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Know-It-All Q&A - Engines

Q.  Do engine temperature gauges read high or low in the winter?

A.  It depends on the gauges and the application but most round analog gauges do not compensate for ambient temperature. They measure the difference between the probe and a reference connection built into the instrument typically assumed to be at 59 degree F.  If it is colder than 59, the instrument will display a reading higher than actual temperature at the probe location.  Your gauges may read high in winter.

Q. Why do most two-cycle powered planes have an exhaust gas temperature (EGT) gauge?

A. Some exhaust systems have an exhaust gas temperature probe and indicating gauge. This probe transmits an electric signal to a gauge in front of the pilot. The gauge reads the signal and provides the EGT of the gases at the exhaust manifold. This temperature varies with power setting, propeller loading, and mixture (ratio of fuel to air entering the cylinders). It’s used to make sure the fuel-air mixture is within engine operating specifications. When there’s a problem with propeller loading or carburetion, the EGT gauge normally will be the first notification for a pilot.

Q. Why do some planes have a fuel strainer or fuel sumps?

A. After leaving the fuel tank and before it enters the carburetor, the fuel passes through a strainer which removes any moisture and other sediments in the system. Since these contaminants are heavier than fuel, they settle in a sump at the bottom of the strainer assembly. A sump is a low point in a fuel system and/or fuel tank. The fuel system may contain a sump, fuel strainer, and fuel tank drains, which may be collocated.   

The fuel strainer should be drained before each flight. Fuel samples should be drained and checked visually for water and contaminants.

Water in the sump is hazardous because in cold weather the water can freeze and block fuel lines. In warm weather, it can flow into the carburetor and stop the engine. If water is present in the sump, more water in the fuel tanks is probable, and they should be drained until there is no evidence of water. Never take off until all water and contaminants have been removed from the engine fuel system.

Q. How does the propeller develop thrust?

A. The propeller is a rotating airfoil, subject to induced drag, stalls, and other aerodynamic principles that apply to any airfoil. It provides the necessary thrust to pull, or in some cases push, the aircraft through the air. The engine power is used to rotate the propeller, which in turn generates thrust very similar to the manner in which a wing produces lift. The amount of thrust produced depends on the shape of the airfoil, the angle of attack of the propeller blade, and the revolutions per minute (rpm) of the engine. The propeller itself is twisted so the blade angle changes from hub to tip. The greatest angle of incidence, or the highest pitch, is at the hub while the smallest angle of incidence or smallest pitch is at the tip.

The reason for the twist is to produce uniform lift from the hub to the tip. As the blade rotates, there is a difference in the actual speed of the various portions of the blade. The tip of the blade travels faster than the part near the hub, because the tip travels a greater distance than the hub in the same length of time. Changing the angle of incidence (pitch) from the hub to the tip to correspond with the speed produces uniform lift throughout the length of the blade. A propeller blade designed with the same angle of incidence throughout its entire length would be inefficient because as airspeed increases in flight, the portion near the hub would have a negative angle of attack while the blade tip would be stalled.

Q. Why is it necessary to de-carbon my two-cycle engine?

A. Two-cycle engines are lubricated by oil that is mixed with the fuel-air mixture. During the combustion process, deposits referred to as carbon can form as the result of unburned oil or undesirable elements of the fuel-oil mix. Different oils and fuels will generate different types of deposits at different rates. The type of oil and fuel used can have a dramatic impact on the amount and consistency of the carbon formation.

Over time the deposits can fill the piston ring grooves to the point that the ring will stick in the groove.  When this happens the ring loses its ability to transfer heat to the cylinder wall and a hot spot is created, which in turn can cause the piston to stick (seize) to the cylinder wall.

Q. Are four-stroke engine exhaust systems critical to the operation of the engine like two-stroke exhausts are?

A. Four-stroke engines are not as sensitive as two-stroke engines because they have exhaust valves and therefore do not need the precision pulse tuned exhaust system. However, directing the exhaust out appropriately and reducing the noise are important considerations. Using the manufacturer’s recommended configurations is required for special light-sport aircraft (S-LSA) and recommended for experimental aircraft.

Q. Should my plane have a fuel mixture control?

A. Carburetors are normally calibrated at sea-level pressure, where the correct fuel-to-air mixture ratio is established with the mixture control set in the full rich position. However, as altitude increases, the density of air entering the carburetor decreases, while the density of the fuel remains the same. This creates a progressively richer mixture, which can result in engine roughness and an appreciable loss of power.

