hey guys,

got in a heated discussion with my friend tonight. he just learnt stick on his vw tdi...argues that lugging the engine isnt bad. just says lugging makes the car accelerates slower.

i argued, well it's definitely not healthy and its makes the engine work harder than necessary.

any other valid points to debate back?

 

The Car Talk guys say it's bad too: Car Talk: The pings and perils of lugging a car's engine

"Tom: When you lug the engine -- when you try to accelerate in too high a gear -- you make the engine work harder than it's supposed to.

Ray: When the engine is overworked, it overheats. The way it overheats is that the temperature inside the cylinders gets too high. When that happens, the gasoline-and-air mixture -- which is supposed to detonate when the spark plug fires -- detonates unpredictably at other times, too. That's called pinging -- when those extra detonations make a sound like marbles rolling around in a coffee can.

Tom: And under the worst of circumstances, you can also get piston slap, which is a "clunky" noise. Because you are getting uneven combustion inside the cylinder (the detonation doesn't necessarily happen at the right time or in the right spot), the explosion can force the piston off to one side when it descends during the power stroke. When that happens repeatedly, the cylinder walls get all scraped up, and over time, your car turns into the proverbial oil-burning heap."

Maybe your friend is just spoiled by his TDI; that engine has a lot of torque at low rpms.

 

^ What he said.

Although, your friend's car is notable in that it's a turbodiesel. Modern turbodiesels are usually mountains of pure torque just off idle, even with just four cylinders and small displacement.

Do note that diesel engines ignite their air/fuel mix not via spark plugs (as gasoline engines do) but by pure compression - much higher than gasoline engines. Their engine blocks are thicker and heavier because they have to put up with at least 25:1 compression. By comparison gasoline engines usually run compression ratios of 9:1-10.5:1.

 

your friend should play with RC cars you build. then he will learn quickly about gear ratio's and how it works and why it's not a good idea to use a higher gear out of torque band and push it.

 

Diesel is different. High compression and a more complete burn keep lugging from doing much of anything.

On a gas engine... high throttle means having to run rich to keep temps down and pinging away. Rich means excess carbon, slow engine speed and high throttle mean high pressure and lots of ignition retarding, and the combination of the two can make lots and lots of carbon to make hotspots, potentially causing detonation and engine damage down the road.

Usually the first sign is going through spark plugs, but sometimes not even that is noticeable.

This is different from say, 70% loading at low rpm where the engine is still in closed loop and running stoich, which doesn't have this problem.

 

Maybe a different way to approach the question is this. Have you ever ridden a multiple speed bicycle (like 10, 12 or 15 speed)? Put it in high gear (large sprocket up front and small sprocket in back) and see how it feels to take off from a stop or go up a slight incline. Would you say it was difficult and hurting your legs? In a gas engine you're putting the same kind of hurt on your rods and bearings. That's lugging. Notice how much smoother and more effortless it is to ride the bicycle when you're in the proper gear. Your legs and your engine will last a lot longer if you don't lug.