| Motoring Discussion > Jake Brakes | Miscellaneous |
| Thread Author: CGNorwich | Replies: 28 |
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New Mon 6 Jun 11 10:10
Jake Brakes - CGNorwich
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| Are these things illegal in the EU? When in Canada often see signs forbidding compression braking by trucks within the town limits and they do make a hell of a noise. Never heard one over here. |
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New Mon 6 Jun 11 10:41
Jake Brakes - Cliff Pope
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Never heard of them, but as usual Wikepedia supplies the answer: en.wikipedia.org/wiki/Compression_release_engine_brake Fascinating stuff. I never thought before about that inherent difference between a petrol and a diesel engine. I have only ever driven a diesel in a van, and I must say I never noticed it lacked engine braking compared with a petrol. |
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New Mon 6 Jun 11 11:06
Jake Brakes - Bigtee
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| Be good to see the layout on a air schematic. |
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New Mon 6 Jun 11 11:13
Jake Brakes - CGNorwich
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This video gives a good idea what they sound like - turn the volume up to maximum! www.youtube.com/watch?v=qocMoTOVn6Q&feature=related |
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New Mon 6 Jun 11 11:18
Jake Brakes - Dog
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>>This video gives a good idea what they sound like<< Is that a 1600E dash :) |
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New Mon 6 Jun 11 11:42
Jake Brakes - sherlock47
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The interesting comment from the Wiki article is that in a petrol engine the added braking is effectively provided by the throttle butterfly valve restricting the flow of inlet air. The relatively lightweight build of most butterfly valve assemblies would, at first thought, not seem to be up to the task of significantly slowing a 70mph 1 ton projectile! Last edited by: pmh on Mon 6 Jun 11 at 11:43
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New Mon 6 Jun 11 11:56
Jake Brakes - CGNorwich
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More technical stuff from the Jacobs web site. www.jakebrake.com/technology/compression-release-brakes/ |
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New Mon 6 Jun 11 12:04
Jake Brakes - henry k
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| I thought the topic was about an F1 presenter. :-) |
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New Mon 6 Jun 11 13:07
Jake Brakes - Dave_
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Our newer M.A.N. lorries are fitted with exhaust brakes, as was every Iveco Cargo I've driven. en.wikipedia.org/wiki/Exhaust_brake A little different in function but with the same overall effect as a compression release brake, in use it more or less doubles the natural engine braking effect. On the puddle-jumpers it's operated by a footswitch by the driver's heel, but there's also a 3-way switch on the dash which can link the exhaust brake operation to either releasing the throttle or applying the footbrake, depending on the driver's preference. Its main purpose on these lorries is not to increase braking power per se but to extend the life of the brake pads. Last edited by: Dave_TDCi on Mon 6 Jun 11 at 13:08
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New Mon 6 Jun 11 16:23
Jake Brakes - Old Navy
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| Do modern lorries have retarders similar to the ones fitted to some coaches? |
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New Mon 6 Jun 11 16:52
Jake Brakes - Armel Coussine
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Either by State law (NSW), national law or a standard municipal bylaw, use of those compression brakes is forbidden in small Australian towns during the hours of darkness, to give the Ockers and their visitors a couple of hours of kip. Of course no one takes a blind bit of notice and the things can be heard giving their sudden loud snores all night long, preventing you from doing the same until you get used to it. Fortunately I am not unduly disturbed by noises when I am asleep, provided they fall into some sort of normal category (police sirens, cocks crowing, things like that). What bothers me is light. Most unfortunate in one who stays awake for much of the night and wakes around 10 or 11 am. |
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New Mon 6 Jun 11 17:31
Jake Brakes - CGNorwich
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| First came across these things when staying in a small out of town motel. Certainly take bit of getting used to. |
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New Tue 7 Jun 11 08:16
Jake Brakes - Old Navy
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>> First came across these things when staying in a small out of town motel. Certainly >> take bit of getting used to. >> Witnessing a loaded Road Train descending at walking pace from the Darling Ranges onto the coastal plain down Toodyay Road or the Great Eastern Highway into the Perth suburbs is a sight and sound to behold! At the moment they are shifting rock from a quarry to a road building project, (in addition to the normal Road Train traffic). An articulated truck with two or three full sized trailers. All loaded vehicles above 22.5 tons must stop in a layby at the top of the descent and there is a truck arrester bed near the bottom, with a sign saying "No Parking!" |
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New Mon 6 Jun 11 20:45
Jake Brakes - Harleyman
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>> Do modern lorries have retarders similar to the ones fitted to some coaches? >> According to wifey (PSV driver) they're exhaust brakes on both. On my Volvo the switch has three positions, varying from gentle deceleration to kissing the windscreen if used in conjunction with low gears and footbrake. As Dave said above their main use is to reduce brake pad wear and avoid overheating on long descents. They are also very useful when you want to slow down without attracting the attention of Mr. Plod, as they don't activate the brake lights. Their effectiveness varies from model to model; usually the bigger the engine the more effective the exhaust brake. The Volvo is pretty good, the one fitted to our little Daf 45 is all noise and no action, bit like the rest of the truck. |
