RansMail #022 May 30th 2004

 

News:-

a)    Brian McCartan wrote:-

"I am enjoying your newsletters.
My Rans flew for 2 hours on Sat 22 (May). It goes very well. I did not do
anything spectacular; just familiarisation, a few stalls and landings etc."

And in response to my comment re current lack of radio in the a/c.  

"I just use a hand held and a headset connection which works fine and no complications".

 

b)   Edward Wallington warned me against being overkeen on fabric replacement, thus:-

" Changing the fabric on ailerons and flaps is a real >pig< of a job. So unless it really needs doing, I would recommend leaving it till you have to. The fabric is pulled tightly over the frame, no heat was used on mine. I made some jigs to do it, even so it was difficult. Graham said I should have used some Mr Sheen on the frames first. Might have helped. The fabric tension holds the ailerons together, so they need to be tight."

 

c)I saw this 'pdf' article mentioned and persuaded the US publisher to send me a copy but to reduce its file size I have eliminated the graphics. Actually I don't agree with all the writer says, but it makes you think !

 

Mike.

 

"Preventing Piston Seizures • Engines by Tom Olenik. 48 JANUARY 2004 www.ULTRAFLIGHT.com

Often when I give a presentation on engines, one of the first questions I ask the audience is how many of them have had engine failures. At least a few hands are raised. The next question is how many people want to have an engine failure in the future. I don’t get many hands for that one. However, I know that many are pushing their luck, whether they know it or not. I would like to make sure you know it when you are running on the hairy edge of having an engine failure or that you at least understand what you can do to reduce your chances of having one.

Most of what I write in my articles relates to preventing engine failures, but I have not yet devoted specific emphasis on preventing some of  the most common types of failures. Let’s look at piston seizures. This is a failure caused from the friction of the piston in the cylinder exceeding the power produced by the engine. The engine stops, leaving you with one of the most sudden, heart-stopping moments of silence you will ever experience. Always remember that if your engine ever quits for any reason other than turning off the  switch, you must check for signs of piston seizure. An engine is not a living organism. It cannot heal. If damage has occurred, it must be repaired. In most two-stroke engines in ultralights and light planes, an engine will restart just seconds after a piston seizure has occurred. It may run for another minute, or it may run for hours before it quits again. However, it will be much easier to quit the next time.

When a piston seizure occurs, small grooves ground into the piston skirts are removed, and the crosshatch of the cylinder wall is filled with aluminum from the piston. These would  normally help hold oil and transfer heat, but after a piston seizure the piston and cylinder can no longer hold oil or transfer heat nearly as well. Thus, it must be repaired even if the engine still  starts and runs seemingly normal. To check for a piston seizure, remove the exhaust manifold. Use a flashlight and look at the sides of both pistons for scuffing. Rotate the engine through so the piston is below the port, and look through to the intake side of the cylinder wall for signs of aluminum on the cylinder wall. Sometimes you can’t see the marks on the piston because they’re outside the exhaust port. In such a case, you can see signs of aluminum on the opposite cylinder wall. If any scuffing or aluminum transfer is detected, it must be repaired. If there are no signs of such failure, install new exhaust gaskets when you reinstall the manifold. Let’s look at three common causes for piston seizure like this.

Lubrication. One cause is simply a lack of lubrication. If no oil is present between the piston and the cylinder wall, the friction will be too great, causing seizure. It takes very little oil to prevent this from happening. While a wise guy might say all piston seizures are caused from lack of lubrications (because it is friction that stops the engine), most seizures do not occur from a lack of oil either in the fuel or being pumped by the pump. However, this type of failure does occur occasionally. Lubrication failure is probably most common in engines that use pre-mix rather than oil injection. It seems oil injection pumps are more reliable than the average person’s memory or mixing procedures. If you have an engine that requires mixing, here are some tips to help prevent lubrication failures if you must mix the oil with the fuel manually. 1. Designate a gasoline jug for mixed fuel ONLY. Clearly mark this jug. Don’t use it for any  other purpose.

2. Use a precise measuring device such as a Ratio Rite. This helps reduce errors.

3. Always put the oil in the jug just before adding the gasoline. Doing it this way, and using the "mixed fuel" jug, there is no question that  there is oil in the fuel.

