This is extracted from our Accu-Sim manual for our P47 Thunderbolt. We have tried to make this simple to understand for those who wish to know the basics of how a combustion engine works.THE COMBUSTION ENGINE
The combustion engine is basically an air pump. It creates power by pulling in an air / fuel mixture, igniting it, and turning the explosion into usable power. The explosion pushes a piston down that turns a crankshaft. As the pistons run up and down with controlled explosions, the crankshaft spins. For an automobile, the spinning crankshaft is connected to a transmission (with gears) that is connected to a driveshaft, which is then connected to the wheels. This is literally “putting power to the pavement.” For an aircraft, the crankshaft is connected to a propeller shaft, and the power comes when that spinning propeller takes a bite of the air and pulls the aircraft forward.
The main difference between an engine designed for an automobile and one designed for an aircraft is the aircraft engine will have to produce power up high where the air is thin. To function better in that high, thin air, a supercharger can be installed to push more air into the engine.OVERVIEW OF HOW THE ENGINE WORKS AND CREATES POWER
Fire needs air. We need air. Engines need air. Engines are just like us as – they need oxygen to work. Why? Because fire needs oxygen to burn. If you cover a fire, it goes out because you starved it of oxygen. If you have ever used a wood stove or fireplace, you know when you open the vent to allow more air to come in, the fire will burn more. The same principle applies to an engine. Think of an engine like a fire that will burn as hot and fast as you let it.
Look at the four pictures below and you will understand basically how an engine operates.
The piston pulls in the fuel / air mixture, then compresses the mixture on its way back up.
The spark plug ignites the compressed air / fuel mixture, driving the piston down (power), then on its way back up, the burned mixture is forced out the exhaust.AIR TEMPERATURE
Have you ever noticed that your car engine runs smoother and stronger in the cold weather? This is because cold air is denser than hot air and has more oxygen. Hotter air means less power.
Carburetor Air Temperature (CAT). Your CAT is the temperature of the air just before it enters the engine. On the P-47, air enters just below the engine, travels to the back of the aircraft, gets powered by a turbo, cooled by an intercooler, and forced back up the fuselage and into the engine. Use your intercooler flaps to control this temperature, you can cool your CAT by opening these flaps. However, the more you open these flaps, the increased drag can slow down your aircraft. The key is finding the magic balance between keeping your CAT low and keeping your intercooler flap drag low. MIXTURE
Just before the air enters the combustion chamber, it is mixed with fuel. Think of it as an air / fuel mist.
A general rule is a 0.08% fuel to air ratio will produce the most power. 0.08% is less than 1%, meaning for every 100 parts of air, there is just less than 1 part fuel. The best economical mixture is 0.0625%.Why not just use the most economical mixture all the time?
Because a leaner mixture means a hotter running engine. Fuel actually acts as an engine coolant, so the richer the mixture, the cooler the engine will run.
However, since the engine at high power will be nearing its maximum acceptable temperature, you would use your best power mixture (0.08%) when you need power (takeoff, climbing), and your best economy mixture (.0625%) when throttled back in a cruise when engine temperatures are low.
So, think of it this way:
For HIGH POWER, use a RICH mixture.
For LOW POWER, use a LEAN mixture.THE MIXTURE LEVER
Most piston aircraft have a mixture lever in the cockpit that the pilot can operate. Forward is usually rich, and backward is usually lean. The higher you fly, the thinner the air, and the less fuel you need to achieve the same mixture. So, in general, as you climb you will be gradually pulling that mixture lever backwards, leaning it out as you go to the higher, thinner air. How do you know when you have the right mixture?
The standard technique to achieve the proper mixture in flight is to lean the mixture until you just notice the engine getting a bit weaker, then richen the mixture until the engine sounds smooth. It is this threshold that you are dialing into your 0.08%, best power mixture. Be aware, if you pull the mixture all the way back to the leanest position, this is mixture cutoff, which will stop the engine.
AUTO-RICH AND AUTO-LEAN
More advanced aircraft may have an AUTO-MIXTURE system, with AUTO-RICH and AUTO-LEAN settings. You simply select which one you want and the auto-mixture system automatically adjusts the mixture for you based on altitude and power setting.INDUCTION
As you now know, an engine is an air pump that runs based on timed explosions. Just like a forest fire, it would run out of control unless it is limited. When you push the throttle forward, you are opening a valve allowing your engine to suck in more fuel / air mixture. When at full throttle, your engine is pulling in as much air as your intake system will allow. It is not unlike a watering hose – you crimp the hose and restrict the water. Think of full power as you just opening that water valve and letting the water run free. This is 100% full power.
In general, we don’t run an airplane engine at full power for extended periods of time. Full power is only used when it is absolutely necessary, sometimes on takeoff, and otherwise in an emergency. For the most part, you will be ‘throttling’ your motor, meaning you will be dictating where its limit is.MANIFOLD PRESSURE = AIR PRESSURE
You have probably watched the weather on television and seen a large letter L showing where big storms are located. L stands for LOW BAROMETRIC PRESSURE (low air pressure). You’ve seen the H as well, which stands for HIGH BAROMETRIC PRESSURE (high air pressure). While air pressure changes all over the world based on weather conditions, these air pressure changes are minor compared to the difference in air pressure with altitude. The higher the altitude, the much lower the air pressure.
