How Does Engine Braking Work?

How does engine braking work?
Experienced mechanics share their insights in answering this question :
Engine braking is the process of using the engine to slow the vehicle down instead of using the vehicle brakes. Engine braking can be used on both gas or diesel engines and is used to slow a vehicle down or control vehicle speed without using the brakes on a steep downhill grade, while also help the brakes from overheating. Engine braking transfers the heat energy to the engine instead of the brakes and releases the heat to the engine cooling system.

The gasoline engine uses engine braking by using the engine’s throttle plates. When you close the throttle plates on the engine, it can no longer pull air in large amounts and the fuel injectors shut off to cut most fuel to the engine. The cutting of fuel and closing of the throttle plates cause the engine to turn by the powertrain instead of the normal engine combustion process of the engine. This in turn will put a drag on the vehicle and slows it down without using the vehicle brakes.

A diesel engine uses different types of engine brakes since it does not have a throttle plate to control how much air goes in the engine and controls how much fuel is injected into the cylinder depending on the need for power. Most smaller vehicles that do not pull heavy weights like a semi-truck and trailer will have a valve that will be in the exhaust outlet of the engine and will shut off the exhaust and fuel of the engine, causing a loss of power and drag on the powertrain versus using the brakes. The other type system, often called a JakeBrake, is a system that will open and close the exhaust valve and fuel injection valve to control the engine backpressure on the powertrain to slow a vehicle, maintain vehicle speed on a downhill incline, or to slow the vehicle to assist the brakes and keep the brakes from overheating.

The overheating of vehicle brakes can lead to dissipation of brake effectiveness, known as brake fading, and can then lead to glazing of the brake pads and brake rotors. Overheating the brakes can also warp the rotors and cause stress cracks in the rotors and pads. When traveling through mountain regions, you might have seen vehicle runoff ramps. These ramps were put there for the big semi trucks that have had brake failure and can no longer stop or slow down their truck. The truck may have had an engine brake system failure and could no longer use the engine to assist the brakes and the brakes got hot to the point of fading. With a heavy load and going downhill, the truck is not be able to control its speed. Therefore, it is important to have a working engine braking system.

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In standard petrol (gasoline) engines, engine braking works by restricting airflow (by releasing the accelerator) which causes a high manifold vacuum that the cylinders have to work against. This has the effect of sapping energy from the engine which is what gives that sudden sense of deceleration and drop in power.
Diesel engine braking

Unfortunately, diesel engines don`t have a throttle valve, so instead they use an exhaust valve that closes during the exhaust stroke of the engine cycle. Back pressure is then created which fights back against the motion of the pistons, slowing them down.

The master cylinder transmits hydraulic pressure to the slave cylinder when the pedal is pressed. When you push the brake pedal it depresses a piston in the master cylinder , forcing fluid along the pipe. The fluid travels to slave cylinders at each wheel and fills them, forcing pistons out to apply the brakes.
In engine braking, the engine acts as a compressor, thus producing the required braking power. The braking power is generated by either reducing the volumetric efficiency or increasing the pressure difference across the cylinder. This is usually achieved by means of exhaust valve lift modulation.
Engine braking is more fuel-efficient than normal braking. This is because when you engine brake, the engine stops consuming fuel. Engine braking only saves a little fuel here and there, but those savings can add up quickly, especially on long trips!
Additionally, fuel injection engines generally do not use fuel while engine braking. This is known as deceleration fuel cut-off (DFCO).
When a semi uses engine brakes you will often hear a loud blat-blat-blat, sometimes referred to as a Jake Bark because the compressed air is forced through the exhaust valve in the engine`s cylinder.
Air brakes are really three different braking systems: service brake, parking brake, and emergency brake. The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
These include disc brakes, drum brakes, and the parking brake. Disc brakes and drum brakes perform the same task of slowing your vehicle when the brake pedal is pressed but use different parts to do so.
BMEP can be thought of as a `Torque ratio` for the engine. An interesting way to think of BMEP is that it tells you how efficiently the engine uses its size (displacement) to produce torque. It is a tool that can help you during the generator system design.
Three different types of brake retarder systems are used on diesel engines: exhaust brakes, engine compression brakes, and variable-geometry turbo brakes.
The power developed by an engine at the output shaft is called brake power and is given by Power = NT/60,000 in kW where T= torque in Nm = WR W = 9.81 * Net mass applied in kg.
Engine braking is basically the process of slowing the car down by releasing the accelerator and shifting down through gears, rather than using the footbrake. In technical terms, it`s using decelerative forces in the engine to slow the speed the wheels are turning at.
Engine braking can be helpful for driving downhill. When driving downhill, staying in a lower gear allows you to maintain a safe speed, whereas going up in the gears will mean you speed up rather quickly – without even engaging the accelerator – which could potentially be dangerous.
Engine braking isn`t necessarily bad for your engine or transmission, but it can be if you do it incorrectly. You have to balance the benefits of engine braking against some other factors: Shifting frequently increases clutch wear on a manual transmission, and can lead to high temperatures* in an automatic transmission.
Whenever you brake harshly, your car`s transmission will have to downshift to lowest gears automatically or manually to regain lost momentum. Lower gears require faster engine revolutions which in turn demand more fuel.
braking consumes fuel indirectly but not directly. As the vehicle speed reduces by braking so the energy is needed again to accelerate. braking means you need to accelerate after sometime. So less braking means less fuel use.
The actual slowing of a gas powered vehicle is caused by vacuum rather than compression. This is the “exhaust” brake for gassers.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

