|
|
| Found this thru a post on WC.
Two-stroke Conventional Wisdom
BY RALPH LAMBRECHT
Everyone in the marine industry and most of its environmental critics are aware that there are now at least four manufacturers of two-cycle outboards with DFI, direct fuel injection: systems that put the fuel directly into the combustion chamber after the intake and exhaust ports close. Such systems eliminate loss of some of the incoming fuel charge out the exhaust ports along with the scavenged products of combustion that occurs with carbureted or EFI manifold injection systems. Nevertheless, conventional wisdom tells most of the critics of the two-cycle engine that it will never be as clean as a four-cycle engine. For this reason they would eventually advocate banning the two-cycle engine from the waterways on environmental protection grounds.
There are even more manufacturers producing four-cycle outboards, including the same manufacturers that make the DFI two-cycle engines. They must seemingly follow down both roads for self preservation, as part of the outboard market is definitely leaning in the four-cycle direction, driven that way by hype, environmental concerns, and certain perceived advantages. We have already considered the ramifications of the increased engine weight for the four-cycles, the potential effect on boat trim, and the possible inability to float the boat level when swamped, as required by federal regulations for outboard boats less than 20-feet long. Then there is also the increased cost and complexity involved with four-cycle power, to be offset by savings realized in fuel consumed and elimination of smoke and oil slicks.
This may be the price of progress, they say. But, is it possible to "have your cake and eat it, too?" Some recent tests run comparing 2002 model two-cycle DFI outboards with four-cycle outboards of equal power rating, mounted on the same boat, would seem to indicate such things are really possible. Comparison tests of two brands of four-cycle 225-hp outboards were made with a current state-of the-art DFI two-cycle 225. On identical 20'7" boats one four-cycle brand produced a best mileage of 4.7 mpg at 27.7 mph while the two-cycle gave a best 4.5 mpg at 28.6 mph. Very close. But, the two-cycle had a top speed of 59.8 mph against 52.4 mph for the four-cycle. At the same 52-mph speed the two-cycle gave better mileage to the tune of 3.2 mpg to 2.7 mpg for the four-cycle. The two-cycle produced better fuel mileage at every speed from 34 mph up and was also better at trolling speeds of 4-7 mph.
When tested against the other 225-hp, four-cycle brand on identical 24' boats, the DFI two-cycle again prevailed overall, delivering a matching best 3.15 mpg at 32 mph. This outran the four-cycle 49.3 to 45.7 mph, getting better mileage (2.58 mpg) at its top speed than the four-cycle (2.44 mpg) at its top speed. It also produced far better mileage in the trolling speed range from 3.5-8 mph.
A third set of tests compared a 135-hp, two-cycle DFI outboard against a 130-hp, four-cycle outboard on identical 20' boats. The two-cycle delivered 4.25 mpg at 20.8 mph against a best 3.97 mpg at 20.4 mph for the four-cycle. Best economy for the two-cycle was achieved at 27.9 mph: 4.45 mpg. It also bested the four-cycle in the 3-8 mph trolling speed range and beat it in top speed 43 mph/3.54 mpg to 37 mph/2.97 mpg.
"Bah, humbug!" you might say. But there are sound engineering internal combustion engine principles for this surprising result. It is true that the typical four-cycle engine may have an inherent advantage in fuel consumed per horsepower. But not when the engine must be designed to produce very high horsepower per cubic inch of displacement at high engine speeds, as it must to achieve even the already heavier weight seen when compared to its two-cycle competitor.
In order to achieve this high-power output, while firing only every other revolution of the crankshaft, the camshaft valve timing must develop considerable overlap between intake and exhaust valve openings and closings, which means it begins to suffer some of the same raw fuel loss out the exhaust problems as the carbureted or manifold injected two-cycle engine. It only has manifold injection, so the fuel and air must mix in the manifold and enter together past the intake valve into the combustion chamber while the exhaust valve is still partly open. The result is Some loss in fuel economy.
