View Full Version : piston engine questions

2nd Mar 2009, 02:47
Hi all,

some questions;

what are the purpose of hydraulic valve tappets for piston engines?

what would happen if there is insufficient tappet clearance in an engines inlet valve?

why is a tappet clearance provided in an a poppet valve engine?

Thank you.

Old Fella
2nd Mar 2009, 05:24
Downwind , assuming that this is a genuine question and not just a windup,

a. hydraulic lifters (tappets), unlike solid lifters, do not require adjusting once set up and generally will run much quieter than solid lifters. They are however suceptible to problems caused by dirty oil.

b. Insufficient clearance on an inlet valve will allow leakage during compression stroke and combustion stroke.

c. Poppet valves actuated by solid lifters and pushrods require clearance to allow for metal expansion and so prevent valve burning and/or gas leakage.

Mach E Avelli
2nd Mar 2009, 07:04
Old Fella, another question with hydraulic lifters. If engine oil pressure gets low due to very low quantity, will it affect the valve operation? The reason I ask is recently I lost most of the oil overboard in a Jabiru engine, but it was an old one with solid lifters. It kept running even though the oil pressure was down to below 30 psi and has suffered no apparent damage. I wondered what my fate would have been with one of the later engines with hydraulic lifters?
Pardon my ignorance, but I grew up on old inverted inline and radial engines etc, not these new fangled things. As long as thre was some oil, they ran fine.

2nd Mar 2009, 07:27
If there's no oil at all, the lifters will slowly bleed down until they won't lift - and then silence .......:eek:

But even a small amount of oil pressure should keep them up.

BTW - what caused the loss of oil in the Jab?

Mach E Avelli
2nd Mar 2009, 07:45
The engine is installed in a Sonex taildragger. I fitted some large diameter wheels to go bush, but did not recalibrate the dipstick to allow for the increased nose-up angle. A combination of false dips, complacency on my part - because until then it had used bugger-all oil - and siphoning out through the oil filler into the collector bottle then overboard, nearly got me on a very long day's flying where I did 9 hours. Early in the day I put half a litre in, not realising that the other half went overboard as I taxied out. A whole lot more probably dumped during the next 3 takeoffs, so several hours later it got rather interesting. What I do know is if the oil temperature starts to decrease on a hot day, you are about to run out of oil, so that was my first clue. I promise to dip oil before EVERY flight from now on.

2nd Mar 2009, 10:22
Quick and dirty answers.

Tappet clearance MUST be provided or the valves will almost certainly NOT close. This means gas leakage past the valve seats and very quickly burnt seats, yes even in cast iron burnt seats, and naturally low compression and hence low power output.

Hydraulic lifters are a means of providing this clearance but doing it quietly. Check the noise from the engine of one of the local hoons who has replaced the hydro lifters with solid lifters.

Oil pressure has next to nothing to do with the operation of hydro lifters. The lifters are ported to an oil gallery but the lifter port closes immediately the cam lobe commences lifting the cam follower. As long as there is a hint of oil pressure the hydro lifters work - the rest of the engine will likely go bang though!!

2nd Mar 2009, 10:39
What I do know is if the oil temperature starts to decrease on a hot day, you are about to run out of oil, so that was my first clue.

First time I have heard that one, Under what mechanism does that work?

Old Fella
2nd Mar 2009, 10:47
As Mustafagander said, low oil pressure will not inhibit hydraulic lifter operation and it is likely that the engine will overheat, seize or run a bearing before the hydraulic lifters stop working. They sometimes drain of oil, evidenced by valve gear clartter on start, but this usually goes away when the oil is returned to the lifter via the oil gallery mentioned by mustafagander.

2nd Mar 2009, 15:27
Years and years back, had the tach drive cable back out of the case of a 985. Tach went crazy but the engine was running fine. Then noticed the oil pressure and temp were both going down. Too much weird stuff going on so feathered the engine. Turns out it dumped the oil so fast it wasn't going bsck into the tank and so the temperature reading went down. Only engine failure I've seen where the temp wasn't rising.

Mach E Avelli
3rd Mar 2009, 01:56
Checkboard, any engine that runs oil through a cooler will indicate a LOWER temperature with REDUCING oil quantity. When it runs almost completely out and the oil is no longer circulating through the cooler, sure it will get hot, but by then the engine is ready to sieze. Think about it.

3rd Mar 2009, 03:21
Back in the day, I drove a Chevrolet Corvair with horizontally opposed six - like a Continental, Franklin or Lycoming. Lifter leakdown overnight was not uncommon. If a lifter was still noisy by the time you got to 35 mph, you were a quart low on oil.

Collapsed lifters (lack of oil) will clatter, and give reduced power at high rpm, but not too serious if they are set correctly with about .030" gap when collapsed. The beauty of them when receiving oil is that they fill up, and follow the camshaft lobes perfectly, rather than solid lifters being hit by the cam lobes, the amount and noise of impact depending on the gap.


Old Fella
3rd Mar 2009, 04:01
I think a few of the statements made have been a bit ambiguous.

Normally, if fitted, three oil system instruments can tell you lots about the situation. Low Oil Pressure accompanied by rising Oil Temperature will almost certainly be accompanied by falling Oil Quantity. Reduced oil quantity means the available oil gets circulated more often which will cause the higher temperature (less oil to carry away the heat and less time to cool) High oil pressure accompanied by low temperature means, usually, oil cooler flap is too far open. It always wise to look at all info available to trouble shoot. Checkerboard's comment about low oil temp means you are about to run out of oil may not necessarily be true, it will all depend on where the temperature sensor is in the system. Some people run their cars out of water and see no rise in water temp. that is because the sensor relies on the temperature of the water in which it is immersed to transfer the heat to the sensor probe. Oil system temp probes work the same in some installations.

