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So...lugging the engine obviously isn't something that is good. But why?

For example when you feather a clutch to get the car moving, it lugs for a quick moment when you hit the catch point (When giving it no gas). Or at least it does in my car (2014 VW GTI).

Is this a really bad habit, I only feather it when im moving slowly in say....traffic jams, or drive throughs and such.

Thanks!

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2 Answers 2

up vote 3 down vote accepted

Lugging an engine is like hammering the engine parts with every explosion in a cylinder. It racks on the rod bearing/journals, makes the pistons slap the side of the cylinders hard, and if is done enough, could probably break piston rings. It also creates a hammer effect all the way through the drivetrain. There are springs in the friction disk (of the clutch) which takes up some of this hammering, but they will only take so much before they wear out as well. This causes premature wear all the way around. It's not like it will wear out tomorrow, but it will cause wear over time. If you want your car to last a while, you don't want to be doing this.

I think the major problem with what you are doing is, you aren't doing it right. It sounds like you are just trying to take off from a stop (or slow roll) without revving the engine at all. You should bring the engine up in rpm to around 1500 to 2000 rpm when starting to slip the clutch. You should get used to slipping the clutch in either situation (stopped or slow roll) no matter where you are at. To get it down, you should press on the pedals like you would a bicycle. Basically, as you press on one, you should allow the other to come up, so: gas pedal down slightly while allowing the clutch pedal to come out; and visa-versa - clutch pedal down while allowing the gas pedal to come out. Over time and experience, this becomes second nature. You will figure out (mainly by feel) where the engine needs to be at in speed in order to get the car moving. This will keep you from lugging the engine at all.

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With experience you will learn how slow you need to release the clutch to prevent the engine from lugging even without stepping on the gas, but generally for starters, you do need to rev the engine up. The trick is to practice on neutral and try to keep the engine at around 1500 RPM. That's generally a good RPM to use from a complete stop to get a smooth ride and not accelerate too quickly. –  Nelson Feb 23 at 8:43

As gasoline burns, it expands. When an engine is operating smoothly and efficiently, the space above the piston will expand at roughly the same speed as the burning gasoline inside. If that happens, the force on the piston will be relatively constant throughout the stroke.

If an engine is rotating more slowly, the much of the fuel will burn before the piston has had a chance to move down very far. This will cause the force on the piston to be much greater near the top of the stroke than nearer the bottom; if the amount of fuel-air mixture in the cylinder is small, however, as when the engine is idling, that isn't a particular problem since the peak force would still be less than when the engine is operating wide-open throttle at speed.

Two related bad things can happen, though, if the engine is operated at low speed without being throttled back. First of all, fuel will burn more quickly at higher pressures; if the engine is running at speed, pressures will be limited because the piston will be moving down as the fuel is burning. If the piston doesn't move down fast enough, however, then not only will the pressure will increase beyond intended levels, but the increased pressure will cause the fuel to burn faster, thus hastening further pressure increase. This is a qualitative effect; either the expansion will be fast enough to limit the rate of combustion, or it won't.

The second thing that can happen is that the an excessive portion of the force from the piston can be transferred into pushing on the engine bearings rather than turning the car. If a crank is at 90 degrees, all of its force will get converted into torque; at 0 or 180 degrees, none of it will get get converted into torque. At intermediate angles, varying amounts will get converted into torque. Ideally, much of the combustion should take place while the crank is significantly beyond the zero-degree mark. If the fuel ignites too quickly, however, that may not be the case. Knocking thus causes a double-whammy of excessive force from the piston, provided at a time when the crank can't use it very effectively. Indeed, because some engines ignite fuel just before the crank hits top-dead center, that peak force could in extreme cases apply torque in reverse. It's unlikely that it could actually make the engine rotate backwards, but applying torque in the wrong direction would stress many components in the engine while failing to do useful work.

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