More cylinders/V-engine = longer engine life than fewer cylinders/Inline?

Any mechanical engineers here?

So my friend is shopping for a used car, and he tells me about this 'rule of thumb' his father told him about, that the effective working life of an engine is approx 50,000 kilometres per cylinder.

So (as a rule of thumb) theoretically, a 4-cyl engine could go 200,000 kms vs. an 8-cyl could go 400,000 kms.

I asked him to ask his dad why, and his dad says (my friend is not mechanically-inclined at all, hence some simplified explanations... and he's russian so hence the spelling errors):

the wear on the pistons and cilinders is much smaller in V6 that is why it lasts longer.

it is also to do with the engine construction of V6 engine, where 2 groups of cylinders (3 in each) are positioned to form a V shape and it is much easier for them to turn the shaft, as opposed to I4 where all 4 cilinders are inline. V shape is much more gentle for cylinders

Click to expand...

I realize that the explanation is somewhat flawed in that the 'rule of thumb' is based on the # of cylinders vs. the layout of the engine, but humour me.

Would an engine with more cylinders theoretically be more durable/last longer than one with less, all things being equal?

And then to that: would a V6 theoretically be more durable/last longer than an inline engine, all things being equal?

I've never heard anything about number of cylinders vs lifespan of the engine. While each piston might work less than a smaller engined car, and therefore have less wear, I really think the difference is negligible.

A 'V' engine makes it easier to produce low end torque, leaves more space in the bay and is cheaper to make.

An inline engine is smoother as it produces less stress on the crank because the pistons aren't firing back and forth towards each other.


I dunno from personal experience though as I haven't had a chance to have a car for that long. Subaru's supposedly have great longevity, but they run a boxer engine (flat 4), so take from that what you will.

But I'd go with the inline 6, I really doubt that the number of cylinders makes much of an impact in the long run.

As an ME, I'll need to consider this a bit more, but first:

The fixed causes of wear on the rings appear to me to be the choice of the rings themselves first, which can vary, and their interference, which can vary, choice of cylinder material, which can vary, the LUBRICATION, which can vary, the cooling, which can vary, as well as all sorts of other factors such as charge density and timing, all of which can vary. But if you hold all those things equal and just consider effects ONLY caused by number of cylinders, then the following occurs to me:

1. The primary remaining factor is the effect of the number of cylinders on the engine's balance.

2. A V-12 and an I6 should then be the best, as they are the only two standard engines that have perfect primary and secondary harmonic balance -- i.e., the motions withstood by the engine and the crank, as well as the high order derivatives of those motions, are balanced.

3. An I4 should be the worst, because it has the greatest harmonic imbalance, and often require balance shafts and other tricks to keep from shaking the engine to pieces.

However:

4. If the output power of the engines are the same, I would suppose that the engine with more cylinders would last longer, because each cylinder would be doing less work. However, one would hope that you get more power from a V8 than from an I4, so that statement's usefulness is of some question.

5. Assuming the V8 puts out more power than the I4, would it be likely that the I4 would be driven harder and therefore wear faster? Possibly. Another statement of limited usefulness.

I will think about this some more.

~ H

My take as an engineer (industrial) is that all the above is correct, however, I'd say more cylinders = more things that can go wrong. My experience with cars would suggest this is true as many V12 cars seem to have pretty limited life-spans vs. 6 and 8 (where I'd say a 6 is better than 8).

Interesting topic folks. My question then is how would this theory work with a W8 or W12 engine?

Interesting thread. It does seem that overall the I6 engines usually are very reliable, more so than the v6 - from toyota to chrysler.

An interesting aside to the thread is that there are many other things that can go wrong before the engine does - so it obviously isn't the only consideration with the engine.

Originally Posted by celery

Click to expand...

I dunno from personal experience though as I haven't had a chance to have a car for that long. Subaru's supposedly have great longevity, but they run a boxer engine (flat 4), so take from that what you will.

All boxer engines, whether they are 2 cylinders or 12 (Testarossa ) are balanced.

W12 is balanced, from what I understand. The VR6 only has a 15 degree bank seperation, and has minimal second order vibrations. The W12 is two VR6s joined together, in a way.

Have any of the F1 teams ran a W config?

Originally Posted by dhc905

Click to expand...

My take as an engineer (industrial) is that all the above is correct, however, I'd say more cylinders = more things that can go wrong. My experience with cars would suggest this is true as many V12 cars seem to have pretty limited life-spans vs. 6 and 8 (where I'd say a 6 is better than 8).
Remember, too, that V-12s are generally quite high performance affairs, and there are not that many of them. 6s and 8s are a dime-a-dozen in any number of output levels.

Originally Posted by crazyquik

Click to expand...

