Would a CVT transmission benefit Low RPM High torque engines the most?

Discussion in 'The Garage' started by fleabag, Jun 18, 2009.

  1. fleabag

    fleabag Banned

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    The main benefit with high HP and consequently high rev limits vs lower hp and lower rev limit engines is that they require less shifting assuming you didn't screw up the torque curve. I'm thinking that the primary reason for why these low revving engines which are undersquare are so slow to accelerate is because they have taller gearing in order to allow them to reach the same speeds as other engines that are high revving and can therefore have shorter gearing. One solution for them in order to increase acceleration speed is to give them more gears that are closer together, but the problem then becomes one of too much time spent shifting.

    So I'm thinking that the solution to low revving engines which are undersquare is for them to be matched with a CVT transmission, that way you can accelerate faster without having to rev to a high RPM and spend time shifting. Now the question becomes, do CVT transmissions behave like a tall and short transmission at the same or are they fixed to a certain amount of acceleration. I remember reading somewhere the specifications of a CVT transmission and it said it had infinite gear ratio for one thing but then like a final drive of 2.5:1 or something of the sort.

    Also was wondering if an engine experiences partial loads with a CVT all the time or for all but idle none of the time. Because it just feels like with a CVT, if you're going up a hill and you have the engine fixed at 2500rpm, you'll lose speed quickly despite you trying to maintain speed, so how would one maintain speed with a CVT without revving the engine higher? Finally, is there anything that could've been done differently to the CVT transmission in the '96 Civic HX in order for that car to boost its mileage figures? I'm not talking about efficiency improvements but instead doing changes like those that have been done to manuals and automatics where they make the transmission taller, giving better fuel economy numbers but not necessarily improving the efficiency of the transmission itself.
     
  2. fleabag

    fleabag Banned

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  3. JDub02

    JDub02 Diamond Member

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    i didn't think anyone had a CVT that would handle a high torque application. that's why they're usually paired to 4-bangers.

    what you're talking about sounds like an ideal version of a semi-truck. those usually come with a 10+ speed transmission .. keep adding more gears and you get a CVT.
     
  4. EightySix Four

    EightySix Four Diamond Member

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    I've always thought it would be neat to have a CVT set-up for racing where the motor always stays at peak power you just change the gear ratios for accelleration and deceleration.
     
  5. sdifox

    sdifox No Lifer

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    I think the biggest CVT now is Nissan, which has it on a 3.5L 290hp Maxima I believe.
     
  6. cheesehead

    cheesehead Lifer

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    Maximum RPMs of an engine is generally a function of two things: piston stroke and how much you've spent on things like conrods and crankshafts. A long piston stroke makes for small size and easy cooling, but will have a far lower redline (and, hence, specific output) than an engine of similar displacement with a shorter stroke and larger bore. This is further complicated by the issue of valves: unless you have some very complicated machinery to fiddle with the timing, a given valve configuration will give a pretty narrow powerband.

    As a result, an engine like the 2.2L Willys Go-Devil Jeep engine produces little power, but is small, durable, and produces loads of torque from idle: the undersquare design results in a design that, because of the very low redline, is sitting almost constantly in its powerband. On the other hand, the 2.2L engine in the Honda S2000 will produce 240HP at over 8,000RPM, but produces very little torque at low RPMs. The former is suitable for pulling stumps, while the latter, though far more powerful, is not.

    (NOTE: The following text is mostly speculation. Most cars with CVTs these days are relatively inefficient, and presumably only used them for increased smoothness and greater acceleration due to constant output.)

    CVTs are often used with small, high-revving low displacement engines. These generally have two modes: "On", in which they run at very high RPMs continuously to stay in the powerband to produce enough power to accelerate, and "off", where the low internal friction allows them to produce the tiny amount of power required to keep moving. This allows a car to use a disproportionately small engine, which can deliver better fuel efficiency (presumably due to lower internal friction.)
     
  7. Zenmervolt

    Zenmervolt Supermoderator<br>The Garage<br>Elite member
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    1) CVTs are infinitely-variable within a given range. For example, a CVT may have a minimum ratio of 3:1 and a maximum of 1.2:1. Such a CVT would allow any ratio between those two bounds, but would not be able to offer a shorter "creeper" ratio, nor a taller "highway" ratio. The range of a CVT is generally less overall range than the range between 1st and 6th in a 6-speed manual transmission.

    2) CVTs are unable to handle large torque loads currently. Nissan's toroidal CVT, which handles higher torque loads than most other CVTs, is also more limited in range than traditional pulley-based CVTs.