The roughness normally is due to spark plug fouling from excessive carbon buildup on the plugs. Carbon buildup occurs because the rich mixture lowers the temperature inside the cylinder, inhibiting complete combustion of the fuel. This condition may occur during the pretakeoff run-up at high-elevation airports and during climbs or cruise flight at high altitudes. To maintain the correct fuel/air mixture, the mixture must be leaned using the mixture control. Leaning the mixture decreases fuel flow, which compensates for the decreased air density at high altitude.

Most ultralights do not have a mixture control because they operate within a few thousand feet of their takeoff altitude and use fixed carburetor jets that must be changed prior to takeoff for the appropriate altitude or season. The Rotax 912 and 914 engines found in many new light sport aircraft use an altitude compensating carburetor that eliminates the need for a fuel mixture control.

Q. What effect does old gas have on my engine?

A. Letting fuel sit for weeks without using it will cause it to go bad. Even if gas does not go bad, it will often lose its octane with time. For those that premix gasoline and two-stroke oil, there is another set of problems. Fuel and oil are normally mixed at a 50-to-1 ratio. If premixed gas sits in a plastic container for a while, the gas will evaporate out, leaving a richer oil mixture in the container. In any case, fresh gas should be used as much as possible.

Q. How does a reed valve work on a two-cycle engine?

A. Reed valves are commonly used in high-performance versions of the two-stroke engine, where they control the fuel-air mixture admitted to the cylinder. Typically, they’re made from thin spring steel or composite material and located in the crankcase just behind the carburetor. The reed valve is designed to open and close with changes in crankcase pressure. As the piston rises in the cylinder a vacuum is created in the crankcase beneath the piston. This vacuum opens the valve and admits the fuel-air mixture into the crankcase. As the piston descends, it raises the crankcase pressure causing the valve to close to retain the mixture and pressurize it for its eventual transfer through to the combustion chamber.

Q. Why is it important to inspect my exhaust system for cracks?

A. Engine exhaust systems vent the burned combustion gases away from pilot and passengers, and in some cases provide heat for the cabin. An exhaust system has exhaust piping attached to the cylinders, and typically a muffler and muffler shroud for cabin or carburetor heat. The exhaust gases are pushed out of the cylinder through the exhaust valve or port and then through the exhaust pipe system to the atmosphere.

For cabin heat, outside air is drawn into the air inlet and is ducted through a shroud around the muffler. The muffler is heated by the exiting exhaust gases and, in turn, heats the air around the muffler. This heated air is then ducted to the cabin for heat and defrost applications. The heat and defrost are typically controlled, and can be adjusted to the desired level.

Exhaust gases contain large amounts of carbon monoxide, which is odorless and colorless. Carbon monoxide is deadly, and its presence is virtually impossible to detect. The exhaust system must be in good condition and free of cracks.

Some exhaust systems have an EGT probe. This probe transmits the EGT to an instrument in the flight deck. The EGT gauge measures the temperature of the gases at the exhaust manifold. This temperature varies with the ratio of fuel to air entering the cylinders and can be used as a basis for regulating the fuel/air mixture. The EGT gauge is highly accurate in indicating the correct mixture setting. When using the EGT to aid in leaning the fuel/air mixture, fuel consumption can be reduced. For specific procedures, refer to the manufacturer’s recommendations for leaning the mixture.

Q. What are the advantages and disadvantages of a two-cycle engine?

A. Generally a two-stroke engine is lighter and has fewer moving parts, so they’re simpler and cost less to manufacture. Because two-stroke engines fire every two strokes, they can develop more power, therefore having a higher power-to-weight ratio than a four-stroke engine.

Two-strokes need to burn oil constantly to lubricate the engine. Due to the way the engine works, they’re less efficient and have higher emissions than four-stroke engines. Due to higher rpm, the piston rings and engine bore are prone to wear, and although the engine is pretty easy to strip and rebuild, a two-stroke in comparison to a four-stroke has a shorter service life.

Four-stroke engines give a smoother spread of power for most of the rpm range, whereas two-strokes tend to produce their peak power in a very small section of the rpm range, usually at very high engine rpm.

Q: I use auto gas. Is the 87 number on the pump the octane of the gas?

