posted 05-08-2005 10:52 AM ET (US)        

Piston Travel
In the past it has been suggested that one possible index to engine wear and tear might be piston travel. Motors of high horsepower which are run at lower operating speeds experience less wear and tear it is claimed in part because the lower speed of operation results in less total piston travel. This seems reasonable. We now take this approach and analyze several current motors to see how they compare in piston travel.

Piston travel is calculated by taking the engine's stroke dimension, multiplying it by two. (One up/down travel of a piston is needed for each crankshaft revolution.) This is the piston travel per revolution. This figure is multiplied by the engine's rated maximum crankshaft speed in RPM. This gives the piston travel per minute at rated horsepower. Let us compute the piston travel per minute of operation at rated horsepower for several engines.

In order to do this calculation, we need the stroke dimension of the engine's piston. These are usually give in millimeters, but we shall convert the final answer into feet (to help our American readers gain a better appreciation of the actual distances involved and the comparison between engines).

We will look at four engines. All are very recent products and should represent the best engineering of their respective designers. The engines are:

Evinrude E-TEC 150
Mercury VERADO 150
Honda BF-150
Yamaha F-150

From the published specifications we located the stroke (in MM) and the rated crankcase speed (in RPM)

Evinrude E-TEC 150 = 66 MM; 5500 RPM
Mercury VERADO 150 = 82 MM; 6400 RPM
Honda BF-150 = 99 MM; 6000 RPM
Yamaha F-150 = 94 MM; 6000 RPM

Taking the stroke, multiplying by 2, multiplying by the rated RPM, and dividing by 12 and 25.4 gives the piston travel in feet per minute at the rated horsepower:

Evinrude E-TEC 150 = 2,382 feet per minute
Mercury VERADO 150 = 3,444 feet per minute
Honda BF-150 = 3,890 feet per minute
Yamaha F-150 = 3,787 feet per minute

On the basis of favoring lower total piston travel, the Evinrude E-TEC 150 engine should have the longer lifespan.

ENGINE STRESS
Another approach to estimating longevity is that engines should be run under low stress conditions. This is often cited as one of the reasons that a higher horsepower engine is preferred. It "just loafs along" at lower horsepower and lower speeds. When comparing engines of similar horsepower, one could try to compare the "stress" placed on the engine to develop that horsepower. One index to "stress" should be how much horsepower is developed per engine displacement. Let us compare this among these same four engines. Their displacement is:

Evinrude E-TEC 150 = 2.589 liter
Mercury VERADO 150 = 1.732 liter
Honda BF-150 = 2.354 liter
Yamaha F-150 = 2.670 liter

All of these engines are rated at the same horsepower (150), so it is simple to figure the horsepower per liter of displacement:

Evinrude E-TEC 150 = 57.9 HP/L
Mercury VERADO 150 = 86.6 HP/L
Honda BF-150 = 63.7 HP/L
Yamaha F-150 = 56.2 HP/L

If an engine's longevity is affected by its "stress", then lower horsepower per liter should indicated higher longevity.

Another index of stress might be the horsepower per cylinder. This is a very simple calculation. All of the engines in our sample are four cylinder engines, except the E-TEC, which is a six cylinder engine: Thus:

Evinrude E-TEC 150 = 25 HP/cylinder
Mercury VERADO 150 = 37.5 HP/cylinder
Honda BF-150 = 37.5 HP/cylinder
Yamaha F-150 = 37.5 HP/cylinder

Again, using the concept that less "stress" means longer life, a lower value of horsepower per cylinder should correlate to longer life for that cylinder.

Finally, it may be interesting to compare the number of power strokes needed to produce the horsepower per cylinder. This is again a simple calculation. In the four-stroke engines there is only one power stroke per two revolutions, while in the two stroke engines there is one power stroke for each revolution. This implies that the power delivered on each power stroke must be twice as powerful in the four-stroke engine as in the two-stroke engine. Thus we could calculate the power per power stroke per cylinder as follows:

Evinrude E-TEC 150 = 12.5 HP/power stroke/cylinder
Mercury VERADO 150 = 37.5 HP/power stroke/cylinder
Honda BF-150 = 37.5 HP/power stroke/cylinder
Yamaha F-150 = 37.5 HP/power stroke/cylinder

RESULTS

The several parameters analyzed above seem to point quite favorably in the direction of the dimensions of E-TEC engine as having the potential for long life span if one accepts these paradigms:

--lower piston travel is better
--lower engine stress is better

These results would appear to say that the E-TEC engine is well designed for reducing what is commonly believed (and advocated) as important parameters for obtaining long life from an engine.

