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Author Topic:   Outboard Engines and Carpenters

jimh posted 03-19-2014 09:22 AM ET (US)
In outboard engines we have two-stroke-power-cycle engines and four-stroke-power-cycle engines. I describe their operation using an analogy of a carpenter driving a nail.
Two carpenters have the same type hammer and want to drive nails into some dense wood. The two-stroke carpenter hits the nail with his hammer on every down stroke, giving it a tap of force F. The four-stroke carpenter hits the nail as often as the other carpenter, but he only takes a forceful swing on every other stroke.
After one minute, both carpenters have driven the nail into the wood the same distance. Both carpenters hit the nail the same number of times.
Since the nail was driven the same distance, we have to conclude that the four-stroke carpenter must have hit the nail with a force of 2xF. This is necessary because he drove the nail as far as the other carpenter, but he only hit it with force half as often.
Returning to the outboard engines, we can apply the same analysis: in the cylinder of the two-stroke engine the force being created on each power stroke must be about one-half the force being created on each power stroke of the four-stroke engine.
Is there any significant error in this analogy and its conclusion?
ANDREW PITCHFORD posted 03-19-2014 03:35 PM ET (US)
These carpenters, Are they left or right handed?
jharrell posted 03-19-2014 04:03 PM ET (US)
Missing some pieces of the puzzle:
1. Number of pistons at the same power, say I2 vs I4 in 60HP range. More pistons spreads out force among greater number of pistons lower the force per piston for same total power.
2. Stroke ratio. For outboards it looks like E-TEC's and Optimax's are all oversquare while typical four-strokes are undersquare. Power is force x distance / time. Undersquare engines have lower force on the pistons since they travel further distance, ceteris paribus. That is they have more leverage.
3. Usable stroke. Two-strokes must devote part of their stroke to open intake and exhaust port openings (blowdown), this reduces their actual usable power stroke, the four-stroke may utilize the entire power stroke to extract work.
This would make your hammer analogy such that they don't have the same hammer. The oversquare two-stroke has a short hammer that can only be lifted a short distance from the nail, while the undersquare four-stroke has a long hammer lifted further from the nail. This has an equalizing effect on the forces exerted given one hammer is striking twice as fast while the other less often with greater mechanical advantage.
Only if rpm, cylinder count, and effective power-stroke length where the same would your original conclusion be accurate. Perhaps you are confusing force with power or torque?
RevengeFamily posted 03-19-2014 04:37 PM ET (US)
Very interesting Jim,
What you failed to include in your analogy is the resistance that the valve train on a four stroke engine has versus the lack of that resistance on a two stroke. What percentage of the carpenters swing powers the camshaft which opens the intake and exhaust valves?
Norm
jimh posted 03-19-2014 06:48 PM ET (US)
The left-hand or right-hand makes no difference. We only observe the distance the nail is moved and number of hits applied with force.
The engine analogy can assume one-cylinder. It is simple: one hammer, one cylinder.
Assume the distance of the hammer swing is the same.
Assume the hammers are identical.
ruest posted 03-19-2014 08:41 PM ET (US)
The problem with the analogy that I see is the assumption of a Tap Force of F in the 2-Stroke.
To correct the analogy let's say the 4-Strokers are "Master Carpenters" that have learned a technique of hitting with with force on every other tap and doing so enables them to apply a constant Tap force of F when they do strike.
The 2-Strokers are the young "Apprentices" that try to outdo the masters by wailing away with force on every stroke. They figure that they hit twice as often and apply force 2F and get done twice as quick. However the speed at which they must swing without that rest/cycle of the "Master Carpenter" means they generate taps with variations in force with taps <F (sometimes a lot less which equates to the narrower power band on 2-strokes that generate maximum power).
