CVTs have many advantages over both manual and automatic transmissions, in terms of both putting the power to the wheels and in potential engine braking. Automatics (unless they have a manual function) can't be used well for engine braking. Manuals can be used easily for engine braking, but after the revvs initially shoot up from the downshift, you have to wait until they drop to downshift again and provide more braking force. The CVT has great potential for engine braking since it is continuously variable, however I have never seen one that lets the driver do so. The CVT engine brake pedal is a third pedal (where the clutch would be in a car with a manual transmission) that, depending on how far it is pressed, adjusts the gear ratio to hold the engine at a certain rpm. It would automatically cut fuel and could hold rpm as high as the redline for a semi-panic stop.-- acurafan07, Dec 24 2006 A Continuously Variable Transmission Website http://cvt.com.sapo.pt/very informative... [BJS, Dec 25 2006] Manual Slider CVT Manual_20Slider_20CVTrasberry re-tart's idea. (I actually think it's better.) [BJS, Dec 28 2006] Great idea. Makes so much sense mechanically. The only problem I can see is a safety issue. With another pedal that is, in effect, a brake operation could be confusing, especially in fast reaction situations. Simpler is better for safety. Remember the Audi "spontaneous acceleration" issue? This was a pedal placement problem. People thought their foot was on the brake, but it was on the throttle. Maybe for commercial vehicles, big trucks, etc. with professional drivers.-- JackyD, Dec 25 2006 True, but the worst people would do is a.) if they were used to a stick shift, think it was the clutch and accidentally slow down or b.) mistake the engine brake for the real brake.-- acurafan07, Dec 25 2006 Good idea. I think it would be best if the system was computer controlled.
And what type of CVT did you have in mind?-- BJS, Dec 25 2006 Any type would work. Nissan's toroidal (uses rollers) CVT seems to work quite well, and doesn't even use any breakable chains or belts.-- acurafan07, Dec 25 2006 What is the advantage of engine braking over conventional braking in "stopping" situations? The only time I see a real advantage is when continuous braking is necessary, such as when descending a hill.-- supercat, Dec 25 2006 One advantage is that it doesn't wear out your brakes so you don't have to replace them as often, and another advantage is that it is more fuel efficient, especially if the fuel to the engine is turned of while engine braking.-- BJS, Dec 25 2006 Exactly. If a light touch on the brakes caused the car to slow down only a little, the full potential of engine braking would never be used. If it were computer controlled and (because of the CVT) could be held at any rpm, the engine brake pedal would be just like the standard brakes.-- acurafan07, Dec 25 2006 Engine braking could reduce wear on the brakes, true, though I would expect the cost to implement it as described would exceed the reduction in brake-maintenance costs.
I don't see how it would improve fuel efficiency, however. Whether energy is dissipated in the throttle valve or the brake rotors/drums, it's still not doing anything useful. Regenerative braking on hybrid vehicles does improve fuel economy, but of course that's already well-baked.-- supercat, Dec 26 2006 Actually the cost to implement it would be very small. The CVT's main purpose is to be able to constantly change the gear ratio. Because of that, the only cost would be the electronics that control it and the pedal itself.-- acurafan07, Dec 26 2006 I used to ride a front-wheel-drive motorcycle with the engine over the front wheel, and a Comet CVT. I can attest that engine braking worked on that setup, with no third pedal needed. I'd often let off the gas quickly just to watch the big flames pop out of the short exhaust pipes. Whee!
I'm saying that the CV could be tweaked with the right centrifugal-weight set, or programming, or however it is controlled, to always try to shift downward when the gas is let off. No pedal needed . . . well, maybe a switch somewhere to change from one mode to another, if you want.-- baconbrain, Dec 26 2006 Sometimes people just want to coast...-- BJS, Dec 26 2006 After all, coasting is one of the easiest ways to increase gas millage. As I've pointed out, you could decelerate very rapidly with a CVT because it could hold the revs as high as the redline. I know that I for one would not want the surprise of lifting off of the throttle thinking I am about to coast only to hear the engine shoot up to 6,000RPM and the car slow down instantly. Yes it is true that it could be implemented so that when you release the throttle, it works to a lesser extent but A.) the full potential would still never be used and B.) I'm guessing that those who have been driving for a long time would find it hard to adjust. The thing about the CVT is that since it can put the engine at any rpm, it has the potential to create different degrees of braking (just like disk or drum brakes). And seriously, what's so bad about a second brake pedal?-- acurafan07, Dec 27 2006 It's the middle of summer here. If you've got an air conditioner on you get more cooling with high engine revs than with normal braking and low engine revs.-- caspian, Dec 27 2006 It's the same with the heater, only the heater gets hotter when more fuel is being burned.-- BJS, Dec 27 2006 Engine braking effect on A/C: The engine goes faster, the engine's connected to the air conditioner compressor, so that goes faster too.
