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A Couple of Q & A's About Powerplant Systems
Jim Davidson, President
Davidson Engineering Resources, Inc., Tucson, AZ
Phone (520) 977-9824
Fax (520) 232-3660
Here is a bit of information that concerns the A&P Powerplant area. Let me know if any of the information is out-dated...
Roller bearings have more roling resistance than ball bearings. This is due to the total contact area presented by the roller versus the ball. Think rectangular contact area versus small cicular area. Ball bearings are used in many high power engine bearing applications. The bearing race can be designed to take axial loads as well as the ususal radial loads.
Grinding either an intake or exhaust valve so that the edge is "sharp" is not a very good idea. You may think it improves airflow, but you'll be surprized what actually happens. Think about heat and how the knife edge you just gave that valve will deal with all that cylinder heat. The knife edge, because of the lack of mass (remember you removed all that metal), now can't get rid of the heat fast enough. The thin edge may begin to glow...and when that air-fuel mixture enters the cylinder head - POW. Preignition. Also, since the edge is so thin, the increase in localized heating at the valve edge will probably cause the valve to burn. How much was that overhaul anyway? Machine to the specs and leave the hot-rodding to your 68 Camaro.
OK, a bit of physics. How fast is the piston going when it is Top Dead Center (TDC)? Hint, the same velocity as at the bottom of the stroke. Another hint, think of how fast the piston is going at the exact middle of the stroke. At exactly 0º TDC the piston is stationary. The piston rod has moved the piston to the furthest point away from the crank and is now starting to bring it back closer. As the piston goes through 90º crank angle, it is moving at it's highest velocity. So, starting from 0º TDC, the piston accelerates until 90º crank angle, when it begins to slow down as it reaches 180º crank angle. Check out a few web sites that talk about basic dymanics, all pretty simple stuff.
Floating Piston Pins:
Sounds like a Monty Python movie, "Small rocks float..." But no. A floating piston pin allows for the relative motion between the piston and small end of the connecting rod. The floating design allows for reduced friction, and the resulting reduction in damage of the piston-connecting rod interface. The floating piston pin is mostly a push-fit installation and kept from laterally moving by some form of retention mechanism. Excessive or extreme lateral movement by a floating piston pin may cause damage to the cylinder wall. So make sure the floating piston pin retention "doo-hickies" are inistalled correctly.
No, it's not how many twinkies you can jam into your mouth at lunch! It is the ratio of the volume in the cylinider when the piston is at the Bottom Dead Center (BDC) to the volume in the cylinder when the piston is at Top Dead Center (TDC). Looking at it another way, visualize the volume of air in the cylinder when the piston is at the bottom of the stroke, this would be the maximum volume. As the piston moves upward the volume of air is squeezed, smaller, and smaller...until, when the piston can't move upward in the cylinder anymore. this would be the minimum volume. Such numbers as 7:1 or 9:1 are said, "Seven to one compression ratio", or, "Nine to One compression ratio". The air in the cylinder is squeezed 7 times and 9 time respectively.
Just as an aside, try pumping a bike tire up with a tire pump and feel how hot the pump cylinder gets. Air, when compressed, heats up. Too high a compression ratio can cause pre-ignition and detonation if the conditions are right (remeber that valve that was grinided to a knife edge).
Cam Ground Pistons:
Why would a piston NOT be manufactured perfectly round, after all the cylinder is pretty darn close to being perfectly round? Think expansion and contraction, and also how the piston is designed. Look at the underside of a piston and you'll see a bunch of metal where the piston pin goes, and not alot of metal elsewhere (other than the actual piston wall. When the piston gets to operating temperatures, what happens to the piston in the direction of that mass of metal holding the piston pin in place? Yup, it expands. More in this direction than in the other. If the pistion was manufactured round - out of the box, it would become an oval when operting at temperature. Reverse that thought, and you can see why the piston is made as a slight oval. The manufacturing process is called cam grinding. thus "cam ground" piston. The reason for all this manufacturing slickness is to provide a piston that seals better when the piston is at operating temperature.
Valve Timing in the 4 Stroke Engine:
There are two vales, an exhaust and an intake in the basic 4 stroke engine. When are the vales open and when are they closed?...Ok, time's up.
The four "strokes" in the 4 stoke engine are: Compression, Power, Exhaust, and Intake.
It is easy to see that BOTH vales must be closed inorder to compress the air-fuel mixture and that BOTH valves must be closed during the power stroke. In the compression stroke any valve leakage will result in a loss of power, and the same applies for the power stroke. In these strokes both the intake and exhaust valves must be firmly seated. The other stokes that follow are Intake and Exhaust. Well, the names of the stroke tell you what the valve is doing - right? During the intake stroke the intake valve MUST be open and exhaust valve closed, inorder for the air-fuel mixture to be drawin into the cylinder(assuming normally aspired engine). During the exhaust stroke the exhaust valve MUST be open and the intake valve closed to get rid of the combustion gasses.