All boxer engines, whether they are 2 cylinders or 12 (Testarossa

) are balanced.
I knew someone would bring up the flats. Ok, this isn't really all that simple. We had a lively debate in Advanced Thermo on this. There are two main kinds of 'flat' engines, the Boxer and the 180 deg V. They are not equivalent in terms of balance. 1) The Boxer, in which the cylinder motion is mirrored across the crankplane, i.e, both cylinders in each bank are at TDC and later BDC at the same time. For this to be possible, imagine the cylinders at BDC, closest to the crank. Where are the con rod big ends? If the cylinders are perfectly opposed the big end bearings would be occupying the same space, which of course cannot be. Therefore Boxers do not have perfectly opposed cylinders in each bank, they are offset slightly, requiring (as the defining feature of a Boxer) separate crank journals. Now even though the pistons move in opposite directions (inducing primary balance), there is a free secondary couple (erm, tied rotational forces) caused by that cylinder offset. That couple acts to rock the engine around the vertical axis (that is, about an axis normal to the cylinder plane). So while all Boxer configurations can have inherent perfect primary balance, that's kind of big whoop. Secondary balance is much more challenging to achieve, and Boxers can by canceling out the couples with enough cylinders. The Boxer-4 is not perfectly balanced. I think the six is. 2) The 180 degree V. The difference between a Boxer and a 180 degree or flatplane V is that the cylinder motion is not mirrored across the crank. One cylinder in a bank is at TDC while its twin is at BDC, resulting in (just like a crossplane V) two big ends per crank journal. Obviously then, perfect primary balance is not achieved for all configurations. Again, add cylinders to fix this problem. The 180 deg V-6 should be (I think), and I know that the 180 degree V-12 is balanced. However, that is for the same reasons that the V-12 is balanced, not because it is a flat engine. I'm also reasonably certain that the Testarossa uses a 180-V and not a Boxer. 3) Explanations like this are when I wish I was a better kinematician.

Originally Posted by crazyquik

Click to expand...

W12 is balanced, from what I understand. The VR6 only has a 15 degree bank seperation, and has minimal second order vibrations. The W12 is two VR6s joined together, in a way. Have any of the F1 teams ran a W config?
No clue about the W's. ~ H

Very awesome response, Huntsman; very interesting. Amazing the types of members we have here. I posted this question on a car forum, only to get a response of, literally, "NO!"

I guess the whole V vs. Inline thing is misguided (i.e. bullshit) since balance is highly determinant on the number of cylinders; assuming that it is harmonic balance that determines engine longevity.

While the 50,000 * # of cyl's rule-of-thumb may be of some significance, albiet it is is more to do with human factors rather than mechanical/physical factors. And up to a certain extent as mentioned with V12's.

Life F1 team ran a W-12 in 1990.

http://www.f1complete.com/index.php?...k=view&id=5860

So beside BMW and Volvo, does anyone still sell cars in the USA with I-6 engines?
Anyone beside Volvo stil sell a 5 cylinder?

Originally Posted by zippyh

Click to expand...

Life F1 team ran a W-12 in 1990.

http://www.f1complete.com/index.php?...k=view&id=5860

So beside BMW and Volvo, does anyone still sell cars in the USA with I-6 engines?
Anyone beside Volvo stil sell a 5 cylinder?
GM has both an inline-6, and a 5-cyl truck motor in their arsenal. I've heard good things about the 6 cylinder, not so good about the 5.

Originally Posted by sonick

Click to expand...

Very awesome response, Huntsman; very interesting. Amazing the types of members we have here. I posted this question on a car forum, only to get a response of, literally, "NO!" I guess the whole V vs. Inline thing is misguided (i.e. bullshit) since balance is highly determinant on the number of cylinders; assuming that it is harmonic balance that determines engine longevity. While the 50,000 * # of cyl's rule-of-thumb may be of some significance, albiet it is is more to do with human factors rather than mechanical/physical factors. And up to a certain extent as mentioned with V12's.
Yeah I've found this is a pretty awesome place to get answers on most anything -- frequently so much better than the specialist forums with their poseurs. As far as longevity, I don't want to give the impression that balance is a big player -- it probably is only in extreme cases. Hey I drive a V-12 that never disappoints at 110k miles, and they routinely go to 200k+ miles. On the other hand, the V6 in my father's '01 Grand Am has 237k and counting with no major issues. It's a complicated thing. ~ H

It's an interesting question, considered in the abstract, all-else-being-equal situation, but in real life, so many other things can affect an engine's longevity. For example, even for the same engine architecture (say an I-6), different manufacturers will do things differently in the implementation of the engine which will affect its longevity.

Speaking of high mileage, in the early 90s, Mobil did a marketing campaign where they ran an E30 BMW 325i for 1 million miles on a treadmill, following just the recommended service intervals, using Mobil 1 oil, and simulating varying speeds, but mostly at highway speeds. Except for servicing, the car ran 24/7 during the 4-year period it took to get to 1 million miles. They took the engine apart after 1 million miles, and found that the tolerances were still like new or well within spec.

When the Lexus IS first came out, it had an inline-6. Those guys shamelessly copied BMW.

--Andre

That's the dumbest rule of thumb I've ever heard. Ever.

I've seen V8s that died sub 100,000 kms and I4s that went well over 300K. There is much more to consider than the shape of the engine.

In theory that rule of thumb is probably right but I do not think that you should ever consider that when you buy a car.

The cars that set the record for longest life are diesel engines from Mercedes and Volvo and those are all 4 cyclenders.

Also consider the fact that the only auto manufactures that offer V12s in their regular production cars are Mercedes, BMW, Ferrari, Aston Martin, Lamborghini...you cannot get a new car with a V12 for under $100k and if you buy a used one you can bet that when they were new they cost the equivalent of $100k today or more.

For instance while I owned a BMW 850I which had a 5.0 litre V12 the engine had only done 40,000 miles but the tune ups were very expensive and any time one of the many electronic items went wrong (which they frequently did) it cost at least $1000 to replace.

If I had a Honda Civic and the 4 cylinder engine blew it would only be a couple of grand to get a new one...so the cost of ownership will always be less with a much simpler car.

The advantage of a V12 from my perspective is that the engine is much smoother than a V8 I6 etc...because each cylinder has to deal with less power...which brings us back to the irrelevant issue of engine wear.