If you had a second CVT you could vary that independently and use air-conditioner braking instead of engine braking but that might be too complex.
I can imagine how engine braking could heat up the engine to make the heater work even with fuel turned off, but I don't know if it would be enough to work properly. The engine would be compressing air, which heats up, and some of the heat could be conducted into the engine.-- caspian, Dec 27 2006 If you have this method of manually controlling the gear ratio during deceleration, why not simply have some mechanism to control the gear ratio manually at any time, like some automatics have? I know that the new Mini with the CVT allows you to choose between 6 stepped "gears," but if you had some sort of dial or lever to control the ratio, that would work better. [+]-- discontinuuity, Dec 27 2006 //but if you had some sort of dial or lever to control the ratio// ...Or pedal?-- acurafan07, Dec 28 2006 //but after the revvs initially shoot up from the downshift// You sound like a car guy, so you *do* match your revs on downshifts, don't you?
(BTW, I'm an Acura fan too. My daily driver is an old NA1.)-- harebrained, Dec 28 2006 Doesn't the Prius kinda work like this?
When in "B" mode it feels like torque is split between the pumping losses of the ICE, and the high-RPM generator. This would be in addition to the braking from the traction motor.
Better to store some of the power rather than just blow it off through the ICE/heater etc.
Technically, the Prius (HSD) doesn't even have a transmission, but to explain it to the masses it's referred to as an electronically controlled CVT.
I find when driving my Camry Hybrid, that I get the best fuel economy when I either leave it in "D" and brake gently, to avoid high I2R losses, or when braking more quickly use "B" so as not to rely on the disk brakes as much.
Great idea, but baked by Toyota I suppose.-- TIB, Dec 28 2006 If I am not mistaken, many hybrids use the electric motor as a generator to harness the power and cause the car to brake. Unless your Prius shoots up in revs when you brake then it is most likely not baked by Toyota. Of course regenerative brakes are the most efficient kind because they harness some of the power that would otherwise be wasted, but for all non-hybrids this would simply not give any benefits. Besides, I've often wondered how much of the electricity from braking can possibly be used to recharge the battery in a very short amount of time.-- acurafan07, Dec 28 2006 Capacitors my friend, Capacitors.-- jhomrighaus, Dec 28 2006 [acurafan07]:
Yep, in "B" mode, the RPMs shoot up. You can think of the Toyota system and a differential installed backwards. Basically, a generator and an ICE are the inputs (where wheels would normally go), and the driveshaft is the output (it has an electric motor on it to assist). Really it's nothing like that, but it helps to explain it this way.
When "engine braking", the computer loads the generator (forget the other electric motor for now) with the kinetic energy of the car. Naturally the differential wants to split the load, so sends torque back to the ICE - hence it suddenly revs up.
This system is almost perfect, but for a car that uses a simple CVT and no hybrid components, your pumping-loss brakes would be a cheap way to save on brake pads. You may need a degree of computer control (not a big deal) to maintain linearity in the feel of the brakes as the pumping losses through the engine may not track directly with RPM.
+ for thinking about stuff I like...-- TIB, Dec 29 2006 [not sure why I'm commenting on a 4+ year old idea, but this just seems obvious to me]
Why not set this up to utilise the first ~25% (or even less) stroke of the brake pedal? Most brake systems have some takeup anyway, all you need is a small encoder (which most modern vehicles will have anyway) to detect the light brake pressure. Added advantage is that your brake lights will come on.
