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Why do it?


old man emu
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Why do people have to "blip" the accelerator of their engines when working on them?

 

You see and hear it all the time. Blokes leaning into an engine bay, frantically opening and closing the throttle of their engine in rapid succession so that the engine alternately revs high then drops back to idle, only to have the cycle repeated. What does that prove?

 

To my mind, if you rapidly open the throttle, there's going to be a tiny hesitation as the fuel/air mixture goes lean and then the engines begins to cope with the mixture it's getting. So, due to the hesitation, the "mechanic" goes on a wild goose chase to find the mechanical cause of the hesitation. Also, whenever does one suddenly increase engine revs from idle to near red line during normal driving? I bet even 1/4-miler drag racers don't even stamp on the accelerator when the lights go green.

 

I reckon that if you are trying to get an engine to run smoothly, you should open the throttle steadily until normal operating revs - about 2000 - 2500 are reached, then run the engine at that speed while diagnosing problems. 

 

What does madly revving an engine up and down prove? That the nut on the throttle linkage doesn't know much about how engines and carburetion work. 

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10 minutes ago, spacesailor said:

It,s

Spacey,

See that key to the immediate left of the ENTER key? The lower symbol is the apostrophe. It's used when we write contractions, like it's for it is. Over there to the immediate right of the key for the letter M, the lower symbol is the comma. It is used to make pauses, or to make a sidestep in a sentence. 

 

QWERTY - Wikipedia

 

15 minutes ago, spacesailor said:

to Hear the sound of ' Bigend bearing clatter ' ( knock  ).

Also the tappit noise.

Good enough, but why is it necessary to do it to an engine that is not suspected of having big end bearing or poor tappet adjustment?  Surely those noises would be apparent at any steady engine speed.

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People used to keep old motors running till they threw a part out the side. Aero engines never get to that stage unless they are powering swamp boats. Wear limits on an aero engine are not large as they are still meant to be able to make rated power the day before they are pulled out of service.

  Older Kero tractors had "Poured" babbit bearings  which could fail and make a loud noise without hurting the  crank unless you kept running it after the bearing had failed.. (usually caused by lack of oil.). 

     Ball bearing motors can have a cage fail and all the rollers fall out producing a large amount of play. 

  General rule.... Main bearings knock under load. Individual big ends can be detected by shorting each plug briefly and it will give a distinct knock when the plug fires again. You will also hear them after a filter change before the oil gets back in the galleries even at idle speed. 

  Loose pistons rattle when cold ,when you increase load slightly. Nev

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The ability to floor the accelerator is needed in some cases to avoid an accident. The petrol auto engine is usually fitted with a means to provide an instant enriching of the engine and also a retarding of the timing. No need for this in an aircraft engine although Lycoming do fit carbs with accelerator pumps to some engines. It does make for easier starting in that case.

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They rev the crap out of engines they're testing to ensure the accelerator pump response is working 100% and there's no flat spot when you try to gun it from a standing start at a T-junction, into speeding traffic.

 

They also rev the crap out of engines, so their garage, the neighbours garage, and the neighbours house, all fill up with stinking petrol fumes - which they alone, love the smell of. The rest of us want to kill the "revver-upperers".

 

Of course, accelerator pumps went the way of the Dodo about 1986 - but no-one's told a number of the "revver-upperers" yet. If you do, they'll say they're clearing out gummy injectors.

 

Edited by onetrack
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I get really annoyed at the number of people who idle engines unnecessarily. I constantly come across people in carparks and other congested areas, sitting there with their engine idling for half an hour or more, generally because they want the A/C running.

But they're filling the area with exhaust fumes while they do so. In many American cities, there's a ban on idling engines after about 3 mins. You can get fined a serious amount for leaving an engine idling for a longer period in these U.S. cities.

 

I think we need to bring this kind of regulation in here, the benefit in emissions alone would be substantial, the savings in fuel would be substantial, and everyone benefits from cleaner air to breathe.

 

In addition, idling engines for extended periods - particularly diesels - is highly undesirable for engine life and health - let alone the emissions problem of poorly combusted fuel. Cummins diesels in particular, with their PT (Pressure-Time) injection system, with injectors operated directly by the camshaft, suffer from poor atomisation of the diesel when they're idling, because the injector is being operated too slowly. Poor atomisation of fuel leads to unburnt fuel and crankcase oil dilution, as the unburnt fuel goes straight past the rings.

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If you are driving long distances and one of Her Majesty's constables stop to chat because you've been a bit naughty with regards to your choice of cruising speed, don't turn off your engine. If you do, the heat in the engine will boil the non-circulating water in the cooling system in about two minutes. 

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WATER

My cars are cooled by  'coolant ' boiling point higher than 100,d Cellcious.

Old man H Ford,

Had the problem with the V6 & V4 motors boiling their heads, whenever they were stopped after an arduous trip.

Turbocharged diesels have to have timers to stop their heads cooking.

spacesailor

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I thought it was already illegal to leave an engine in a car idling without the driver being in it.

I don't understand how atomization can be affected by the speed of the engine. I am not up with the very latest technology, but all the injectors I came across relied on pressure to start and stop the injection and atomization. The injector would not open until the pressure lifted the pintle.

As for accelerator pumps it seems that people nowadays have no idea of what they are.

I have seen people starting diesels with a couple of stabs at the throttle pedal, supposedly to prime the engine. My daughters partner had a problem with the choke on his Nissan diesel bus. I fixed it for him, by fitting a knob on it, but I don't think he believed me when I told him a diesel doesn't have a choke and what he had was a hand throttle. Yes I know a diesel doesn't really have a throttle, but that is what the go faster pedal is called.

 

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Spacey, your diesel engine fuel injection pump rack position is operated by an aneroid arrangement (a diaphragm, which alters position according to manifold vacuum or pressure) - typical of Japanese light diesels. 

 

Older, simpler diesels had a direct throttle connection from foot pedal to the rack. On the little Jap diesels, the rack position is operated indirectly by the diaphragm position.

 

So you plant your foot, the manifold vacuum decreases with the butterfly opening, and the diaphragm alters position to pull the rack open, and inject more fuel.

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Yenn, the people I'm complaining about as regards excessive idling are the ones sitting in their vehicle for 1/2 - 3/4 hr with the engine idling so they can have the A/C working.

 

The Cummins PT system injectors are operated by pushrods from the camshaft, so their speed of injection is related to camshaft and engine speed.

 

When a Cummins engine with PT injection is idling, the injector is only opening relatively slowly, so the injected fuel is not getting the rapid squirt needed, to ensure full and proper atomisation of the diesel fuel.

 

In addition, the slowness of combustion at idle, coupled with low combustion temperatures, means not only is the fuel not combusting fully and properly, it is also forming an increased amount of sulfuric acid, which is exceptionally bad news for engines.

 

Here's an official U.S. Govt Dept of Energy publication related specifically to excessive engine idling. It notes that all IC engine manufacturers recommend against extended idling periods.

 

https://info.ornl.gov/sites/publications/files/pub61263.pdf

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