Avoid Engine Brakes

[W. K. Lis]

I seen these signs on highways and wondered what are they talking about?

Avoid Engine Brakes​

Essentially, engine braking is when you down shift the transmission in order to slow down your vehicle. For example, you’re going down a road in fifth gear, you see a red light ahead and instead of slowing your car by using the breaks, you down shift into third gear.

While down shifting is an easy way to take it easy on your brakes, it’s also a great way to needlessly waste gas.

Anytime your engine revs and the RPMs increase (which is what happens when you down shift), you will use more gas than you would at the same speed but in a higher gear (lower RPMs).

So, instead of using your engine to brake go ahead and ease on to your brake pedal and smoothly come to a stop.

In trucks, it's something more. Check this out.

Turns out that on trucks it could go out of control, if used improperly. An engine brake is designed to assist the service brakes in controlling the speed of a loaded tractor trailer as it descends a steep grade, and should be used only for this purpose.

Apparently, the signs are for the trucks not to use their engine brakes, especially if there is a curve ahead and they have to slow down.

 

[CDL.TO]

Turns out that on trucks it could go out of control, if used improperly. An engine brake is designed to assist the service brakes in controlling the speed of a loaded tractor trailer as it descends a steep grade, and should be used only for this purpose.

Apparently, the signs are for the trucks not to use their engine brakes, especially if there is a curve ahead and they have to slow down.

An interesting topic. I didn't even know that engine braking can refer to something you can do in your car.

I'm 99% certain that the main reason you see 'Avoid Engine Brakes' signs is that Engine Brakes are extremely noisy (almost a machine-gun type noise). In rural Ontario, you often see 'Avoid use of Engine Brakes' signs on the way into towns and villages.

Other names for engine brakes are Jake brakes and compression brakes.

 

[ShonTron]

I see this in some urban areas on major roads as well. I know what engine brakes can sound like at night, and yes, they can be very annoying. I didn't know about the gas consumption of engine brakes, I thought truck drivers did it to save gas.

 

[RedRocket191]

Nope, trucks use it to save the air brakes.

Air brakes take time to recharge after you make an application, so applying and releasing the brakes too many times too frequently might result in less air in the pipes when you need it.

 

[hkric88]

Engine brakes for diesel engines operate completely different from the analogue in cars. This is because diesel engines do not have a throttle valve in the intake, which when closed in a gasoline engine (foot off the gas), it causes resistance to the air flowing into the engine. Therefore there is resistance to the engine turning, and that causes breaking. In a diesel engine, there is no such throttle, since power is controlled by the amount of fuel injected into the cylinders, as opposed to the amount of air. Therefore if a diesel truck simply down shifted and eased off the gas, nothing would really happen, engine resistance would be minimal. Remember that the energy requirement of compression during the compression stroke is returned during the power stroke, with or without the addition of fuel.

To allow diesel engines to employ an engine break, a mechanical process known as the 'Jacobson Break' uses some form or another to open the exhaust valve of each cylinder during the top of that cylinder's compression stroke (Top Dead Centre before beginning of the power stroke). Therefore, resistance incurred by the compression stroke is not returned during the power stroke.

When the 'Jake Brake' is activated (and it must be deliberately activated), the resultant process just described causes an explosive releases of air into the exhaust manifold as the exhaust valves open at the height of engine compression.

This is noisy.

Most people describe the sound as being similar to a machine gun. And it is precisely this noise generated that is the reason for those signs you see. It is so the residences don't have to listen to those noisy engine breaks all night and all day.

 

[Degnaw]

I see this in some urban areas on major roads as well. I know what engine brakes can sound like at night, and yes, they can be very annoying. I didn't know about the gas consumption of engine brakes, I thought truck drivers did it to save gas.

I do it regularly in my car, because if the rpm'age is above 1200 or so the engine consumes basically no fuel. Ok so its not the same concept, but still...

 

[wyliepoon]

The only place where I've ever seen an "Avoid Engine Brakes" sign is Highbury Avenue in London, ON heading north, just before the expressway section ends and the speed limit drops from 100 km/h to 50 km/h. Unfortunately it's hard to avoid engine brakes there sometimes because the expressway drops somewhat steeply just before the red light at Hamilton Road where the expressway ends.

 

[W. K. Lis]

I usually do the downshifting in winter during a snowstorm when I approach a stop sign or red light. I can hear the engine revving up as I do so to slow down, but I don't slide as I do using only the brakes. That's about the only time I do so with my car.