On a standard day (59 F), the air pressure at sea level is 29.92Hg BAROMETRIC PRESSURE. To keep things simple, let’s say 30Hg is standard air pressure. You have just taken off and begin to climb. As you reach higher altitudes, you notice your rate of climb slowly getting lower. This is because the higher you fly, the thinner the air is, and the less power your engine can produce. You should also notice your MANIFOLD PRESSURE decreases as you climb as well.Why does your manifold pressure decrease as you climb?
Because manifold pressure is air pressure, only it's measured inside your engine's intake manifold. Since your engine needs air to breath, manifold pressure is a good indicator of how much power your engine can produce. Now, if you start the engine and idle, why does the manifold pressure go way down?
When your engine idles, it is being choked of air. It is given just enough air to sustain itself without stalling. If you could look down your carburetor throat when an engine is idling, those throttle plates would look like they were closed. However if you looked at it really closely, you would notice a little space on the edge of the throttle valve. Through that little crack, air is streaming in. If you turned your ear toward it, you could probably even hear a loud sucking sound. That is how much that engine is trying to breath. Those throttle valves are located at the base of your carburetor, and your carburetor is bolted on top of your intake manifold. Just below those throttle valves and inside your intake manifold, the air is in a vacuum. This is where your manifold pressure gauge's sensor is, and when you are idling, that sensor is reading that very low air pressure in that vacuum.
As you increase power, you will notice your manifold pressure comes up. This is simply because you have used your throttle to open those throttle plates more, and the engine is able to get the air it wants. If you apply full power on a normal engine, that pressure will ultimately reach about the same pressure as the outside, which really just means the air is now equalized as your engine's intake system is running wide open. So if you turned your engine off, your manifold pressure would rise to the outside pressure. So on a standard day at sea level, your manifold pressure with the engine off will be 30”. So how can an engine produce more power at high altitudes where the air is so thin?
Since the power an engine can produce is directly associated with the pressure of the air it can take, at some point during your climb (above 10,000 feet or so), that engine will be producing so little power that the aircraft can no longer climb. This is the point where the engine can barely sustain level flight, and is considered the aircraft’s service ceiling. A supercharger can raise this ceiling.SUPERCHARGING
The supercharger has a powerful fan installed in your intake system that forces more air into the engine. As you fly higher and the air pressure decreases, your supercharger will help to compensate and keep air pressure higher than it would be otherwise.
Let’s say while air pressure at sea level is 30”, it is 21” at 10,000. At 10,000 feet, your supercharger fan pushes in more air to increase your manifold pressure to 30”. Now your engine will produce the same power at 10,000 feet as it would at sea level. It would feel every bit as strong as it did when you took off.
However, even a supercharger has its limitations. At some point, it will hit its own limit of how much air it can force and manifold pressure will again start to drop off. Some aircraft include a second stage supercharger, this is basically a HIGH / LOW gear. Some planes may automatically kick into HIGH at a certain altitude. When you hit this altitude, you will notice a nice punch of power. Other planes, like the P-47 Razorback, use both a turbocharger and a supercharger. A turbocharger does the exact same thing as a supercharger, except while a supercharger is driven directly off the engine by mechanical gears, a turbocharger is driven by the power of the exhaust pressure. This is where the term ‘turbo lag’ comes from. Turbo lag is the time delay after you apply power and before the exhaust has enough pressure to spin the turbo charger hard enough to push more air into your engine. The turbo, being driven off exhaust, is only applying power when the engine is producing power. So the turbo process is a cycle – engine power produces more turbo power that produces more engine power and so on. It’s like rolling a snowball down a hill, this is your turbo ‘spooling’ up. Since the supercharger is gear driven, it moves perfectly in step with engine RPM – it’s there and ready when you apply throttle.
While turbo and superchargers can be used to compensate for lost air pressure up high, they can also be used to over-boost the power at sea level. This is called “ground boosting.” Ground boosting adds more air pressure (and power) at sea level than would normally be available.
If you add power and see your manifold pressure rise above 30”, then you have some form of supercharging or turbocharging adding more air into the engine than would normally be available. A normal engine that is producing 1,000 horsepower at 30” will produce 2,000 horsepower at 60”, since it is twice the pressure. 45” produces 1,500 horsepower and so on.
SUPERCHARGING HEATS THE AIR
The downside to supercharging is heat. The more you compress air, the more the temperature increases, therefore more supercharging = higher CAT temperatures. The increase in temperature can be extreme. -40 degree air coming into the intake system can be 100 degrees hotter after it exits the supercharger. This is where your INTERCOOLER comes into play. The INTERCOOLER is a heat exchange, and is basically a radiator taking heat out of the incoming air. Use your INTERCOOLER FLAPS to transfer heat out of your intake manifold and out the flap doors. The more you open your intercooler flaps, the more heat you remove. Use your intercooler flaps to keep CAT temps nice and low for a strong and healthy running engine.IGNITION
The ignition system provides timed sparks to trigger timed explosions. For safety, aircraft are usually equipped with two completely independent ignition systems. In the event one fails, the other will continue to provide sparks and the engine will continue to run. This means each cylinder will have two spark plugs installed.