A: The number which is posted on the automobile service station pump isn’t a true octane number. It’s what is called an antiknock index number (AKI). This number is the average of two octane numbers arrived at by two different kinds of tests. One is called the ASTM Research Method and is often abbreviated R or RON. The other is the ASTM Motor Method, M or MON. The antiknock index number on the pump is then this average, or R + M divided by 2 = AKI. A rule of thumb is that the Motor Method octane number (MON) is approximately five points less than the AKI. The significance of the MON is that this is identical to the octane number for aviation gasoline.

Specification D-4814 (previously D-439) for automobile gasoline requires a minimum of 82 MON when the posted number is 87 AKI or more. When the EAA requested approval from the FAA for the auto gas STC, the request was for an AKI number of 87 to ensure a safety margin of 2 octane numbers over the aviation gasoline approved rating for 80 octane engines.

Q. What’s the function of the engine oil system?

A. The engine oil system performs several important functions:

  • lubrication of the engine’s moving parts
  • cooling of the engine by reducing friction
  • removing heat from the cylinders
  • providing a seal between the cylinder walls and pistons
  • carrying away contaminants.

Q. What is the difference between a wet-sump and dry-sump oil system?

A. Reciprocating engines use either a wet-sump or dry-sump oil system. In a wet-sump system, the oil is located in a sump, which is an integral part of the engine. In a dry-sump system, the oil is contained in a separate tank and circulated through the engine by pumps.

The main component of a wet-sump system is the oil pump, which draws oil from the sump and routes it to the engine. After the oil passes through the engine, it returns to the sump. In some engines, additional lubrication is supplied by the rotating crankshaft, which splashes oil onto portions of the engine.

An oil pump also supplies oil pressure in a dry-sump system, but the source of the oil is located externally to the engine in a separate oil tank. After oil is routed through the engine, it’s pumped from the various locations in the engine back to the oil tank by scavenge pumps. Dry-sump systems allow for a greater volume of oil to be supplied to the engine, which makes them more suitable for very large reciprocating engines.

Q. What are the advantages and disadvantages of a four-stroke engine?

A. Four-stroke engines are very common in most aircraft categories and are becoming more common in light planes. A number of advantages include reliability, fuel economy, longer engine life, and higher horsepower ranges.

These advantages are countered by a higher acquisition cost, lower power-to-weight ratios, and a higher overall weight. The increased weight and cost are the result of additional components (e.g., camshaft, valves, complex head to house the valve train, etc.) incorporated in a four-stroke engine.

Q. I’m flying with a two-cycle engine that is fitted with a choke to help the starting process. If I add a plunger primer, will it be easier to start?

A. It will be easier starting as long as you don’t overprime and flood the engine. The fuel plunger primer is used to draw fuel from the tanks to supply it directly into the cylinders prior to starting the engine. This is particularly helpful during cold weather when engines are hard to start because there isn’t enough heat available to vaporize the fuel in the carburetor.

Q. I haven’t flown for four months, and my gas is at least that old. I use auto gas with premixed two-cycle oil. Is it safe to fly with this gas?

A. Probably not. It’s likely that your gas has turned bad. Letting fuel sit for months without using it will cause it to go bad. Even if the gas doesn’t go bad, it will often lose its octane with time. Lower octane can cause detonation and engine damage. Fresh gas should be used as much as possible.

Q. Why do some engines have a reduction drive gearbox?

A. Gearboxes are used on many lightweight reciprocating engines to take the rotational output of an internal combustion engine which is turning at a high rpm and convert it to a slower (and more useful) rpm to turn the propeller. Gearboxes come in different gear ratios depending on the output speed of the engine and the needed propeller turning speeds.

Some examples are a two-stroke rpm reduction from 6,500 engine rpm with a 3.47-to-1 reduction, resulting in 1,873 propeller rpm. A four-stroke rpm reduction could be from 5,500 engine rpm with a 2.43-to-1 reduction, resulting in 2,263 propeller rpm. A gearbox is a simple device that bolts directly to the engine, and in turn, has the propeller bolted directly to it.

A two-cycle engine gearbox is kept lubricated with its own built-in reservoir of heavy gearbox oil. The reservoir is actually part of the gearbox case itself. The gearbox oil has to be changed periodically since the meshing of the gears will cause them to wear and deposit steel filings into the oil. If the oil isn’t changed, the abrasive filings cause even more wear. Some gearboxes have a built-in electric starter motor. When activated, the motor turns the gearing which cranks the engine.

