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 Post subject: Duckster's take on Engine Lugging
PostPosted: Sun Feb 16, 2014 10:56 am 
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Pressure drop in a CV carb is primarily split across 2 component devices:

1. the butterfly (disk on a shaft) throttle valve controlled directly by the handlebar throttle. this is on the outlet of the carb,
2. the vacuum operated slide valve controlled by venturi vacuum (proportional to air flow through the carb). This slide is near the inlet of the carb above the main jet.

Older motorcycle carbs often had only the slide valve controlled directly by the handlebar throttle, but on a CV carb it is controlled indirectly by the airflow venturi vacuum keeping the air velocity through the venturi nearly constant by changing the cross sectional area as the slide lifts.


Normal WOT (wide open throttle) operation results in minimal differential pressure across the carburetor and maximizes manifold pressure (MP).
Manifold Pressure (in inches of mercury absolute pressure) is commonly used on aircraft engines to define the degree to which the engine inlet is pressurized by incoming air/fuel mixture. The higher the MP the more effort you are demanding of the engine.
Normally the carburetor drops MP at part throttle operation by taking a pressure drop across the butterfly throttle and the CV vacuum slide allowing the operator to reduce engine effort. The pressure drop across the carburetor subtracts from atmospheric pressure to give the effective MP applied to the engine inlet.

Obviously at full MP (approximately 29 inches Hg at sea level) the maximum fuel/air charge will be stuffed into the cylinders of the engine, and when ignited. the maximum explosive pressure will result. Of course the explosive pressure takes a finite time to develop due to the time needed to fully ignite the charge.

At high RPMS, the maximum explosive pressure typically occurs once the piston is well into the power stroke and the con rod is able to develop a high torque on the crankshaft. The peak cylinder pressure is less than the theoretical maximum for the charge (at top dead center) because the piston has moved down to increase the combustion chamber volume by the time combustion is complete enough to produce the maximum standard volume of hot gases.

In the case of a lugged engine, the maximum cylinder pressure is occurring very early in the power stroke before the con rod is in a position to maximize torque on the crankshaft. Cylinder pressure becomes extreme and stays high for a longer time because the engine is turning too slowly to effectively convert the extreme pressure to shaft power. This excessive cylinder pressure is the definition of engine lugging, and it creates very high temperatures and extreme con rod loads.

Lugging can happen at various throttle settings and RPM combinations of course. All that's necessary is a very high cylinder pressure in the early part of the power stroke of the engine. The engine cannot effectively convert the pressure to shaft power because it is turning too slowly. High temperature and high bearing pressures result, which if allowed to continue will result in rapid engine wear and early failure. The problem is made worse by the low oil pressures and flows that are a result of low engine speed. Bearing loads are higher, but oil temperature,pressure and flow are lower so oil provides less engine protection.

Notice that wide open throttle and maximum combustion pressure is fine as long as RPM is high enough that excessive cylinder pressures don't result. Also, low RPM operation is also fine as long as cylinder pressures are kept low by reducing throttle to reduce the quantity of fuel and air mixture that explodes in the combustion chamber.

to summarize. Engine lugging is extreme cylinder pressure and temperature resulting from too high a manifold pressure for the RPM of the engine.

Any questions or criticism?

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 Post subject: Re: Duckster's take on Engine Lugging
PostPosted: Sun Feb 16, 2014 5:27 pm 
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Joined: Jun 18, 2009
Motorcycle: 1999 Rebel CMX
Rebel: 250
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Greetings Duckster,

That is a Spot On Technical Description of Lugging the Engine. To me, it covers the Topic very in depth. But I have a small Advantage, over 80% of the Members and Visiting Guest, that they don't have. 50 years of Wrenching.

I'm thinking of the School of Hard Knocks, that I went through. When asking a question, on a problem that I had with a motor vehicle. During my learning years, not having the experience of the other wrench. The answer that I was given, was over my knowledge level at the Time. Over the years, as my knowledge and experience grew. I was the one being asked the questions.

Early on, I was reminded of my learning years. I didn't want the young and inexperience wrench, to go through the same learning curve as I did.

I don't consider myself as a Motor Vehicle Instructor. I consider myself as a Transfer of Information person. I try and answer a question, on the Skill and Knowledge of the Person that is asking the Question.

I Respect and Admire the Individuals of High Learning. That is one of the few Regrets that I have.

Again Duckster. A Very Great Description of Lugging the Engine.
.

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 Post subject: Re: Duckster's take on Engine Lugging
PostPosted: Sun Feb 16, 2014 5:48 pm 
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Don't sell yourself short Soul Searcher. Practical knowledge gained from self-guided study and experience is the best kind in my opinion. Many are educated in the theory of things but Those who are not afraid to get their hands dirty doing maintenance and repair can be more effective in achieving practical results,and becoming a source of practical knowledge for others.

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 Post subject: Re: Duckster's take on Engine Lugging
PostPosted: Sun Feb 16, 2014 8:40 pm 
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To the explanation you gave of engine lugging, which often occurs when the load on an engine is too high for the engine to pull, there are 2 other topics very closely connected to.
Pinging, and pre-detonation (pre-ignition).
Both can be just as catastrophical.

Generally speaking though, an engine will not lug, when you don't step on the pedal (or rev that throttle) at extreme low RPMs.

Another issue that's connected to it:
When the engine has very low RPMs, the oil on the cylinder walls sometimes does not allow the piston to glide over it due to too slow speed, and a too high lateral forces (usually when acelerating very fast at extreme low RPMs, like 1000-2000RPM region.

All of these cause engine wear.
Perhaps it would be interesting if you further elaborate to the signs of lugging an engine.
As for instance, not having a constant acceleration power could be very similar (and often mistaken) with a cold start of an engine, and rolling off the choke, while trying to accelerate while the engine is still very cold.
In these conditions, the engine also sometimes does not accelerate linearly.


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 Post subject: Re: Duckster's take on Engine Lugging
PostPosted: Sun Feb 16, 2014 9:55 pm 
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Joined: Feb 23, 2009
Motorcycle: Rebel 250 plus a few others
Rebel: 250
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City: Fredericton
Pinging is similar to lugging in that there is an extreme cylinder pressure due to the rapid burning of low octane fuel in a high compression engine. Low rpms makes it more obvious and serious.
Detonation is just pre-ignition of the fuel before the spark occurs. It's sometimes caused by carbon deposits in the combustion chamber. Again, excessive cylinder pressure happens. Low RPM is not necessary .

Lugging will be obvious to most riders because the engine will not accelerate in the gear selected. It vibrates and refuses to go faster at the selected throttle setting. Experienced riders would not mistake this for the misfiring of a cold engine because the engine does not stall or misfire when lugged. It just vibrates and makes noises most of us don't want to hear.

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2004 BMW R1200GS
1996 Ducati 900SS
1973 Norton 850 Interstate


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