With mechanical devices it is of utmost importance that they be well constructed, too, so the actual longevity depends in the main on the actual realization of the device. Quality material and craftsmanship are a must for long life in a mechanical device. On the other hand, if one assumes a relatively equal quality, then a device which has inherent advantages in its design may prove to be more long lasting.

posted 05-08-2005 01:02 PM ET (US)            

I used to subscribe to the theories that you suggest and I am sure that they are probably true if applied to exactly identical engines or when applied to the same engine but at different operating conditions. For example the same 2 engines, one run at 50% output should out last one operated at near maximum output.

But, when comparing different engine manufacturers, consider a Toyota vs. a Chevy. The Toyota will come out looking very poor using any of the criteria that you suggest but I would wager that on average that the Toyota will have twice the useful life of the Chevy engine. The difference has to be in the manufacturing tolerances and other engineering considerations.

Probably the best indicator is past history but we all know that corporate goals and strategies can change. They are not going to tell us if they decide to reduce quality to improve profits. Also, from my investigations, it does not appear that any outboard manufacturer has held an edge in producing more reliable engines.

That said, I don’t think that you are not going to be able to predict who’s outboard is going to have the longest life. The consuming public just doesn’t have enough information or expertise.

posted 05-08-2005 04:13 PM ET (US)            

Evinrude E-TEC 150 = 91 MM
Mercury VERADO 150 = 82 MM
Honda BF-150 = 87 MM
Yamaha F-150 = 94 MM

Again it looks like the two-stroke E-TEC is a larger diameter piston than most.

In the two-stroke engine there is always the view that there are twice as many power strokes, so that implies twice the duty-cycle. On the other hand, each power stroke is only about a third the load on an individual cylinder when compared to the four-strokes (with only four cylinders).

Let's look at the power stroke on a cylinder-to-cylinder basis between the Verado and the E-TEC. The piston and rings of the E-TEC have to absorb about 12.5 HP on each power stroke, and spread this load across a piston ring that is

(91 X 3.14) = 285.9 MM in circumference.

This implies a loading of (12.5/285.9), or

E-TEC = 0.044 HP/MM of piston ring

On a VERADO we have each cylinder producing 37.5 HP per power stroke, and spreading this across a piston ring that is

(82 X 3.14) = 257.6 MM in circumference.

This implies a loading of (37.5/257.6) or

VERADO = 0.146 HP/MM of piston ring

If we compare these, we see that the loading on the VERADO piston ring is higher by a factor of

0.146/0.044 = 3.32 times.

The piston and rings of the VERADO have to absorb about almost three and a half times greater load than the piston rings in an E-TEC during a power stroke. But the E-TEC has twice as many power strokes, so let's divide that difference in half

3.32/2 = 1.66

Thus we ought to be able to infer that the load on the piston rings in the Verado is 66-percent greater than the load on the piston rings in an E-TEC. The other engines fall somewhere in between.

posted 05-08-2005 10:38 PM ET (US)            

E-Tec 150: 150 X 396,000 / 5500 X 158 = 68.35
Verado 150: 150 X 396,000 / 6400 X 106 = 87.59
Honda 150: 150 X 396,000 / 144 X 6000 = 68.75
Yamaha F150: 150 X 396,000 / 163 X 6,000 = 60.74

This formula supports the Yamaha as the longest expected engine life by a small margin over the Honda and E-TEC The Verado falls somewhat further back primarily due to its lower displacement.

Somewhere in my archives is the article which would explain what the basis is for the 396,000. As it is already packed for moving I can't even search for it now.