Since the four-cycle engine has the same radical valve timing at low engine speeds, it suffers even more when compared to the two-cycle DFI engine at trolling speeds. The only way to fix this problem in the four-cycle engine is to go to direct fuel injection into the combustion chamber after the valves close, like the DFI two-cycle, or have a system providing variable valve timing with engine speed, conservative timing at lower speed and radical timing at higher speed. Such systems are now being developed for future automobile engines. Such things would add complexity, cost and weight, to an already more expensive and heavier product.
Then there is the factor of acceleration from idle to planing speed. On the 241 boat the 225-hp, two-cycle DFI went from zero to 150 feet in 7.06 seconds while the four-cycle took 7.76 seconds. On the 20' boat the 135-hp, two-cycle DFI went zero to 150 feet in 6.2 seconds while the four-cycle took 8.7 seconds for the same distance. Acceleration from zero to 30 mph on the 20'7" boat for the 225-hp two-cycle DFI took 5.77 seconds compared to 10.7 seconds for the 225-hp four cycle. This demonstrates the better low-end torque and fast-rising power curve of the two-cycle, firing every revolution of the crankshaft. The four-cycles are quieter at low engine speeds, but this advantage goes away at the higher engine speeds.
So, the conclusions are that the state of the art two-cycle DFI outboard can match or beat the four-cycle in fuel economy, top speed, and acceleration. What about exhaust emissions, which brought on the whole move to four-cycle outboards in the first place? These two-cycle engines can match or beat the four-cycles there, as well. It matches pretty much with the fuel economy story. The more fuel the engine consumes at a given boat speed, the more exhaust emissions that come out the other end. With precise microprocessor control and direct injection of the fuel into the combustion chamber after the ports close, the two-cycle DFI can better the most stringent exhaust emission requirements now proposed out to 2007. The four-cycle can do no better.
After more than five years of testing and field experience the 2002 two-cycle DFI outboards have been developed to have quality durability, economy and environmental friendliness to match or beat the four-cycles, and at lower weight and cost. Both can exist and be successful in the marine market but no one should sell the two-cycle engine short on its ability to survive and prosper long into the future. It just has too many good things going for it. You might even see it on some future stern drives.
Ralph Lambrecht is an engineer with more than 50 years of experience in the marine industry and marine safety standards development.
Lambrecht, Ralph. 2002. ?Two-stroke conventional wisdom.? Boat & Motor Dealer. April. 34-37
| |
| | |
| I found this artcle after I made my above post, but it looks like I was pretty close anyway. I worked for a dealer for a couple years just as Yamaha brought out the HPDI and got to run one for the 2 years. I also tried to pay as close attention as I could to the general market, hence my comments in the above post. In a big hp motor, I'd go with the Yamaha HPDI for overall usage and the EFI for max performance. | |
| | |
| OK, I’ll bite, or strike, as we say in the musky biz…..
This is the 2nd time that this article has been brought to my attention in a month. The first time, my wife found it prominently displayed in the ladies room @ a marina!!
I’m not an engineer, not even a mechanic, but as an interested observer of the 2 stroke vs 4 stroke debate, the first question that jumps to mind is “Who signs Mr. Lambrecht’s paycheck?”
I have no data to present, though I notice that he failed to mention who supplied his. Here are a few other thoughts that occurred as I read.
A 4 stroke’s camshaft allows asymmetrical tuning of intake & exhaust timing for tuning flexibility not possible in a 2 stroke motor, regardless of induction method. The 2 stroke’s timing is fixed, by ports cast into the cylinder walls, & symmetrical w/regard to TDC.
Variable valve timing has been used in production cars (Honda) since the late ‘90’s & is now finding its way into marine application.
ALL lubrication used in a 2 stroke is discharged into the water. Though I confess a certain callousness, as I negotiate some of the large impoundments surrounded by a million other boats, it sometimes pricks @ my conscience while on my favorite 200 acre potholes.