To follow from the previous section, what do you think causes a back-fire?
If the intake valve opens a bit too early at the end of the exhaust stroke - visulaize what happens. Hot combustion gas is pushed by the partially open intake valve and the hot gas travels into the induction system. The induction system is ripe with fuel and air and just waiting for somoe form of energy (spark or heat) to set it off. Well, the hot exhaust gas will do the trick. If a back-fire is heard, follow the maintenance manual checks to assure all the induction system components are not harmed.
No, it's not when the intake and exhaust valve touch each other - that should never happen! Valve overlap is defined in terms of crankshaft degrees traveled when BOTH the intake and exhaust valves are off their seats. The induction system and valve timing are designed to take advantage of the momentum of the intake charge coming towards the intake valve. This principle can be seen in drag racing cars that use "tunnel ram" intakes. The length of the "tunnel" is determined by the cam / valve train component system design. The longer "tunnel" allows the intake charge to gain momentum as it nears the intake valve as a result of the suction developed by the intake stroke, which actually allows for as much air-fuel mixture to be "crammed" into the cylinder before the combustion stroke.
Piston Ring Stagger:
After a night drinking the piston rings...NO, not that. If the piston ring gaps are aligned, then more "blow-by" occurs. The differential pressure compression check, when performed and the piston ring gaps are aligned, will indicate a "worn piston ring" reading. So, when rebuilding a piston / cylinder or an entire engine, remember to stagger the piston ring gaps...and follow the engine maintenance manual when performing rebuilding operations.
Piston Ring Stagger:
Math done in tight spots - not really. Remember a few items above the compression ratio was discussed. Here we'll put a few numbers into the equation.
If the volume of the cylinder, when the piston is at BDC, is 70ci (cubic inches), and the volume of the cylinider when the piston is at TDC is 10ci, what is the compression ratio of that cylinder? Yup, you got it... divide 70ci by 10ci and get 7. Note that the dimensions of "ci" cancel each other, giving a number that is dimensionless. The answer is usually stated as a ratio, in this case 7:1. See, no claculator needed.
The Stokes and What They Do:
Deja Vu, all over again.
What are the 5 events (one event and the 4 strokes) that occur in the 4 stroke engine, in order and what happens during each stroke?
1. Intake: The air-fuel charge, passing the open intake valve, fills the cylinder as the piston moves from TDC towards BDC.
2. Compression: The piston reverses direction moving from BDC towards TDC and during this transition, the air-fuel volume in the cylinder is squeezed into a smaller volume. Both intake and exhaust valves are closed. Remember that the air-fuel volume heats as it is coompressed, but not so much as to cause pre-ignition or detonation.
3. Ignition: As the piston reaches the TDC location (actually the spark is "timed" to occur at a preselected piston location, which is described in terms of "degrees before TDC". The engine spec sheet will tell you what this setting is. Ok, so as the piston reaches "that certain point near TDC", the magneto releases it's energy in the form of a voltage potential across the spark plug gap. The spark ignites the fuel-air mixture pretty darn close to the maximum "squeeze" time and when the piston is starting to move back down towards BDC. (Again, actual spark timing should occur before TDC - refer to the engine manual for the specification).
4.Power: The air-fuel mixture is, for lack of a better defintion, EXPLODING. But since the volume is allowed to increase as the pressure increases the event is more like a controlled explosion. The force of the expanding combustion event forces the piston downward in the cylinder. In physics, the term Work is defined as a Force acting through a Distance. here ya go! The expanding gasses increase the cylinder pressure. This pressure is the force (Pound per Square Inch) acting on the surface of the piston top. This Force acts as the piston moves from near the top of the cylinder to the bottom of the cylinder. This is the work that drives the propeller, well, most of it anyway. There are several "losses" of energy due to friction, etc, before the work perfomed is translated into prop rotation...but I'll stop for now.
5. Exhaust: What to do with all the used up stuff? Push it out through the exhaust port. Yup, after the piston is driven downwards by the combustion process, the next step is to move back up toward the top of the cylinder. The air induction system and fuel metering system is designed to stuff the cylinder with a certain amount of "energy" - the air-fuel mixture. The mixture should be fully comsumed by the combustion process by the time the piston reaches the bottom of the Power Stroke so that when the piston moves upwards again and the exhaust valve opens, the combustion by-products are all that is left to be pushed out of the cylinder.
Crankshaft Counter Weights:
To reduce the vibrations caused by the Power strokes or by the occasional mis-fire, crankshafts can have counterweights installed on them. The counterwieghts are moveable and installed on certain crank throws. Another name for these counterweights is dynamic dampers. Owing to the function they provide, they dampen the torsional vibrations by moving, like pendulums, to counter the unwanted vibrations at the crankshaft.
