Question about engine speeds/work
 

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.

I agree with you, I would be more apprehensive of buying a car that in alot of stop and go low speed traffic rather than a porsche thats driven on a super smooth usually not stop and go traffic autobaun.

I think you are asking two different questions.

Is it better to buy a car that has been driven mostly highway miles at high speeds vs one that has the SAME NUMBER OF MILES but driven in stop or go?

To me, take the highway miles.

Your example is NOT valid I think. The car driving at 130 MPH is benefiting from the mechanical advantage afforded the gearing but he has to overcome resistence far greater than the other guy. That may make it a wash.

Again, if were are talking the SAME NUMBER OF MILES, I would still pick the highway car, ALL THINGS BEING EQUAL, which of course, they never are.

I also agree, but I dont 100% agree. If your in 1st and going 5 grand steady and you push the gas it will go like hell to the rev limit, but if your 5 grand steady at 130 and you push the gas all the way down it will slowly gain. My analogy is that you can bench press 40 Lbs 10 times a minute and you can also bench press 100 Lbs 10 times a minute but the heavier weight (gearing and resistence in 5th gear) will be more stressfull.

thats true bogg, you run into wind resistance ...BUT however the engine would have far better oil pressure in 5th gear at 130mph @ 5K rpms than 1st gear 5k rpms...More load equals alot more oil pressure...i think. You can hear it in the engine, Try coasting at 5k RPMs in first gear and then try coasting 5k RPMs in 3rd Gear....3rd gear sounds way more suttle and easier on the engine than 1st gears irratic engine noise...

1st gear may have high volume like all the other gears but less pressure under these conditions we are discussing.

Rail, I just want to say that your Spanish continues to improve. Great work! :)

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.

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.

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.

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.

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.

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.

Hi,

I agree with you totally...

Happy Motoring!... Jim'99

apologies; didn't read all the posts

agreed...higher gear = higher stress
 

...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

Thanks for all the input...next time I am drunk and having this discussion I will make the same argument however, I will preface it with "You are driving in space at 5k rpms....."