Talk:Retarder (mechanical engineering)

Each section could use its own illustration, for clarity. -- Beland 22:26, 24 May 2007 (UTC)

We are talking about several types of engine brakes here and some trucks use some of these types of brakes together. For example; The Ginaf X5376T truck uses a ZF retarder which is like explained earlier, oil hitting a rotor rotating in an opposite direction and thus slowing down the output shaft. It also has the "butterfly" valve in the exhaust pipe which closes when applied, not letting the exhaust gases go out freely, so the engine has to "work" the gases out. And then there is the "Jake" brake. A "jake" brake uses camshafts with a different 3 cammed cam (a normal one is "eggshaped") but this one has two extra "bumps" in them. So what happens; The valve rods are bi-parted with, during normal operation, oil between the two parts and just one of the cams is applied on the exhaust valve (only to let the gasses out) When you apply the jake brake, the oil flows from between the bi-parted rods allowing the two other cams to actuate on the valve rod. So first at TDC the exhaust valve opens to let the compressed air out in order not to allow this compressed air to help the piston go down. The truck will use the rotation coming from the gearbox to bring down the piston. The second step is that when the piston is all the way down, the second cam opens the exhaust valve again in order to let compressed air (from the exhaust pipe with the butterfly valve closed) into the cylinder again. As a result the piston goes up with already compressed air in above it, making it even more difficult and thus creating an amazing braking force. I have tried to write this as easy as possible without any "technical" terms so anyone can understand (I hope)  —Preceding unsigned comment added by 201.17.145.130 (talk) 17:17, 26 March 2009 (UTC)

Clarification
Something need to be cleared up in this article - I'm not sure what the correct answer is though.

The Retarder page indicates that engine braking in a gasoline engine primarily occurs due to intake manifold vacuum, and secondarily due to internal friction: The retardation effect is not caused by friction in the engine (although that does make a contribution), but by the fact that with the throttle closed, air cannot enter the cylinder on the intake stroke of the pistons. Essentially, a partial vacuum is being created at each intake stroke, and the energy required to create this partial vacuum comes from the transmission, hence retarding the motion of the vehicle. While the Exhaust brake article blatantly contradicts that: the intake vacuum, is commonly mistakenly believed to create the slowing effect felt in gasoline engines when they are going down a hill with the foot off the gas. Whilst this does occur, it is to a particularly small extent since even at a hypothetically perfect vacuum condition, the force acting on the underside of the piston would be at most atmospheric pressure. Given that modern engines often operate with less than atmospheric pressure in the crankcase to assist in lower emissions/oil consumption, the effect is marginalised even further. The braking effect is typically predominantly due to engine friction. This can be easily enough demonstrated on cars where the fuel metering can be independently turned off, and the car will still decelerate with wide open throttle but no combustion occurring. My understanding is that the negative intake manifold pressure is the primary explanation, although internal engine friction does play a role in the prosses, an engine inherently produces enough power to keep itself running with the throttle closed (idle), and other forces of friction such as air resistance, drive train friction (bearings, gears, etc), and tires (gravity) would play similar roles. However I believe that the first article is slightly mistaken as well, as it is not the vacuum on the intake stroke that slows down the engine, it is the vacuum created on the combustion stroke where the valves are closed (no air can be sucked past the IAC or throttle body) that slows down the engine. The intake manifold pressure does play a role though, because no air is being let past the carburetor needles or MAF, there is very little or no fuel getting to the cylinders, very little or no combustion, and thus a large vacuum on the combustion stroke. (shorter?)

Theshadow27 07:27, 17 June 2007 (UTC)


 * I think they are all factors. I don't have enough experience to say which type of internal resistance does the majority of the braking, but I think that in lieu of consensus or hard data, we should avoid making any conclusion about which one is the most influential factor at all. My personal feeling is that combustion engines vary so much in design that the "major factor" in engine braking is actually different for different engine types, and perhaps it is even different on the same engine depending on engine conditions like RPM and temperature. Would a 50cc go-kart engine have the same braking percentages from each factor as a F-1 engine operating at 20,000 rpm? Would it be the same as the 109,000 bhp Sulzer Diesel on Emma Maersk? I doubt it. If we can determine that it does in fact vary from engine to engine I think that simply saying that it varies would be a more valuable contribution to the article than trying to pin down any specific "major factor". But in either case, we should not be stating potentially inaccurate information in either article. --Cecilkorik (talk) 20:10, 16 March 2008 (UTC)

When you engage an exhaust brake, you are trying to 'choke' the engine and hence stall/ prevent it turning (incidentally, you need to be careful if the valve sticks... stalling the engine loses PAS and can be quite dangerous). Likewise if you build a negative pressure in the inlet manifold, the relative pressure between intake and outlet is the same effect- you are denying the engine the fuel it needs to keep turning.

My concern with the article is that I can see his point with a carbuerretor driven engine, but since a fuel injected petrol ('gasoline') engine works (in THIS one point) in the same way as a diesel engine his arguement between diesel and petrol falls down.

Basically, if you prevent the petrol engine developing more power than it requires to overcome all friction of the engine, transmission, tyres, wind resistance, it is going to slow down.

