Twin Engines: DFI Leads to Easier Synchronization

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Author Topic:   Twin Engines: DFI Leads to Easier Synchronization

jimh posted 11-05-2006 11:54 PM ET (US)
This weekend I had an opportunity to spend about 30-minutes at the helm of a twin engine boat with DFI outboards. I have quite a bit of experience piloting twin engine outboard boats, as I had one myself for several years. I found that the DFI nature of the engines allows for easier synchronizing of engine speed than I had experienced previously.
Previously, running a pair of 70-HP Yamaha outboards with conventional two-stroke carburetor design, maintaining engine speed synchronization was a bit of a chore. If one engine was lagging or leading the other in speed, an adjustment to its throttle would tend to result in that engine changing speed, but the speed of the other engine would also be affected. For example, if the PORT engine were running at 3,900-RPM and the STARBOARD engine running at 4,100-RPM, advancing the throttle on the PORT engine to increase its speed would often result in both engines gaining RPM. The effect of the lagging engine increasing in speed would tend to permit the other engine to run slightly faster, too. The pair might speed up to 4,200-RPM and 4,100-RPM.
The next step would be to reduce the faster engine slightly. Eventually, the two throttles could be adjusted to synchronize the engines. The speed of a carburetor engine tends to react more to load changes. As the second engine came up in speed and took more of the load, the faster engine ran even faster.
In contrast, a DFI engine seems to run at the speed set by the throttle, and it does not seem to hunt around much as the load changes slightly. When synchronizing the two DFI outboards, it was typically possible to reach synchronization with just one adjustment of one engine's throttle. Changing the engine speed on one engine by a few hundred RPM seemed to have little or no effect on the other engine.
After about 30-minutes of operation, I got my ear tuned to the sound of these DFI motors. At that point I could ease into really fine synchronization by listing for the heterodyne or beat frequency resulting from the difference in engine speeds between the two. This was somewhat harder to hear than on my old Yamaha outboards because the sound level of these DFI engine was lower. Their pitch was lower, too, because they were running at lower speeds.
In this case the DFI motors were running at 3,000-RPM. This is a crankcase rotation speed of 3000/60=50 Hz. There are two power strokes per rotation, so the fundamental pitch of the motor is 100-Hz. In contrast I was usually running my Yamaha carburetor motors at 4,000-RPM, which is a crankcase rotation of 66.66-Hz, and a fundamental pitch of 133-Hz. Also, the Yamaha motors had a raspier tone, and probably contained more harmonics. The ear is most sensitive to pitch variations at frequencies somewhat higher in pitch (around 1,000 to 3,000-Hz) so the more harmonics in the sound signature, the easier to hear the heterodyne when the engines are out of synchronization.
The reason a DFI engine runs so steadily, I believe, is related to the very comprehensive electronic controls. The "throttle" at the helm is really more of an electronic lever which signals the engine at what speed to operate. A throttle position sensor converts the mechanical position of the throttle into electronic instructions about how much fuel to supply.
By the way, you can really see this effect when you yank back on the throttle of a DFI engine. Unlike a carburetor engine, which will tend to slowly float down in speed when the throttle is pulled back, a DFI engine will drop speed like a falling rock, very quickly going from high engine speed to idle. The result can be very rapid boat deceleration. If you are not anticipating it, the sudden change in engine speed can throw you forward with some force.
The ease at which DFI engines can be synchronized in twin engine applications was a pleasant surprise. I had seen some of this behavior in EFI two-stroke outboards, too. They also tend to be easier to synchronize than twin carburetor engines, however, I don't think they have quite the precision of the DFI in maintaining engine speed.
Of course, the ultimate in twin engine control is to move the throttle into an electronic realm, and to use software to accomplish synchronization. I had the pleasure of piloting a friend's 55-foot twin engine diesel motor yacht a few months ago. It was equipped with electronic throttle controls, and the 900-HP engines could be run with a single throttle control, to which they would respond in unison and maintain their speed in very close synchronization.
There are some modern outboards available with fly-by-wire throttle controls and, in some cases, their electronic controls offer advanced synchronization features for twin engines. Those features do not come without added expense. In one case, the upgrade to digital throttle is $1,000 per engine and the controls needed are also expensive.
For simpler installations on smaller boats, the more rigid throttle behavior of DFI engines offers a nice bonus in twin engine installations.
(The boat I drove this weekend was GAMBLER and the engines were 90-HP E-TEC two-stroke motors.)

crabby posted 11-06-2006 10:08 PM ET (US)
Nice article, Jim.
One nitpicking point: wouldn't a 2-stroke 3 cylinder motor have 3 power strokes per revolution?
