Dangers Of Low RPM extreme ECO Driving

"Why's it bad to lug your engine, & what does it mean?"

Low RPM driving, often referred to as "lugging" the engine, can actually damage your car over time (wear and tear is gradually increased). People often do this to save fuel, assuming that a car will use less fuel at low rpms.

This means that they will often drive in too low a gear whilst accelerating or adding load where the fuel saving is doubtful and save up a host of other problems which I'll cover in this article.

Just to open the discussion it is worth noting that modern engines are pretty good at protecting themselves in this instance, but a little mechanical sympathy will go a long way to ensure reliability and maximise your fuel economy especially if you are driving an "older car".

Please Watch Dangers Of Lugging explanatory video guide on my channel for a good overview".

Low RPM with no load ie cruising along causes very little damage or wear and tear (with a few exceptions as I'll cover later), the problems happen when you start to add throttle load at low RPM.

What is a Low RPM?

It depends but you as a driver can determine this if you are accelerating and the engine is struggling. A low RPM figure will vary depending on the gear you are in, if you are going up hill or needing to accelerate and fuel type/air temps etc...

This will largely depend on your engine and how it was designed, especially if it has a mechanical crank driven water and oil pump, but it is always good to vary your driving to include high rpms and even a red line visit now and then to blow out the soot that builds up.

Let’s dive into why this happens and see why this myth of driving all the time at low RPM can actually run up some pretty scary bills further down the line.

Two areas of concern, full throttle at low rpms, and driving at low rpms to maximise fuel economy at light throttle, let's focus on full throttle at low rpms first. ECO drivers often do this as they need to pick up speed quickly but they are in too low a gear to do so.

Increased Stress on Engine Components

Driving at low RPMs means your engine isn't working in its sweet spot and you'll get more vibration. The region the manufacturer designed it to operate in for maximum reliability economy and efficiency is generally at the top of the bottom third of the RPM range.

Excessive driving at close to tick over speeds can lead to extra strain on parts like the crankshaft, bearings, and pistons not to mention the dual mass flywheel primarily due to vibration.

Imagine you are riding a bike uphill in a high gear – it gets difficult if not impossible and puts a lot of pressure on your legs. In the same way, low RPM driving forces the engine to work harder than it should, stressing out its mechanical parts especially when you start adding a load (like a hill or accelerating).

What happens when you are lugging the engine? The combustion event is fairly consistent, but at low rpms say 1000 rpm most of that energy happens in a small part of the stroke, often where the rod angle is not best suited for so much power effectively adding a pressure wave against the slower moving piston.

This can, especially when the engine is cold, result in piston slap where the engine rings scrape the cylinder wall and will wear the rings on one edge, it can also affect the cylinder walls and even the pistons resulting in poor compression, burning oil and bad running.

At 6000 rpm the energy is released through much more of the stroke so causes less stress and the shockwave or pressure wave is spread out through greater piston travel.

In some cases of low RPM load the piston moves so slowly that the combustion event will try to spill past the pistons into the crank case known as blow by and this messes up your oil quality, the engine seals and causes more oil vapour to flow through the PCV into the intake.

An engine will try to correct problems in the burn by trimming the fuel or adjusting the timing, at 1000 rpms you have less opportunity to do this than at say 5000rpm - effectively giving 5 times more opportunity to correct every 60 seconds. (You get greater resolution of data coming into the ECU and it can make more adjustments as there are more rotations).

Low speed pre ignition is also a potential problem here, where vapours present inside the combustion chamber ignite too early but I think this deserves it's own article and video as the causes are complex & heavily debated.

At low RPM the combustion event is generally triggered later making more risk for detonation or knock.

Basically then, flooring the throttle at low rpm means the engine is straining for long periods of time, effectively having to work against mechanical advantage and isn't revving high enough to make more power.

You won't accelerate and are just adding more effort and work to the engine components.

Poor Lubrication

Engines need proper lubrication to stay happy. In most engines the oil pump is connected to the crank via a belt, chain or gearing system, so the faster the engine revs the faster the oil pump works and the higher the oil pressure will be.

Hopefully the manufactures have set things up to cope well at constant low RPM but I have seen exceptions to this such as on the VW 3.0 V6 engine designs for a while.