So in the analogy the Nail is driven the same distance when these 2 formulas are equal:
4 Strokers = H*F (Hits x Force)
2 Strokers = SUM(F(1)...F(2*H))
Taking the analogy further the Master 4-Strokers can work much longer before lunch (better MPG) while the Apprentice 2-Strokers flailing away at those nails but take a rest and snack break (Lower MPG).
jimh posted 03-19-2014 09:58 PM ET (US)
I don't have much interest in further analogies or for calculations of the force based on anything other than the total distance the nail is moved and the number of hits with force.
As I observed earlier, it seem that each hit in the four-stroke engine made with force must be of twice the force of each hit in the two-stroke. I don't see any ration conclusion that could be otherwise.
ruest posted 03-19-2014 10:27 PM ET (US)
In a simplified analogy yes the logic is sound. If the nail is driven the same distance on half the hits then the hits have twice the force, and that explains the basic difference in operation between a 2-stroke and 4-stroke.
The added complexities would be needed to explain real world scenarios like how a 4-stroke acan be so much more efficient at idle vs only around 10% at WOT.
macfam posted 03-20-2014 06:20 AM ET (US)
Let's assume a 2 cylinder 2 stroke vs. 4 stroke.
In the 2 stroke, each time a cylinder is on it's power stroke, the other opposing cylinder is on a compression stroke. That "resistance" of the compression stroke has to be taken into account.
On the 4 cycle, each time a cylinder is on a power stroke, the opposing cylinder has it's exhaust valve(s) open and is not compressing, resulting in far less, if any compression or "resistance".
This could account for the 4 stroke having 2x the power in each of it's stokes.
However, there is often a marked difference in torque between the 2 stroke and the 4 stroke. Although they may have the same horsepower at high rpm's, the torque or "twisting power" is greater at lower rpm's in the 2 stroke.
jcdawg83 posted 03-20-2014 09:53 AM ET (US)
I'm not smart enough to do all the physics, but if the whole "four stroke power stroke is double the two stroke" concept is correct, wouldn't the four stroke be able to generate the same horsepower with half the displacement? I haven't done the research, but I don't think that is the case.
Also, I really don't see the point of the discussion. 90hp is 90hp regardless of whether it is two or four stroke.
jimh posted 03-20-2014 10:05 AM ET (US)
It is clear to me that in the four-stroke-power-cycle engine, when the power stroke occurs, there must be twice as much force being applied compared to the power stroke of the two-stroke-power-cycle.
jimh posted 03-20-2014 10:18 AM ET (US)
The analogy can be further expanded. Before expanding the analogy, I will give the carpenters names so we can refer to them more easily. The two-stroke-power-cycle carpenter is Ole, and the four-stroke-power-cycle carpenter is Sven.
We can consider that Ole and Sven might not always hit the nail at the same pace. One might hit the name more frequently in one minute than the other, but in the same amount of time they drive the nail the same distance. This would be analogous to the cycle speed of the engine, or its RPM.
Let us assume Ole hits the nail a total of 500 times in a particular time period, and every one of those is a power stroke, and Sven, who works a little faster, hits the nail 550 times in that same period, only half of them power strokes. But they both drive the nail the same distance. Now we can compare the force they created to hit the nail on the basis of number of power strokes in a certain time period. Ole moved the nail with 500 power strokes, but Sven moved it the same distance with only 275 power strokes. We must conclude that the force used per power stroke was in a ratio of 1.82:1.
AZdave posted 03-20-2014 11:14 AM ET (US)
The initial assumption of a 2:1 ratio may not always be accurate. I did an extremely limited analysis of two outboards that would be appropriate to my boat. The Evinrude Etec 90 hp. is a three cylinder that produces rated horsepower in the range of 4500-5500 rpm. The Mercury Fourstroke is a four cylinder that is rated at 5000-6000 rpm. Thus for this pair of outboards running at the same rpm, the ratio would be 3:2. In the extreme case that the Etec might reach rated horsepower at 4500rpm and the Mercury reached rated horsepower at 6000 rpm, the rpm of the Mercury is 4/3 that of the Etec. When that is multiplied in, the 3:2 ratio becomes 3:2.66. I'm not suggesting that this would be true for all, or even most outboards.
seahorse posted 03-20-2014 11:14 AM ET (US)
Your analogy is about an object that is hit with a force, moves a distance, then STOPS may not be the best type of example.