An added advantage to having two sparks instead of one is more sparks means a little more power. The pilot can select Ignition 1, Ignition 2, or BOTH by using the MAG switch. You can test that each ignition is working on the ground by selecting each one and watching your engine RPM. There will be a slight drop when you go from BOTH to just one ignition system. This is normal, provided the drop is within your pilot's manual limitation.ENGINE TEMPERATURE
All sorts of things create heat in an engine, like friction, air temp, etc., but nothing produces heat like COMBUSTION.
The hotter the metal, the weaker its strength.
An aluminum rod at 0 degrees Celsius is about 5X stronger than the same rod at 250 degrees Celsius, so an engine is most prone to fail when it is running hot. Keep your engine temperatures down to keep a healthy running engine.CHT (Cylinder Head Temperature)
CHT is a measurement of the temperature in the back of the cylinder head. The combustion is happening right inside the cylinder head, so high power will increase temperature rapidly. The key is to watch and manage your cylinder head temperature by being aware of the outside air temp, keeping your speed up, and using your cowl flaps to control how much cooling is applied. The largest CHT rise will come from sitting on a hot ramp, just after takeoff, or in a slow and steep climb.LUBRICATION SYSTEM (OIL)
An internal combustion engine has precision machined metal parts that are designed to run against other metal surfaces. There needs to be a layer of oil between those surfaces at all times. If you were to run an engine and pull the oil plug and let all the oil drain out, after just minutes, the engine would run hot, slow down, and ultimately seize up completely from the metal on metal friction.
There is a minimum amount of oil pressure required for every engine to run safely. If the oil pressure falls below this minimum, then the engine parts are in danger of making contact with each other and incurring damage. A trained pilot quickly learns to look at his oil pressure gauge as soon as the engine starts, because if the oil pressure does not rise within seconds, then the engine must be shut down immediately.
Below is a simple illustration of a crankshaft that is located between two metal caps, bolted together. This is the very crankshaft where all of the engine's power ends up. Vital oil is pressure-injected in between these surfaces when the engine is running. The only time the crankshaft ever physically touches these metal caps is at startup and shutdown. The moment oil pressure drops below its minimum, these surfaces make contact. The crankshaft is where all the power comes from, so if you starve this vital component of oil, the engine can seize. However, this is just one of hundreds of moving parts in an engine that need a constant supply of oil to run properly. MORE CYLINDERS, MORE POWER
The very first combustion engines were just one or two cylinders. Then, as technology advanced, and the demand for more power increased, cylinders were made larger. Ultimately, they were not only made larger, but more were added to an engine.
Here are some illustrations to show how an engine may be configured as more cylinders are added.CARBURETOR ICING
You may think carburetor icing is most problematic in cold weather, but in many cases, the opposite is true. First, you have to understand what carburetor icing is: When the air enters your intake manifold, the passageway narrows, and the air is forced to speed up. This creates lower air pressure, and in turn, reduces the air temperature. Your Continental A-65-8 engine has an unusually long intake, which means the air pressure and temperature drop is larger than in many other aircraft. Also, the carburetor on the Continental A-65-8 engine is further away from the engine block than on many other engines, so it does not receive much heat from the rest of the engine. The temperature drop at the throttle plate can be as much as 40 degrees Fahrenheit, or more. What this means is, even on a 70 degree day, the air as it enters your carburetor can be below freezing, and therefore ice can build.
Now, you have to understand that air contains water, and the hotter the air, the more water it can hold. You may notice, when the sun goes down after a hot, humid day, grass, cars, bikes, windows, or anything outside can become moist. Also, first thing in the morning, you may see dew on the lawn. This is simply because the previous warm, saturated air was cooled, and since the cooler air cannot hold as much water as warmer air, it was rung out like a sponge.
So, if you know this, then you will understand that, on a hot, humid day, the air is thick with water. It is this hot, humid air that is most dangerous when it enters your carburetor if the air is cooled to the point of freezing. This hot, humid, water-soaked air can deposit water inside your carburetor like running water, and in a very short time, literally minutes, your carburetor can be loaded up with ice.
It is well to understand that the air is being cooled most when you pull the throttle back in flight (in a power-off glide. Greatly cooled air is then rushing in through the slightly cracked throttle plate, and this is why you are required to use carb heat every time you pull that throttle back in flight. At normal throttle settings you should not experience carb icing. However, if you are experiencing lower RPM or expected power than usual at a given throttle setting, you may have carb ice. If you suspect you do, turn on your carb heat immediately and advance the throttle slowly. If ice is the cause of the power loss, you should see normal power resume rather rapidly as the ice breaks up and dissipates.
So, when you fly your Cub, and it is hot and visibility is low, regardless of the temperature outside, think, “this is dangerous carb icing weather.”