Teach me oh wise ones

Why is driving city miles more wearing on the engine and engine oil then driving highway miles?

I understand the wear on brakes and suspension - stop and go to wear the brakes and potholes/uneven pavement to wear the suspension.

But why is the repeated running the engine from idle to 4k (driving two blocks then stop) wearing? Why are long idles wearing? Why would idling the enging for a long time be more wearing than driving 100 miles on the freeway?

I guess there would be less air blowing throught the engine compartment and therefore more thermal increase under the hood, but shouldn't the engine be running at a relative constant temperature? Isn't that what the coolant system is for?

This is something I've never understood.

 

I would think that in stop and go driving, the "stop" part is relatively light on your car compared to the "go". My guess is it takes a lot more energy to move something that is stationary (due to inertia) compared to something that is already moving. As such, if you are doing the stop+go thing, your car's engine has to work very hard to move it after you stop. It will take your engine a lot more effort to increase your speed from 0mph to 1mph compared to 50 to 51 mph.

I have never heard of long idles being bad for the engine though... I've heard of idling being bad for your gas mileage, and idling leading to carbon monoxide poisoning in certain dumb cases (eg idling in your enclosed garage), but never idling wearing on your engine more than driving.

As for air helping to cool the engine compartment, most modern cars are liquid cooled, not air cooled, so I would hazard a guess that flowing air performs only a minimal cooling duty on the engine/radiator. Unless you have a faulty cooling system, in which case all bets are off.

 

The answer is rather simple:

1) Engine works more hours and you get very little miles if you drive in the city, whereas on highway, it takes less time to cover the same amount of miles. Time is a factor of tear and wear.

2) Stop and go city traffic makes your engine work at higher RPMs(well depends how you lunch), which produce more tear and wear.

I think reason number one is the bigest.

 

Think of a engine as a "Heat Generator" yes the coolant keeps it from over heating but rev'ing creates more heat than steady state. These hot spots cause the hydo carbons in petro oil to break down and build up (coke) on various surfaces, causing more hot spots. Thus the heat will break down bearing surfaces. Using syn oil pretty much eliminates these mechanisms. I had to KISS this answer because books have been written on this subject.

 

So its the delta (change) in rpm that wears the engine?

In response to russianDude's #2 - higher rpm is more wearing than lower. Is this really true (under redline)? Is is really more wearing to run the engine at 4K then at 2K (assuming all systems are good). russianDude's #1 makes no sense - you've misunderstood my question. I'm not asking about overall car wear, just engine wear.

D73's statement about engine work to move the mass makes sense (moving from 0 - 1 mph is more work than 50 - 51mph). But isn't that what the transmission is for - adjust the work produced by the engine for the inertia already 'produced' by motion.

fender4 said books are written about this. Do you have any recommendations?

 

> So its the delta (change) in rpm that wears the engine?
I'm guessing yes, rpm change does wear the engine. But of course it's not the only thing. Like fender4 said, heat will eventually break down your engine oil and create hot spots. That's why we change engine oil. Or at least I hope you change your engine oil.

> In response to russianDude's #2 - higher rpm is more wearing than lower. Is this really true (under redline)?
I'm guessing it is. Kinetic energy (KE) is 1/2 * m * v^2, and So if you're doing twice the rpm, you're generating four times the KE. And this KE has to come from SOMEWHERE (ie fuel), and due to inefficiencies, excess energy has to go SOMEWHERE (ie friction, heat etc).

> But isn't that what the transmission is for - adjust the work produced by the engine for the inertia already 'produced' by motion.
When an object is at rest, in order to get it moving, you have to overcome static friction + inertia. Once it is moving, you just have to overcome inertia. That's why it hurts the car more when you go from 0 to 1 than it does from x to x+1 (x>0).

I'm sure there's more "car oriented" explanations out there, but I'm hardly a car nut.

 

Regarding #1: I was talking only about the engine wear. Think of it this way: Lets say you drive 1000 miles on perfect highway with constant speed 65mph ( this will give you approximatly 2000-3000 RPM ). You will cover 1000 miles in approximatly 15 hours. So basicly your engine will work for 15 hours. That;s 15 hour of tear and wear for engine.
Now, lets say you drive the same amount of miles in the city, with average speed 10 mph and lets assume RPMS are also between 2000-3000. This will give you total of 100 hours of engine tear to cover 1000 miles in the city.

Conclusion: In both city and highway approximatly the same RPM was used and the same amount of distance was covered (1000miles). However, it took more work hours for engine to cover 1000 miles in the city, therefore more TEAR was done to the engine in the city.
Sorry if I was not clear the first time.

PS. Engines do not live forever. Engineers even calculate average life of an engine measured in total hours of work given average RPM and resistance. Yes, Idling slowing kills your engine.

 

That's truth. Can't argue with that.

 

Sorry. I see what you are saying now. I was looking at the equation backwards. You are right - it's quite simple.

Resaid: for an equal amount of MILEAGE put on the car the highway driven car will have less HOURS then the city driven car.

Thanks

 

Unfortunatly when you try to sell your car, nobody will belive you that you have highway miles or city miles. People typicly look at the number of miles, the lower the better.

 

Correct. And I would agree with you in that a city driven car with, say, 10,000 miles may be more worn than a car with 20,000 only used for transcontinental rides.

I think that they measure airplane engine use in hours, not miles - which may be a more acurrate method than mileage in automobiles.