Question about engine speeds/work

[blue2000s]

Originally Posted by Rail26

I was having a discussion over a couple shots of Don Julio and some chicken mole enchiladas about how hard an engine is working at different speeds. My argument was the following:

Hypothetical: If an engine is turning at 5000 rpms and going 30 mph in first gear or turning 5000 rpms and going 130 in 5th gear...the engine is experiencing the same amount of stress. This was brought about by my buddy saying he would never buy a German car from Germany that was driving routinely on the Autobahn. What do you guys think? I don't think the engine would be anymore less worn out then if you purchased one in the states.

So are you asking if a car is driven for say 50,000 miles at 5000rpm in 3rd gear is better or worse off than one driven for the same miles and rpm in 5th gear? If this is the question, the answer is, it depends.

Engine speed is far from the only thing that effects engine wear. Things like engine load (essentially, how wide open is the throttle), fuel quality, the outside conditions like temperature, humidity, dust content, how well are the oiling and cooling systems maintained, and I'm sure other things that I'm forgetting.

If, instead you're asking if a car that's driven in mostly city miles vs one that's mostly done highway miles, 9 out of 10 times the highway engine is going to be in better shape. Constant changes in engine speed and load and consistant short trips will decrease engine life due to mostly friction and water accumulation.

[Rail26]

Allow me to elaborate on my question....I don't think I asked it very clearly. Is the engine under the same amount of load cruising at 130mph in 5th gear at 5000 rpm or crusing at 35mph in 1st gear at 5000rpm? The engine is still turning at the same speed and isn't engine wear a function of load and speed? If I cruised around at 130 mph everywhere would the engine show more wear if I cruised around at 35 mph with the above conditions? Let's also assume they are all highway miles with no stop and go.

[blue2000s]

Originally Posted by Rail26

Allow me to elaborate on my question....I don't think I asked it very clearly. Is the engine under the same amount of load cruising at 130mph in 5th gear at 5000 rpm or crusing at 35mph in 1st gear at 5000rpm? The engine is still turning at the same speed and isn't engine wear a function of load and speed? If I cruised around at 130 mph everywhere would the engine show more wear if I cruised around at 35 mph with the above conditions? Let's also assume they are all highway miles with no stop and go.

Strictly speaking, if the loads and engine speeds are the same and there are no other differences but vehicle speed, there would theoretically be no difference in the wear on the engine given a set amount of time. The total miles on the car would be different, but that wouldn't matter to the engine.

[MNBoxster]

Originally Posted by blue2000s

Strictly speaking, if the loads and engine speeds are the same and there are no other differences but vehicle speed, there would theoretically be no difference in the wear on the engine given a set amount of time. The total miles on the car would be different, but that wouldn't matter to the engine.

Hi,

That's just it, it's the loads which make the difference, not the revs. 5k RPM on level ground places much less stress on the engine than 5k RPM on an incline.

The car going 5k RPM on level ground will also experience a lot better MPG, because it's doing less WORK than the one going up the incline at 5k RPM.

Now, all this assumes that the gearing is the same for each. Because it usually isn't, any less rolling resistance/friction/mechanical inefficiency experienced by the car at highway speeds than the one operating at 35MPH in a shorter gear, will be offset by the increased drag which rises by the square of the speed the car is traveling.

The reasons a Highway driven car is preferable to a street driven one is that: a.) It has less time/stress on it per mile than the street driven one. b.) It has spent more of it's life with the Coolant Pump, Oil Pump, (both Crank Speed dependent) operating at their most efficient speeds. Also, Radiator Airflow, Alternator output are also in their sweet spots. And, these conditions remain constant throughout the drive, or at least much more so than for a street driven car...

Happy Motoring!... Jim'99

[blue2000s]

Originally Posted by MNBoxster

Hi,

That's just it, it's the loads which make the difference, not the revs. 5k RPM on level ground places much less stress on the engine than 5k RPM on an incline.

The car going 5k RPM on level ground will also experience a lot better MPG, because it's doing less WORK than the one going up the incline at 5k RPM.

Now, all this assumes that the gearing is the same for each. Because it usually isn't, any less rolling resistance/friction/mechanical inefficiency experienced by the car at highway speeds than the one operating at 35MPH in a shorter gear, will be offset by the increased drag which rises by the square of the speed the car is traveling.

The reasons a Highway driven car is preferable to a street driven one is that: a.) It has less time/stress on it per mile than the street driven one. b.) It has spent more of it's life with the Coolant Pump, Oil Pump, (both Crank Speed dependent) operating at their most efficient speeds. Also, Radiator Airflow, Alternator output are also in their sweet spots. And, these conditions remain constant throughout the drive, or at least much more so than for a street driven car...

Happy Motoring!... Jim'99

I think that's what I said in my original post.

[MNBoxster]

Originally Posted by blue2000s

I think that's what I said in my original post.

Hi,

I agree with you totally...

Happy Motoring!... Jim'99

[Brucelee]

To add to Jim's list on Highway miles, all things being equal, the brakes will last longer, ditto the transmission, motor mounts, u-joints etc, tires.

Stop and go kills.

[insite]

Originally Posted by blue2000s

Strictly speaking, if the loads and engine speeds are the same and there are no other differences but vehicle speed, there would theoretically be no difference in the wear on the engine given a set amount of time. The total miles on the car would be different, but that wouldn't matter to the engine.

engine load is what matters. if you're at 5K in 2nd and accelerating full throttle, this is different than cruising at 5K in second (stable speed). that having been said, the load on the motor is higher at 130mph because power required to overcome the air resistance at this speed is significant, increasing engine load.

[blue2000s]

Originally Posted by insite

engine load is what matters. if you're at 5K in 2nd and accelerating full throttle, this is different than cruising at 5K in second (stable speed). that having been said, the load on the motor is higher at 130mph because power required to overcome the air resistance at this speed is significant, increasing engine load.

My original post addressed this.

[insite]

Originally Posted by blue2000s

My original post addressed this.

apologies; didn't read all the posts

[racer_troy]

...just from the mechanical advantage alone the engine has to work harder...and then factor in wind resistance...in the real world, 5k in top gear will ask the engine to work harder, but may not be wearing it out faster...

make sense?

--troy

[blue2000s]

Originally Posted by Rail26

Allow me to elaborate on my question....I don't think I asked it very clearly. Is the engine under the same amount of load cruising at 130mph in 5th gear at 5000 rpm or crusing at 35mph in 1st gear at 5000rpm? The engine is still turning at the same speed and isn't engine wear a function of load and speed? If I cruised around at 130 mph everywhere would the engine show more wear if I cruised around at 35 mph with the above conditions? Let's also assume they are all highway miles with no stop and go.

Sorry, re-read this. I didn't get that you were asking IF loads were equal. I thought you were saying what if they were equal.