Four-stroke propeller reduction gearboxes use oil from the engine oil system for lubrication. Some gearboxes come with a built-in centrifugal clutch and others have allowances for installation. A centrifugal clutch is very useful in a two-stroke engine because it allows the engine to idle at a lower speed without the load of the propeller. Otherwise, two-stroke engines can generate a great deal of vibration at low rpm when loaded. As the engine speeds up, the centrifugal clutch engages and smoothly starts the propeller spinning. When the engine is brought back to idle, the clutch disengages and allows the engine to idle smoothly again; the propeller stops when on the ground and windmills when flying.

Q. How does carburetor ice affect my engine?

A. One disadvantage of the float-type carburetor is its icing tendency. Carburetor ice occurs due to the effect of fuel vaporization and the decrease in air pressure in the venturi, which causes a sharp temperature drop in the carburetor. If water vapor in the air condenses when the carburetor temperature is at or below freezing, ice may form on internal surfaces of the carburetor, including the throttle valve.

The reduced air pressure, as well as the vaporization of fuel, contributes to the temperature decrease in the carburetor. Ice generally forms in the vicinity of the throttle valve and in the venturi throat. This restricts the flow of the fuel/air mixture and reduces power. If enough ice builds up, the engine may cease to operate. Carburetor ice is most likely to occur when temperatures are below 70°F and the relative humidity is above 80 percent. Due to the sudden cooling that takes place in the carburetor, icing can occur even with temperatures as high as 100°F and humidity as low as 50 percent. This temperature drop can be as much as 60°F to 70°F. Therefore, at an outside air temperature of 100°F, a temperature drop of 70°F results in an air temperature in the carburetor of 30°F.

The first indication of carburetor icing in an aircraft with a fixed-pitch propeller is a decrease in engine rpm, which may be followed by engine roughness. In an aircraft with a constant-speed propeller, carburetor icing is usually indicated by a decrease in manifold pressure but no reduction in rpm. Propeller pitch is automatically adjusted to compensate for loss of power. Thus, a constant rpm is maintained. Although carburetor ice can occur during any phase of flight, it’s particularly dangerous when using reduced power during a descent. Under certain conditions, carburetor ice could build unnoticed until power is added. To combat the effects of carburetor ice, engines with float-type carburetors employ a carburetor heat system.

Q. Why is it necessary to warm up my two-stroke engine prior to flight?

A. Two-stroke engines must be run at low rpm and warmed up because they’re made of different metals that expand at different rates as they are heated. When heating steel and aluminum, the aluminum parts expand faster than the steel parts. This becomes a problem in two different areas of many two-stroke engines.

Typically, the cylinders have steel walls that expand slowly, compared to aluminum pistons that expand quickly. If an engine is revved too quickly during takeoff before warming up, a lot of heat is generated on top of the piston. This action quickly expands the piston, which can then seize in the cylinder. A piston seizure can stop the engine abruptly.

Another area of concern is in the engine around the crankshaft. This is an area where parts may get too loose with heat rather than seizing up. The crankcase has steel bearings set into the aluminum case which need to expand together or the bearings could slip. Many two-stroke engines have steel bearings that normally hug the walls of the aluminum engine case. The crank spins within the donuts of those steel bearings.

If the engine heats too quickly, the aluminum case out-expands those steel bearings and the crank causes the bearings to start spinning along with it. If those steel bearings start spinning, it can ruin the soft aluminum walls of the case, which is very expensive. If heat is slowly added to an engine, all parts will expand more evenly. This step is done through a proper warm-up procedure.

Many two-stroke engines are best warmed up by running the engine at a set rpm for a set amount of time. Follow the instructions in the engine operator’s manual; however, a good rule of thumb is to start the engine initially at idle rpm, get it operating smoothly at 2,500 rpm for 2 minutes for initial warm-up, and then warm the engine at 3,000 rpm for 5 minutes. The cylinder head temperature or coolant temperature for liquid-cooled engines must be up to the manufacturer’s recommended temperatures before takeoff. This may require running the engine at higher rpm to reach required temperatures on some engines.

Q. How does carburetor jetting affect my 2-cycle engine?

A. Carburetors are normally set at sea-level pressure, with the jets and settings determined by the manufacturer. However, as altitude increases, the density of air entering the carburetor decreases, while the density of the fuel remains the same. This creates a progressively richer mixture, same fuel but less air, which can result in engine roughness and an appreciable loss of power. The roughness is usually due to spark plug fouling from excessive carbon buildup on the plugs. Carbon buildup occurs because the excessively rich mixture lowers the temperature inside the cylinder, inhibiting complete combustion of the fuel. This condition may occur at high-elevation airports and during climbs or cruise flight at high altitudes. To maintain the correct fuel-air mixture, you can change the main jets and the mid-range jets setting for base operations at a high-density altitude airport. Operating from low-altitude airports and climbing to altitude where the mixture becomes rich for short periods is okay.

