EMS/Jomar Engine Sync Unit
 

When you order the EMS/Jomar twin engine sync unit you get a small populated circuit board about 1 1/4" square, a box to put it in, two Hall effect sensors, four magnets, a hank of wire, and a little bit of wire solder.

You have to make 14 solder connections to the circuit board, and except for the sensor wires, you have to supply all the wire and connectors.

If you assemble it as indicated in the instructions, you can cut one leg of a long wye connector and hard wire the unit into the cut leg, then with the sensors hard wired to the board this is the least expensive way to set it up.

The airplane I got the unit for has plug in wings, so hard wiring was not an option. Went to Radio Shack and got two miniature stereo jacks and plugs, used these for the sensor connections. I glued two female servo connectors to the top of the box, the result is a fancy wye connector with places to plug the sensors in. See first picture.

The second picture is one magnet, and the sensor. I had intended to put a size reference in, but forgot. The magnet is 1/4" diameter and 1/4" long. The sensor, as you see, is much smaller.

How it works:

The unit gets the throttle signal from the RX through a wye connector, with the "Master" engine servo on the other leg of the wye. It has no effect on the master engine throttle servo. Its other inputs are the rpm of the master and slave engines, its output is the throttle signal to the slave engine throttle servo to keep the engines at the same speed.

More to follow.

Bill.

RE: EMS/Jomar Engine Sync Unit
 

I'm building a Palmer OV10 to be powered with a pair of 24.5cc conversion engines. I want to put the EMS/Jomar Sync unit on this airplane if it works. Keep us posted on how the system works out.

RE: EMS/Jomar Engine Sync Unit
 

EMS suggests gluing the rpm sensors to the front of the engine mounting rail. I rejected that without much thought, the three tiny wires would be in the open, easily damaged. If the unit, operating normally, loses the rpm signal from the master engine it becomes a feed-through device, giving both engines the same servo signal. If the signal loss is from the slave engine though, the slave servo is driven to full throttle. This is not a good idea.

I soldered the leads to the sensors, then bent the leads to a right angle and potted the assemblies in a short length of shrink tube. If you do this be sure the flat side of the sensor without writing is on the outside.

Making up the sensors this way I only have to drill a hole in the cowl ring behind the spinner to mount them. The wiring is all reinforced, small chance of damage. See pictures.

More to come.

Bill.

PS: For a size reference, the lines in the first picture are 1/4" apart. Small stuff. wr.

RE: EMS/Jomar Engine Sync Unit
 

Here are two pictures of the inside of the box, as I said there are 12 soldered connections to make. Not difficult, the instructions are clear, but be careful.

If you do it as I did, three of the connections have two wires in them. THis is to have the master throttle servo plug into a jack on the box, if you suspect a problem the entire assembly can be replaced with a normal wye harness for testing.

Finally, a picture of the connectors on the box itself. Normal "J" connectors for the servos, the round jacks below are to plug the rpm sensors in.

Operation and fine tuning next.

Bill.

RE: EMS/Jomar Engine Sync Unit
 

The system works by reading the rpm of both engines and comparing them. It then changes the servo signal to the salve to match the master. If it loses the master rpm signal it drops out and sends the receiver signal to the slave, giving you direct control of the slave rpm. This will cover failure of the master speed sensor, in which case you are flying as though the sync unit were not there. If it loses the slave signal with both engines running you have a problem. The slave will go to full throttle, giving you control of the master engine only. I've not had either problem, but it needs to be considered.

The instructions say to start the slave engine, tune it, then shut it down and start the master. When the master is set restart the slave. BUT. this makes the slave start at full throttle, then the sync unit will close the throttle on the slave to match its rpm to the master. I do not like starting at full throttle.

Several ways to get around this. Start the slave, and without shutting it down start the master. This is the easiest way. Or you can set the throttles to starting point, and turn the airplane's radio off. Start both engines and turn the radio back on. I've used both ways, they work, and are better than starting the slave at full throttle.

The unit works, and works well. I didn't like the response time on quick throttling, the slave always has a little delay following the rpm of the master. The instructions mention the possible need of more than two magnets for precise operation on slow turning engines, so I went to four magnets on each engine. Now the delay is so short as to be no problem, so long as the engines will spool up as quickly as you give the higher throttle signal. With two magnets the airplane would yaw a little, took about 3/4 second to match speed. There is still a delay with four, but almost imperceptible.

Summary:

The sync unit at $90, the specially cut Tru-Turn spinners and adapter nuts for another $90, and roughly $20 for the additional wire and connectors added $200 to the cost of the plane.

Is it worth the cost? I might well have gotten the spinners, but the original Du-Bro plastic spinners were less than $20 the pair, much less cost than the Tru-Turn, but the metal spinners are a lot prettier, and actually weigh less than the Du-Bro. The weight savings was lost with the other added parts though.

I guess my opinion of the worth is best illustrated by my not bought a second one.

Nice, yes. Needed? No.

Bill.