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Home> Continental & Lycoming TroubleShoot
High Cylinder temperature
1. Spark plugs of improper heat rating installed.
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| The heat rating of a spark plug is the measure of its
ability to transfer heat received from the firing end of the plug into the
cylinder and engine cooling system.
A cold plug transfers heat rapidly away from its firing end into the cooling system and is used to avoid core nose heat saturation where combustion chamber or cylinder head temperatures are relatively high. A hot spark plug has a much slower rate of heat transfer and is used to avoid fouling where combustion chamber or cylinder head temperatures are relatively low. The primary means of adjusting heat range are by varying the length of the core nose and the alloy material used in the electrodes. Hot plugs have a relatively long insulator nose with a long heat transfer path. Cold plugs have a much shorter insulator nose and thus, transfer heat more rapidly. The correct heat rating for the engine design ensures that the plug operates cool enough to prevent preignition but warm enough to resist the accumulation of conductive plug-fouling deposits. With Champion plugs the higher the number the hotter the plug Today, the term "hot and cold" is commonplace in general aviation — especially when related to engine spark plugs. With the introduction of high compression, and high horsepower engines, a need for improved spark plugs was eminent. Spark plugs used in low compression, low horsepower engines were not compatible with the new, more sophisticated powerplants. The non-compatibility factor of existing plugs with new engines resulted in development of spark plugs capable of operating efficiently at high compression ratios and high power settings. Many aircraft operators have come in direct or indirect contact with the term "hot and cold" during the course of conversation with other pilots or mechanics. Its meaning and relationship to engine operation was sometimes rather vague. What do we mean by "hot and cold" spark plugs? What is the relationship between an engine and spark plugs? How important is it to smooth engine operation? These are but a few questions we will try to answer in this article. Both spark plug and engine manufacturer working together determine the proper type spark plug suitable for each engine model These plugs can be either fine wire or massive electrode type. Before being released for production, each new type plug is checked in the laboratory and under actual flight conditions. They are tested through a wide range of operating conditions and at different power settings, and only after both engine and spark plug manufacturer are completely satisfied with test data are plugs released for production. To eliminate any possibility of error in spark plug selection, both manufacturers provide spark plug charts as a guide for proper plug selection. Final authority concerning proper plugs for a specific engine is the engine manufacturer. When selecting spark plugs, be sure to also consider the spark plugs’ heat range. Operating temperature of the spark plug insulator core nose is one factor that governs formation of troublesome combustion deposits. To help overcome this problem, selection of spark plugs with the proper heat range should be made. Spark plugs are susceptible to carbon deposits when the operating temperature of the core nose insulator is at or below 800o F, but an increase of just 100o F is sufficient to eliminate formation of these deposits. Also, lead deposits form because the bromide scavenger contained in tetraethyl lead is nonactive at low temperatures. At 900o F temperature, the bromide scavenger is fully activated, disposing of lead deposits with combustion gases during exhaust cycle. In this case, an increase of just 100o F was sufficient to make the difference between a smooth and rough running engine. To eliminate or keep this problem at a minimum, avoid prolonged idling at low RPM, avoid power-off let downs, and after flooded starts run engine at medium RPM before taxiing. Deposits formed between 1000o F and 1300o F are low in volume and electrical conductivity and are least apt to cause spark plug fouling. This is the reason for selecting a plug that will operate within the aforementioned temperature range at all power settings. Now let’s get back to the term "hot and cold" as related to engine spark plugs. Normally, a hot plug is used in a cold engine — low horsepower, and a cold plug in a hot engine — high horsepower. In actuality, these terms refer to the plugs ability to transfer heat from its firing end to the engine cylinder head. To avoid spark plug overheating where combustion chamber or cylinder head temperatures are relatively high, a cold plug is recommended, such as in a high compression engine. A cold running plug has the ability to transfer heat more readily. A hot running plug has a much slower rate of heat transfer and is used to avoid fouling when combustion chamber and cylinder head temperatures are relatively low. From our discussion, it is clear to see that there is more to spark plugs than just buying a set and installing them in your engine. Be sure you know what type of spark plugs to use with your engine. Also, good spark plug service and maintenance is as important as proper plug selection. Take care in selecting and maintaining your plugs, it can result in many additional hours of smooth engine operation. Additional spark plug information is always available from the engine or spark plug manufacturers and other service organizations. |
2. Cooling baffles missing, broken or bent. Check this one out especially
if you have high cht's after maintenance. On the IO550 A make sure that the inner-cylinder baffle (smaller one that
goes around the head flanges is not mounted incorrectly (180 degree turned). Each engine model has a slightly different
arrangement of inner-cylinder baffle and it is easy to install them incorrectly.
