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Anyone who’s crisscrossed the country has likely seen posted signs that read, “No Engine Brake,” “Residential Area Do Not Use Engine Brakes,” and “Engine Braking Prohibited.” These restrictive messages are aimed at semi-truck drivers who have disturbed the region with obnoxiously loud engine braking, but engine braking isn’t reserved for semi-trucks.
Though long-haulers use it more than others, it’s also used by mechanical souls who continue to drive with manual transmissions. Shifting braking responsibility from the brake pads and rotors to the engine can increase safety, decrease maintenance costs, and prove helpful in various situations.
The Drive’s crack info team has driven all over the country and is well versed in the art of engine braking. For your hungry minds, we’ve put together a quick lesson to explain what it is, how it works, and how it affects your car. Let’s jump in.
What Is Engine Braking?
Engine braking is the act of using your engine to absorb energy and slow the vehicle down rather than using the brake pedal and friction brakes to absorb energy and slow the vehicle down. This is achieved by releasing the throttle while in gear.
Methods of Engine Braking
Different vehicles use different methods of engine braking. We break it down below.
In an average consumer vehicle with an automatic transmission, engine braking is rarely used, but it’s technically possible to do so by switching from Drive (D) to Low (L). L keeps the vehicle in low gearing, so it will drag the vehicle’s speed down when you let your foot off the gas pedal. Like neutral-dropping while driving, don’t ever do this while driving at speed. It could safely be used by shifting in L before descending down a mountain or hill.
Some automobiles with automatic transmissions that feature the capacity for the driver to change gears, either via paddle-shifters or a console-mounted gear lever, can also use engine braking. All the driver has to do is downshift to a lower gear and, once again, let the lower gear drag the vehicle’s speed down.
Far more common is engine braking in a vehicle with a manual transmission. While in gear, releasing the gas pedal closes the throttle body. Without air to suck into the cylinder, a vacuum is created. This vacuum slows the vehicle down.
Harsher braking can be achieved by downshifting into lower gears, but this can only be done if you really know your car, its rev ranges, and its gearing. Shifting into too low of a gear while traveling at too fast of a speed could shoot the needle above the redline and damage your engine. If you choose to engine brake with a manual transmission, allow the car to slow into the proper range, shift down one gear at a time, and make sure the people behind you know you’re slowing down.
Diesel semi-trucks most commonly use two different methods of engine braking. One technology uses a valve on the exhaust of the engine to create backpressure. This backpressure pushes against the positive motion of the engine and slows it down. The other technology is called a Jake Brake.
What Is a Jake Brake?
Contrary to popular belief, the jake brake has nothing to do with Jake from State Farm. The jake brake is a valve timing technology used by diesel trucks to slow down the vehicle using the engine. In a normal combustion cycle, the process starts when the intake valve opens to let gas into the combustion chamber. The piston compresses the air, spark ignites the air, and the piston goes back down to move the vehicle. The exhaust valve then opens, and the piston goes up to push the exhaust air out of the chamber.
With Jake Brake tech in action, the exhaust valve opens after the first compression stroke to release the air and energy. Without energy to push the piston back down, the system is retarded and the vehicle slows down.