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New Mon 6 Jun 11 21:30
Jake Brakes - Dave_
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>> the one fitted to our little Daf 45 is all noise and no action, bit like the rest of the truck I like it HM :) It also comes in handy in a 7.5t mostly to add that extra little bit of slowing power, when a mimser overtakes you and then gently slows down to 1mph less than you were doing before. I hadn't thought about the comparative lack of engine braking in a lorry (see Cliff's Wiki link above: No throttle = no engine vacuum to provide back-pressure on the overrun) but it makes sense. That'd be why the Daf's exhaust brake has minimal effect then, 2x not very much is still not very much. |
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New Mon 6 Jun 11 21:40
Jake Brakes - MD
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| Are these like de-compressors on older trials bikes?? |
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New Mon 6 Jun 11 22:11
Jake Brakes - Old Navy
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I was thinking more of these electrical devices, I know they are fitted to busses, I wondered about lorries. www.electricbrakesystems.com/howdoesitwork.php Last edited by: Old Navy on Mon 6 Jun 11 at 22:11
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New Mon 6 Jun 11 22:19
Jake Brakes - Dave_
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>> I know they are fitted to busses, I wondered about lorries. bit.ly/myp3TW (Scania) bit.ly/mU2mYu (M.A.N.) These both seem to focus more on electronic integration of the service brake, exhaust brake and cruise control, rather than any different retardation technologies. Last edited by: Dave_TDCi on Mon 6 Jun 11 at 22:24
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New Mon 6 Jun 11 22:26
Jake Brakes - Rudedog
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| I just wondered how you can have any kind of braking system that doesn't activate the brake lights, surely that can't be safe. |
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New Mon 6 Jun 11 22:41
Jake Brakes - Dave_
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>> I just wondered how you can have any kind of braking system that doesn't activate >> the brake lights, surely that can't be safe. The engine braking effect in a big lorry is minimal compared to that in a petrol car - the momentum of the lorry means the slightest downhill incline will cause the lorry's speed to increase. Anything that can mitigate this effect without cooking the brakes is a good thing. Even if the lorry brakes are 10x as powerful as a car's, they have twice the work to do. Although I'm not licensed to drive the big boys, part of my job entails manoeuvring them around the yard. Even at a crawl with a trailing throttle they will mount a kerb and just keep rolling, that's what 40 tonnes of weight does for you. The kinetic energy of a 40 tonne lorry at 2mph is likely to be the same as in a 2 tonne car at 40mph...* You don't see lorries braking sharply or coming to a complete halt with no brake lights showing. The retarder does exactly that, it merely aids slowing rather than providing all braking effort. *Number_Cruncher will be along now to point out some error in my maths there, but still it's a lot more than a car. Last edited by: Dave_TDCi on Mon 6 Jun 11 at 22:45
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New Tue 7 Jun 11 00:04
Jake Brakes - Number_Cruncher
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>> *Number_Cruncher will be along now to point out some error in my maths there, but >> still it's a lot more than a car. >> Sorry, but, yes! The effect of the v squared term in the expression for kinetic energy is quite dramatic. For the car to have equivalent kinetic energy, it would only need to be doing about 9 mph!! |
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New Tue 7 Jun 11 00:33
Jake Brakes - Dave_
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>> The effect of the v squared term in the expression for kinetic energy is quite dramatic. >> For the car to have equivalent kinetic energy, it would only need to be doing about 9 mph!! I thought so :( It's still noticeable that in circumstances where a car would bounce off a kerb, a lorry will drive up it like it's not there though. Our yard has a 6ft wide kerbed grass verge down both sides, the metal fence around the boundaries bears the scars from where I and others have failed to take the lorries' momentum into account when reverse parking. bit.ly/m5BIP4 (Overhead photo is about 4 years out of date, and taken in the daytime when most trucks were out.) Last edited by: Dave_TDCi on Tue 7 Jun 11 at 00:34
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New Mon 6 Jun 11 22:44
Jake Brakes - Number_Cruncher
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As PSVs are generally not as weight sensitive as trucks, hanging a large eddy current brake off the propshaft doesn't affect the vehicle's payload. The compression brakes and exhaust brakes add only minimal extra mass to an engine. As a broad rule of thumb, you can obtain about 30% of the power the engine could produce when driving as braking power. I first saw these systems in the form of exhaust brakes fitted to Mercedes trucks in the 1970s, where, as mentioned above, they were very useful in reducing brake wear and brake heat build up. On a V6, V8, or V10 installation, the driver's foot switch would operate three pneumatic cylinders, one exhaust butterfly for each bank of cylinders, and one pneumatic cylinder to force the injector pump to the no fuel position - thus, the exhaust brake switch would provide a fuel cut off function, and would be also be used to shut the engine down. Owing to wear, one of the exhaust brakes stuck on on one of my father's V6 engines trucks, and it limped back to base - at a walking pace up some of the hills! Jake brakes are interesting because they point towards one of the mechanisms for engine braking in a petrol engine and the major mechanism in a diesel engine. In a diesel engine, if the compression and expansion process were 100% efficient, there would be no engine braking, all the pressure built up by the piston on compression would simply drive it down again on what would have been the power stroke. On a petrol engine, when the injection system is in over-run fuel cut off mode, there would be nothing technically to stop a Jake brake working. That petrol engines don't do as much work against compression, and as passenger cars brakes aren't as marginal as HGV brakes means there simply isn't the need. |
