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What Happens When You “Lug” Your Engine for Too long?

What Happens When You “Lug” Your Engine for Too long?

Let's start by thinking about what it means to lug the car. From the very word itself “Lugging”, the Oxford dictionary defines it as carrying or dragging (a heavy or bulky object) with great effort. And when it comes to engines it is the word used to describe when you putter in a higher gear than you ought to be in, and the engine is turning low RPMs (revolutions per minute). So you're straining the engine. Low RPMs are okay when you're either cruising or slowing down, but when you’re starting to speed up low RPMs put the engine at a gearing disadvantage.

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For an average gasoline engine anything that is below 1750rpm in 3rd or higher gears is lugging. With diesels the RPM range is from to 1250rpm or so. You’re lugging your engine if you hear grinding, gurgling or other unusual noises that reduce when the RPMS have increased. It can also be considered “lugging” when in gear and if you accelerate and the car is unable to accelerate. Or in basic terms, it means operating with a full-throttle engine at a very low speed. Most drivers force down the throttle when going down the road in a higher gear than the vehicle needs, which hurts any engine


Most people don't even know that they're lugging their engine when on the road. Most beginner drivers have the tendency of doing this because more often than not they still don't understand what the engine is doing and what speed they should be in a particular gear. But even seasoned drivers still unknowingly lugs their car. 


So what would you do if you’re cruising along the road and the car in front of you slows down and you decide to change lanes to pass? Will you decide to downshift or simply leave the car in gear and stomp the throttle? 

Picture a 10-speed bike going through a steep hill and trying to climb it in 10th gear. You're riding smoothly, having shifted up into the higher gears. Suddenly you accelerate and in trying to turn those pedals it follows that you find it takes a lot more effort than it would if you were in the lower gear. You will be putting more effort  than necessary just to be able to get that bike moving again. The bike would be straining, too, and might even “stall.” That's because, like a vehicle's engine, it’s not using the mechanical advantage that the lower gears are designed to do. The engine has to work a lot harder to get the vehicle up to speed.


In this instance, you're lugging your body because you're the engine of the bike, and you can perceive that it does not feel good to do that, so why do it to your vehicle? Why not be in the right gear to make things sweet and simple. 


One question is if it would damage the engine if you lug the engine once in a while for just a few seconds, shift to a lower gear and then end up correcting it. The answer is no. But what happens when you “Lug” your engine for too long?


Placing your car in a high gear and trying to accelerate is against any practical sense because it means you're also going to have less wheel torque as a result of the gear disadvantage. It would make more sense to downshift if you're trying to accelerate. Otherwise you're placing your engine at a disadvantage because you're telling it to do more work than it can and it will result in it being less efficient. And if it’s running in a less efficient range it follows that it’s going to heat up more because efficiency matches up with heat. 

You are going to have more heat going into the engine block itself rather than it going into useful work. As you push to overwork your engine temperatures elevate tremendously inside the cylinders and then once things start to heat up inside the cylinders you begin to get unpredictable timing. You may end up getting a ping or knock and if you have a hot spot somewhere in there it can mess up the timing.

You will have uneven combustion that's occurring in different locations or at bad times that will cause devastating damage to the piston. A piston slap might occur. If there’s pressure from where it's not supposed to come from it can force the Piston up against the cylinder wall.  That can start to scrape up the cylinder walls and over time wear out the seal. By then you'll end up burning all kinds of oil. In addition to that you wouldn’t be able to have as good pressure eventually causing damage on pretty much any engine. 

What’s taking place inside the engine is an ugly picture. So if you get in the habit of shifting too soon, and lugging the engine after each shift that’s when things will start to get screwy. It may not be the kind of catastrophic overheating you see when steam comes out from under the hood but when you lug the engine and your engine has to work harder it leads to nothing but sure engine malfunction.

While lugging is not good for normally aspirated engines, it's even worse for the turbocharged ones. The distinct feature of turbocharger is that it sends pressurized air inside the cylinder of the engine. So when the engine begins lugging, it also creates pressure. So in effect the pressure in the engine is doubled because of the turbocharger coupled with lugging. This damages the engine and the turbocharger which will leave you with nothing but a very expensive repair or replacement. Also with a turbocharged engine, you also have to  think about low-speed pre-ignition, a phenomenon that can cause damage to your spark plugs and even crack your pistons.