At least some food for thought?!?!?!

posted 05-09-2005 09:04 AM ET (US)            

To put this in the real world, we have four fine engines in this comparison and it appears theoretically that the
E-TEC comes out best with Merc, Honda, and Yamaha following in order. If the guy with the Yamaha takes better care of his engine than the E-TEC owner, all this comparison goes out the window as the Yamaha will likely last longer. Before anyone starts scratching their head wondering what I am getting at, how many boater do we know who take horrible care of their engines, do not make sure they get serviced when required, etc.? There are plenty of boaters who do, but also plenty who run the crap out of their equipment and only service it when they think of it, or worse. You see it in cars every day. I am one of those guys who can get over 20 years out of his automobiles. I am meticulous with maintenance and proper servicing.

posted 05-09-2005 10:45 AM ET (US)            

When comparing two-strokes to four-strokes in endurance, the valve train needs to be in the equation, The piston is the valve in most two-stroke engines. It is also accepted that fuel lubricity offsets piston wear in two-stroke engines compared with four-stroke engines. ("Detroit Diesel Allison" [perhaps an allusion to the two-stroke diesel engines made by that firm--jimh.]) Provided the air source is 100% clean.

I bet the engine with the best, most-protected electronics is the winner of the real life endurance test! The lion's share of failures will be due to human error!

posted 05-09-2005 01:09 PM ET (US)         

I think the general perception is that the four-stroke engine will last longer, in spite of its higher piston travel figures due to higher RPM and lower pitch prop useage. So far, with four-stroke engines on the market for some time now, the results are encouraging. People think well of their Hondas, on the market for the longest time. There is also some evidence (a lot of failures!) that DFI technology stresses the engine more than conventional two-stroke fuel induction systems. Today's conventional EFI four-stroke technology is pretty low stress, from what I can tell. It's too soon to know about supercharging.

posted 05-09-2005 02:31 PM ET (US)            

The only way one can (in a fair manner) compare endurance based on this is in the same exact engine type but with different bore and stroke configurations. You can not figure it out on paper; it's fun to try. There is still a lot to learn.

posted 05-11-2005 12:03 AM ET (US)            

An analysis like this cuts across any brand preference. You just look at the dimensions of the engine and see what they tell you. What I see:

The piston in a 150-HP Verado, a Yamaha, or a Honda travels a lot more per minute at the rated horsepower than a piston in an E-TEC.

The load on the cylinder in each power stroke is much higher in a 150-HP Verado, a Yamha, or a Honda than in an E-TEC. The difference is 3:1.

Maybe factors like this are part of the reason that these other engines need to be continuously bathed in a stream of lubricating oil in order to run. That, or course, overlooks all the additional components they have in motion, such as the cam shafts, the valves, the balancing shafts, the super chargers, etc., which all need to be kept lubricated when running with a stream of oil.

Let's look at another dimension: stroke. The longer the stroke, the more weight in the crankshaft. The longer the stroke the bigger the crankshaft has to be, and the more rotating mass and angular momentum there is. Let's compare stroke:

Evinrude E-TEC 150 = 66 MM
Mercury VERADO 150 = 82 MM
Honda BF-150 = 87 MM
Yamaha F-150 = 96.2 MM

posted 05-11-2005 08:59 AM ET (US)            

Jim--Too many assumptions on the rotating assembly. Stroke has other effects, but to say the rotating inertia is greater with longer stroke is wrong! I have a 60-lb. crank in one engine with a 3.5-inch stroke and a 39-lb. crank in another with a 3.48-inch stroke, and the 60-pound crankshaft has less inertia but more mass because most of the weight is in the center.

Now let us talk about crank dampening. It is accepted that the heavier the damper the better the endurance of the rotating assembly. A super light rotating assembly with a super light damper making high horsepower might win races, but endurance will suffer! However, longer rods that are usually heavier can add endurance.

Do you have piston and connecting rod weights from any of these engines? Most outboard two-stroke engines have heavy pistons and pins compared to high horsepower four-strokes. I have 700-HP four-stroke engines running with 310 gram pistons.

I have not seen enough mechanical information here--not even close--on any of these engines to even make a guess as to which will live the longest "by design."

Many engine desgins are tested on a Spintron to get combustion out of the equation and just see how long things will go up and down and round and round.

posted 05-11-2005 09:01 AM ET (US)            

Two engines - both 350 cubic inches (5.7L). Engine #1 had this displacement designed and built into it by using big bore and relatively short stroke. Engine #2 had this displacement designed and built into it by using smaller bore and longer stroke.