How about something like a merry-go-round with kids on it that gets propelled for about 1/2 of a revolution only on alternate revolutions compared to one which is pushed for about 1/4 of a revolution on every revolution?
That simulates the dwell and the frequency of the powerstroke in a 4-stroke or 2-stroke engine.
jimh posted 03-20-2014 11:18 AM ET (US)
I like my nail analogy because everyone has driven a nail with a hammer and understands that the nail only moves when you hit it with force.
The merry-go-round is a mixture of flywheel motion and limited resistance. Once the merry-go-round is up to speed the load is only the load of overcoming its rotational friction and wind resistance to keep it going.
jimh posted 03-20-2014 11:22 AM ET (US)
AZDave--your analogy seems to be mixing in two variables:
--number of cylinders
--speed of engine
I considered engine speed in the analogy. See above.
As for number of cylinders, you could consider this to be like a carpenter having a hammer in each hand. We might say Sven hits the nail with two hammers in quick succession, while Ole uses one hammer. But, for the moment, let us keep the analogy simple: one hammer or one cylinder. Whatever we find can later be scaled in proportion to cylinders.
Binkster posted 03-20-2014 11:30 AM ET (US)
The analogy of using carpenters is not accurate in real life. As a lifelong carpenter I can tell you that driving nails in dense wood takes a certain feel for the wood. Hitting the nail too hard will only result in bending the nail, and if these two Swedish carpenters took a minute to drive a nail and had to hit that nail 500 times they would be more like woodpeckers than carpenters.
rich
padrefigure posted 03-20-2014 04:25 PM ET (US)
Taking Jim's analogy at face value and assuming that all else is equal (same number of cylinders, same displacement, same rpm, etc.)boh engines are doing the same amount of work, but one is doing it with half as many power strokes as the other. Mathematically, this means that a four stroke engine must produce slightly more than twice as much power on each power stroke as the two stroke engine. Slightly more because the exhaust stroke in a four stroke engine consumes power that the power stroke must provide to arrive at an equal output. But of course all things are not equal which, I think, is the point of discussion.
jimh posted 03-21-2014 01:12 AM ET (US)
ASIDE:
Sven and Ole are putting some siding on the ice fishing shanty. They're working on opposite sides of the little shed.
Sven: Hey, Ole, ha'f of dese nails I pick up havda head on the wrong end.
Ole: Jeez, Sven, dose are da ones for my side.
saumon posted 03-21-2014 07:44 AM ET (US)
Too bad those scandinavians brothers doesn't have access to Thor's hammer (aka Verado) to drive their nails...
Peter posted 03-21-2014 09:47 AM ET (US)
Thor's Verado hammer would be too heavy for the carpenters to lift. ;)
contender posted 03-21-2014 03:03 PM ET (US)
They did not use nails they drove in wooden pegs
jimh posted 03-21-2014 03:58 PM ET (US)
Both Ole and Sven have to lift their hammers away from the nail before swinging them toward the nail on the power-stroke. If you consider that they are driving nails horizontally instead of vertically, then gravity does not affect the sideways motion (at least not very much for this simple analogy). But we do see that Sven has to move the hammer twice as much as Ole for each power stroke. Sven must be using more energy in moving the hammer than Ole. Ole's power-stroke on every swing toward the nail seems to be inherently less wasteful of energy than Sven's. Is that a fair assessment?
knothead posted 03-21-2014 04:42 PM ET (US)

Who has the framing hammer vs who has the finish hammer?
Who wins in the real world when the hammer is down?
regards---knothead
jimh posted 03-21-2014 08:25 PM ET (US)
So far the hammer has been kept constant. There may be some element of the analogy in which the weight of the hammer could be linked to something analogous in the engine.