At low RPM, the oil pump might not circulate engine oil effectively, which means parts don't get the lubrication they need. This causes friction and wear between moving parts and over time, this wear can lead to significant damage.

Generally speaking manufacturers build excess capacity into an oil pump and dump the excess pressure which helps maintain a constant pressure, but we do see variation in oil pressure at different RPM's on some engine designs.

At say 6000 rpms you might see the oil pressure rise to 75psi but in the same engine at idle speeds it could drop to 15-20psi, so making an engine work hard with poor lubrication is a bad idea.

Even some engines with variable flow oil pumps will struggle to properly lubricate an engine that is driven consistently at very low rpms causing accelerated engine wear and other problems.

Theoretically this shouldn't happen, but manufacturers are pushing fuel economy as far as possible and some have set the low rpm oil pump circulation to the bare minimum - the later VW 3.0 V6 TDI engines seem to suffer from this issue, although it can be mitigated with an ECU setting change to drive the oil pump harder.

Incomplete Combustion

Another issue with low RPM driving all the time (under load) is incomplete combustion. This leads to carbon buildup inside the engine, especially on the valves, piston rings, and spark plugs. We did a video recently on this problem suggesting ways to avoid it altogether, but let's look into the specifics of low RPM driving on this problem.

Some engine designs are more prone to this, but when you strip down an engine that has been grannied everywhere you will usually find much more soot, or carbon particles than in one driven hard regularly.

Too much carbon building up can cause misfires, lower performance, and eventually, engine damage. If the engine can’t "breathe" properly, it will lead to inefficiency and cause faster carbon build up.

Reduced Efficiency

Driving at low RPM all the time reduces fuel efficiency and increases emissions in most engines, again when under load. If there is no load it doesn't seem to matter where the RPM sits, little fuel is used to maintain the equilibrium.

Engine vibration is often higher at these very low rpm ranges and the flywheel is working harder to smooth this out. Note that the operating range is what the engine was designed to operate in, going outside of this optimum causes problems and sticking to the bottom or top end all the time will cause problems in most cases.

Over time, this inefficiency can hurt the engine's performance and lifespan and you'll usually find that the fuel economy also suffers compared to driving at a higher RPM figure.

Catalysts and particulate filters are also more likely to clog up thanks in part to the extra soot produced and partly because the flow of exhaust gases is too slow to push anything through these filters.

Diesel engines in particular need to be driven hard, and they only have a narrow RPM range so produce plenty of low end torque, which is probably why many drivers just sit at these low RPM figures all the time instead of giving it a decent run.

Potential Overheating

Low RPMs can also mean your engine doesn't cool properly. Many water pumps depend on engine speed to circulate coolant effectively much like the oil pump. So if the coolant isn’t moving around as it should, the engine can overheat.

Also lower rpm will often mean lower speed so there is less air flowing through the engine bay to cool the engine but you could argue less heat is being produced by the engine.

Interestingly in some cases the engine temps can drop to below the ideal during winter months when sticking to low RPM with no to little load.

In reality the engine is often running hotter, especially if you are using more throttle at these low rpms, because it is working inefficiently and this harder work leads to higher cylinder temps that can cause uneven combustion.

This prolonged overheating can warp engine components and cause severe damage, similar to a computer that overheats without proper ventilation and starts to malfunction.

Wrapping Up

Occasionally driving at low RPMs won’t wreck your engine overnight, but making a habit of it can cause a bunch of problems. To keep your engine running smoothly, try to stay within the manufacturer’s recommended RPM range.

Even lugging a modern engine will usually just add a little engine wear, perhaps reducing engine life a little, but much can be mitigated with proper servicing and observing a proper warm up and cool down cycle on your engine.

This helps your engine stay well-lubricated and avoid unnecessary stress. Just like any machine, your car performs best when used as intended and it is folly to ignore the whole rpm range.

So, next time you're behind the wheel, keep an eye on that tachometer and give your engine the care it deserves. It’ll thank you with years of reliable service!

This article is flagged for an update, so please pass on specific tips below that I can include. All articles, and content was written by me, Waynne Smith TorqueCars founder. Some outlines are just too general so I appreciate your feedback and suggestions. This entry was filed under Articles, Car care, Driving. You can leave a response below or join our forum to discuss this article and car modification in detail with our members.

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