Highway/city engine life

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May 12, 2003
OK, I've heard this for years and now I'm want to get schooled by the best of them, you guys. "Highway miles are easier on an engine and it will last longer than city miles."
1. Doesn't the engine run higher rpm's on the highway as opposed to city traffic?
2. Oil is still being pumped throughout the engine, i.e. pumped through the rods, crankshaft, bearings, etc. etc.
3. Oil is still being sheared as the engine is churning out rpm's.
Like I said, something I've always heard and now, I want the facts from the pros.
It's because on the highway your rpm's is more stable where as in the city your going and stopping more work for the engine means more wear on the engine.
E.g If you were running down the street would it be harder on you if you ran realy hard for 10 sec then jogged for 10 sec. Then you repeated this all over.
Or on the highway you just ran gracefully for the time you were on it.
As Bob has pointed out before, he said something about when you are at a light idling, your engine is only pumping oil at idle... then you put a load on your engine and then the oil pressure has to catch up to that... I would like to add the fact that when you are on the highway your engine is not really under a load in the way that the car has already built up its momentum and the engine is only working enough to maintain the speed. When you are at a light and you start moving, the engine is having to work to build up the momentum as well as having to maintain it.
Another angle: At constant highway speeds the engine is drawing as close to the theoretical ideal "stoichometric" air/fuel ratio of ~15:1 as it gets. Conversely, stop-and-go city driving is constantly feeding a fuel rich mixture charge everytime you accelerate from slow speed or a dead stop. That over-rich condition results in increased fuel dilution of the engine oil and its resulting undesirable oxidative reactions of the oil from the inevitable blow-by past the piston rings.
In response to question #1, remember that at highway speeds the engine is coupled to the pavement at the direct or overdrive ratio, so while the rpm's at a give instant are higher, the total revolutions required for a given distance are at the minimum.
This all makes perfect sense. Can relate.
Another way to look at it- inertia. A body in motion tends to remain in motion, a body at rest wants to remain at rest. So, your engine has to work a *lot* harder in stop & go driving than it does at a steady highway cruise. Probably one reason my car's doing so well at 140,000 miles.
Hours and cycles are how we measure aircraft life, not distance traveled. Maybe an engine hour meter and something that counts cold starts would be a better measure than miles in a car or truck. Newer GM trucks have an engine hour meter built in.

Originally posted by Jimbo:
Hours and cycles are how we measure aircraft life, not distance traveled. Maybe an engine hour meter and something that counts cold starts would be a better measure than miles in a car or truck. Newer GM trucks have an engine hour meter built in.

yeah, this is an important fact.
consider this: you drive for 5 miles at 60MPH, so the engine gets 5 minutes of use.
You are in the city, and you drive a mile at 30MPH, and then stop at a red light for a minute, and then you drive another mile at 30MPH, and then stop at a red light. You've gone 2 miles, but the engine has again seen 5 minutes of use.
So when you look at the odometer of a used car you may plan to buy, if the owner tells you it's mostly highway miles, it's probably seen less runtime than if it was largely city miles.
If I was looking to buy a used car and I had two to choose from, both identical except one had 30k on it and was all city driving, and one had 100k on it and was all highway, I'd choose the 100k car, it probably has more life left in it.
I think there are too many variables to make a blanket statement like that, Patman. I would ask to see the 30,000 mile city car's service history. If it was documented through invoices that the car received routine oil/filter maintenance every 3,000 miles or 3 months (whichever's less), and was operated in a mild climate like southern California's, it might just be the equal or even the better of the two.
OK, maybe Patman is hyperbolizing a bit, I'm sure he knows as well as anyone that *somewhere* out there is someone that has done 100,000 miles and adding gas is 95% of the maintainance done....
BUT I am rather surprised that in this discussion, no one has pointed out the variance in gas mileage between city and hiway. City is always poorer. Same set of reasons. To me they are one of tyhose things that is very 'obvious' yet really hard to put into words. All the more reason to post the question here!

Originally posted by TooManyWheels:
...at highway speeds the engine is coupled to the pavement at the direct or overdrive ratio...

AND with the torque converter locked for vehicles less than 20 years old with automatic transmissions.
OK, can relate to all the inputs. But if engine oil, good oil that is, protects the engine, would it really matter if the miles were city or highway? I was thinking along the lines of van der Waals forces and polarizability. Since the engine block itself is grounded, wouldn't there be an attraction of the oil to the engine, meaning that there would be oil covering all the metal parts which would be protecting it? I really believe that is why moly is so important in an engine oil. It has the ability to stay attached to the metal parts because of its electrical charge, its affinity. Kind of like what Mercruiser does with it's outdrive corrosion protection, they used charged cathodes that disrupt the waters ability to corrode these metal parts that are immearsed in water.
Anyway, my point is that even though the engine works harder in city traffic, the engine would still be well protected if the engine oil is in good shape. But after reading some posts, there are things that I didn't consider to factor in. Basically, more work=less longivity.

