The Pros And Cons Of Carbureted vs. Fuel Injected Engines
There are two main types of fuel induction in airplane engines, through a carburetor or through fuel injectors.Each has its benefits and drawbacks, so it's important to understand how they work in comparison to each other...especially if you fly both types.
Let's start with a basic systems overview.
The carburetor houses a float-type chamber where gas is collected and distributed to the fuel induction system.
By using the venturi effect, where air speeds up in the manifold due to a narrowing of the chamber, fuel is vaporized and mixed with air entering the engine. The volume of air flowing through the induction system is the primary means of fuel metering. The throttle controls how much air goes into the engine, while the mixture controls how much fuel goes into the mixture.
This fuel/air mixture then flows together through the induction system to the engine's cylinders where it's ignited by spark plugs to produce power.
Fuel Injected Engines
A fuel injected system uses a fuel pump to push fuel through a metering system, at which point the fuel flows through injector lines to each cylinder.
Keep in mind, there is no air mixed with the fuel at all through the metering system. A servo regulator measures airflow into the engine and meters fuel accordingly to gain the proper mixture.
At the cylinders, each fuel injector typically sprays fuel just outside of the cylinder head and is vaporized/mixed with air before entering the cylinder for combustion. Fuel injected engines often have electric fuel pumps as a backup to ensure fuel can be pushed through the metering system even if the engine driven pump fails. However in some aircraft, the electric backup pump doesn't provide enough pressure on its own to keep the engine running. Your POH will tell you which system you have.
Starting Your Engine
Cold starts are relatively easy for both carbureted engines and fuel injected engines. While priming a carbureted engine, it's possible that only one cylinder is priming, but it can be any number dependent on your engine setup. It's more common in fuel injected engines for each cylinder to be primed at once, usually by an auxiliary fuel pump.
Starting a hot, fuel injected engine can be tough. When you park a fuel injected airplane after running, there is no longer cooling airflow over the engine. As a result, fuel still in the injector lines can vaporize. Once you try to restart the hot engine, the cylinders initially may not receive the right amount of fuel in the mixture for combustion because it's in a gas state. Hot start procedures for fuel injected engines are usually more extensive than for those with carburetors.
In carbureted engines, there's a risk of carburetor ice forming, something that's led to hundreds of engine failures and crashes over the years. Ice in the carburetor is formed by air expansion and fuel evaporation in the venturi of the carburetor, both of which cool the surrounding are to a sub-freezing level.
Surprisingly, you don't need to fly through icing conditions to get carburetor ice. High humidity or visible moisture and temperatures between 20 degrees and 70 degrees fahrenheit are the most common causes of carburetor icing.
You'll recognize carb ice forming as there's a drop in RPM with a fixed pitch propellor or a drop in manifold pressure with a constant speed propellor. So what should you do? In carbureted airplanes, the corrective action is to use carb heat. Hot air is taken from around the exhaust shroud to pilot controls valves that let hot air into the carburetor. This hot air enters intake manifold and melts any formed ice. The hot, unfiltered air now entering the engine will result in a further drop in RPM or manifold pressure. After the ice has melted, RPM and manifold pressure will rise again and you can turn off carb heat.
But where does the ice go? Through your engine, making it cough, wheeze and shake until the ice is gone. It's not fun to hear, but stick with it, because it will eventually get better. There are countless NTSB report where pilots turned off carb heat because they thought they were making the situation worse, only to totally lose the engine shortly after. You don't want to be one of those statistics.
If you fly a fuel-injected airplane, this is irrelevant to you because there is no carburetor, no venturi, and thus no risk of carburetor icing. However, you'll still need to be concerned with induction icing or a clogged filter for both types of engines. Just like the icing that can build up on your wings, you can have ice from visible moisture form on your induction intake or air filter. On nearly all aircraft, there's an alternate air intake just for this reason.
Carbureted and fuel injected engines have their pros and cons, so there certainly isn't one "right" type of engine to use. Now that you know a little more about the difference between the systems, flying both types should be a little more simple.