Bogging/Lugging (How can you tell when you are doing the engine harm?)
 

How can you tell when you are doing the engine harm? I have a 2.5 L 170 hp Mazda3 and I usually shift at 2.5k at 3k then go straight to 6 and cruise on 6th gear in cities going 55. I don't accelerate then by stepping hard on the throttle but rather lightly if i ever needed to get to 65 which would take a pretty long time.

Anyways I just wanna know if 1200 rpm would be too low to cruise at? I do this too even if the engine is cold.

Thanks

I've heard, but can't verify the truthfulness of the claim, that modern engine control computers won't allow you to damage an engine. They supposedly shut the engine down, rather than allow it to run under too much load.

My SE-R's engine/transmission combo is similar to yours, a 2.5L, 170 hp, 6 speed. I use 6th gear from 40-45 mph upwards, on level ground. I have noticed my cruise control will kick me out when I'm going up steep hills at 50 or 55 mph. IIRC, the engine is still on, and I'm left in 6th gear under minimum load, with speed steadily dropping, forcing a downshift.

1200 rpm is probably too low an engine speed for cruising. Your engine's BSFC graph should point you to where you want to be. Usually, your best efficiency is between about 1500 and 2200 rpm for 2 liter class engines. Rather than cruise at 1200 rpm (30 mph in 6th?), I'd use a lower gear to accelerate past that speed, and coast in neutral or EOC through that 1200 rpm dead spot.

The way I'd handle a 30 mph cruise, I'd pulse from 25 mph to 35 mph in 4th gear (about 1500-2000 rpm), shift to neutral (either NICE or EOC), and glide back to 25. I wouldn't risk lugging the engine in 5th or 6th gear at 30 mph. 4th gear gives you more acceleration ability.

You might find this article interesting for ideas on how to wring best mileage out of your 3.

Oh Okay, thanks for the reply. I hope it`s true your car wont let you damage your engine cause I`ve been doing this for a while, but usually I`m around 1400 and the engine seems pretty comfortable at 1200 unless i try to accelerate which then I`d down shift. and damn what mods do you have on your sentra thats a high mpg

No mods. I drive slow - either cruise control at 55 mph, or pulse and glide between 52 and 65 mph on highways. In town I use the speed limit as the upper end of my pulse and glide range. I get my mpg gains by modifying my driving technique.

Some of the best advice you'll get here.

Yep, now I'm just more carefull about my low revs. I really hope i did no harm to my engine by cruising at 1.2k sometimes :( my car still only has 3.5k km so i don't wanna mess it up!

Regarding loading (given air/fuel ratios remain constant), as rpm's decrease turbulence within the combustion chambers decrease, flame front speed decreases, and required timing lead increases. The end result - more negative work applied to the reciprocating assembly, more wasted work than driving higher rpm. This is a general rule with 1000's of variables, but there is a window where your loading/rpm relationship yields the lowest bsfc. Finding that window is the hard part.

It's doubtful you've done any measurable damage to your engine unless you've been driving it like that when it's cold for a while.

I sometimes do, however I'm not always under 1.5km either. And if I am i don't really step on the gas pedals so I don't hear any vibration, im barely stepping on it, you think that should help?

I regularly pulse starting as low as 1200 rpm. My driving is all pulse & glide, as well, so that happens often. Cruising is much lighter load - I sometimes cruise down below 1000 rpm with light load. 175,000 miles and my engine is doing fine.

I like to look at the rpm and gear selection from a miles / engine revolution perspective. The higher the gear you can run, the fewer engine cycles per distance traveled.

If you are pushing in the gas and the engine is shuddering your RPM is too low. The oil film that hangs out in your bearings can get compressed out of the bearings and you will have increased wear. I lope along at 1000 rpm frequently l, 5th gear and mph, the engine is smooth so no worries. If I try to increase my speed and the engine vibrates it means I am applying too much pressure to the accelerator or I need to down shift.

An engine is a very robust thing, you really have to be abusive to do much damage. If you are being abusive to it, it will complain if you are listening for it.

True, but potentially misleading. (It needs qualification.)

Your best efficiency might be in that RPM range when accelerating/climbing and you want high load. But it's not correct to say you'll get better fuel economy cruising at that RPM when you could use a higher gear.

(I realize you then went on to describe pulse & glide, but I just wanted to bring this up in case someone doesn't make the connection.)

For someone who doesn't want to pulse and glide, driving with load (DWL) at low engine RPM is a good alternative.

I'd have no problem cruising at 1200 RPM provided the engine will do it without complaining (and by that I mean physically juddering). Bestclimb's post above describes lugging nicely.

My experience: I've yet to find a car where (in real world circumstances) the best cruising fuel economy isn't in the highest gear possible without lugging. I don't think the "best" engine RPM/load window is hard to find.

At cruise, the rule of thumb is: upshift, unless it lugs.

What about when it shifts into open loop or sub stoichiometeric operation? Seems like an economy loss would be eminent once that occurs.