    3) Regarding hills. Imagine going uphill in your car without downshifting. You would simply press the gas pedal down more. The engine would not rev any higher, but would be generating more power. If the engine could not supply enough torque at that engine speed, a CVT would reduce its ratio, resulting in higher RPM, just like downshifting.

    4) Regarding the Civic, yes, the final drive could have been swapped for a taller final drive (though this would hurt mileage in town) or the CVT could have been designed with a range that covered taller ratios at the expense of shorter ones.

    ZV
     
  8. Jumpem

    Jumpem Lifer

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    I want to test drive an Outback CVT.
     
  9. simbasat

    simbasat Junior Member

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    Zenmervolt,
    You are correct about CVT's having limited range (with the exception of Infinitely Variable Transmissions (IVT's) that have no limit to input speed). IVT's are unique, so are distinctly termed 'IVT', but are a theoretical subset of CVT's, not practically commercialized yet. You are also correct about CVT torque limits because all commercial CVT's transmit power through traction of smooth steel surfaces without gear teeth to couple high torque; imagine getting car driving traction with steel tires on a smooth steel road surface coated with lubricant! Traction IVT's are an unlikely dream because their infinitely low gear output speeds are accompanied by infinitely high output torque with slippery coupling traction.

    High torque in Traction CVT's will greatly compromise their durability and efficiency.

    If an IVT could have gear-like coupling of torque, it would have excellent 'creeper' ratios all the way down to zero vehicle speed! Its final drive ratio in a car would only limit the car's top speed, but a step-up increaser in series with its transmission or its high inherent uppermost ratio limit would raise the top speed without lowering its lowest (zero) speed!

    Regarding hill-climbing, a CVT can be controlled to either maintain constant engine speed at various vehicle speeds, or vice-versa, or a combination of the two options. Since all transmissions change the speed/torque ratio of given power, and since power generally increases with engine speed, many combinations are possible. Hill-climbing and acceleration would be enhanced if CVT control systems were programmed to allow a rise in engine speed, but most commercial CVT-powered cars defer to low engine speeds for fuel economy enhancement instead. It's all about raising engine power through higher revving, and then controlling the tranny ratio to increase torque to the wheels by sacrificing wheel speed; in effect raising the wheel torque by applying two amplifying operations, and getting hill-climbing enhancement as a result. The transmission enables the driver to 'harvest' greater torque from the higher power output of its higher-revving engine. Engine power=engine torque times engine rpm.

    See my response to 'fleabag' for more.
     
  10. EightySix Four

    EightySix Four Diamond Member

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    That is one hell of a first post my friend.
     
  11. simbasat

    simbasat Junior Member

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    fleabag,
    Your comments are pretty old, but I'm responding as commentary for recent members too.

    Your analysis in your first paragraph is right-on. Undersquare engines, called 'torquers' and 'strokers' have torque curves that are relatively high at low engine speeds, requiring less frequent shifting in general, and are often coupled to low numeric differentials (oddly, they are 'high' geared) to deliver better fuel economy at the expense of good acceleration. They are called 'flexible' engines because they give the driver flexibility in widely changing vehicle speeds without shifting gears; the low numeric differential extends their flexibility further too.

    Your speculation about CVT's isn't quite right. First, CVT's do not have infinite gear ratios, they have infinite choices within a limited range of ratios. Second, a CVT does not inherently keep engine speeds low; it is only their control systems that are deliberately programmed for low engine speeds for better fuel economy that limit higher engine speeds that a driver otherwise could use for better acceleration & hill-climbing. In fact, using a torquer engine with a CVT is a mistake. 'Peaky' engines that only produce a lot of power at high rpms could take much better advantage of CVT's, and give the driver even better fuel economy and higher power on demand as a result.

    A huge advantage of the CVT is that the driver (or the automatic tranny) doesn't have to shift gears! This means that installing 'peaky' engines, that would normally require very frequent gear-shifting, would be very desirable. Here's why: peaky engines produce very low power at low rpm, therefore use very little fuel for moderate driving; yet can deliver lots of power on demand if only the CVT control system would let the engine rev up when the accelerator pedal is floored! It's that simple!

    The dirty little truth is that engine designers have been designing fuel-inefficient engines because they've had to accomodate shifting (discretely-geared) transmissions. the Auto companies have not been wanting their customers (or their automatic transmissions) to do a lot of frequent shifting because it's annoying to drivers and destructive to the durability of gear train components. Unfortunately, they got in the habit of designing engines that way with few exceptions.

