What Is Engine Braking, and When Should You Use It?
Avoid premature brake wear by making your engine do the deceleration work.
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Simply put, engine braking is a way of reducing a vehicle’s speed using the powertrain instead of (or in addition to) the mechanical brakes. It’s something you might do to minimize brake wear when descending steep declines, and it gives the driver more control over the car’s speed when going downhill, which is especially useful if you’re carrying or towing a heavy load.
How Does Engine Braking Work?
Engine braking works by restricting airflow. In a vehicle with a gas engine, when you lift your foot off the accelerator pedal, the cylinders stop receiving air and fuel, but the engine’s spinning parts continue to rotate. As a result, the pistons keep trying to suck air into the cylinders but end up working against a vacuum because the throttle valve is closed, allowing very little air into the system. As the pistons pull against this vacuum, they slow down; this has a braking effect from the crankshaft through the transmission and on to the wheels.
Downshifting to a lower gear increases how quickly the pistons move, creating even more vacuum pressure and a greater braking effect. If you find yourself in a situation where you’d have to ride the brakes to maintain the posted speed—say, while descending the Davis Dam grade in Arizona—choose a lower gear to give them a rest and let your powertrain do the work. This is exactly the scenario that a vehicle’s tow/haul mode is designed for: Turn it on when you’re towing or hauling and the transmission will hold a low gear for you. Many automatics have an L shifter position that will do the same thing. Others have a manual-shifting mode where you can drop a gear or two by toggling the shifter or shift paddle.
Engine Braking in Diesels and Semis
Diesel engines don’t have intake throttles, so to achieve the braking effect, you have to restrict airflow somewhere else. To do this, modern passenger-vehicle diesels use an exhaust brake downstream of the engine, which cuts off the path of the engine’s exhaust and creates backpressure in the cylinders. In some vehicles, the exhaust brake is a simple butterfly valve; in others, such as GM trucks with the Duramax diesel, a variable-vane turbo fulfills this task in addition to its other duties. Either way, the result is the same: With the valve or turbo closed, would-be exhaust air can’t leave and is instead compressed, slowing down the engine.
Those “engine brake prohibited” signs you see on highways in and around towns? They’re aimed at truckers who use compression brakes on their big rigs. These work a bit differently from exhaust brakes, as they allow air to compress in the cylinders (not in the exhaust system) and then let it out when the piston is at the top of the compression stroke. This happens repeatedly in every cylinder and produces a pretty loud and annoying noise—some liken it to a machine gun firing—hence the signs and restrictions.
What About Hybrid and Electric Vehicles?
Things are a little different when electric motors are involved. To slow down off-throttle, hybrids often use regenerative braking, where an electric motor recaptures energy and uses it to charge the battery, but the engine is still capable of providing braking force. Some hybrids have an L or B setting that increases regen and employs engine braking if necessary.
In an electric vehicle (EV), there’s no engine, but similar to a hybrid, braking happens both mechanically at the rotors and via regen from the motor(s). Given that most EVs have only one gear, you can’t choose a different transmission mode, but the driver can often increase the default level of regen so you don’t have to use the friction brakes as much. And some EVs have a paddle that, when pulled, will boost regen on demand.
