Talk:Retarder (mechanical engineering)

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

message: 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)
replies: text: 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)
thread_title: Clarification

message: 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)
thread_title: Engine brake