4. While pumping gasoline into the jug with oil, slosh it around. Pay close attention to the amount of gasoline you pump. If you put oil into the jug for five gallons, don’t pump in five and one half. This might seem obvious, but a fivegallon jug often holds five and one half gallons, which would change your mix ratio if you had oil  for only five gallons.

5. When done filling the jug, close it and violently shake it one minute. Many modern fuels do not have a good solvency. More thorough mixing is required to ensure the oil is evenly mixed. Most oils are dark and will change the color of the gasoline when mixed in. If the color did not change, it’s not mixed well enough.

6. NEVER, NEVER, NEVER mix two different oils even if they are the same brand. If two different oil formulations are mixed together, reactions can occur which can cause the oil to become  solid. For this reason, I discourage switching oils unless it’s immediately following a thorough disassembly and cleaning of the engine. If you are running oil injection, the chances of  oil injection pump failures are extremely low. However, the pump cannot pump oil that is not there. The only lubrication failures I see on engines with oil injection is from the user simply running out of oil or otherwise creating a situation where oil cannot get to the pump. I cannot recall a single oil injection pump failure in all the years Rotax has produced these in the UL series engines. However, lubrication failures can occur for other reasons than failure of the pump. Here are a couple tips to make sure your oil injection pump and engine stay fed with oil.

1. Always make sure you have enough oil in the oil injection tank for the amount of fuel you have in your fuel tank. If your engine is a 50:1 engine, you have 10 gallons of fuel. This should       be a minimum of 25 ounces of oil in the injection tank with a little extra for some safety margin. I would want to have a minimum of two pints. Anything less for 10 gallons of fuel is cutting it too close.

2. Make sure you keep the venting of the oil injection tank clear of restrictions and out of  low-pressure areas. The oil is gravity-fed to the pump and it must be able to flow freely to the pump. If the vent in the tank is plugged, the oil cannot flow causing the pump and engine to be starved for oil.

3. NEVER, NEVER, NEVER mix two different oils of even the same brand. If two different oil formulations are mixed together, reactions can occur between them, which can cause the oil to become solid. For this reason, I discourage switching oils.

Over-heating

Another cause of piston seizures is that the piston actually melts. If the piston gets too hot, it can melt causing pieces of aluminum to build up on the piston skirt and the cylinder wall. Clearance is then gone and the piston or cylinder no longer holds oil. The friction becomes too great and the engine stops. In Rotax two-stroke engines this is less common. In 2si two-stroke engines, it is probably the most common form of piston seizure. Most of the time when the piston melts, it is from pure over-heating caused by a cooling system failure or extremely high  exhaust gas temperatures, indicated by a lean fuel/air mixture. If a fanbelt slips or breaks in a Rotax 503 or 447, we will sometimes see this failure. This type of failure often gives plenty of warning in a high CHT (Cylinder Head Temperature). If you don’t have CHT’s on your light aircraft, you should. This instrument reading is sometimes neglected in Rotax 582 installations, but it needs to be there. CHT and coolant temperature DO NOT always correlate. A coolant temperature reading is not a replacement for a CHT reading.

Expansion

The third cause of piston seizures is from the piston simply getting larger than the bore in the cylinder. This increases the friction causing the engine to stop. This is the most common form of piston seizure in the Rotax two-stroke engine. However, it is unusual in the 2si two-stroke engine. A "cold seizure" would fall into this class, but the term cold seizure does not fully encompass this seizure. To understand how the expansion seizure occurs, we need a little physics lesson. In one physics lab, we heated and measured several different rods made of different metals. We learned that different types of materials absorb heat at different rates and expand differently. When the same amount of heat is applied, aluminum absorbs heat faster and  expands more than cast iron. In many two-stroke engines cylinders have cast iron sleeves and pistons are made of aluminum alloy. If we apply the same amount of heat to the piston and the cylinder, most pistons expand faster and more than the cylinder.

So how does this cause failures and what can we do about it? In an internal combustion engine, heat is generated from the  burning of fuel in the combustion chamber. The combustion chamber is made up of a cylinder, a cylinder head, and a piston assembly. These components are all directly in contact with the fire and heat produced from the combustion process. These are often made of different materials. Remember different materials absorb heat and change size at different rates. In a two-stroke ultralight engine, we likely have a cast iron cylinder sleeve and an aluminum alloy piston. All Rotax and 2si two-stroke engines are this way. Hirth is a little different. Aluminum absorbs heat faster and expands more than cast iron. So the piston can actually become larger than the cylinder. This is when expansion seizure occurs.

