A Scientific Definition of Lugging Engine

[sudz]

Hi all,

A Precursor:

I am not a mechanic, an engineer, or anything of the sorts. I'm handy, and get a fairly decent grasp of the workings of items.
I know what Lugging the engine is, I know that it isn't optimal for your engine, and I know that bad things can happen if you repeatedly and consistantly lug your engine.

However I got into a discussion (read: Argument) with my brother about it.

Leading up to the "discussion"

He let me drive his 2002 BMW M3 to dinner. It is a really nice car, 6 spd Stick. Its a large engine compared to my Mazda3, and has much more HP and torque. (obviously)

However, when I am crusing down a road at 60kph, I have MY mazda3 in 4th(of5) gears at 2000 RPM. I did the same with his car (4th of 6 gears), and he mentioned that I was lugging the engine. As it was his car, I didn't argue and I downshifted to 3rd and cruised along at roughly 3000rpm.

It got me thinking about the definition of Lugging.

Lugging is bad:

Yes it is. Don't do it. Not going into those details, but there are a few resons.

What is lugging?

A lot of people say/Think "anything below XXXX RPM is Lugging" but I've always seen it a bit differently, but never really been able to understand why.
This post is me looking around ont he interwebs trying to apply logic to what my brain things "lugging" is.

My initial definition:

when you drive the car in a way where you asking the engine to accelerate or maintain a speed faster than it is able to.

It sounds wordy, But I'm not a mathematician or scientist - But I'll try to figure it out now anyway.

The Maths:

I know enough to know that Torque and Horsepower are actually fairly similar, and completely related.

Torque is just a method of applying POWER (horsepower) in a rotational fashion. So, When I do say "torque" I really am referring to the overall "power" that the engine is producing.

Horsepower has a mathematical relation to Torque and RPM's of the engine - Its also why a proper dyno ALWAYS has the HP and torque cross at around 5250RPM - every time. If a dyno doesn't have the two curves cross at that point, some wizardry (bullshit) is occurring.
There is no machine that measures horsepower - that was just an arbitrary unit of measurement created by someone to see how much "power" a horse had. Its actually calculated via Torque and RPM. Its pretty much right up there the imperial system in my books. (Go Metric system!)

The maths for Horsepower is simply this: Horsepower = (Torque X RPM) / 5250

If my car produces 135 Torque @ 4400 RPM, its producing ~113 hp.

Now, I started to think - well if its a simple mathematical equation, how come dynos are all jittery, and vary quite a bit from car to car? and how come HP and Torque fall off at different points?

There are a number of factors, and To be honest, I don't understand them all.

Other impacts I can think of while on the Dyno: Variable valve timing, Air Flow, Air Speed, Turbulence and eddies in the intake/valves, etc at various intake air speeds (basically fluid dynamics?), Environmental conditions, Tires, friction from all of the above, etc.

I believe the biggest effect is that dynos are all done in the same gear, but that gear ratio changes. The torque to the wheels is all over the place depending on what gear you are in. More on this soon.

So, Back to "lugging"

The transmissions job is to keep the engine in an optimal Power band. Mostly.
Another fact, is that transmissions AMPLIFY torque. Very much like a pulley system, an engine can produce torque at the crank, but we measure it at the wheels. The amount of torque at the wheels varies greatly per gear, its why we use a single gear (3rd? I think?) on a dyno - It gives us the best way of comparing vehicles to each other, and a more uniform way of measuring the output of the engine. Crate engines (ones you buy for engine swaps etc) are generally measured at the crank, avoiding all this - its just raw output of the engine - no gearing wizardy to get in the way. Throw that crate engine in a car, and the torque numbers vary greatly, and the Horsepower calculated will take a hit from drivetrain loses (friction, tires, weight, etc)

Back to Lugging - again.

So, If my car weighs 3,000 pounds, The calculation needed is beyond me, but basically, (not accounting for friction) you need Power (torque to the wheels) to accelerate this weight. (plus my fat ass)

The Current Output of the engine and the current gear ratio needs to be able to apply enough torque to accerate the vehicle at a rate that you have called for via the throttle. If your throttle input exceeds this amount in excess, You are lugging your engine.

So, to rephrase my initial "what is lugging" definition:

My understanding is that "lugging" the engine consists of is when you call for a greater rate of acceleration than the engine & current transmission ratio can apply efficiently, rather than a set RPM. (although the Set RPM rule is reasonably helpful in deterring lugging in those learning)

How I avoid Lugging:

It helps a lot (IMO) to have the "current fuel consumption" feature on your car.

Cruising/Maintaining speed:

Use the gear that gives you the best fuel econ for cruising. When I go up the mountain every day, If I'm at 100kph, I have to be in 4th gear, and my car consumes 9.4L/100km. It can maintain speed in 5th, (Barely) but the fuel consumption goes to ~14L/100km It struggles.

Acceleration:(For general driving (not spirited/track day))

use the acceleration pedal in a linear way - Don't just mash the pedal down - As the car is able to accelerate, apply as much throttle that makes a difference. If you are at 2000RPM and mash the pedal, you're going to accelerate slowly anyway. So instead of mashing the pedal, add throttle slowly, applying more as the car can "use" it. I understand these aren't technical terms, but its the best way I know how to describe it.

I make a conscious effort to avoid applying more throttle than my car can currently make use of.



Hoping you all find this useful. If you have anything to add, I'd be glad to add it to this point. I'm not even sure if this is correct - Its just my opinion, based on some things I've discovered..
Also, if anyone one here has a big brain and can do the actual mathematical calcuations for the relation of Weight, Torque, Gear Ratios and Acceleration, I feel it would add greatly to this thread. I may add it later as I gain greater understanding..

Some footnotes/Questions:

By "my" definition, Small engine/low power cars lug a lot. (oh, and every rental car ever rented :) I also have the impression that 4 bangers in big cars don't live as long. They work harder to move the bulk around. They work too hard. In a way, Lugging - Constantly trying to accelerate mass faster than they are able.

So, Let me know what you guys think!

[stock3]

Your brother is nuts for thinking that 2k RPM is lugging the engine.

Now, you accretion that if the engine cannot maintain speed at certain RPM or it cannot accelerate means that the engine is lugged is also wrong. It simply means that the engine doesn't have enough power/torque to maintain speed for that gear and RPM. Now, if in that situation you do nothing but keep the gas pedal floored and the RPM drop too low, the engine will be lugged.

What lugging simply means is too low engine RPM for a given load and gear ratio. The engine may be able to maintain speed or accelerate, but still be lugged. In the past excessive lugging caused lubrication problems, pre detonation and knock. These days with knock sensors and electronic injection systems pre detonation and knock are not a concern. The only concern today is sufficient bearing lubrication.
You see, engine oil pump is driven directly from the crankshaft, which means its output is dependant on RPM. Low RPM means low oil volume and high RPM means high oil volume. When the engine is at very low RPMs, like 800-1200 RPM (and that figure will still vary from vehicle type to vehicle type), the gear is high, like 3rd or 4th and heavy load is demanded from the engine, there won't be enough oil volume to provide adequate oil flow into the crankshaft bearings. This may cause the bearings to have metal to metal contact and you may spin a bearing just like that.

Essentially in today's, electronically controlled cars it is very hard to truly lug the engine to where damage will happen. You will also feel when the engine is lugged as it will sputter, shake and jolt trying to deliver what is asked of it. If the RPM rises smoothly without any hiccups, you're not lugging the engine.