Talk:Retarder (mechanical engineering)

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

This article appears to contradict another article. Please see discussion on the linked talk page.

[edit] 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)

[edit] 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 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.

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)