When I'm slowing using compression braking (I drive a diesel) - out of courtesy (and precaution) I lightly press the bake pedal enough for the lights to come on, so anyone behind me can see that I'm slowing. This idea then becomes a logical extension of that. In fact you don't need a CVT, this could work with variable exhaust brakes as well. Light brake pedal pressure means increased engine braking, increase pressure and the vehicles' actual brake system comes into play.
Someone tell me this is baked already?-- Custardguts, Jan 17 2011 [CustardGuts]: One thing I've wondered about with diesels: I know that throttling down the air intake on a diesel engine would prevent ignition, but would there be any problem with throttling as a means of braking if fuel injection was suspended during throttling (as it is during compression braking)? I would think it'd be a lot less noisy than jake brakes (which functionally convert engine momentum into intermittent spurts of high pressure air, aka loud noise)?-- supercat, Feb 16 2011 What [custardguts] said. Surely there's no need for an extra pedal.
There may be a small fuel saving, since during engine braking, some extra power is going to the alternator (and possibly the air conditioner).
I also have a question: When engine braking with the ignition turned off, is fuel still being used? My guess is in a fuel injected engine: no; in a carburetted engine: yes, until the carburetor is empty.-- spidermother, Feb 16 2011 //When engine braking with the ignition turned off, is fuel still being used?//
Not in a diesel with a mechanical fuel pump - the fuel solenoid would be closed. In a common rail diesel - the electric pump would be off, so once again, no fuel.-- Custardguts, Feb 17 2011 Sorry, I meant non-diesel engines (hence the mention of carburetors). It's a question vaguely related to this idea, but mostly to satisfy my own curiosity. I would also guess that vehicles with a mechanical fuel pump and a carburetor would just keep sucking fuel, and blowing it out the exhaust pipe unburned.-- spidermother, Feb 17 2011 This could probably be implemented for more than just the first 25% stroke of the pedal; the CVT's engine braking range would effectively be off idle to redline. Engine braking at higher rpms would probably be equivalent to signifcant travel of the brake pedal and thus as long as it is programed to have a linear braking feel and have the regular disk brakes engage with ABS if the wheels lock, engine braking could probably account for more like 60-70% pedal travel with the last amount of travel for emergency braking (which could also work in conjunction with engine braking to even further reduce brake wear).-- acurafan07, Feb 17 2011 Spider, many carburettors have a fuel shut-off solenoid. The Weber on my Landrover does, but the SU's on my Spitfire don't.
Switching off ignition in a moving vehicle is a really bad idea. Finding the steering locked is not pleasant, if you have ABS it won't work, electric PAS won't work, some types of brake servo won't work.
Using VVT to pump air from the engine could be made to work, but storing any significant quantity of compressed air will involve large, heavy tanks. Is it really worth carrying all the extra hardware?-- Twizz, Feb 18 2011 Thanks, [Twizz].
However, I don't agree that //Switching off ignition in a moving vehicle is a really bad idea.//, in all cases. In many small cars, there is no PAS or ABS, it's impossible to activate the steering lock unless the steering is nearly at full lock (in which case you are already in trouble!), and the vacuum assist for the brakes can be kept topped up by allowing the engine to turn over at intervals. It gives you one more thing to think about, and is therefore in the same danger class as talking to a passenger, but less dangerous than, for example, eating or smoking or fiddling with the sound system while driving, IMO.-- spidermother, Feb 18 2011 [Twizz] Your compression idea has unfortunately bee baked (at least on trucks). Sometimes known as a 'Jake brake' (Jacobs is a big brand in this kinda thing) the compression brake allows the engine to intake air and compress it, but around the time when you would normally inject the fuel (or fire the spark on an SI engine) the exhaust valves are opened, dumping the energy used to compress the intake charge into the axhaust system and out of the tailpipe. This is far more effective at high rpm, as you're pumping more air.
On the Cummins ISX (15l truck engine) this technique along with an exhaust brake/VGT brake can produce nearly the same braking power as propulsion power (if run as a pump around 15. times it's rated speed).
The engine is still turning, so all your accessories will still work.
This kind of system is only implemented on large commercial vehicles due to the 'on cost' for the technology being offset by the high milage and heavy loads run by the vehicles meaning they would cost a fortune in brake pads/disks otherwise. On a passenger car this isn't such an issue due to the vehicle weight and light use.-- Skrewloose, Feb 18 2011 random, halfbakery