Lugging an Automatic
 

So, I have modified the automatic to lockup the Torque Converter at the flick of a switch -- if started in 2nd, it has been possible to stall the engine and override the ECU/TCU.

With that said, I have wondered the same about lugging. Typically, I can get the revs down to 1100, which maxes engine load and has been noted to warm-up the vehicle quicker than slipping with the slush (or even lower with 2nd-gear start). The best feature is the throttle lift-shift that generally produces more of a vibration than "lug". To engage full lockup, it requires significant throttle input -- which is a give-and-take. Transmission warmup is faster as well.

FE has improved with the mod, but with Winter, the percentage is hard to estimate. The benefit of essentially having a "heavy manual" is nice.

RH77

Very good point - at low load, you can go all the way down to idle. When you open up the gas, though, it starts shuddering at that same rpm.

I concur.

Also a good point. I know, from the Scangauge, that my car goes into Open Loop at >95% LOD - pedal all the way to the floor. I monitor that gauge and keep it below that point.

Good points by Darin (metrompg) and Andrew (pale). I agree. Best cruise mpg is during lowest cruise speed in your highest gear.

I just had the "opportunity" to lug my engine due to a clutch problem, and retract my original impression. It is, indeed, possible to damage your engine by lugging it too low in rpm at too low a speed. I had mine shuddering due to starting without a clutch in 4th gear. It's possible, and you should avoid it.

So the concensus seems to be: it's OK to cruise at low RPM and low load.

If you need more power, you should downshift if:

1) the engine lugs (shudders) when you push the accelerator further, or

2) you cause the engine to go into open loop (typically at very high loads - ie. pedal near the floor), which can be monitored with a ScanGauge

Those vibrations can get really bad really quickly if you're loading up the engine like that, so I'd have to agree. Plus, when you're making the engine vibrate wildly, you're wasting fuel... that vibration energy has to come from somewhere, eh? ;)

I can put my truck into 5th gear at 20 MPH and still get up to speed, but it takes forever and a very light right foot. I rather do a 2/4/5, and I've only had it for a week, and driven it just over 100 miles, so beyond "first impressions", I'm not an authority on how to drive my truck, by any means. I'm still waiting to get it fixed up and put the MPGuino in, so I can have some real-time feedback and try a few different things.

I also think that the tenet we endorse which has people accelerating at high load/low RPM needs to be further explained because of this. It should rather state that accelerating at the highest load you can use without creating excess vibrations or shuddering/detonation at the lowest RPM for your speed (highest gear) may be the most efficient. This introduces a new idea, that maybe the absolute highest gear you can use isn't necessarily best for your situation, and you should adjust accordingly, because even though you're at a higher RPM/lower load in a lower range, you may be closer to "optimum" for the shorter time you're accelerating.

I know it seems like common sense, but that's not nearly as "common" as the name would imply, and people do follow suggestions to the letter, as though they were directives.

I think that for acceleration, you're still best going by the best BSFC window, not lowest RPM at which you can accel without bogging.

I guess I kind of labeled the thought incorrectly. I should have said - "That has been tossed around". After posting that, I was looking around for it, and could only find general mention of it, not necessarily anyone saying that it works/doesn't work.

I'm kinda working on trying both ways, just to see if there is an obvious difference. With 4.9L of displacement when you're used to 2.4 or less, it seems almost monumentally important to keep the RPMs as low as possible, and that may not necessarily be the best way to do it.

Just as has been said before if you're lugging the engine too much you'll know it by the way it responds to acceleration. Shuttering and vibration would be signs you're putting too much strain on the engine at low rpm's.

Try using royal purple oil, It has a higher film strength which is great for protecting your valvetrain and your lower end. I use it because I lug mine around at 1k rpms from time to time. http://www.youtube.com/watch?v=kqvhRi7-iMA

I've seen many premature problems with engines that have been badly lugged. Usually it's older people who don't seen to understand how to downshift when they want to speed up. I frequently see bad scouring on the piston skirts and resulting oil consumption.

Lugging is also not optimal from an efficiency/pollution point of view. High cylinder pressures and low RPMs mean a much higher chance of detonation. As a result carmakers tend to run a lot less timing in the 1500 heavy throttle range. A lot of them tend to richen up the mixture because that gives the engine a nice feel when you free rev it. This can be OK because it means if you stay in closed loop the car will learn a leaner fuel trim and run leaner than stoich for a few seconds after you return to normal driving - less so on newer cars.

If it sounds bad and feels bad it's usually bad. Lugging itself long-term will also tend to wear your rod bearings as it puts a lot of pressure on them for an extended period of time - and it's also the worst time to do it as at lower RPMs your engine has the lowest oil pressure. Some cars are much worse in this area than others.

Finally, fuel saving modes in your engine (such as EGR and lean running mode) won't come on at really low RPMs. For example a lot of the Mazdas and Mitsubishis the EGR won't activate until 1800-2000 RPM. (differs by engine/year/market)

-Michael