In my own personal experience, (Sorry, no studies or data available) 2 stroke piston & ring life are comparatively shorter. I offer these possibilities: First, the accumulation of carbon in the ring grooves is accelerated by the combustion of lubricating oil. Second, the rings are hammered, each time they bulge out into the port windows & then are slammed back again onto the cylinder wall.
Finally, as my friendly mechanic says, “You can fix almost anything if you want to spend the money.” How much money are we willing to spend to upgrade old technology to increasingly stringent emissions standards? If memory serves, when the 2006 standards were announced - ’99 I think - none of the 2 strokes could pass but every current Honda already would have - no microprocessors needed! Following the scramble to bring 2 stroke DFI to market, one manufacturer was seriously depleted by a grievous shakedown period after introduction.
The hallmark of 2 strokes has always been simplicity & light weight. As 2 stroke tech has become more complex & 4 stroke tech becomes more lightweight, I believe that manufacturers & consumers will see that we’ve reached an economic point of diminishing returns. Witness the proliferation in the 4 stroke lines of most manufacturers & the corresponding elimination of many 2 stroke models.
I believe that in the next several years, barring a miraculous breakthrough in 2 stroke tech or a staggering reversal in environmental legislation, 2 stroke engines will be relegated to applications where lowest weight is the rock bottom criterion – weedeaters, chainsaws, motocrossers, etc., & high horsepower outboards for which there is not a corresponding 4 stroke model – currently, motors exceeding 225 HP.
For the record, I own (2) 2 stroke outboards, one old & one brand new, but they will be the last.
Steve Petree
West Chester, OH
| |
| | |
| Oh, I love this debate. 2-stroke vs. 4-stroke is second only to the Hughes River Bait topic on the bbds if you ask me.
I'm no mechanic, far from it. But I have made a few simple observations about outboards over the years:
1) 4-strokes are heavy
2) 4-strokes use considerably less gas
3) 2-stroke motors are loud, and smoke like a chimney fire
4) 4-stroke motors are expensive
As far as why this is the way it is, I have no idea. It will be interesting to see if 2-stroke technology catches up to 4-strokes, or if 4-strokes can match up with 2's.
| |
| | |
| I heard a rumor that best fits in this thread.
Have any of you heard that Jim Wilson is starting to make Hughes River Baits in BOTH 2-stroke and 4-stroke? I hope so - I have noticed following muskies that lose interest in my Hughes River 2-strokes because of the trailing oil slick. I am not sure if it is the smell or if they actually lose sight of the bait because of the oil!
But seriously - I do love this debate in general and specifically now as I contemplate the purchase of a new motor for my boat and look at all these factors.
I can't wait to here more opinions.
Kevin | |
| | |
| One of the reasons that I posted this, besides another veiw of an ongoing debate that maybe some guys haven't seen, is that it pretty much parralels my personal veiws. Much of what I think about the motors comes from running all the motors mentioned.
Of course everyone has run carborated motors and yes, they smoke a lot and are loud. The EFIs aren't a lot better, but are powerhouse motors. I ran a DFI motor for 2 seasons and it was substantially quieter, and had almost no exhaust smell with good fuel economy, compared to the carb and EFI motors.
I test ran several to many 4-strokes and they vibrate a little less,but not by much, they are a lot quieter at lower rpms and also at lower rpms they have better fuel economy. Not so different at WOT.
For a kicker motor, pontoon or small boat, the 4-strokes are kick-b-tt motors, but anything 75hp & up, I'll stay with the 2-stroke motors.
The bigger 4-strokes are just too heavy and you have to sacrifice either hole shot or top end. They just don't put out enough to have both.
In my opinion, for what it's worth.So, in your experience, what would you run if you had the choice?
| |
| | |
| Where is Tim Taylor when you need him...[:p]
Seriously,
Give me more power, then we will talk. LOL
Jim | |
| |
|
2-stroke V 4-stroke article