Why do cars shudder at low revs?

Something that’s even more noticeable in my Dad’s diesel SAAB, when driving around at low revs in a high gear, there’s a shuddering throughout the car. What causes this?

And does it damage the car in any way? I sometimes leave the car in a higher gear so that the revs drop to almost stalling for a few seconds. Dad says this is a bad idea and strains the car unnecessarily, does it?

Short answer - yes, it is bad for the car. In a high gear at low revs, when there is a load on the transmission (i.e. you are driving along a flat or uphill road), the engine has to struggle because it is not operating efficiently. This puts unnecessary stress on the engine and transmission, potentially shortening its life. The judder is when the load on the engine becomes almost too great for the power it is producing to handle, similar to when you are cutting long grass with an electic mower and the resistance of the thick grass slows the motor down. Shifting down increases the revs in the engine so it can produce sufficient power.

Long answer - wait for Rick or GaryT to show up and explain it properly.

Hmm - my short answer is a lot longer than my long answer!

My first guess would be too low of RPM for the gear. I know you stated it in the op but thats what happens in all vehicles that I’ve driven that have a manual trans gas or diesel. Other than that maybe age of the vehicle, engine mounts worn, something along those lines.

I am teaching my daughter to drive in a diesel, so I know the shudder well :smack:

As I understand it, the resistance of the drive train slows the pistons down, and at some point, the compression in the cylinder is not sufficient to trigger ignition. However, due to mechanical wear not all the cylinders reach this point at the same time, so you get uncoordinated firing, short chains of ignition as the vehicle lurches and eventually a stall.

It is bad for the engine - get the clutch in and change down (or, get the revs up cause you are in first/reverse).

Si

I can’t explain what’s going on inside the engine but this is called “lugging the engine” and is a bad thing.

Yes, I don’t know who taught you how to drive a stick shift, but my brother and dad would holler at me “Don’t lug the engine !!!”. This was considered not only a faux pas in our family, but a sign of inferior driving skills. Since we siblings all wanted to look like Stirling Moss in the driver’s seat, we avoided lugging the engine (among other things).

Interesting, thanks.

I’ll not tell Dad that he was right or that the Dope told me, I’ll just drop a gear and pile on the revs :smiley:

From a mechanical standpoint you’re trying to apply more hp against the resistance of the higher gear. think in terms of the mechanical advantage of the gears. Picture a very large gear connected to the engine trying to drive a very small gear.

Each cylinder of an engine operates in a 4 stroke cycle. 3 of theses strokes use up power, slowing the engine and one produces power, speeding it up. To smooth this out, the engine has a flywheel, and the torque converter or clutch add additional moment of inertia to this. In order to be effective at smoothing the engine’s power pulses, a certain minimum rpm is required. Go below this, and the engine nearly stops at the top of each compression stroke, and then speeds again.

The reasons such lugging is bad for an engine:

-The lurching itself creates peak loads that are much higher than the average load you’d have at higher engine rpm.

-For a given amount of power, you can have high torque, and low speed, or high speed and low torque. Most of the wear producing forces in the engine are related to torque, assuming you stay well below the rev limit of the engine (near the rev limit, speed related forces become a problem).

-Most engines use plain oil lubricated jurnel bearings. To a large extent, these depend on hydrodynamic forces to keep the metal parts from contacting each other. The moving parts “water ski” on a film of oil. Below some critical speed, this no longer works. This is exacerbated by the fact that oil pressure is lower at low rpm due to the greater effect of leakage when the oil pump is operating slowly.

I don’t think I have ever been asked to explain this before beyond the don’t lug the engine it’s bad for it stage, but I will give it a shot.
There are two parts to this explanation. First from the engine side, the second about the gears.
As Kevbo said each piston fires once every 4 strokes. Intake, compression, power, exhaust or if you prefer suck, squeeze, bang, whoosh. only the power/bang stroke speeds up the crank shaft. The rest slow it down to a greater or lesser extent.
So let’s assume a 4 cylinder engine. Each piston fires once every 2 revolutions, so all the pistons fire over the course of 720 degrees of crank rotation.
At the end of the crankshaft is a flywheel to keep the engine smooth. The heavier the flywheel the smoother the engine runs. There is a downside to having a heavy flywheel, for which you can blame Issac Newton. Heavy flywheels gain momentum slowly and when you shut the throttle, they tend to maintain the momentum for a long time. It’s that whole object in motion thing.
Anyway car designer tend to not put any heavier flywheel on the engine than is necessary.
The converse of the heavier the flywheel the smoother the engine is, the lighter the flywheel is the less smooth the engine is.
So your average car has a very light flywheel to make it preform better, but this makes it only run smooth at higher revs.
now for the gearbox side. Have you ever tried to pedal a bike up too steep a hill in too high a gear? Your pedal cadence slows down until the power comes in fits and starts. Same thing here. Your car does the same thing. The power is inadequate for the engine to run smoothly. Add that to the lighter than it could be flywheel, and you have an engine that lugs if the revs aren’t high enough.

If this explanation makes no sense, say so, I will come back tomorrow sober and give it a second shot.

No, that sounds quite comprehensive, thanks. I’ve never had to work on a car so much of the innards are a mystery to me.

I’m a suck, squeeze, bang, blow guy myself.