USER: Guest. 22/10/07 11:42 GMT —Preceding unsigned comment added by 88.110.252.42 (talk) 10:42, 22 September 2007 (UTC)

deceleration/retardation in specificly automotive truck application happens from a fatorial standpoint of newtonian fundamentals. speed (inirtia) resistive frictional forces ,dynamical resonant inductive forces - akin to the diffrence between a resistor and a inductor in electronics .The frictional forces braking have a simplistic and easily understandable properties, conversly the dynamic force vectors of a recipocal engine is dependant of frequency of reciprocation and latent forces such as internal friction and vacume A Gasoline Motor (engine) is spinning at a highter average R.P.M and as such cycles thru a reciprocation more often than a diesel. lower overall average compressionraitos of gasoline motors offers less overunning inirtia and flywheels designed to allow for rapid spool ups offer little in the way of freewhelling to the gasoline motor. in conclusion the motor cycles more often and loses more energy from not friction as much as inifecient energy conversion, it is convienient to qualigy this friction/reciprocal loss as internal friction. the dynamic differntial force of compressed intake charge with low total cumbustive force combined with a loss of overunning in the uptake cycle provides a deceleritive total force vector (you slow down). 1st wickipost, badboybilzer@yahoo.com —Preceding unsigned comment added by 98.148.157.227 (talk) 07:53, 30 January 2009 (UTC)

Engine brake
I think further clarification is needed as to the term "engine brake". Whereas "engine braking" is used to describe slowing down a fast moving vehicle by shifting in a lower gear, or using the lower gear to slow the vehicle down during a long downward slope, "engine brake" refers to something else.

In old diesel trucks the engine kept running even after the electrical power is turned off, because diesels are self-propelling. That's why there was a lever that had to be pulled, which activated a clamp on the flywheel in order to stop the engine. In some heavy trucks the engine brake is electrical and is operated by a foot-operated button located on the door side of the driver's seat (left for right-hand traffic). In modern trucks there's an electrical valve incorporated in the fuel pump, which cuts off the fuel when power is turned off.

'That's why there was a lever that had to be pulled, which activated a clamp on the flywheel in order to stop the engine' one has never come across a 'clamp on the flywheel' as an engine stop before, finding that older diesels are normally stopped by holding the fuel rack(s) shut manually or mechanically, the same may be done with the decompressor. Also engine brakes fitted older commercial vehicles are used as an engine stop as they both slow the engine and shut the fuel rack.

That electrical engine brake can be used for slowing the truck down when fully laden and descending on a long slope. If not fully laden, and not on a slope, it would completely stop the engine. However, this is completely different from a retarder. To my knowledge (and I've been a professional heavy truck driver for 9 years), retarders on manual transmission vehicles operate by cutting off the fuel and limiting the aperture of the exhaust valves, thus making the kinetic energy of the vehicle work for creating pressure in the cylinders. This results in a distinctive noise from the engine. Blocking the exhaust manifold has the same effect, and it's up to the manufacturer to decide which solution to use.

as a man who works on much agricultural and industrial machinery, i would like to note that some diesels have a 'throttle', which is a butterfly valve on the air intake that forms part of a vacuum operated governor where the vacuum created pulls the fuel rack shut against its spring. This is found on some older Perkins and Dorman engines, and from driving Hudswell Clarke D558, with its giant dorman 4lb engine with an enormous flywheel, that this 'throttle' produces a high amount of engine braking. — Preceding unsigned comment added by 90.209.35.122 (talk) 13:43, 25 May 2014 (UTC)

Cutting off the fuel is necessary, because otherwise exhaust gases would build up in the manifold and cylinders, and choke down the engine when you need it to work normally again. However, fuel supply is restored as soon as the retarder is disengaged, and the kinetic energy of the truck is enough to start it right away. It's similar to the concept of cutting off the fuel when suddenly lifting off the throttle, while (in gasoline engines) maintaining the spark to ensure quick start when the throttle is stepped on again. I don't know the technical term for this in English... in Bulgarian it's called forced-idle fuel cut-off.

On automatic gearboxes there's another implement, since the automatic gearbox disconnects when there's too much difference between the speed of the engine and that of the wheels. There's an electrically-driven hydraulic pump which works against the rotation of the driveshaft, thus slowing the vehicle down. I've been using this on a Mercedes Connecto, where it can be linked to the brake pedal for faster stopping, or to use the retarder before the friction brakes on icy roads. --Lasombra bg (talk) 18:15, 16 May 2008 (UTC)

RE: Engine braking in general Three words, compression and compression ratio! Engine braking is using the rotational energy from the driveline (turning wheels) to compress the air in the engine cylinders. When engine braking the engine is pretty much turned into an air compressor, instead of combusting air/fuel it is now compressing the air/fuel(which it was already doing, now whithout large combustion). Why do you think diesel engines engine brake so much more effectively, because they have huge compression ratios. A larger compression ratio means a larger volume of air is being compressed into a smaller space, requiring more energy from the driveline. This is also why a high performance naturally aspirated car engine brakes better than your nana mobiles and turbo cars as they have low compression ratios. Also how would this vacuum that brakes the car you talk about above magically appearing. When the engine is in its intake stroke the intake valves are wide open and the throttle is NOT closed. If the throttle was to close when your foot was off the gas pedal your car wouldn’t idle. The throttle is always open slightly, never closed shut! Therefore no truck stopping vacuum cleaner engine cycles. 203.97.48.81 (talk) 21:40, 2 July 2008 (UTC)CAM(Mech Eng) '