So what does low-speed pre-ignition mean? LSPI also known as stochastic pre-ignition (SPI) is a pre-ignition event that can happen in gasoline vehicle engines in the event of a premature ignition of the main fuel charge. As mentioned it is most common in turbocharged direct-injection vehicles operating in low-speed and in high-load driving conditions. 

So what are the possible sources for this low-speed pre-ignition? It comes from something combustible and something large enough to cause this from happening like oil droplets, which could have entered into your combustion chamber through the positive crankcase ventilation system. It could also come from deposit peelings from the cylinder walls and oil dilution that can happen when you get gasoline mixing in with the oil and then having that oil misted back up into the combustion chamber from the sides of the piston as they're coming up and getting some oil droplets in there. Those things can heat up and cause the pre-ignition when you’re lugging your engine.  

When the engine is not able to push down on the pistons because of the unrealistically high load placed incomplete combustion follows and so are increased emissions.  Drivers must be aware of the required engine speed to be used to prevent smoke emissions. 


Overloading a vehicle or running an engine ‘on the governor' (i.e. at full RPM) for long periods when the prevailing conditions demand less may generate smoke emissions. It can also cause detonation, worse mileage, carbon deposits in the engine, and issues with the engine's timing. So how do you stop this from happening? The easiest way is to just not floor it when you're at low engine RPMs. 


It's not that pre-ignition can't occur at higher engine RPMs but it's far less likely to happen. Even if there was presence of that deposit floating around in there at the right RPM you can get away with having those deposits and not have pre-ignition cause damage to your engine.


So is the same true for Modern Engines?


The justification for not wanting to place a lot of load on the engine at a low speed is because the oil pressure of the engine is lower at such speed. This is important because the main bearings on the crankshaft and the connecting rod bearings are wholly hydrodynamic bearings. 


The oil pump channels oil into the bearings to hold the oil coating between the surfaces. In the power stroke , the piston and therefore the connecting rod exert a very strong stress on the bearing due to the weight of the gasses burning in the cylinder. If the oil pressure happens to be too low, this oil film may be squeezed out and the metal surfaces may be in contact with each other. A small amount of oil on the surface has anti-wear additives to secure the bearings, but more damage can occur


Engines have been made to produce slightly more elevated oil pressure at idle since around the 2000s to be able to operate different valve timing mechanisms as well as piston cooling oil jets to be able to tolerate a bit higher load. However, the significance of this remains debatable. Older cars are still being used at very low engine speeds particularly in parts of the world where fuel is more costly and there’s no evidence that engines being driven in that manner are failing any sooner, otherwise it would have been evident already.

Modern engines are becoming more and more low-rpm optimized as car manufacturers chase higher fuel efficiency, and we are now able to see automatic transmissions allowing heavy acceleration barely above 1000rpm, and shifting below 1000rpm in some instances. However, it goes without saying that the car manufacturer would not put a warranty on any car if this were not okay for the engine.

In addition if this in fact was a real problem there are numerous methods to limit the torque and bearing load at such low rpms, namely delayed ignition timing, even with older engines whose ECUs cannot manage the throttle itself.

In summary, it's difficult to conclude that the manufacturer would let the engine produce more power at low rpm than the bearings could handle, unless it's a really old vehicle with the absence of an electronic ignition timing. Truth be told, most people who drive manual transmissions do not even really know how to downshift correctly anyways and will lug their engines every now and then, and the manufacturer undoubtedly has taken this into account when forecasting the acceptable amount of wear when “lugging.” With that, if you wish to save fuel, don't step on it at 1000rpm, even if the engine can handle it because the engine’s efficiency can drop like a rock.

So how do you identify that you’re already lugging your engine.

You ‘d be able to spot the engine lugging, as the vehicle would start vibrating and trembling. In turn, the engine will start feeling weak and there will be no pick-up and engine response. There will also be no response from your accelerator pedal. Occasionally, you ‘d even be able to hear the sound of the motor shaking, which is not a positive indication at all, because that means you ‘re seriously damaging the motor.


How to avoid lugging your engine.


Avoiding engine lugging is very simple. After you've found that your engine has started lugging, you just need to push your gear down before your revs rise. It should ensure that the optimal quantity of fuel and air mixture is met in the tank, and so that there are further revs to guarantee that the combustion is carried out at the appropriate rate.


There are many ways you can take care of your engine. Ensuring regular and timely maintenance are key but there are also driving habits that are very easily avoidable  that can cause premature wear and tear to your car, specifically your engine. With all things said, one significant bad habit is that of lugging your car for too long. Being aware of every driving decision you make is key to making the most out of your engine life.