What is the output difference?

Engine #1 will develop peak power at a higher RPM value than engine #2. It will have less low-end power (torque) but great high-end power.

Engine #2 will develop peak power at a lower RPM value than engine #1. It will have greater low-end power (torque) but not as much power in the upper RPM range.

posted 05-11-2005 09:54 AM ET (US)            

It now has to do with cylinder pressure and where in the power event of the cycle it is occuring and at what rate it is applied.

For years it was easier (still is) to make low end torque with a long stroke because designers had much more time (the stroke ) to get it done. The stroke would mask poor design very well.

posted 05-11-2005 12:41 PM ET (US)            

I don't have experience with two-cycle engines, but detailed and extensive experience on four-cycle engines, and, having been raised and working in the oil patch, I have seen the difference in wear and longivity caused by different lubricating oils and maintenance. Using cleaner oils and keeping the filters clean vastly extends the lifetime of an engine.

For example, all lube oils are doing at least a good job of lubricating, but some oils are notably cleaner than others.

And I suspect that the cleanliness is, at least, part of the difference in decreased lifetime noted of dirt bikes.

posted 05-11-2005 02:53 PM ET (US)            

My two-stroke engines are the most reliable, longest lasting things I own (except for my series 60 Detroit), and my four-cycle engines are only fair in the endurance category. Use plays a big role here.

Let us get back to which outboard is the endurance king. I think that is the question, and I am willing to learn, if someone can come up with a scientific way of figuring this out. But so far for every heave there is a hoe, kinda like engine designing huh!

Jerry, I almost missed [commenting on your discussion of the effect of oil lubrication quality on engine endurance]. Sorry. And "Amen." [The imposition of an oil rating schedule called] TCW was supposed to level the field here.

posted 05-11-2005 04:55 PM ET (US)            

But, I have not heard anything about recent/current improvements to the Wankel, but suspect that someone out there is working on them. What is the status of the seal problem? Wish I had one to play with and test.

Thanks for the information on the filters.

posted 05-11-2005 06:18 PM ET (US)            

The wankel ; I had to learn about them because when I was a mechanic in the MC [possibly "motor cycle"] industry there was mucho money to be had if one could understand, tune, and repair them. It was pretty short lived, though, but man could they be smooooth, and what power. We were talking recently with a well-to-do customer about making a prototype outboard for his skater [perhaps a brand of boat?]. I don't even know if it is feasible. Money talks and it is quiet at this time. It has been years since I touched one.

posted 05-12-2005 08:32 AM ET (US)            

Displacement = 2.507-liter
Bore = 89 MM
Stroke = 67 MM
RPM = 5550

These calculate to

HP/Liter = 59.8
HP/Cylinder = 25
HP/Cylinder/Power Stroke = 12.5
Piston Travel at Rated HP = 2,417 feet/minute

This two-stroke 60-degree V engine block has been around for many years and has proven to be a reliable and strong engine for Mercury. It has demonstrated longevity in conventional two-stroke engines. The problems that Mercury had with their OptiMax low-emission engines should not detract from this basic engine block design and its proven reliability.

posted 05-12-2005 12:45 PM ET (US)         

Am I correct in assuming that the 3.3 liter 90 degree V-6 block of the 200-250 E-tecs, is just a displacement increase on the older V-block, that also provided the 2.0 liter V-4, the 3.0 liter V-6, 3.0 liter V-6 Ficht, and 4.0 liter V-8?

Regarding DFI vs four-stroke longevity, it would seem hard to believe that companies like Mercury and Yamaha would bring out new four-stroke technology, Verado or F-series, at huge R & D cost, and not design them for longer life expectancy than their current, lower cost, DFI two-strokes. Otherwise, what reason at all to develop them? Only to compete with tiny market share Honda or Suzuki? I think it's much more than that.

If BRP had the financial R & D resources, they would have had their own larger four-strokes on the market for some time now. They are presently too small to be able to compete in this four-stroke outboard market place with the others since being spun off from parent Bombardier, into a new company.