--pm
jimh posted 11-06-2006 10:15 PM ET (US)
I believe the way these 3-cylinder outboard operate is that two cylinders move in unison and one moves 180-degrees out. So there are two cylinders firing at once, then a single cylinder firing, each rotation. Thus there are two power strokes in each rotation. The firing is a Ba-Booom-Ba-Booom-Ba-Boom sort of sequence.
If the 3-cylinder engine had a 120-degree spacing between pistons in the cylinders, then there would be three separate power strokes per revolution of the crankshaft. This would be a boom-boom-boom sequence, and give a different pitch to the engine's note.
Peter posted 11-07-2006 07:37 AM ET (US)
On the in-line 3-cylinder 90 E-TEC, one of the 3 pistons reaches top dead center (TDC) every 120 degrees of rotation of the flywheel. This is easy to see by looking at a drawing of the crankshaft which is available on-line. Thus, there are three power strokes per revolution. The firing order appears to be from top to bottom.
If two cylinders moved in unison and one 180 degrees out of phase with the other two, it would seem that the motor would be out of balance. What you describe regarding unison of movement is typically done in an in-line 4-cylinder motor where pistons move in pairs to keep the motor balanced.
jimh posted 11-07-2006 10:21 PM ET (US)
If Peter is correct, then:
At 3,000-RPM the crankcase is turing 3,000/60 = 50-HZ
Three power strokes per revolution: 3 X 50 = 150-Hz fundamental pitch of the engine.
(I am going to check with a third party for a tie-breaking vote.)
jimh posted 11-12-2006 12:38 PM ET (US)
I checked with an OMC/Bombardier expert--Dave Zammit at Lockeman's Boat and Hardware. He confirmed the three cylinder E-TEC engines fire at 120° crankcase angles, so there are three power strokes per crankcase revolution. Thus the fundamental pitch of such a motor is:
(RPM/60) X 3
and at 3,000-RPM that would be
(3000/60) X 3 = 150-Hz fundamental pitch of engine
I don't recall where I got the incorrect notion that the three-cylinder E-TEC ran as I tried to describe above.
One of the reasons that four-stroke motors seem to run more quietly than two-stroke motors is their fundamental pitch is always half that of a two-stroke. So a four-stroke motor with a similar arrangement as above would have a fundamental pitch of only 75-Hz. The ear is less sensitive to low frequencies like that, so the sound of a four-stroke tends to be perceived as slightly lower in volume. This may make synchronization of their speed by ear even harder to judge.


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Author	Topic: Twin Engines: DFI Leads to Easier Synchronization
jimh	posted 11-05-2006 11:54 PM ET (US) This weekend I had an opportunity to spend about 30-minutes at the helm of a twin engine boat with DFI outboards. I have quite a bit of experience piloting twin engine outboard boats, as I had one myself for several years. I found that the DFI nature of the engines allows for easier synchronizing of engine speed than I had experienced previously. Previously, running a pair of 70-HP Yamaha outboards with conventional two-stroke carburetor design, maintaining engine speed synchronization was a bit of a chore. If one engine was lagging or leading the other in speed, an adjustment to its throttle would tend to result in that engine changing speed, but the speed of the other engine would also be affected. For example, if the PORT engine were running at 3,900-RPM and the STARBOARD engine running at 4,100-RPM, advancing the throttle on the PORT engine to increase its speed would often result in both engines gaining RPM. The effect of the lagging engine increasing in speed would tend to permit the other engine to run slightly faster, too. The pair might speed up to 4,200-RPM and 4,100-RPM. The next step would be to reduce the faster engine slightly. Eventually, the two throttles could be adjusted to synchronize the engines. The speed of a carburetor engine tends to react more to load changes. As the second engine came up in speed and took more of the load, the faster engine ran even faster. In contrast, a DFI engine seems to run at the speed set by the throttle, and it does not seem to hunt around much as the load changes slightly. When synchronizing the two DFI outboards, it was typically possible to reach synchronization with just one adjustment of one engine's throttle. Changing the engine speed on one engine by a few hundred RPM seemed to have little or no effect on the other engine. After about 30-minutes of operation, I got my ear tuned to the sound of these DFI motors. At that point I could ease into really fine synchronization by listing for the heterodyne or beat frequency resulting from the difference in engine speeds between the two. This was somewhat harder to hear than on my old Yamaha outboards because the sound level of these DFI engine was lower. Their pitch was lower, too, because they were running at lower speeds. In this case the DFI motors were running at 3,000-RPM. This is a crankcase rotation speed of 3000/60=50 Hz. There are two power strokes per rotation, so the fundamental pitch of the motor is 100-Hz. In contrast I was usually running my Yamaha carburetor motors at 4,000-RPM, which is a crankcase rotation