EPC light on 2004 Jetta. Code said brake switch faulty, but brakes lights worked. Replaced brake switch, now no brake lights.
ANSWER : Hi there. The first thing that pops in my mind beyond of what you have listed is perhaps the electrical harness attached to the brake switch. Perhaps the harness itself is damaged or you have an exposed wire, loose ground wire somewhere. You might want to have a professional mobile mechanic come to your location and complete a brake lights not working inspection to help you diagnose this issue.

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Honda Civic 2008. AC stopped working suddenly and engine check light came ON for a day.Next day AC worked and no engine light.
ANSWER : Hi there. In both cases, when an AC system is not working and a warning light is coming on the dashboard, typically a professional mechanic will complete the same initial inspection, which is to download the stored error codes that triggered the warning light initially. This tends to give them an idea as to what is causing the problems. This could be something as simple as an electrical relay that is faulty or loose or an indication of a computer related "glitch". Either way, the best way to move forward is to have a professional mobile mechanic complete a check engine light inspection.

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When I put my foot on the brake my back left brake light goes off, when I take it off the brake the brake light comes on
ANSWER : I would suggest trying to disconnect and reconnect the wiring to the rear lights. I have seen these older Toyota vehicles rear lighting short out on the lights themselves due to the way the light circuit was made on the lamp housing. Look at the housing circuit on the light to see if one is touching another, and that could tell you why it is malfunctioning. There could also be a problem with the brake light switch. If you are not comfortable dealing with wires, I recommend getting in touch with a certified mechanic who can look at your brake light issue for you to see what’s going on.

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Low brake pedal and anti-lock brakes are not kicking in.
ANSWER : The brake pedal is working to stop the vehicle, but there may be air in the controller unit causing the ABS brakes to not function. I recommend bleeding the brake system from the farthest location from the master cylinder to the master cylinder including the ABS unit. If the brakes are still spongy after a full bleed, then the controller will need to be replaced. If you need further assistance with your brake pedal being spongy, then seek out a professional, such as one from Your Mechanic, to help you.

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Okay so when I turn my lights on at night my brake lights do not work, when I don’t have my lights on they work fine.
ANSWER : Hi there. Based on your inquiry, it’s highly probable that you have a damaged light switch or some electrical fault inside the turn signal relay. However, it’s also possible that the brake switch above the brake pedal could also be the source of your problem. Due to the fact that multiple electrical components might be causing this issue, it’s a smart idea to first have a professional mobile mechanic complete a brake lights are not working inspection, so they can determine what’s causing all of these issues and recommend the right repairs.

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An inconsistent starter motor that occasionally works perfectly now will not work. Then engine will not crank at all.
ANSWER : Hi there. It is possible for the starter to fail. But before replacing the starter, make sure that the battery cables are tight and clean. Then check the starter relay and make sure that the relay is clicking. If the relay is not clicking, then check for power to the relay. If there is power to the relay but there is no click, then the starter relay would need to be replaced. If there is no power to the relay, then either the wiring from the relay to the ignition switch has a break in it or the ignition switch is failing. If there is power to the relay and power to the starter motor, then the starter motor needs replaced. If you need further assistance with your no start issue, then seek out a professional, such as one from Your Mechanic, to help you.

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Supportive bar attached to brake pedal interrupts proper braking process
ANSWER : It’s possible to have someone alter the brake pedal or adapt a brake pedal from a different vehicle. You may call around to some customization shops to see if anyone is interested in taking on the project. But first, I would try reaching out to the selling dealer with your concern – especially since it is a new model. Good luck.

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I replaced the brake light switch. Gear lever unlocked because it had locked. Now the brakes feel hard.
ANSWER : Hi, thanks for writing in. You need to check the brake switch install adjustment. You may have the switch adjusted too tight and it is holding the the brake pedal partly down. This will cause the brake master cylinder to not release the brakes when you let off the pedal. Readjust the switch away from the pedal until you feel some free play again in the pedal. This should fix the problem. If you would like help, consider having an expert automotive technician from YourMechanic come to your home or office to inspect and diagnose this issue for you, and make or suggest any repairs as needed.

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