics till you get it figured out.

  • avatar

    Either multiple sequential turbos ($$) or CVTs (!!) or just be patient til it spools up. I get this every time I leave a stoplight with my VW 1.8 turbo sludger. The guy behind me in his burly V6 shoots up on my bumper for the first second, then gets dusted off when the boost builds. Probably annoys the heck out of him. Shrug. Soon the EPA will force him to join us on the dark side.

  • avatar

    Now I’m wondering if it’s the dual-mass flywheel that makes my Focus SVT drink so much gas.

  • avatar

    Why not just use a mild hybrid for the job?  Start/stop is a good idea anyway, and electric motors are great at providing torque at low speeds.

    • 0 avatar

      A hybrid sounds like the perfect solution to me. Personally the slight lag of turbos never bothered me, I’ve got a 1.8T VW and T5 Volvo and had an Eclipse turbo back in ’97. Its a small price to pay to get the other benefits of boosted powered, I actually enjoyed the rush once the boost built up and the car took off. The Volvo is 5 cylinder running less boost so it doesn’t lag nearly as bad as the VeeDub, however its a heavy car so it does take a bit to get going. I also read that in order to mange torque steer its power is limited in 1st gear but not sure if that’s true. 0-30 mph isn’t impressive, but from 30-90 mph its “on like donkey kong” as a friend of mine likes to say – the car bolts off like it suddenly lost 1,000 lbs. Due to smooth torque you can do the 30-90 run in 3rd gear if desired.

      • 0 avatar

        I don’t know if it’s wise to use start/stop in combination with a turbo, because of what happens when the engine stops (along with oil flow).  I can’t say this for a fact, but it would seem to me the oil life would dramatically shorten, due to oil cooking around hot turbine bearings, unless provisions were made to circulate the oil even after the engine powered off.

      • 0 avatar

        The cooling can certainly be managed.  I drove an Audi A2 TDI with stop-start more than 10 years ago — and while the model was never a big commercial success, neither did it ever experience any epidemic engine problems.

  • avatar
    Dr. Kenneth Noisewater

    BMW’s got a small inline-6 that should have perfect primary and secondary balance..  1.6l and 160hp naturally-aspirated, presumably you could decrease the stroke to the same volume as a 3cyl..

    • 0 avatar

      This is the one thing i don’t get. Why not lower displacement, but leave the number of cylinders. Why does a turbo  3.5L V6 have to replace a 5.7L V8? Wouldn’t a turbo 3.5L V8 be a better answer?

      • 0 avatar

        I’m guessing that there would be more friction and parasitic drag from doing so.  Recall the old Mazda 1.8 litre V6?  Pretty cool, good power for the day, but rather poor mileage.

      • 0 avatar

        The Mazda 1.8 6 cyl in the Mazda MX-3 was a very unique offering in a pretty cool car.  But I believe the complexity and cost of offering more cylinders (along with the resulting reduction in torque as compared to fewer cylinders in an engine of the same displacement) outweighs the benefits in smoothness.

      • 0 avatar

        Yes M. With all the technology out there an efficient, small displacement V8, with or without a turbo should be possible.

      • 0 avatar
        Brian P

        An engine with more cylinders is unavoidably going to have more surface area for heat loss and more friction. Even the “displacement on demand” GM and Chrysler V8’s are still dragging four cylinders worth of friction and heat transfer along for the ride when they are running in 4-cylinder mode, and that solution comes with its own set of NVH issues to deal with.
        Want to make an engine with few cylinders run smoothly? Make the engine even smaller (to make the reciprocating parts lighter) and spin it faster. I have a small motorcycle with a single cylinder engine (counterbalanced) and at its usual 6000 – 9000 rpm operating range, it’s fine.

  • avatar
    Uncle Mellow

    Dual-mass flywheels are fitted to most diesel cars these days , and give a lot of trouble in cars that spend a lot of time in traffic. They are very expensive to replace as well.

    • 0 avatar
      Diesel Fuel Only

      Yup.  My Golf TDI has a DMF and pretty big counterweights on the crankshaft in addition to a balance shaft (if I’m not mistaken).  Earlier versions did not have the DMF, but the weren’t as powerful and vibrated more.
      Add to the DMF hydraulic motor mounts, variable vane turbo, a decent (not flawless, but very smooth nonetheless) MT and all the gods of “over-engineering” do rear their ugly heads from time to time.