of 66.66-Hz, and a fundamental pitch of 133-Hz. Also, the Yamaha motors had a raspier tone, and probably contained more harmonics. The ear is most sensitive to pitch variations at frequencies somewhat higher in pitch (around 1,000 to 3,000-Hz) so the more harmonics in the sound signature, the easier to hear the heterodyne when the engines are out of synchronization. The reason a DFI engine runs so steadily, I believe, is related to the very comprehensive electronic controls. The "throttle" at the helm is really more of an electronic lever which signals the engine at what speed to operate. A throttle position sensor converts the mechanical position of the throttle into electronic instructions about how much fuel to supply. By the way, you can really see this effect when you yank back on the throttle of a DFI engine. Unlike a carburetor engine, which will tend to slowly float down in speed when the throttle is pulled back, a DFI engine will drop speed like a falling rock, very quickly going from high engine speed to idle. The result can be very rapid boat deceleration. If you are not anticipating it, the sudden change in engine speed can throw you forward with some force. The ease at which DFI engines can be synchronized in twin engine applications was a pleasant surprise. I had seen some of this behavior in EFI two-stroke outboards, too. They also tend to be easier to synchronize than twin carburetor engines, however, I don't think they have quite the precision of the DFI in maintaining engine speed. Of course, the ultimate in twin engine control is to move the throttle into an electronic realm, and to use software to accomplish synchronization. I had the pleasure of piloting a friend's 55-foot twin engine diesel motor yacht a few months ago. It was equipped with electronic throttle controls, and the 900-HP engines could be run with a single throttle control, to which they would respond in unison and maintain their speed in very close synchronization. There are some modern outboards available with fly-by-wire throttle controls and, in some cases, their electronic controls offer advanced synchronization features for twin engines. Those features do not come without added expense. In one case, the upgrade to digital throttle is $1,000 per engine and the controls needed are also expensive. For simpler installations on smaller boats, the more rigid throttle behavior of DFI engines offers a nice bonus in twin engine installations. (The boat I drove this weekend was GAMBLER and the engines were 90-HP E-TEC two-stroke motors.)
crabby	posted 11-06-2006 10:08 PM ET (US) Nice article, Jim. One nitpicking point: wouldn't a 2-stroke 3 cylinder motor have 3 power strokes per revolution? --pm
jimh	posted 11-06-2006 10:15 PM ET (US) I believe the way these 3-cylinder outboard operate is that two cylinders move in unison and one moves 180-degrees out. So there are two cylinders firing at once, then a single cylinder firing, each rotation. Thus there are two power strokes in each rotation. The firing is a Ba-Booom-Ba-Booom-Ba-Boom sort of sequence. If the 3-cylinder engine had a 120-degree spacing between pistons in the cylinders, then there would be three separate power strokes per revolution of the crankshaft. This would be a boom-boom-boom sequence, and give a different pitch to the engine's note.
Peter	posted 11-07-2006 07:37 AM ET (US) On the in-line 3-cylinder 90 E-TEC, one of the 3 pistons reaches top dead center (TDC) every 120 degrees of rotation of the flywheel. This is easy to see by looking at a drawing of the crankshaft which is available on-line. Thus, there are three power strokes per revolution. The firing order appears to be from top to bottom. If two cylinders moved in unison and one 180 degrees out of phase with the other two, it would seem that the motor would be out of balance. What you describe regarding unison of movement is typically done in an in-line 4-cylinder motor where pistons move in pairs to keep the motor balanced.
jimh	posted 11-07-2006 10:21 PM ET (US) If Peter is correct, then: At 3,000-RPM the crankcase is turing 3,000/60 = 50-HZ Three power strokes per revolution: 3 X 50 = 150-Hz fundamental pitch of the engine. (I am going to check with a third party for a tie-breaking vote.)
jimh	posted 11-12-2006 12:38 PM ET (US) I checked with an OMC/Bombardier expert--Dave Zammit at Lockeman's Boat and Hardware. He confirmed the three cylinder E-TEC engines fire at 120° crankcase angles, so there are three power strokes per crankcase revolution. Thus the fundamental pitch of such a motor is: (RPM/60) X 3 and at 3,000-RPM that would be (3000/60) X 3 = 150-Hz fundamental pitch of engine I don't recall where I got the incorrect notion that the three-cylinder E-TEC ran as I tried to describe above. One of the reasons that four-stroke motors seem to run more quietly than two-stroke motors is their fundamental pitch is always half that of a two-stroke. So a four-stroke motor with a similar arrangement as above would have a fundamental pitch of only 75-Hz. The ear is less sensitive to low frequencies like that, so the sound of a four-stroke tends to be perceived as slightly lower in volume. This may make synchronization of their speed by ear even harder to judge.