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The MVVS 3094iW Gas Engine
By: Pé Reivers, March 2002
This review in fact covers two engines, and is spearheaded around the ability
to fly a decent plane, rather than give some dry test bench figures. The first
engine is a standard (first) production run engine, and a second is a production
run engine with graphite plated piston.
When the first engine came in, I was a bit disappointed with the packaging of the engine. The thin cardboard box was damaged by the spark plug, and the crank-shaft end. The engine however was fine. What the heck, after all, I fly the engine, not the wrap! All external engine surfaces were excellent in their finish, and the case was glass bead blasted. The ignition Hall sensor is fixed in it's position. The ignition fires just before TDC, and advances 30 degrees when the engine gains rpm. All this is processor controlled, and needs no user inputs. The result is, that the engine is a pussycat to hand-start, without any vices.
Upon inspection, the engine was quite tight and felt rough. I thought: "This
baby should get a lot of oil for the first runs."
I dismantled the piston and sleeve. The connecting rod has needle bearings in wrist pin, and lower end. To my astonishment, the hardened steel liner is chrome plated! All parts look very well designed and stress relieved, but all edges are razor sharp, so I ever so slightly slightly chamfered all edges, except the gas flow outlets.
This engine looks very much like the glow engine that we are all used to. Fitting in the plane is very straight-forward.
The head has a bowl shaped combustion chamber with a slanted squish band, glow engine style. The spark plug is a standard garden appliance affair with 14mm short reach thread and a 60 degrees conical seat. This is a very nice detail, because on the Moki I needed 10mm thread spark plugs of $18.00 each, and I needed to drive to the end of the world to get them.
The ignition looks very handsome, and is extremely well shielded. The casing is the size of a film roll comtainer, in fact, it fits snugly inside such a container. The lower plug boot chrome plating however was flaking off, and a small flake had damaged the plug seat. The leak caused by this slight damage would later vanish. (sigh of relief). I removed all loose flash. The plug wrench is dimensioned to centre the plug boot around the plug when tightening. It is important not to let the lower flange of the plug boot catch between the plug threads.
After a generous rinse in lamp oil and an oil bath, I assembled the engine
again to check the ports timing.
Exhaust: 150 degrees
Transfer: 112 degrees (19 degrees blow-down)
Boost: 108 degrees
Induction: from 45degrees ABDC to 45 degrees after TDC (180 degrees total)
Carb dia.: 7mm (.276") This is very small, and adds to the excellent running properties!
The timing of this engine is very conventional, and nothing fancy. It makes one wonder about the power claims of MVVS. More about that later.
The first starts were very uneventful. The engine was set rich on a 15:1 oil
mix, and needed leaning. First runs were made using an 18x6 prop for rotating
mass and light engine loads. When slightly wet, the engine would start on the
first flip. When too wet, it would need a couple more flips to clear the lungs
first. The final, still quite rich needle settings were 1.5 turns open for both
needles. From there, the tweaking begins. Both needles influence each other,
so each change needs a couple of full throttle runs to settle, and check the
mixture again. As long as the engine accepts throttle well, lean the idle needle
little by little. Then open up 1/16th of a turn on the last good setting. I
did the same with the high end. Cross checking every time is required. Once
set, very little tweaking, if any, is needed, but I needed to re-adjust at the
field during the first runs. In a plane, the engine behaves a bit different.
After the first two runs (two minutes each), I diluted the mix to 30:1, using Valvoline Syn-Power two-stroke oil. That is the mix I kept using troughout further tests.
I used a back plate for mounting, fastened to the rear of the engine casing using slightly longer M4 bolts, and rubber grommets for vibration isolation without the customary shaking at idle.
First flights were done with the standard muffler, in a 17 lbs high-wing plane
of 86" span (Multiplex Big-lift), using a 18x8 APC prop. The plane has
a decent lift-off after about 50 yards, and a 30 degrees climb out. Cuban eight
and Immelmann are easy, loops are quite large and round, when up, and at energy
levels. The carb needs some more tweaking, to a bit richer settings.
By now, the engine gets a silky feel when turned over.
Fuel consumption is about half an ounce (15cc) per minute.
After one gallon through, which are many flights, I disassemble the engine
again to check the inner workings.
The engine still is not quite run in, and the ring does not bear completely in the bore. All is well inside, and the liner has a high gloss, except for a few spots that need to run in some more.
I take the risk to loose warranty, and modify the scavenge ports to slant upwards towards the rear, add a window in the piston to line up with the boost port, and increase exhaust duration somewhat. A new exhaust pipe is made up, combined with a tuned pipe for a 2.1 (35cc) engine.The header is quite long. I also add a larger carb of 8mm.
The effect is startling! The plane now has near vertical performance, and lift off occurs in a couple of yards.
I fly this mode until another gallon is gone. With this engine that is many flights with spectators in awe! In the meantime, I changed to a 30 oz tank, so I need not refill during the day. Just flip-n-go! The gas engine flight gets the better of me, and my glow starter batteries are left unattended. During the cold winter days of the test flights, every one was working up a sweat to get their glow engines started, while I would just prime, flip the prop a few times, switch on the ignition and go!
Due to the modifications I made, it was not possible to get any standard prop rpm values any more, except my standard 18"x8" prop, which turned 7300 before the mods, and up to 7800 after. The 18x10" Menz prop turned 6700 rpm before mods, and was not measured later, but seemed to turn in excess of 7000. A 20x7 wooden prop ran wel, and came up to good working rpm, but broke on landing, not to be replaced because of expenses. Alas, no rpm figures were taken either.
Then came the time to switch to the new graphite coated piston type engine,
and test it. I must restrain myself to not try to do any mod's, until I have
accomplished the complete run-in, and then some.
With the standard 15:1 mix for five minutes run-in, I changed again to 30:1, and dumped her into the flight test program. The next reasonable day saw a lot of loops, wing-overs, cuban eights etc. with fly-by's and one wheel touch-and-go for seasoning. The prop was a Menz-S 18x8, and the engine turned a happy 7350 on the ground, while unwinding a lot when aloft.
The running in process did not add one bit of rpm. The engine seemed more stable than the previous one, and did not loose any power at all when heated up. In my humble previous opinion, all two strokes loose some power when the crankcase heats up. This engine just looses 50 rpm, thats it! With one litre (one quart) through the engine, I started piggy back hauls with a 10 foot glider. All went like on rails, with about a 20 degree climb out. The total weight of the combo is in excess of 22 lbs. Take-off to the left, then a turn to the right, past the pits, a turn to the left, and when in front of the pits again, the glider combo is small, and up to release height. No thermal searches were done yet to spare the engine long flights. After all, it is not run in yet.
Prop rpm and thrust test data after these excersizes:
|prop size as sold||size as measured(tangent)||designation||rpm||thrust in grams|
|16x12||16x12||wood, wide blade||7050|
|18x6||18x7||Airflow wood||7500||6300, noisy|
|18x8||18x9||APC, narrow type||7800||5800|
|18x10||18x8||Menz, thinned and cambered||7250||6500|
|20x10||not measured||Menz-S wood||4500||off tuned pipe|
The APC prop, while upped in RPM, had a disappointing thrust, with the engine nearer it's peak power. Thrust figures however confirmed behaviour in the air, where the Menz-S 18x8 is just great!