Operating an aircraft at a lower-altitude airport with the jets set for higher altitudes will create too lean of a mixture, heat up the engine, and can cause the engine to seize. The pilot must be aware of the jetting for the machine to adjust the mixture. Consult your pilot’s operating handbook for specific procedures for setting jets at different altitudes.

Q. Other than reducing noise, what does the exhaust system do for my two-cycle engine?

A. In two-stroke engines, the exhaust system increases the fuel economy and power of the engine. The two-stroke exhaust system is an integral part of any two-stroke engine design, often controlling peak power output, the torque curve, and even the rpm limit of the engine.

The exhaust system must be tuned to produce a back pressure wave to act as an exhaust valve. When hot spent gases are vented out of the exhaust port, they are moving fast enough to set up a high-pressure wave. The momentum of that wave down the exhaust pipe diffuser lowers the pressure behind it. That low pressure is used to help suck out all of the residual, hot, burnt gas from the power stroke and at the same time help pull a fresh fuel-air charge into the cylinder. This is called scavenging and is an important function of a tuned two-stroke exhaust system.

The design of the exhaust-converging section causes a returning pressure wave to push the fresh fuel-air charge back into the exhaust port before the cylinder closes off that port. That is called pulse-charging and is another important function of the exhaust system. Tuned exhaust systems are typically tuned to a particular rpm range. The more a certain rpm range is emphasized, the less effective the engine will operate at other rpm’s. Vehicles like motorcycles take advantage of this with the use of transmissions. Motorcycle exhaust pipe builders can optimize a certain rpm range, and then the driver shifts gears to stay in that range. Aircraft have no transmission, and thus do not have this ability.

On an aircraft, an exhaust pipe has to be designed to operate over a broad range of rpm’s from idle to full speed. This is part of the reason that simply putting a snowmobile engine on an aircraft doesn’t always work well.

Overall, the two-stroke exhaust system for an aircraft is a specific design and must be matched to the engine to operate properly and obtain the rated power. It also reduces noise and directs the exhaust to an appropriate location. Exhaust silencers can be added to reduce noise, but additional weight, cost, and slight power reduction are the by-products.

Q. What are the differences between a climb and a cruise propeller?

A. The climb propeller has a lower pitch, therefore less drag. Less drag results in higher rpm and more horsepower capability. This increases performance during takeoffs and climbs but decreases performance during cruising flight.

The cruise propeller has a higher pitch, therefore more drag. More drag results in lower rpm and less horsepower capability. This decreases performance during takeoffs and climbs but increases efficiency during cruising flight.

Q. Over the winter I will not be flying for a couple of months. I have been told that every couple of weeks I should start my two-cycle engine, warm it up, allow the oil to circulate over the bearings, and then shut it down. Some people say I should also cover the intake and exhaust openings to seal off the engine.

A. This is a question that will bring different answers depending on whom you ask. I have learned from experience, and it’s been confirmed by engine manufacturers, that this is really more harmful than helpful to your engine. The problem is that when you heat up the engine and cool it down you are introducing moisture into the engine. Also, when you seal up the engine you are trapping in the moisture. Trapped moisture in the engine will lead to rust in critical engine components, such as crankshaft ball bearings and connecting rod roller bearings. Once rust is started on these critical components failure is likely to follow.

If you are not going to operate the engine for an extended period, you should fog the engine with quality two-cycle engine-fogging oil, typically sold in aerosol spray cans. Most engine manufacturers specify a procedure to follow. I have found that it works best to remove the air filters, run the engine at a fast idle, spray the oil into each carburetor for 10 seconds, and then shut down the engine. Remove the spark plugs and spray the fogging oil into each cylinder for five seconds, ground the plugs and hand-prop the engine through several times, and then replace the plugs.

If I am using 100LL fuel, I am done. If I am using auto fuel, I will drain the tank, fuel lines, and carburetors. This has worked for years without any issues. This way, I am assured that the crankshaft bearings, rings, and cylinders are coated with oil through the winter months.

By Jim Leon, owner/operator of The Ultralight Place, an authorized FAA Part 145 repair station and independent Rotax repair center.


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