Inlet port open to exhaust - is this normal on some engines ?

[edholly]

Have posted something under "Another ML Midge" in the work in progress forum about a possible design anomaly. However the questions I ask are a bit lost there so have started this thread in the hope that those answers might be considered.

The quote below from there is self - explanatory ... however to cover it briefly .... I found that the piston comes down below the inlet port hole by a few thou and maybe this is why the engine lacks some power.

In the plans it is designed for a 4 thou opening - just wondering if this is normal practice - my thoughts are it would not be ...

Presently making a new conrod .015" longer to see if there is any improvement in power.

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Well the Midge was finished some time ago and has now had about 2 hours running.

I must say I was a little disappointed with it, as although the fits are very good, it is a trifle cantankerous not wanting to start easily on anything smaller than 8 x 4 props and it is down a full 1000 revs on a DC Merlin on the same prop at 6000 versus 5000.

Now I know a sideport won't give the same power as an advanced timed rotary port - but I thought it should do better than this - and as I am 80% finished a Veron Cardinal just for this motor - I feel that it will be very marginal on power.

Another intriqing thing it has is that it wants to bite badly on tying to start on small props and I use a chicken stick as a result. It also has little suction and will actually spit out any prime from the venturi instead of sucking it in ! Clearly not good for performance when its trying to trying to throw the incoming fuel/air mixture back out the venturi.

So this morning I took the inlet venturi off and with a stong torch checked what the piston was doing at tdc - well maybe I found the cause of all these little problems. The piston was uncovering the inlet by about 10 thou as best I can judge. Obviously any uncovering of the inlet will allow residual exhaust pressure to escape down the venturi reducing inlet flow and thereby reducing power. It also has this very annoying habit of oscillating back and forth, rather than rotate when trying to start.

So I went back to the drawing to see what the specs were, and I think that maybe the problem is in the original design - unless it was meant to be this way.

I would welcome any comments pro or con ...

The way I see it is this - at BDC

Centreline Crankshaft to datum land on cylinder (all in inches) 0.906
Conrod length 0.787 plus piston above 0.197 = 0.984 less 1/2 crank rotation 0.200
therefor top of piston is 0.784 above CS centreline

The inlet hole is 0.157 below datum and 0.078 diameter so top of hole is 0.118 below datum or
0.788 above CS centreline

So therefore the piston uncovers the inlet by 0.004 according to the drawings as far as I can see - and if a small error occurs in maching to these tolerances then it can "fix" the problem or exacerbate it.

Over to the boffins now - do you think that this would rob the engine of some power?

Also ... if I made a new conrod say 10 thou longer - would this help ? I know it will throw out the timing but what I lose in timing might that be made up for by better inlet tract flow?

 

[Mechanicboy]

Yes, it is normal both exhaustport and inlet port is open, but first important of the pressure of combustion must be reduced before inlet port is opened by piston to prevent the crankcase is pressured by pressure of combustion. Also the exhaust port is higher than inlet port i the cylinder sleeve.

Read this: http://modelenginenews.org/techniques/frv_time.html#2

 

[edholly]

Firstly thanks Mechanicboy for your input ... The Midge is a sideport, so unlike a front or rear rotary valve engine any overlap of inlet and exhaust allows the exhaust to force some of the incoming charge to be pushed back against the incoming fuel/air mixture. This exercise shows that this is most critical during the starting phase - once the engine is running it would appear the "dam" of in rushing air counteracts this tendency.

A new conrod 15thou longer was made and installed, and the engine has now run with this new component. There is NO uncovering of the inlet port now, yet the piston comes nicely to the bottom of its port.

The difference in handling is terrific - the engine now starts very easily and in about a dozen starts not once did it go into that oscillating mode. And it has lost it's harshness, I dispensed with the chicken stick with the 7x4 prop.

It has not actually picked up much power, maybe 100 revs on an 8x6 Master Airscrew from 4300 to 4400, but it now is happy to start and run on a 7x4 APC prop and runs out to 6,000 on this. So still not as much power as I had hoped, so we will see in about a week how it goes with the Veron Cardinal.

Must say it has been an interesting exercise, and another bit of learning in the mysteries of model diesel engines !

 

[Mechanicboy]

Use degree disc to learn out how much is the timing in the intake port, inlet port and exhaust port. The ideal timing for the model diesel engine with sideport: 90-95 degree for intake port, 120-130 degree for inlet port and 150-155 degree. Be careful to adjust the height of ports until the engine will run fast and easy to start the engine. Too much degree of the timing period will make difficult to start the engine and loss of effect. The Side port engine is not a hi-reving engine, between 4500 and up to 7000/8000 depending on timing and long or short stroke engine. The long stroke engine is not a hi-reving engine and need low pitch and large propeller diameter to make good thrust. In case your airplane can not rise out of ground, replace the propeller to low pitch and large diameter. You can use the program ThrustHP to measure the effect and thrust of your engine when the propeller size is selected. Download ThrustHP here: http://www.hoppenbrouwer-home.nl/ikarus/software/thrusthpv20d.htm