Mach E Avelli
3rd Mar 2009, 06:46
Old fella, surely it depends on the total capacity of the oil system and efficiency of the cooler? The 2200 Jabiru engine set-up in my homebuilt aircraft certainly runs cooler on 2 litres than it does at the full 2.4, by a good 10 degrees. Therefore I run it at 2 litres in hot weather, a bit more in winter.
Some more advanced types - e.g. turboprops and jets - usually run the oil through a cooler that itself is cooled by fuel (it doubles to heat the fuel and prevent icing at high altitudes - very neat solution). In this setup, a drop in oil temperature will either precede or confirm a low quantity warning, often well before any pressure drop. I have personal experience of this on a Bae146 which happened to be the one in the fleet with no oil quantity gauges. Coming out of Port Hedland on a very hot evening, during the climb the oil temp started to steadily decrease. At the first flick of the associated pressure gauge, we pulled it back to idle, gave it a minute or two to stabilise and shut it down. There was some criticism from uneducated quarters in the Company at the time that this was an over-reaction because no limit had actually been reached, but they shut up soon enough. A bearing seal had let go and dumped most of the oil. Saved a huge repair bill, we did...

Old Fella
3rd Mar 2009, 09:02
Mach E Avelli. I have spent many thousands of hours monitoring engine and system performance as a Flight Engineer and I can honestly say that I have never known an oil system with the appropriate level of oil in it to run hotter than it would with less oil in the reservoir. Why your Jabiru apparently does is odd, in my view, to say the least. I have operated JT3D-3B's and RB211's as well as Allison T56 engines. Fuel cooled oil coolers which double as fuel heaters have rarely caused problems as distinct from air cooled types such as fitted to the Allison. Temperature control is maintained by varying the oil cooler flap position. My experience of piston engines has been varied and includes R985, R1830 and Bristol Sleeve valve Hercules as well as GA light aircraft engines of various configurations. I would genuinely be interested to know why your Jabiru reacts in the way it does if others have an explanation. As for the oil temperature decreasing with decreasing quantity on the 146, I can only assume that the oil temperature sensor must be similar to many motor vehicle engines and as the quantity fell below the level of the sensor there was no means to transfer the heat from the oil to the sensor. You did not say whether the oil pressure increased at all when you initially noticed the decrease in oil temperature. It would have been a normal reaction for the pressure to increase with falling oil temp. BTW, you obviously saved a lot of money for your company by conducting the precautionary shutdown. Sometimes it does pay to act before any limit is exceeded. Happy Landings.

Mach E Avelli
3rd Mar 2009, 09:12
In my Jabiru situation, the temp went south first, then the pressure shortly after. I did not notice a rise at any stage, but then wasn't looking for a rise. With the 146 situation, I recall the pressure was constant as the temp dropped, then the gauge just gave a flick or two and I decided that was enough. With that one I had the advantage of prior experience in a similar 146 which happened to have quantity gauges. What happened with this one was a chip detector had not been properly secured and fell out. As the oil quantity gauge decreased, so did the temperature - long before any noticeable change in presssure. We had our two indications, so shut it down before any damage occurred.
Obviously I defer to your FE background, but those have been my experiences with oil loss. Another which was not oil loss was a RR Dart which had just been changed (F27) and they forgot to remove a blank from the oil cooler. That one was a no-brainer - the temp went ballistic shortly after takeoff and I did not even need to cross refer to pressure for the second confirmation - after all the engine was straight out of the shop.
We shut down and came home. Again, no harm done to the engine; just some embarrassment in the hangar.
What a pity they don't pay us bonuses when we save them money!

3rd Mar 2009, 14:39
Most engines have plenty of oil pressure with a relief valve for regulation. The pressure has to drop below relief pressure to make a change on the gauge.


3rd Mar 2009, 18:12
MarkerInbound (http://www.pprune.org/members/206121-markerinbound) -
.....so feathered the engine.

OK.....picky, picky, picky time. I think you mean you feathered the PROP, not the engine? I've "caged" an engine or two, but never feathered one!!:8:8

3rd Mar 2009, 18:25
Sully's Geese that "feathered the engines".

3rd Mar 2009, 18:26
No.....I think they tried to goose the engines!:8

Big Pistons Forever
16th Mar 2009, 19:53
Don't forget high oil temp followed by low oil pres, can be caused by several problems other than low oil level. It is often the first sign the engine has suffered a major internal mechanical failure. Certainly in large radials a sudden rise in oil temp preceeded the start of two "you know you are having a bad day when" flights for me :uhoh:. This does not include the time we were climbing out of Billings Montana in the DC 6 and I noticed the No 4 oil cooler door was wide open while the No 3 was 3/4 closed. (the oil cooler doors move automatically to maintain a constant oil temp). Sure enough within a few mins the oil temp started to climb but we were light and the old Douglass tri motor got us uneventfully home.

As an aside I still consider the DC6 my favorite aircraft to fly. It is truely a gentlemans carriage:ok:

17th Mar 2009, 00:48
As an aside I still consider the DC6 my favorite aircraft to fly. It is truely a gentlemans carriage

Well, having flown the Connies and the 6/7, I'll take the Connie any day. Might fly a little like a "truck", but it's all airplane. Probably why I like the DC-8 so much: kinda reminded me of the old Connie.:)