Edit: Or was I wooshed somehow? :smiley:

My problem with that is, a lugging car has a definite frequency, bump-bump-bump around two “lugs” per second, or even slower. Even if you’ve tortured the engine down to 300 revs (which you can’t, but it makes the math easy) thats still at least a couple of revolutions per lug, four power-strokes per lug in your 4 cylinder example. It seems to me that lugging isn’t as simple as slowing the engine down to the point where you can feel the bumps of the individual power strokes - you’d stall long before then.

The exact mechanism of “lugging” has always puzzled me, and it still does.

This is due to resonance. What is resonance? Resonance is the tendancy of a system to vibrate maximally at certain frequencies called “resonance modes” or “natural frequencies”, which are all whole fractions of the first mode or prime natural frequency. You experience resonance any time you pluck a stringed instrument or ring a bell; the system (the taut string or the rigid metal of the bell) will respond by making its characteristic sound at that frequency. If you were to apply an external driver at the same frequency or whole fraction thereof the amplitude of the vibration would increase progressively until the object breaks or deforms, or it enters a regime where its frequency response is no longer linear; that is, that the natural frequency of the system will change with vibrational amplitude.

Building off of kevbo’s and Rick’s excellent explanation of what the engine is doing, we can see that lugging increases the vibrational amplitude of the motor. The motor is connected to the chassis via motor mounts, rubber (sometimes gel- or liquid-filled) blocks which exist to damp out the vibration of the motor, which both improves occupant comfort (as anyone who has had a motor mount “go out” can attest) and protects the chassis from excessive and damaging vibration. Like any other system, these mounts and the chassis they connect the engine to have a family of natural frequencies which result in excess oscillation. These are designed so that the frequency is much lower than what they would normally see from the engine; however when the engine is lugging down you have a high displacement (vibrational amplitude) of the motor plus approaching one or more low fractions of the natural frequencies of the system; in some cases, with damping, you can get force coupling so that the damping actually exacerbates the vibration.

Solution? Don’t lug down the engine; not only is it bad for NVH (noise, vibration, and handling) but it also gives poor efficiency and performance of the engine. Your car engine has an optimal range of engine speed under load, and you should aspire to keep it in that range.

Stranger

Lugging occurs when you attempt to add hp against a higher force. Scenario: you’re driving 20 mph and it takes 4 hp to maintain that at 600 rpm. The engine is making enough hp to coast at this speed but if you apply more fuel the engine cannot rev up quickly because of the rolling mass. You have effectively attempted to add more than the 4 hp but because of the rolling mass there is no place for the energy to go because of the resistence of the piston. The rolling mass of the car acts like a brake against the crank. The extra energy is transmitted to all the other parts in the form of heat and mechanical energy.

Heh. Reminds me of my old statics professor - “Every couple has its moment”.

Example of resonance helping to destroy a structure: Link

Related to this phenomenon (possibly)…

Why is it that when you’re on a bus or coach, the whole vehicle judders and vibrates when the engine is idling, but the vibration disappears as soon as the driver puts it in gear and starts moving?

That certainly is resonance.

I’m not sure I buy the phenomenon of lugging as resonance from a vibrating engine, though. I’ve always experienced lugging as a pretty strong forward-and-back pulsation of the vehicle, almost whiplash-inducing. Any vehicle; straight four, straight six, flat four… forward-and-backward every time. (And in the flat-four case, the engine didn’t have resilent mounts - it bolted straight onto the transmission. A VW bus, same arrangement as the Beetle. Pistons moving laterally, crankshaft parallel to vehicle axis. How does that resonate the bus forwards and backwards?) Power pulsations through the drive train fits better, to my mind. It’s almost as if the engine repeatedly stalls and restarts.

I’ve been Googling the subject, and there’s a lot of anecdotal reports of a lugging engine knocking; audibly pinging, although I’ve never observed that personally. If it’s true, it suggests the ignition is too far advanced during lugging. That might be so even at the minimum spark advance, since you have a mostly open throttle (dense charge, giving fast flame propagation) combined with very low revs. Don’t know how that relates to pulsating power though, or even if it does at all.

I don’t think that’s just lugging you’re experiencing. It sounds more like the violent jerking you get from a newbie’s poor clutch control, particularly when getting under way in first gear. In my experience it may start as lugging or poor clutch control that induces an initial jerk in the car. It then gets reinforced by the driver leaving their foot on the accelerator. The uneven movement of the vehicle makes their foot move which makes the vehicle chug more. Lugging is a bit more subtle and is initially felt as a gentle shudder through the vehicle which may get worse if the revs continue to drop, it’s not necessarily whiplash inducing.

Ah, that takes me back to my driving lessons! But no, no newbie stick-shift driver here.

Truth is, getting a modern car to lug the way I described is pretty difficult. They either run, or they stall. But back before the days of engine management, egr valves and anti-knock sensors, let the revs get too low and they’d lug as I described. Clutch fully engaged. “Whiplash inducing” is an exaggeration, but it’s that sort of motion.

From another messageboard entirely:

“ask somebody to grab you around the neck and yank your head back and forth very fast for a few seconds. thats how your car will feel when your lugging. once you lug, you will know exactly what it is.” Page 2, 6 posts from the bottom. That’s what I’m talking about. More than just a labouring engine resonating at low revs.