[ May 20, 2003, 08:44 AM: Message edited by: Schmoe ]
It's been a long time since I heard the term "Van der Waal's"!

The other thing to keep in mind is, how will you be able to trust the person who told you it was "highway miles"? Obviously if the car is 3 years old and has 120K on it, they probably were but it's usually not that clear cut. I'm thinking the only way I would assume they were highway is if I knew the guy I was buying it from.

Given the way the used car market is (cheap), you could probably make out OK buying any car (get it inspected by a mechanic) and dumping it after a few years if it starts to give you trouble (not my personal philosophy though).

I think that the difference in benefits between doing the minimum maintenance and no maintenance is huge, but doing the minimum vs. doing extra (synthetic oil, religious 3K changes with Dino) is not big enough to worry too much about.
All of the above are big factors in the differences. Someone covered my point but let me take it a step farther.

The best lubrication protection you'll get is by creating a wedge of oil between the rod bearing and the crank. This is done with say 40lbs of oil pressure, which when the piston blows downward to move the crank, it maintains the wedge of oil, kinda like a hydraulic effect, piston/ bearing pushes oil-- oil pushes crank.

Now, at idle, say 28lbs of oil pressure exists at the light. When you get on the gas, oil has to be pumped up to the 40lbs to the bearings. Here's what I'm seeing.. oil pump immediately jumps to 40lbs of pressure between the oil filter and the pump(still have 28lbs at the rod bearings to the filter)... now depending on the filters ability to allow flow, oil has to pass through/by the filter to get to the rod bearings to increase the oil wedge(40lbs) at the rod bearing. All the while, as the oil is traveling to the bearings, you are effectively shearing the oil wedge as it isn't completely up to pressure yet. So, instead of using the oil to push down on the crank, the oil is only coating the bearing and this is when the barrier additive such as moly comes into play for the last line of defense for engine protection. Now, this all happens so fast that it doesn't seem like much but after a constant stop and go situation, the oil is sheared more than on the hwy where it maintains that constant wedge. Add to that, the load factors of moving a car from a dead stop vers a car already moving increases the demand of load to the bearing where the oil pressure is under it's norm. So, here it is, heavier load, less oil, causes more wear and more heat. Less load, more oil, causes less stress on the oil and less heat, less oil shearing, and less wear metals due to the shearing. Coupled with the idle time/ gas/blow by as pointed out earlier.
Liquids are notoriously incompressible. (which is a comforting thought whenever you need to apply your brakes...) I don't believe sudden acceleration would result in an instant 12 psi differential between the oil leaving the pump and the oil film in the big-end rod bearings unless you have one honkin' air bubble trapped in the filter. Even if what you say is true, that oil film in the bearing clearance doesn't just suddenly collapse and leave the affected bearings high and dry. If it did, just starting your engine would be disastrous. Yet we all know that, despite the greater wear generated at startup, engines are still easily capable of service life exceeding 150,000+ miles with nothing more than routine oil/filter maintenance.

With all due respect to Mr. Van der Waal's forces, petroleum oil is not composed of polar molecules. With no defined positively charged "end", I doubt that the negative charge on the engine would demonstrate any more attraction on the oil film than that exhibited with the battery's ground cable completely disconnected. What oil clings to the engine during shutdown is more likely the result of simple capillary attraction and the fact that metals' natural porosity traps and holds at least a minimal thickness oil film. (Sintered bronze "oilite" bearings are a good example.)
So, when you say shearing, your actually explaining tearing, right? The molecules are being torn apart that make the carbon chain. But wouldn't they reform through dipole interaction? What I guess I'm getting at is imagine the engine churing out 2,3,4 or even 5K rpm's. That's like anywhere between 33 to 83 revolutions per second. It would seem that the oil would shear like crazy, then add that up to 3000 miles. Wow.
A couple of points. Shear actually breaks the chains of atoms that make up the oil molecules. I suspect the molecules of the viscosity improvers are even more vulnerable. When the molecules break up, you do not end up with charged particles. One fragment may be unsaturated, but it still has very little distribution of charge. At least in dino, it is all carbon and hydrogen forming molecules where the electrons are evenly distributed. Some of you Synthetics have some oxygen and other polar materials in them, and may be attracted by dipole moment, but such bonds are much weaker than the original covalent bonds that held the molecule together.

Back to the original point. One measurer of how hard the engine is working is how much gas it uses. Your modern engine management and fuel injection have reduced the wasted gas that never went into engine power. Your city millage is still much worse than highway because the engine does more work, sometimes at low RPM and high loads, lugging. Sometimes at peak RPM where you at least have full oil pressure.

I once saw figures that showed cars got their best millage at about 40. Perhaps with modern aerodynamics it would be a little higher now. Steady cruising at 40 might be the easiest of all on your engine. Unfortunately the cops frown on it in the city, and you run substantial risk of being rear ended on the highway.
Ok labman or anyone else, here's a chemistry quiz question: If you mechanically shear a paraffin (let's keep it simple), what is the chemical structure of the cleavage product molecules? Hint: the answer has something to do with sludge formation.
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