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New Mon 6 Jun 11 23:15
Jake Brakes - bathtub tom
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>> Are these like de-compressors on older trials bikes?? De-compressors (or valve lifters) are fitted to large, single cylinder 4-stroke bikes to enable weedy, 12-stone runts to kick them over. They also prevent 16-stone elephants breaking their leg. IIRC they open the exhaust valve to allow the engine to get past TDC on the compression stroke. Gentle prodding of the kick start will then allow the rider to turn the engine to a point where a generous kick will let inertia overcome compression. They were also fitted to 2-stroke mopeds with centrifugal clutches. Starting these involved furious pedalling. Without a decompresser, they had a tendency to 'bounce' the engine either side of TDC. Decompressing them built up sufficient inertia to get past TDC. |
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New Tue 7 Jun 11 04:47
Jake Brakes - Pat
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I've been a bit late getting to this thread and number cruncher seems to have covered it nicely. The confusion arises from the use of the correct name and the two most often mixed up air the exhaust brake and the Jake brake The exhaust brake is common on all lorries and works extremely well on some models and is useless on others. Usually the ones that work well are actually Jake brakes. The retarder is a different thing altogether and a basic description is a picture of a clamp on a propshaft that slows the speed at which it turns. It is an excellent piece of equipment, only ever supplied as an extra and works extremely well. Most lorries that do dedicated European work have them fitted and disregard the additional tare weight in the interests of safety and brake wear. They operate usually from a lever behind the steering wheel and make mountain roads so much safer to descend. My only foray into Europe in a lorry was a double manned trip to Valencia and the regular driver insisted taking me the pretty way over Pamplona mountain. I was driving and learned first hand just how good a retarder is...I never did see the view:) Any auxiliary braking system in a lorry will always rely very much on the way the driver uses it and the good old exhaust brake has been responsible for more than one loss of grip on ice and snow. There are two distinct camps and there have been many heated arguments over a pint on a night out, as to whether you should use it on ice or not. I switch mine off and rely on what I can feel through my foot pedal in direct relation to my wheels....most older drivers do that. Perhaps number cruncher can tell us if exhaust braking is the cause of losing grip on ice or not? It would be nice to have a technical explanation next time I defend my action to a young whippersnapper, who has just passed their test:) Pat Pat |
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New Tue 7 Jun 11 09:21
Jake Brakes - CGNorwich
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So to answer my original question Pat, you are saying that Jake Brakes are fitted to some UK and European Trucks? If that is so why don't they make the infernal machine gun noise that North American trucks using them make? Is it just a question of silencing? |
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New Tue 7 Jun 11 09:39
Jake Brakes - Number_Cruncher
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The difficulty with exhaust brakes and Jake brakes on slippery surfaces is that they are on/off devices, the level of braking you get is not really under the control of the driver, and the braking is all on the drive axle. Harking back to the thread on jack knifing, we know that losing grip on the rear axle of a tractor unit, the drive axle, is the cause of those types of crashes, and should be avoided at all costs. Contrast this with careful use of the footbrake - the driver has more control on the level of braking being applied, and that braking is distributed among all the axles of the vehicle - it isn't distributed evenly, but, the axles do all contribute. One difference between car brakes and truck brakes is that in a car, the pedal does act almost directly, via servo and master cylinder onto the brake fluid, and the driver can feel the pedal reaction during light brake applications. In a commercial vehicle, the driver's foot pedal is operated a so-called "foot valve" which acts to produce a varying brake force (i.e. a varying air pressure in the pipework which leads to the relay valves by each axle) depending upon the foot pressure applied - as the driver is essentially pushing against a spring, and the outlet pressure acting on a piston, it's extremely difficult to feel light brake applications in the same way as you do in cars. So, although you can bring these vehicles to rest feathering the brake to give a smooth stop, it takes quite a bit more skill than it does in a car. Just like cars, some trucks are better than others in this regard. |
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New Tue 7 Jun 11 09:52
Jake Brakes - bathtub tom
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I thought the advantage of eddy current retarders was that they didn't cause loss of traction on slippery surfaces. IIRC the retardation is proportional to the speed of rotation of the shaft. I never could get my head round how the same device that worked an old mechanical speedo could slow a commercial vehicle. |
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New Tue 7 Jun 11 11:02
Jake Brakes - Number_Cruncher
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Eddy currents can be remarkably strong. Interestingly, the effect becomes much stronger in conditions of extreme cold - especially with superconducting metals. It's a mechanism I've used in light duty dampers (on a specialised test rig - not for vehicles), where I've taken advantage of the non-contact nature of the mechanism, and have also avoided needing to enclose fluids like a conventional damper with the sealing and reliabilty problems that entails. In vehicle applications, aside for the bearings which support the driveshaft as it passes through a retarder, they are pretty much maintenance free, and are generally very reliable devices. |
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