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Author	Topic: Outboard Engines and Carpenters
jimh	posted 03-19-2014 09:22 AM ET (US) In outboard engines we have two-stroke-power-cycle engines and four-stroke-power-cycle engines. I describe their operation using an analogy of a carpenter driving a nail. Two carpenters have the same type hammer and want to drive nails into some dense wood. The two-stroke carpenter hits the nail with his hammer on every down stroke, giving it a tap of force F. The four-stroke carpenter hits the nail as often as the other carpenter, but he only takes a forceful swing on every other stroke. After one minute, both carpenters have driven the nail into the wood the same distance. Both carpenters hit the nail the same number of times. Since the nail was driven the same distance, we have to conclude that the four-stroke carpenter must have hit the nail with a force of 2xF. This is necessary because he drove the nail as far as the other carpenter, but he only hit it with force half as often. Returning to the outboard engines, we can apply the same analysis: in the cylinder of the two-stroke engine the force being created on each power stroke must be about one-half the force being created on each power stroke of the four-stroke engine. Is there any significant error in this analogy and its conclusion?
ANDREW PITCHFORD	posted 03-19-2014 03:35 PM ET (US) These carpenters, Are they left or right handed?
jharrell	posted 03-19-2014 04:03 PM ET (US) Missing some pieces of the puzzle: 1. Number of pistons at the same power, say I2 vs I4 in 60HP range. More pistons spreads out force among greater number of pistons lower the force per piston for same total power. 2. Stroke ratio. For outboards it looks like E-TEC's and Optimax's are all oversquare while typical four-strokes are undersquare. Power is force x distance / time. Undersquare engines have lower force on the pistons since they travel further distance, ceteris paribus. That is they have more leverage. 3. Usable stroke. Two-strokes must devote part of their stroke to open intake and exhaust port openings (blowdown), this reduces their actual usable power stroke, the four-stroke may utilize the entire power stroke to extract work. This would make your hammer analogy such that they don't have the same hammer. The oversquare two-stroke has a short hammer that can only be lifted a short distance from the nail, while the undersquare four-stroke has a long hammer lifted further from the nail. This has an equalizing effect on the forces exerted given one hammer is striking twice as fast while the other less often with greater mechanical advantage. Only if rpm, cylinder count, and effective power-stroke length where the same would your original conclusion be accurate. Perhaps you are confusing force with power or torque?
RevengeFamily	posted 03-19-2014 04:37 PM ET (US) Very interesting Jim, What you failed to include in your analogy is the resistance that the valve train on a four stroke engine has versus the lack of that resistance on a two stroke. What percentage of the carpenters swing powers the camshaft which opens the intake and exhaust valves? Norm
jimh	posted 03-19-2014 06:48 PM ET (US) The left-hand or right-hand makes no difference. We only observe the distance the nail is moved and number of hits applied with force. The engine analogy can assume one-cylinder. It is simple: one hammer, one cylinder. Assume the distance of the hammer swing is the same. Assume the hammers are identical.
ruest	posted 03-19-2014 08:41 PM ET (US) The problem with the analogy that I see is the assumption of a Tap Force of F in the 2-Stroke. To correct the analogy let's say the 4-Strokers are "Master Carpenters" that have learned a technique of hitting with with force on every other tap and doing so enables them to apply a constant Tap force of F when they do strike. The 2-Strokers are the young "Apprentices" that try to outdo the masters by wailing away with force on every stroke. They figure that they hit twice as often and apply force 2F and get done twice as quick. However the speed at which they must swing without that rest/cycle of the "Master Carpenter" means they generate taps with variations in force with taps <F (sometimes a lot less which equates to the narrower power band on 2-strokes that generate maximum power). So in the analogy the Nail is driven the same distance when these 2 formulas are equal: 4 Strokers = HF (Hits x Force) 2 Strokers = SUM(F(1)...F(2H)) Taking the analogy further the Master 4-Strokers can work much longer before lunch (better MPG) while the Apprentice 2-Strokers flailing away at those nails but take a rest and snack break (Lower MPG).
