A reader recently sent me the following interesting question, which I will do my best to address:
“You often write about the fact that a great deal of harm may be done, especially to a diesel engine, by “lugging” (working the engine at revs that are too low for the load). Sometimes the result may be felt in what I have heard referred to as “torque agony” – a severe and noisy vibration from the drive train. My question is: at what level does one run the risk of harming the engine? I normally try to keep the engine running reasonably close to the revs that deliver the peak torque, but is there a good rule of thumb apart from this?”
I’ve come across one or two motoring journalists who seem to think that the dangers of lugging are just a myth. The only way to answer the question is to look at the way stresses occur in an engine.
The level of stress inside an engine arises from various sources. The combustion process creates pressures that varies from very little at small throttle openings to well over 10 000 kPa at full throttle. This is always accompanied by temperatures of well over 2200C. These two stresses are known as load stress and thermal stress.
In addition, piston movement creates an inertial stress because it has to come to a complete stop twice per revolution. At each TDC (top dead centre) and each BDC (bottom dead centre) it has to stop before turning around and speeding up again.
At 5000 r/min this implies that the piston has to go from zero to its maximum speed of about 70 km/h in 0,0008 seconds, giving an acceleration of well over a 23 750 m/s2 (2 375g). Since force is equal to mass times acceleration, the crankshaft will have to pull the 800g piston with a force of 0,8kg x 23 750 m/s2 = 19 000 Newtons just to keep the revs at a steady speed.
This inertial stress increases as the square of the engine speed. This means at 3000 r/min it is four times higher than at 1500 r/min (3000/1500)2 = 22 =4) and at 6 000 r/min it is 16 times higher than at 1 500 r/min (6 000/1 500)2 = 42 = 16).
So what does all this maths mean in practice?
The load and inertial stresses are opposed to each other in direction, and this has some very interesting consequences.
At low engine revs the load stress dominates, and its value depends on the throttle opening. A high load stress always implies a high thermal stress (high pressures are always accompanied by an increase in temperature). The harmful effect of both pressure and temperature on metal depends not only on the magnitude in kPa and degrees Celsius, but also on the duration of the stress. At 1500 r/min (25 revs/second = 1/25 = 0,04 second per rev) the power stroke lasts for 0,02 seconds or 20 milliseconds. At 6000 r/min the power stroke lasts for 0.005 or 5 milliseconds. This means that when applying full throttle at 1500 r/min the engine is exposed to high pressure four times longer per revolution than at 6000 r/min. This is one reason why lugging is harmful.
In addition, the variation in loads plus the movement of the pistons set up a torsional vibration in the crankshaft that gets transmitted throughout the engine and drivetrain. This vibration, which you correctly called torque agony, is at its worst at low engine speeds and large throttle openings, but it diminishes as the revs increase. It is also a lot worse in diesel engines because the pistons are heavier, and especially in turbodiesels, because the low-speed torque output is often more than twice as high as in a naturally aspirated petrol engine.
At high engine speeds the inertial stress dominates, and its value depends on engine speed. This explains why one should never rev an engine to near its maximum speed when it is not under load (when the car is stationary with the gearbox in neutral). On many modern engines the vehicle computer will not allow you to do this.
At cruising speeds, the two stresses cancel each other out to some extent. This means that the average engine can cruise at 120-150 km/h for a long time without experiencing any significant wear.
Each particular engine design will have its own lowest comfortable full-throttle speed. A mechanically sensitive driver will feel when an engine starts to work hard. A good rule is to change down whenever you know that any further throttle movement will not produce any acceleration (unless you’re flat out in top gear!)
This means that part-throttle operation in a lower gear is usually preferable to full throttle in a higher gear. There’s no need to be pedantic about it. Your choice of ratio will also depend on how long you intend staying in any particular combination of throttle opening and gear ratio.
Finally, an automatic gearbox is an engine-saver, since it will not allow lugging.