Lugging


#1

I have a son who has “gone green”. He is passionate about the environment, recycles religiously, and is very focused on GAS MILEAGE. He owns a '98 Honda Civic. He shifts to the highest gear possible, as soon a possible. (Technical note: On a flat stretch he may shift to fifth gear at 25 mph at 1,000 rpm, causing the engine to shudder).

Consequently when he’s driving me somewhere in his car, our conversation goes something like this:

Me- “Why are you lugging the engine?”

My son- “Because it saves gas”.

Me- “What’s the point of saving gas if it destroys the engine?”

Which one of us has the right idea?


#2

You are correct, but I can guarantee you that your son will not be convinced until he has to pay for a new engine or a new car as a result of his bizarre shifting habits. He personifies the expression, “Penny wise, dollar foolish”.


#3

Your son’s face is going to turn green when he gets the bill for replacing the engine.

While it’s good to get up into higher gears as soon as possible (so the engine isn’t revving excessively), he’s carried it too far. If the engine is lugging, it’s being damaged. Tell him one last time, and don’t offer to chip in for the replacement engine when he ruins this one.


#4

Having raised two kids of my own, and having been one myself, I can tell you that VDC answered perfectly. Your answer is right, but he’ll have to learn through experience. And this too shall pass.

Ah, sweet memories!


#5

It’s hard to tell if he’s lugging the engine or not because you don’t know the throttle opening. The less the throttle is open, the lower the rpm that lugging occurs at. At zero throttle, the engine is actually braking the car at that speed in fifth gear.
When I let my manual Yaris coast down in fifth gear, I can feel a mild bump, a sudden decrease in engine braking at 17 mph in fifth. I believe that is when fuel injection starts injecting fuel again. A 15 mph, I can clutchlessly shift into neutral because the road speed matches the engine’s idle speed.
Full throttle in fifth at 25 mph is definitely lugging the engine, but if he’s just barely opening the throttle, maybe not.

If short shifting is so hard on the engine, my old '91 Geo Metro certainly had a funny way of showing it, 280,000 miles.


#6

when he has to replace the CV shafts, the clutch and the bearing hubs, in all likely hood it will be because it was just a lemon.

OR NOT

but he will be shown a valuable lesson. but will he learn?


#7

Next Christmas or birthday or whatever, give him a Scan Gauge II, which gives real time readouts of mpg, gallons per hour, or whatever of many parameters you choose to have it display. It’s a hyper-miler’s best friend. When he notices that his fuel consumption actually goes up if he shifts too soon, he may start driving differently.

http://www.scangauge.com/


#8

do we all share the same thought on ‘lugging’?

it has always been by experience that when the engine ‘shudders’ or bogs down that is to the point of shaking the car that is too much abuse on the engine.

this doesn’t sound like short shifting or shifting without the clutch is going on here, but shifting WAY below the normal shifting rpms. i have always felt that a ‘smooth’ shift(er) is when you can’t even feel the change in gears while shifting.


#9

Little engines don’t like to be stressed like that. They were designed to spin much faster. You can’t save any money by stressing the engine in fifth gear at 25 MPH. Economic pressure turns otherwise sensible people into cashews. Pressure at work turns them into mixed nuts. Dating…honey roasted peanuts.


#10

For as long as I have been driving I have heard of dire warnings against lugging an engine but have never heard of one ruined prematurely because of lugging. Agreed, if your son is not pressing hard on the throttle and I don’t believe that he is if gas mileage is his goal, light lugging should not be a problem. If your son’s engine had a heavier flywheel or more cylinders, the perceived lugging speed would go down.

Another comment: I seriously doubt that your son is saving gas to any significant extent. Allowing the engine to run more freely at a light throttle opening in a lower gear will use a similar amount of gas as a heavier throttle opening in a higher gear.


#11

causing the engine to shudder

That tells us that the engine is lugging and the engine is subject to damage.

What your son is not considering is that while most people don’t shift soon enough for best mileage, as few people, like your son, shift too soon and are also loosing mileage.

For each car and situation there is an ideal shift point. If you are feeling lugging, you are on the wrong side of shifting too soon.

Top mileage is a dance of matching the car, road conditions, traffic conditions, traffic lights, stop signs, gears, etc. to come up with the best combination of safety, economy (greenness), convenience etc. You son is off balance at the opposite side than most people who are off balance on the other side.

There are no easy rules as it is an art form, not a science. But lugging the engine does not save gas, it uses more. Then again most of us are use to being on the other side so what we might perceive as lugging the engine may not be hurting the engine at all and may be at that engines sweet spot. So go easy, he may be closer to that sweet spot than you or I think.


#12

Lugging an engine is simply operating at a high load point that is near the maximum torque the engine can produce at the given RPM.