To prevent this type of piston seizure a few steps can be taken. Make sure the cylinder is heated up thoroughly before you run the engine at increased power settings, and make sure it stays heated during all of its operation. If the cylinder is at the height of its operating temperatures before you run the engine at increased power settings, it has already fully expanded. This allows the piston to expand rapidly when power is increased without expanding past the size of the bore. If you start a cold engine and immediately go to full power, the piston heats up faster than the cylinder and you risk a seizure from the rapidly expanding piston. By doing a proper warm-up of the engine, you are giving the cylinder a head start. The goal is to let the cylinder finish the race before the piston starts. The cylinder is then expanded all the way before the piston absorbs more than a minimal amount of heat.

However, even if the cylinder is fully warmed and expanded, an expansion-type seizure can still occur. Remember aluminum expands more than cast iron. At some point, if both the piston and the cylinder are heated, the piston will get as large as the bore. If we run the piston and the cylinder hotter, we can still have this type of failure. Most of the time in a Rotax two-stroke engine, we will reach this point where the piston expands more than the cylinder bore before the piston actually starts to melt. This is why this type of piston seizure is more common for the Rotax engine. Other engines, such as the 2si, often reach melting point of the piston before the piston expands beyond its confines. That is why heat seizure is more common in those engines. This distinction is due to the engine design.

What we can do is keep the piston from getting so hot that it expands enough to take up all the clearance to the cylinder wall. We can also make sure the cylinder stays in an expanded state. To do that, monitor the instrumentation and make adjustments accordingly. Most of you have an assortment of instruments that you use for monitoring various temperatures relating to the engine. However, do you know what all those readings mean? I mentioned this in an earlier article, "Beginner’s Guide to Ultralight Engines," but here is a quick review:

CHT: Cylinder head temperature is taken from under the spark plug. It is the best indication of the thermal condition of the cylinder itself.

EGT: Exhaust gas temperature is taken from a probe placed in the exhaust system at a specified point to measure the temperature of the gasses leaving the engine. This is the best indication of the temperature of the fire inside the engine and the amount of heat being applied directly to the piston, cylinder and cylinder head.

Coolant Temperature indicates the condition of the cooling system of the engine. Some installations use this as a replacement for CHT, but it should not be used as such. If the cylinder is cooler, yet the cooling system is weaker, a higher coolant temperature may show, while the cylinder may actually be too cold. One cylinder may be too hot, but since the cooling system is a combined indication of all the cylinders, you may never notice this problem until it results in a failure. This can happen with cylinder head gasket leaks, partial blockages in the coolant passages, and for other reasons. If your cylinders are cooled with liquid coolant, coolant temperature readings do not take the place of cylinder head  temperature readings.

Since the piston absorbs the heat from the combustion faster than the cylinder, we must pay attention to the temperature of the fire we are producing. The best way is with good exhaust gas temperature readings. If you run a higher exhaust gas temperature, your chances of any type of piston seizure are greater because your engine components are hotter. This gets them both closer to melting and closer to a point where all the clearance between the piston and cylinder wall is gone. Pay attention to the thermal condition of the cylinder to make sure it stays warm. Do this with cylinder head temperature, not coolant temperature. Ideally, you want to keep the cylinder head temperature at the hottest temperature possible within specified normal operating ranges to prevent expansion seizure. This allows the cylinder bore to give the piston the most room to expand. However, if it gets too hot, you risk heat seizure.

The trick is for the cylinder to not be too hot or too cold. cold. Similarly the temperatures of your engine can be either too hot or too cold. For almost every specification given for your engine’s temperatures, there is both a minimum and a maximum. Most of us are conditioned to pay attention to the maximum limits, but we often ignore the minimum. This leads to many failures because the minimum limit is just as important or sometimes more important than the maximum. For best reliability, give the minimum limit just as much respect as the maximum. You will be less likely to be one of those holding up your hand at my seminars when I ask about piston seizures. By understanding the hows and whys of piston seizure, you are better equipped to recognize when you have a potential problem that could lead to failure. However, be sure to remain respectful of the fact that a two-stroke engine is prone to sudden failure. Many times you will have no warning even if you know what to look for. It has happened to me and to just about any other engine expert at some time. Respect the fact that the engine can quit without warning. "