Prop Speed (RPM) and Engine Speed (RPM):
A couple of competing physical characteristics fight against eachother when it comes to engine RPM and Prop RPM. One of the characteristics is engine horsepower. It is proportional to engine RPM, so the faster the engine turns the more horsepower it produces. Another physical characteristic is propellor RPM as it relates to what happens at the propellor tip. The faster the prop turns the closer the tip speeds get to the speed of sound; and a resulting loss of propellor efficiency. So, the relationship between engine RPM and prop RPM is that one increases (horsepower) while the other decreases (thrust). A reduction gear set is used between the engine and the propellor. This mechanical arranagement will allow the engine to rev higher (produce required horsepower) while keeping the propellor speed (RPM) within it's efficiency range and provide the necessary thrust.
Fuel-Air Mixture and Ignition Timing:
This was touched on a bit ago. If the fuel-air mixture is correct and the ignition system is set and working correctly, and depending on the specific timing settings of your engine, the combusion process starts at about 20 to 30 degrees before TDC. Remember that the combustion process is not instantaneous. The flame front progresses across the piston top, starting at the ignition source (spark plug) and continues until the combustible materials are consumed. All this released energy pushes the piston downward during the power stroke.
At the propellor, the horsepower measurement is called "Brake Horsepower". The Prony brake is named after the inventor Gaspard de Prony. Prony invented the Pony brake in Paris in 1821 to measure the power of engines. The device used mechanical friction to move an indicator proportional to the amount of horsepower generated by engines. Today a dynamometer is used, which places a load on the engine by hydraulic means, to measure horsepower generated.
Nitriding Cylinder Walls:
Nitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of steel at a temperature range (500 to 550°C, or 930 to 1020°F), while it is in the ferrite condition. Nitriding is similar to carburizing in that surface composition is altered, but different in that nitrogen is added into ferrite instead of austenite. Because nitriding does not involve heating into the austenite phase field and a subsequent quench to form martensite, nitriding can be accomplished with a minimum of distortion and with excellent dimensional control. Ok, what does this mean? Bottom line is that the cylinder walls are hardened so that their life span (resistance to wear), is increased. Note, another method of hardening the cylinder walls is to chrome plate them.
Every person that has riden an old Harley will tell you about the vibration (great feeling huh!). That feeling is produced by the two cylinders doing their thing to generate power. Now, have you heard how smooth that 12 cylinder Jag engine is? There is a smootheness factor related to the number of cylinders in an engine (and yes, what their angular displacement is from the crank centerline as well as other design considerations). The greater the number of cylinders the greater number of power strokes per crank revolution occurs (think higher frequency of power pulses).
One way to control the temperature of the engine oil is by the use of a "floating control thermostat". This device provides oil temperature regulation by modulating an air cooling exit door. The door is moved towards open or towards closed based on the oil temperature between a desired set of limits.
Metal Shavings In Oil:
Bad Stuff!!! An engine that is "making metal" is one that needs immediate attention. Any metal shavings found in engine oil is cause for concern - such a concern that the aircraft needs to be grounded and diagnosed before being released for future flights.
Oil Pressure Guage Fluctuates:
The most probable cause of the oil pressure indicator "needle" fluctuating over the entire range of the gauge is low oil quantity. On the ground, with the engine at idle the oil pressure may indicate properly. However, as the aircraft is moved, whether on the ground taxing or in flight during manuevers, the oil will slosh away from the oil pickup and the engine "sucks air". The air in the oil system will cause the oil pressure indication to vary, and this would be seen on the oil pressure indicator. Check the level of the oil (you did this during pre-flight didn't ya?), and make sure it is to the level specified for that engine type. While you are nosing arouond the engine cowls, check for oil leaks - or oil streaks on the bottom of the fuselage. the streaks will indicate that there is a leak or the person that added oil last had a really bad aim and got oil all over the engine.
Back to Metal in the Oil:
Ok, you see sparkles in the oil pan; now what are they? where are they from? Grab a magnet and swish it in the oil. If the magnet comes out looking like it has grown a beard (think Grizzly Adams or ZZ Top beards) - the metal is NOT from bearings, but most likely from engine parts. If the metal particles are not attracted by the magnet, think bearings or pistons.
Don't Burn Your Fingers...But:
A good way to determine which cylinder is missing is to touch test the exhaust stacks. If the engine is missing on both left and right magneto positions the touch test is a good way to find the cold cylinder. Once the cylinder is identified, check the spark plug (fouling, burned electrodes, damaged electrodes), check the spark plug wires (correct fit, no loose connections). The analysis will continue on from there to find the reason for the miss.