As long as you don?t overheat the engine, cause it to misfire, etc, it should be ok.

The fueling level for an engine increases with both RPM and load, so by lugging the engine you are not saving any fuel; The engine is operating at a high load condition and therefore must run rich, similar to WOT. So not only is he burning more fuel, he is increasing the tailpipe emissions.

This is yet another example of why modern automatics offer better real work performance; it?s very difficult to lug an engine in and automatic, and it always optimizes shift points which results in better emissions and better fuel economy.


#13

I would suggest this as a baseline for a 4 cylinder:

On level ground, accelerating gently, shift at 2000 rpm reaching 5th gear at about 40 mph. For a slight incline shift at 2500 rpm and higher rpm’s when more power is needed.

This pattern has worked for 217k miles on my '88 Accord.


#14

I gotta chime in on this one . . . a few weeks ago I had my Mazda RX-8 in to the dealer for service. The “old guy” Mazda mechanic, who has worked on rotary engines for years . . . told me that I was doing HARM to the engine by shifting at low rpm. I usually shift at about 2000 . . . and have done so “by ear” since I started driving 34 years ago. In fact, much like circuitsmith above I have an old Honda Accord . . an '89 . . . with over 460,000 miles on it. And I shift just as he described . . . at 2000 on level or 2500-3500 on a grade. Seems to work. So I tried a test with my Rotary . . . and ran 8 tankfuls of gas through it, driving it like I stole it . . . shifting at 4000-6000 almost every time. Guess what? Same mpg. No oil loss (the Rotary is expected to “use” oil every oil change, and mine is almost nothing). I have looked for a “sweet spot” for both motors, and the Honda seems to get the best mpg at 55 mph, turning 2000 rpm. The RX seems to get the best mpg at 62 mph, turning 2100 rpm. So I feel that a bit of experimenting is in order here for the Son . . . all cars are not created equal . . . all can achieve better than EPA estimated mpg (both the Honda and the Mazda get at least 15% better than the estimates) if you follow a few logical rules . . . discussed here many times. Keep your car in tune . . . plugs/wires/cap/oil changes/air filter . . . watch your tire pressure, keep it a pound or two over the psi suggested by the CAR MANUFACTURER, not the tire manufacturer . . . drive sensibly, and experiment a bit. My RX likes to ZOOM-ZOOM . . . my Honda likes to take it easy. But experiment . . . lugging isn’t the answer, nor is over-revving (for piston engines anyway) . . . but experiment and find the “sweet spot” . . . and enjoy your driving experience. Rocketman


#15

Here is my take on the engine’s sweet spot.

Your engine is a machine that converts heat into mechanical energy by allowing hot air to expand which turns some of that heat energy into kinetic energy. Every BTU of heat that escapes by transferring from the hot air to the cylinder head, piston crowns, and cylinder walls, ultimately to be disposed of by the cooling system, is a BTU of heat that the engine will not be able to convert into 778 ft-lb of energy. One way to minimize that thermal leakage is to expand that hot air before it has time to cool off, thus high rpm is good for efficiency. But, at high rpm, you have high intake vacuum which means high pumping losses and low effective compression ratios which also lowers the engine’s efficiency. The sweet spot is a trade-off between these two opposing factors.

Small engines have more combustion chamber surface area per cubic inch than big engines do so heat leaks out of their cylinders faster which favors higher rpms.
One way to minimize combustion chamber surface area is to use a long stroke design. Long stroke (undersquare) engine tend to have a lower rpm sweet spot than short stroke (oversquare) engine designs.

I can’t think of an engine design that maximizes the surface area presented to the expanding hot gasses more than a Wankel engine does. This more than anything else is why I believe this engine design is an evolutionary dead end and why these engines must be revved to be efficient, (or less inefficient).

If you look at current car engines, you may notice that the highly oversquare engine design that spelled “modern” in the '60 era V-8s is dead. Most of today’s engines are strokers, especially the ones in all of the econoboxes. I think there is a good reason for this.


#16

Interesting points B.L.E. . . . I never thought of it that way before. Im wating (daily by the mailbox) for a “working” model of a Wankel engine I ordered in April to help me to understand the workings of my RX-8’s “Renesis” rotary . . . mostly 'cause I have a hard time understanding the eccentric aspect of the combustion process. But your points about energy loss and the surface area of the Wankel are interesting. Thanks! Rocketman


#17

Been a long time, but…

The oversquare design has a more compact chamber shape at and near TDC, closer to spherical. I imagine chamber shape is most critical early in the expansion stroke, when temperatures are highest.

Imagine a very undersquare case. At TDC the chamber would be shaped more like a fat coin.

I figure variable valve timing totally changes the game regarding high RPM breathing of oversquare…