  • avatar
    johnny ro

    I believe they are talking about vibrations not turbo lag. May I respectfully suggest dual balance shafts? My 1987 Yamaha 250 single had a balance shaft, 24 years ago. Its smooth. Smooth means less structural mass is needed to strength and you save weight.

    On the other hand I am 100% sure they know about balance shafts. 

    On the small six idea, you lose out with extra friction from the extra parts especially rings and valve train.

  • avatar

    Does anyone know the fuel economy of K-cars? To be honest, I think we’re not thinking big (or rather small) enough. Certainly in Hawaii, maybe 10% of the population needs anything more than a K-car.
    I think ultimately hybridization will overtake turbocharging as the best means of dealing with these problems. Unfortunately, I forsee a lot more people taking the bus in future unless fuel economy really goes up like crazy, even in industrial and heavy-duty applications where CAFE does not apply. Gas isn’t cheap.

  • avatar

    Just give the Fiesta over to Cosworth and let those guys figure it out.

  • avatar

    As a torque junkie, driving some little car with no off idle torque is like being the victim in some kind of real life horror movie. Yeah, it’s fun once it gets some boost going, but in real life traffic, it’s not a pleasant experience. I’ve borrowed a few cars that were like that over the years, and I had to ask myself, “Would I buy something like this?” Nope. Not me. Gas would have to be a lot more than $5 a gallon to force me into some little deathtrap with a turbo.
    Anything is better than riding the bus though. I did it for 4 days in a row once, and it made me feel really sorry for people who had to do it all the time. Three times as long to get to the same place, plus the hassle of waiting for it in the cold/heat/rain, and then the idiots riding with you….

  • avatar

    Take a look at the little 897cc TwinAir motor from Fiat, it has counterbalance shafts and such and despite a bit of a raspy growl upon startup and under acceleration, it’s quite a little refined motor and is I think slightly larger in displacement than 2 large Colt 45 beer cans in size and what it has going for it is light weight and low mass, which also means less friction and it CAN get very good mileage but flog it and not so much, but it is capable of very good mileage, I think in Europe, one is looking closer to 50mpg highway or perhaps a bit more than that. The car currently is lightly turbo’ed with 85HP but in full turbo mode, it’ll do 105HP and I hear there is almost no turbo lag with that little motor and it scoots off the line very well, much like their larger 4 Cyl. motors.
    Sadly, it’s not available here but the performance in Europe is largely mated to manuals so don’t know how it’ll do with an automatic, or dual dry clutch but I’m sure something will be worked out with with mating such an engine with an automatic of some sort.

  • avatar

    I think the perfect solution is already here from the combined efforts of Rotrex and Torotrak.  They’re joined in a venture called (obviously enough) Rotrak.  The best supercharger made meets a small CVT drive.  This setup could accomodate multiple power/boost profiles depending on what the situation calls for, including a torque mode that would keep the compressor spinning at decent speed just off of idle.  From what I understand making it more affordable is where the rest of their efforts lie.

  • avatar

    Audi seems to have it right with their 2.0T. The A4 is generally faster than the 328i off the line, and is also faster 30-50 and 50-70.

    It does definitely lose in smoothness though, the turbo 4 is definitely nowhere near as smooth in operation as the straight 6.

    • 0 avatar

      You just pointed out the reason BMW has done so much with straight sixes. It’s the most balanced engine design. The old Chrysler slant six was not only smooth, it’s long stroke produced ample torque at low rpms. That gutlessness off a standing start is the result of low torque. A small displacement long stroke inline six seems like a solution for NVH and turbo lag, but probably makes the most sense in a RWD compact/midsize car.

      • 0 avatar
        Diesel Fuel Only

        Not to mention that once you go to a V-8, or even a V-6, you have double the number of heads, twice the head gaskets (in theory), the crank now needs counterweights and balance shafts, double the number of cams, two timing belts/chains, all unnecessary moving parts and unnecessary weight compared to a Straight-6.  That weight so far forward also ruins your handling.
        Agreed that the slant-six was a great engine.  The cars were economical on fuel and had good mechanical reliability at a time when so many four-cyl. engines, even years later, were downright crude.
        Aside: many European commercial engines are also I-6 diesels.

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