Every engine needs several things to produce power. Air, fuel, and spark are the main ones. Based on this, every single engine out there is susceptible to losing power due to ice, whether it's fuel injected or not. The above posters were correct in that both carb engines and fuel injected engines can have the primary air filter (in the front) ice up. This will result in either you pulling the alternate air valve, or it will automatically open, depending on what you're flying.
If that standby mechanism somehow fails, you've got a boat anchor instead of an engine (assuming the primary air filter is completely iced over instead of partially). If the standby mechanism works as designed, you will have slightly less power due to no ram air and warmer air being drawn from inside the engine compartment as previously stated.
That is not the only way ice can affect engines though.
FlyingNole said:
The new 172SPs are fuel injected, and therfore have no carb, right?
If so, is the intake/filter on the nose just an air filter where the air passes and goes directly to the cylinders? Or to where?
The airflow in a fuel injected engine goes from the airfilter (assuming it's not iced up) into a throttle body. This throttle body is directly connected to your throttle control and is basically a valve, controlling the amount of air entering the engine. From the throttle body, the air is routed to each of the cylinders, hopefully evenly. Notice that no fuel has been added at this point, and the air has gotten all the way to the cylinders. Once it's in the cylinders, the fuel injectors inject the correct amount of fuel directly into the cylinder. This is supposed to create a more equalized distribution of fuel throughout the 6 cylinders. With this scenario it is difficult to create a blockage in the system somewhere down the line from ice. This is where icing is differnt for a carb engine.
The airflow in a carb engine goes from the airfilter to the carb. As you probably know, the carb is basically a tube the air flows through, with a valve in it, just like the throttle body has. The difference is, the tube in the carb gets a little more narrow in the middle. For the air to fit through the narrow part, it has to go faster. This results in the venture effect, which creates lower pressure, or suction. This suction is used to suck fuel into the airstream. Therefore, if you open the valve in the carb with your throttle control, more air flows through, causing more suction, which automatically makes more fuel come in. Pretty handy, except that the lower pressure also causes lower temperature, making water particles in the air more likely to condense and possibly freeze. Additionally, all the fuel is vaporized as it goes into the airstream. Adding enough heat to the fuel to vaporize it requires taking that engergy from somewhere else, namely the air. These two things combined can decrease the temprerature significantly. This is why a carb engine needs something to heat the air. If enough ice forms on that valve, it can shut the engine down completely, and if the carb is iced up, the only way to get air flowing through again is to get the ice off....no alternate option available here. That's why it's important to use the carb heat before it's too late.
Never hesitate to ask a question if you aren't 100% sure.