No Engine braking does not affect your transmission, whether it may be automatic or manual. It is good but not as effective as brakes.
What does engine braking mean
There is a lot of confusion when it comes to engine braking, some people use the clutch and shift down the gear and slipping the clutch is engine braking. But that is not engine braking and it is a bad idea to do with clutch cause it will wear your clutch and disrupt the balance of your car.
Actually engine braking is achieved when you just lift your accelerator pedal and your are in gear. As you’re in gear it slows your car down.
How does it work?
So what is going on when you lift your foot from the accelerator pedal. When you lift your foot there is a throttle valve which is closed because there is no acceleration. This valve sucks in air when you press the accelerator. So when the valve is closed there is a vacuum created between your engine and throttle valve this slows your car down.
Is it good?
Yes, it is actually beneficial. When you use brake you convert the kinetic energy of you car into heat energy. But in engine braking you are using the engine to convert the kinetic energy to heat energy and your putting more heat into engine.
The heat in the engine would not have any issues because it has dedicated cooling system using the radiator and the coolant which is circulated in the engine. This extracts the heat out of the engine.
Benefits of Engine Braking
Most of the cars now-a-days are Fuel injected. So when you lift your accelerator most of the cars will shut off the fuel injection. So no fuel is used and it improves your efficiency.
Less brake wear cause you will be using your engine to slow your car down. And you will not have to replace your brake pads and rotors more often. If you are in long steep descent and you use your brakes the brakes gets overheated more. Engine braking us advisable in long descent.
So it is not that you only use your engine braking cause it will definitely not slow you down as hitting brakes. Engine braking depends on the gear you are on and the RPM at which your engine is rotating.
If you are on 4th gear at 2000 rpm engine braking will not be that effective. But if you are on the 2nd gear and around 3000 rpm you will definitely slow your car at a faster rate. It is mostly advised to use only on long downhill. It depends on the driver also if your are comfortable with it do not use it, better use brakes.
In some modern diesel cars Jake Nett brake (engine braking) is installed which open the exhaust valve after the compression cycle is completed. It releases the compressed air from the cylinder and slows your car down.
Refer to this video and see how much it is effective
Also published on Medium.