    So, the answer to your stated problems is not to change the differential 'final' ratio or to accomodate torquers, but to rethink how CVT technology can vastly improve our use of fuel and simultaneously enhance the driving experience. New, more efficient 'peaky' engines should be designed with high-rev durability in mind. Hopefully better CVT's are on their way too. We can now free ourselves from the bondage of torquers.
     
  12. Jeff7181

    Jeff7181 Lifer

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    Another thing to keep in mind with cars equipped with CVT's (at least Nissan's) is that they're also equipped with a throttle-by-wire system. So when you're pressing on the accelerator, you're not actually manipulating the throttle, you're telling the computer, "I want 25% acceleration" and the computer adjusts the throttle to provide that. So if you're going uphill, you're still asking for 25% acceleration by moving the accelerator 25% into it's range of motion from idle and the computer will do what it needs to do with throttle and gear ratios to give you that acceleration. This is also true when you have the air conditioning on... you can't feel it because the computer is adjusting the throttle and gearing to give you the acceleration you want, not the amount of throttle you want.
     
  13. SJP0tato

    SJP0tato Senior member

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    Because of this point, I'm guessing we won't really see any significant gains with CVT technology for another 5-10 years. It requires a whole re-design of the engine to work only with CVT. Right now I think Nissan is the forerunner of major car makers going with CVT for the majority of their cars, but most still allow usage for manual/traditional auto transmissions which means a huge tradeoff for the CVT.

    Also it plays into the marketing strategy of incrementally making each car generation slightly better than the last. Why raise mileage by 15-20 mpg across the board in one generation when you can make it 3-5 better each generation convincing people that it's worthwhile to "trade up" to the newest model after only a few years.
     
  14. Bignate603

    Bignate603 Lifer

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    Not really, optimizing for a single performance point can be done and it is arguably easier than trying to make an engine with a very wide power band. One issue that they're running into though with these CVT equipped cars is that often times there are non-CVT versions sharing the engine. That means that the engine needs to still have a wide powerband and will have inherent compromises to get it which will reduce efficiency.

    The way the chevy volt gets high gas mileage after it runs down the batteries is that its engine is designed for running extremely efficiently at a single performance point. The engine is no longer connected to the wheels so its RPM is completely independent of anything else. Additionally, it doesn't need to be optimized for different power levels because it just runs at its most efficient point until the batteries have a decent charge and then it turns off until the batteries are run down again.
     
  15. Jeff7181

    Jeff7181 Lifer

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    I've always said... diesel-electric trains exist for a reason. Do it on a much smaller scale and you have the perfect passenger vehicle.
     
  16. Zenmervolt

    Zenmervolt Supermoderator<br>The Garage<br>Elite member
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    There are several reasons that make it expedient for locomotives to use a diesel-electric system that have nothing to do with efficiency.

    1) The immense cargo loads carried by locomotives require huge amounts of power. It is vastly simpler to use an electric motor than it is to devise a clutch and transmission mechanism that is robust enough to handle the power required by a locomotive.

    2) The torque characteristics of electric motors are far better suited to hauling heavy loads than even diesel engines are. In fact, the overall power characteristics of electric motors (which include peak torque at 0 RPM and the easy ability to reverse rotation) are quite similar to those of reciprocating steam engines.

    3) The ability to have dynamic braking is a huge benefit for vehicles as massive as a loaded locomotive.

    I'm not saying that what you describe isn't a good idea, just pointing out that efficiency itself is not really the primary reason that such systems are used for locomotives.

    ZV
     
  17. simbasat

    simbasat Junior Member

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    I wrote, 'The dirty little truth is that engine designers have been designing fuel-inefficient engines ...'.

    As an IVT inventor, I have a vested interest in promoting CVT/IVT technology. As a citizen of this planet, I have a more intensely vested interest in promoting efficiency of fuel use. So, I invite thoughtful discussion of this topic of engine design, considering the potential to optimize efficiency.

    The internal combustion engine is actually a pretty faulty concept that we have perfected to a high level, making it practical. We'll have to live with it. Alternative prime mover technology will not likely supplant it for years, if not decades. So, it's important for us to dig deep and analyze how we are wasting fuel with these cards (engines) we are dealt.

    CVT's offer us opportunities that may be missed by engine designers stuck in old ways of thinking. Although a CVT rarely averages more than 85% transmission efficiency compared to 92%-94% for the discretely-geared transmissions, CVT's improve vehicle systemic efficiency because they improve engine operating efficiency synergistically in vehicles. This is true for spark-ignition engines and torquer Diesels. Both torquer and peaky engines can both benefit from CVT's, although in different ways.