Currently DFI two-strokes only have 15% of the total outboard marketplace. Conventional two-strokes, mostly by Mercury, Johnson and Yamaha, have 35%. The question is which way will the 35% go next year, and to whom. With Mercury having the lions share of this 35%, it would seem that those users would stay with Mercury, moving to Optimax or four-stroke EFI/Verado. Same with Yamaha, and actually, probably the same with the Johnson buyers moving to E-tec. So, in all probability, where the 35% two-stroke marketshare is now, is pretty much where it's going to end up next year, brand wise.

Right now, unless your boat is badly underpowered (something you DEFINITELY do not want to do with conventional four-stroke due to acceleration problems), a four-stroke engine is already enhancing the re-sale value of your Whaler.

posted 05-12-2005 04:04 PM ET (US)            

It would seem that all of the investment made in developing four-strokes was done at a time when almost everyone, except a few, believed that the emissions regulations would drive two-strokes into extinction. That didn't happen and now, interestingly enough, it seems the table has turned and the two-stroke technology is putting pressure on the four-stroke technology on the emissions front. I have read somewhere recently that Suzuki is developing two-stroke outboards again.

This whole longevity question between four-strokes and two-strokes could very well become a moot point in 10 years if the emissions regulations tighten.

posted 05-12-2005 05:44 PM ET (US)         

The fact that Suzuki might be working on DFI is also of interest, as they were essentially a non-player in the two-stroke market.

Another indication that two-stroke DFI's may be here to stay will be whether Mercury and Yamaha bring the larger current 2 star models up to 3-star ratings. Mercury just introduced a 3 star 225 3.0 liter Pro Max I understand, which indicates it can be done if needed or wanted.

We'll just have to see what shakes out. I have always leaned toward the two-stroke, but of late, the Verado has caugth my interest in 4 cylinder version. When you see them in action, they are impressive. None of the other four-strokes interest me at all.

posted 05-12-2005 09:36 PM ET (US)            

I am still waiting to see what people think of 150-250 HP four-stroke outboards when they are 10 years old. Right now they are all less than 1-3 years old, and everyone loves them. They have not needed any major work. All of those valves and cams are still working. All of those oil passages are still free from sludge and clogging. Let's wait a few years and see how the four-stroke guys feel when they need a $3,000 overhaul on the valve train.

As for Bombardier, I was looking at some satellite photographs of their plant location and it sure looks like there is some room for expansion:

http://google.com/maps?q=10101+Science+Drive,+Sturtevant,+Wisconsin+53177&spn=0.031071,0.068836&t=k&hl=en

posted 05-13-2005 09:07 AM ET (US)            

The initial high-horsepower four-stroke engine was the 135-HP Honda outboard. This is a very heavy engine. It produces quite modest horsepower for its size and weight. I have seen it used in a number of commercial applications where the operating speed of the vessel was not going to be very high. For example, I have seen it pushing around large barge-like vessels at displacement speeds.

In contrast, we now have four-stroke engines of similar size and weight but with the horsepower rating increased to the 225-250 range. These engines are now in use on much higher speed vessels, and are being run at 30-MPH or more for long, sustained runs. It will be interesting to see if the same qualities of the four-stroke are maintained when they are put to this use.

If a motor is going to troll all day at idle speeds, there is no question that a four-stroke will perform better than an old two-stroke engine. Running at idle speed has always been a problem for a two-stroke. Running all day at 3500 RPM has never been a problem for a two-stroke.

If I ran my automobile engine at 3500 RPM, I would be going about 85-MPH. After 4,000 hours of use, I would have traveled 340,000 miles. I have a lot of respect for the light weight aluminum four-stroke engine in my car, but I have to wonder how it would survive that sort of treatment. Also, I would have changed the oil 68 times (at a 5,000 mile interval) or perhaps as much as 113 times (at a 3,000 mile interval). Considering the high-speed application, the later is probably recommended for long life. Changing the oil 113 times is quite a bit of additional expense and down time for the four-stroke.

posted 05-14-2005 04:59 PM ET (US)            

As far as complicating things with multi's, NO2, air, water, air, air, and so on, that is a different story. When considering a turbo or supercharger in an endurance calculation you must add back the negative load on intake stroke.