If the rear cylinders are normal but the forward cylinders are hot what is happening is that the cooling air rushes in through the inlet, follows the bottom of the cowling as it swoops upward at the back till it hits the vertical rear baffle where this high velocity air is abruptly slowed down, raising its pressure. On the top side of the two rear cylinders, the lowest pressure exists due to proximity to the cowling outlet and the scavenging action of the prop. There is high pressure under the rear cylinders, low pressure on top and, presto, most of the cooling air flows through and around the rear two cylinders leaving the forward cylinders with less cooling air and much higher temperatures.
3. Partially plugged fuel nozzle.
4. Fuel lines of improper I.D. being used ( Lycoming aircraft engines). Primer
lines have the same threaded connections as fuel lines but are of much
smaller I.D. The only exception to this rule is the IO540 -E1B5 used in
the Aero Commander 500S. Fuel lines on Lycoming engines should have an
inside diameter of .085 inch to .090 inch. Check fuel line ferrule braze
joint and surrounding area for cracking, evidenced by fuel stains.
5. Aircraft engine improperly timed. Magnetos timed too far in advance can
cause high CHT's and a decrease in EGT's
note: higher CHT and oil temperature with Laser ignition systems.
6. Engine operated excessively lean. If this condition is suspected,
check combustion chamber for carbon deposits. If there are none, this is
sufficient reason to be suspicious of a continuous lean-engine operation.
7. Mixture control improperly rigged. Even though mixture control lever
at pilot seat appears to have full travel, be sure to check at carburetor
or injector to insure that lever is going against full rich and idle cut-off
stops.
8. Exhaust gas leak onto cylinder. Inspect for signs of exhaust leakage;
burnt paint and whitish deposits. This can occur when the exhaust gasket is
leaking and blowing onto the cylinder
9. Loose cylinder head temperature leads. Intermittent contact will
give false readings. High cylinder head indications can be a result of
the cylinder head temperature lead passing too close to an exhaust stack.
10. Inaccurate or incorrect CHT gauge or probe.
11. Taking CHT from a non-reference point on the cylinder. For example, reading CHT using a spark plug thermocouple when the manufacturer references CHT temperature from the thermocouple boss.
Taking the CHT temperature from the spark plug boss places the thermocouple in spark plug's heat path. The temperature is more indicative of spark plug temperature than actual CHT temperature. The only engine that I am aware of where the manufacturer lists CHT for two reference points (thermocouple boss and spark plug boss) is the Continental E225 engine. Maximum CHT at the spark plug boss is higher than at the boss.
Some engines such as the Continental A & C series, O-200, O-300, GO-300 do not have a thermocouple boss. Airplanes that used these engines did not have a CHT as factory equipment.
12. Incorrect carburetor. Carburetors are matched to the engine and
airframe. Most carburetors are jetted richer than best power at full throttle
for engine cooling. Full power fuel flow may be the culprit. Check fuel flows
against those published in the Lycoming operators manual for your engine. If
your full power fuel flow is low it will cause high CHT temperatures
13. Insufficient air cooling caused by high power and insufficient flight speed.
14. Piston scuffing against cylinder wall. May also be evidenced by burnt paint on the cylinder barrel or if you scrape the paint from between some of the cylinder barrel fins you might see some metal bluing. See Cylinder Barrel Cooling Requirements.
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Scuffed piston skirt Operating the engine with 50W oil on a cold 40 degree morning will scuff the cylinder walls. Common practice where I live! You don't see the damage, your mechanic doesn't see the damage but we sure did in our cylinder shop |
15. Improper fuel flow on Continental TSIO series engines with particular attention to the fuel pump settings and performance. You may have to find a shop (only a few in the country) who have a portable flow bench (porta-test) unit to measure fuel flows. Clarksburg Air Repair in Sacramento Ca. has such equipment. If you know of anyone else then let me know and I'll add them to my list.
16. Increased power output. Power requires heat and more power produces more heat. Some of this excess heat finds its way into the cylinder head. If your new "tuned exhaust" is really producing 25% more power you should have higher CHT temperatures.
17 Cylinder cooling fins blocked.
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Cylinder Cooling Fins Blocked Notice how cooling fins don't extend all the way through on this FAA/PMA aftermarket cylinder for a Lycoming engine. These fins are suppose to be open for air to flow thru. When you purchase cylinders take a minute or two and give it a good look over. Take it home where you can spend some quality undisturbed time with your cylinder. Compare it with one of your old cylinders. |
| What is high CHT? The manufacturer's CHT limitations are usually found in the Maintenance Manual and the Engine Type Certificate. Generally, in Lycoming non-turbocharged engines, desirable CHT is <400 F and red line 500F. |
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CHT probe used in many Continental and Lycoming engines. Part number 177S34. This is a screw-in type probe. Some aircraft use a bayonet style probe as shown below. |
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CHT bayonet adapter part number 28202 used with bayonet style CHT probes |
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