    Diesel engines will gain U.S. market share for passenger vehicles because they offer about 20% better fuel economy than spark IC's, newer cleaner fuels are becoming available for them and their drivability is improving. Diesel operating rpm bandwidth is very limited, so they can eliminate frequent shifting only with CVT's, and operate in their 'sweet'spot' for their ultimate efficiency most of the time.

    Compared to Diesels, spark-ignition IC's have wide rpm bandwidth, potentially very wide. CVT's can take advantage of this bandwidth by dialing in low engine speeds most of the time for excellent fuel economy, while still allowing high power at high engine speeds on demand. This paradigm is vastly different from the Diesel/CVT approach, yet it proves that a CVT is potentially better for seemingly opposite reasons!

    Using these two scenarios (torquer/CVT vs. peaky/CVT), the question of which would win a fuel efficiency contest arises. Americans are hooked on power (in more ways than this), so the marketability attributes of spark IC's may have a domestic bias; Diesel-powered sports cars are rare as hen's teeth. Diesels have greater inherent adiabatic efficiency; sparkies have greater rpm bandwith for power harvesting by the CVT.

    Following is my most ardent argument. Peaky engines are best. Take a 302 Z28 engine for example. Cut it in half to make it a four-banger and attenuate its valve overlap with variable camming. Then, shrink it to one liter displacement. Give it a proportionately more massive flywheel. Oversquare, short stroke, low reversing torque, very high critical speeds, low impact forces, high durability, high power at high rpm. It lives easily at 8400 rpm, and it idles at 700 rpm (12:1 bandwidth). Connect it to a wide-ratio CVT. Call it a Z14 (Z28/2). see: http://www.hotrod.com/techarti...o_test_tune/index.html for background.

    Drive the Z14 at about 1200 rpm engine speed to give you superb fuel economy in city & stop-and-go traffic. Accelerate the Z14 at 8000 rpm engine speed with low CVT ratios to merge with stubborn freeway traffic at the on-ramp. It's drivable! 5-10 H.P. city; 200 H.P. on-ramp!

    Here's the key. Engines and electric motors are most efficient when nearly at full load conditions. Q: Why? A: Especially with engines, the power produced is partially used, partially wasted; the more you use, the less you waste. Classic examples of waste are illustrated by gunning your engine at a stoplight when fuel consumption is high without any motion or acceleration; driving slow in 'stop-and-go' with a 170 H.P. engine is similar. It only requires about 20 H.P. to maintain a passenger vehicle at 65 M.P.H. in high gear (discrete gears). We consumers want 170 H.P. (@~4500 rpm) cars so we can accelerate sufficiently with our discretely-geared transmissions, but ignore the fact that a lot of fuel is wasted in the balance.

    Generally fuel consumption is proportional to engine power. That's just plain physics. Power is the rate of energy conversion per unit time. Do we want to convert most of this fuel energy to vehicle propulsion or to heat generation? Why not use an engine that only produces the power that we want to use at any specific moment under any specific driving condition? Wouldn't this engine be the Z14 with a CVT? Wouldn't we want engines with the very widest range of power production?

    The argument that Automotive marketing strategy is based on incremental improvement is not entirely true. The success of the Prius is an example of an exception. If we consumers demand a dramatic change instead of passively accepting the status quo, car companies will respond. Supply and Demand. Supply response to my argument. Demand results from the Auto industry! If you think about the status quo result of our complacency, it has been petro-terrorism. :disgust: It is not sufficient to merely respond yourself, you must discuss this with your friends and neighbors who will think and act too.

    I invite readers here to initiate heated discussions about the prospects I propose. This and other ideas brought to open debate will break open a hard nut that's been protected by idle minds. It's not too late to stop the nonsense. Worldwide, vehicles account for about half of all energy uses, so thoughtful ideas that result in changes to auto business are direly needed.

    Your thoughts?

     
  18. Bignate603

    Bignate603 Lifer

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    I work with a guy that worked for GE locomotive for a few years in diesel engine design. One of the biggest reasons they switched to diesel electric was for the efficiency. However, everything that mentioned weighed into the equation too. For locomotives a diesel electric setup is pretty much perfect.

    For cars it's a bit harder because they have more erratic power requirements which more or less dictates that they need to have some sort of energy storage. It's very feasible but the system will be more complex.