Hissing In the Exhaust:
Ok, this may take two people to accomplish, becasue I can't think of a way to turn a prop and have your ears next to the exhaust stack at the same time. Before pulling on the prop make sure all the electrical power is removed - all the engine switches in the OFF position. An engine cranking when you don't expect it to can ruin your day big time. OK, as the prop is moved you hear a hissing sound in the exhaust, what does that indicate? The hissing is coming from the un-seated, cracked, burnt, other wise BAD exhaust valve. Remember the only time the exhaust valve opens is during the exhaust stroke. So during the intake, comopression and power strokes the exhaust valve should be tightly seated - no leaks.
Oil Dilution System:
Or how in the heck do those Alaskan Bush Pilots start their engines on cold days??? Some engines have a system that, when the engine is shut down, allows a small amount of fuel to be mixed with the oil. This "dilution" will make the oil a bit less viscous during the next cold start. Now, if the dilution system goes on the blink during normal operation, the oil will become thinner or diluted. The oil pressure indication will read low and the oil temperature indication will read high. A check of the oil dilution valve will determine if it is functioning properly.
Engine Operating Flexability:
what do you want from your aircraft engine? Smoothe acceleration and deceleration, specified performance throughout the certified engine operating range and speeds...Well, that is what is called Engine Operating Fexibility.
Metallic Sodium is placed in the hollow valve stem of valves to help reduce the valve temperature. During engine operation, the metallic sodium inside the hollow valve melts. When the valve opens, the sodium splashes down into the valve head and collects heat. Then, when the valve closes, the sodium splashes up into the valve stem. Heat transfers out of the sodium, into the stem, valve guide, and engine coolant. In this way, the valve is cooled. Sodium-filled valves are light and allow high engine rpm for prolonged periods.
Oil Consumption Gremlin:
Worn valve guides will allow oil to migrate between the valve and the valve guide - and you'll notice this when you finid that the oil levels are frequently low. Another way to determine that the engine is buring oil is to analyze the spark plugs. Oil fouled plugs mean that excessive oil is getting into the cylinder somehow...Maybe its time to check the valve and valve guide clearance???
Dead Cylinder Indcations:
What if your engine is running rough, and when you perform the mag drop out check the drop is the same for both mags? Also, you check the manifold pressure and it reads higher than normal throughout a given RPM range...What could this indicate? This is an indication of a dead cylinder! (RIP!) The thing to take away here is that dead cylinders won't be indicated by mag checks. In addition, the higher manifold pressure is the result of the need to advance the throttle further to obtain the desired RPM - compensating for that dead cylinder.
Push Rod Length:
If the engine youo are working on has hydraulic lifters and the valve clearance just can't be set to specs, you can install a push rod of a slightly different length to get the correct settings. Remember to only use parts that are approved for that specific engine. Check the IPC, or MM for your engine and see what your options are before installing any parts. Unapproved parts - just don't do it!!!
Oil Flow at Cold Engine Temperatures:
Oil is not going to be apt to flow easily when it is cold (or even cool). If the engine is throttle up immediately after starting and left at the higher RPM engine damage will most probably occur. Why? The cold oil is not going to flow properly to all the bearings and other parts. Without oil lubrication the parts will operate for a time - metal on metal (ok - there is some minimal oil on the parts but not enough to provide the protection at the higher RPMs). Start the engine and set the idle at the specified setting - let her warm up a bit before advancing the throttle.
Altitude an Pressure:
The higher the altitude the lower the air density. Another way to think of this is to remember that there are fewer oxygen molecules the higher you fly, and the combustion process depends on oxygen to occur. So, if the engine has a carb, the air-fuel mixture will richen (more fuel than oxygen) as you gain altitude. of course if your engine has a turbo you'll not need to worry about this. Adjusting the mixture (leaning it out) as the aircraft gains altitude is necessary to keep pthe air-fuel ratio in the correct range.
What you'll notice is that for a constant RPM and manifold pressure the engine will produce less power on humid days as comopared to less humid days. Again, it is all a part of the combustion process and the balance between the amount of air and fuel needed to produce power.
Newly Installed Engine:
Ok, you completed the engine overhaul and slathered on all that molly grease right (or bought a new engine) and installation is complete. Fire that puppy up!!!??? NO - STEP AWAY FROM THE IGNITION. Sure, you have filled the engine with the proper amount of oil, but think for a second where all that fresh, clean oil is. In the sump. The new oil is nowhere near the crank bearings, the valves, the lifters, all the parts that need lubrication to operate and live a long healthy life. The one thing you need to do before starting the engine is to "pre-oil" the engine. Motor the engine at alow RPM's until oil pressure is indicated. Even this can still can potentially cause some wear since at the low RPM's the oil doesn't reach the top-end quickly. Just don't start a newly rebuilt engine and rev her up!!!