jimh	posted 03-19-2014 09:58 PM ET (US) I don't have much interest in further analogies or for calculations of the force based on anything other than the total distance the nail is moved and the number of hits with force. As I observed earlier, it seem that each hit in the four-stroke engine made with force must be of twice the force of each hit in the two-stroke. I don't see any ration conclusion that could be otherwise.
ruest	posted 03-19-2014 10:27 PM ET (US) In a simplified analogy yes the logic is sound. If the nail is driven the same distance on half the hits then the hits have twice the force, and that explains the basic difference in operation between a 2-stroke and 4-stroke. The added complexities would be needed to explain real world scenarios like how a 4-stroke acan be so much more efficient at idle vs only around 10% at WOT.
macfam	posted 03-20-2014 06:20 AM ET (US) Let's assume a 2 cylinder 2 stroke vs. 4 stroke. In the 2 stroke, each time a cylinder is on it's power stroke, the other opposing cylinder is on a compression stroke. That "resistance" of the compression stroke has to be taken into account. On the 4 cycle, each time a cylinder is on a power stroke, the opposing cylinder has it's exhaust valve(s) open and is not compressing, resulting in far less, if any compression or "resistance". This could account for the 4 stroke having 2x the power in each of it's stokes. However, there is often a marked difference in torque between the 2 stroke and the 4 stroke. Although they may have the same horsepower at high rpm's, the torque or "twisting power" is greater at lower rpm's in the 2 stroke.
jcdawg83	posted 03-20-2014 09:53 AM ET (US) I'm not smart enough to do all the physics, but if the whole "four stroke power stroke is double the two stroke" concept is correct, wouldn't the four stroke be able to generate the same horsepower with half the displacement? I haven't done the research, but I don't think that is the case. Also, I really don't see the point of the discussion. 90hp is 90hp regardless of whether it is two or four stroke.
jimh	posted 03-20-2014 10:05 AM ET (US) It is clear to me that in the four-stroke-power-cycle engine, when the power stroke occurs, there must be twice as much force being applied compared to the power stroke of the two-stroke-power-cycle.
jimh	posted 03-20-2014 10:18 AM ET (US) The analogy can be further expanded. Before expanding the analogy, I will give the carpenters names so we can refer to them more easily. The two-stroke-power-cycle carpenter is Ole, and the four-stroke-power-cycle carpenter is Sven. We can consider that Ole and Sven might not always hit the nail at the same pace. One might hit the name more frequently in one minute than the other, but in the same amount of time they drive the nail the same distance. This would be analogous to the cycle speed of the engine, or its RPM. Let us assume Ole hits the nail a total of 500 times in a particular time period, and every one of those is a power stroke, and Sven, who works a little faster, hits the nail 550 times in that same period, only half of them power strokes. But they both drive the nail the same distance. Now we can compare the force they created to hit the nail on the basis of number of power strokes in a certain time period. Ole moved the nail with 500 power strokes, but Sven moved it the same distance with only 275 power strokes. We must conclude that the force used per power stroke was in a ratio of 1.82:1.
AZdave	posted 03-20-2014 11:14 AM ET (US) The initial assumption of a 2:1 ratio may not always be accurate. I did an extremely limited analysis of two outboards that would be appropriate to my boat. The Evinrude Etec 90 hp. is a three cylinder that produces rated horsepower in the range of 4500-5500 rpm. The Mercury Fourstroke is a four cylinder that is rated at 5000-6000 rpm. Thus for this pair of outboards running at the same rpm, the ratio would be 3:2. In the extreme case that the Etec might reach rated horsepower at 4500rpm and the Mercury reached rated horsepower at 6000 rpm, the rpm of the Mercury is 4/3 that of the Etec. When that is multiplied in, the 3:2 ratio becomes 3:2.66. I'm not suggesting that this would be true for all, or even most outboards.
seahorse	posted 03-20-2014 11:14 AM ET (US) Your analogy is about an object that is hit with a force, moves a distance, then STOPS may not be the best type of example. How about something like a merry-go-round with kids on it that gets propelled for about 1/2 of a revolution only on alternate revolutions compared to one which is pushed for about 1/4 of a revolution on every revolution? That simulates the dwell and the frequency of the powerstroke in a 4-stroke or 2-stroke engine.
jimh	posted 03-20-2014 11:18 AM ET (US) I like my nail analogy because everyone has driven a nail with a hammer and understands that the nail only moves when you hit it with force. The merry-go-round is a mixture of flywheel motion and limited resistance. Once the merry-go-round is up to speed the load is only the load of overcoming its rotational friction and wind resistance to keep it going.
jimh	posted 03-20-2014 11:22 AM ET (US) AZDave--your analogy seems to be mixing in two variables: --number of cylinders --speed of engine I considered engine speed in the analogy. See above. As for number of cylinders, you could consider this to be like a carpenter having a hammer in each hand. We might say Sven hits the nail with two hammers in quick succession, while Ole uses one hammer. But, for the moment, let us keep the analogy simple: one hammer or one cylinder. Whatever we find can later be scaled in proportion to cylinders.
Binkster	posted 03-20-2014 11:30 AM ET (US) The analogy of using carpenters is not accurate in real life. As a lifelong carpenter I can tell you that driving nails in dense wood takes a certain feel for the wood. Hitting the nail too hard will only result in bending the nail, and if these two Swedish carpenters took a minute to drive a nail and had to hit that nail 500 times they would be more like woodpeckers than carpenters. rich
padrefigure	posted 03-20-2014 04:25 PM ET (US) Taking Jim's analogy at face value and assuming that all else is equal (same number of cylinders, same displacement, same rpm, etc.)boh engines are doing the same amount of work, but one is doing it with half as many power strokes as the other. Mathematically, this means that a four stroke engine must produce slightly more than twice as much power on each power stroke as the two stroke engine. Slightly more because the exhaust stroke in a four stroke engine consumes power that the power stroke must provide to arrive at an equal output. But of course all things are not equal which, I think, is the point of discussion.
jimh	posted 03-21-2014 01:12 AM ET (US) ASIDE: Sven and Ole are putting some siding on the ice fishing shanty. They're working on opposite sides of the little shed. Sven: Hey, Ole, ha'f of dese nails I pick up havda head on the wrong end. Ole: Jeez, Sven, dose are da ones for my side.
saumon	posted 03-21-2014 07:44 AM ET (US) Too bad those scandinavians brothers doesn't have access to Thor's hammer (aka Verado) to drive their nails...
Peter	posted 03-21-2014 09:47 AM ET (US) Thor's Verado hammer would be too heavy for the carpenters to lift. ;)
contender	posted 03-21-2014 03:03 PM ET (US) They did not use nails they drove in wooden pegs
jimh	posted 03-21-2014 03:58 PM ET (US) Both Ole and Sven have to lift their hammers away from the nail before swinging them toward the nail on the power-stroke. If you consider that they are driving nails horizontally instead of vertically, then gravity does not affect the sideways motion (at least not very much for this simple analogy). But we do see that Sven has to move the hammer twice as much as Ole for each power stroke. Sven must be using more energy in moving the hammer than Ole. Ole's power-stroke on every swing toward the nail seems to be inherently less wasteful of energy than Sven's. Is that a fair assessment?
knothead	posted 03-21-2014 04:42 PM ET (US) Who has the framing hammer vs who has the finish hammer? Who wins in the real world when the hammer is down? regards---knothead
jimh	posted 03-21-2014 08:25 PM ET (US) So far the hammer has been kept constant. There may be some element of the analogy in which the weight of the hammer could be linked to something analogous in the engine.