A Staff Report from the Straight Dope Science Advisory Board

What are "jake brakes" and why are they prohibited in some locations?

April 12, 2008

Dear Straight Dope:

I was driving around town one day when I noticed a traffic sign that the city put up on a route heavily frequented by trucks. The sign said, "NO JAKE BRAKES." Since my knowledge of automobiles pretty much consists of "put gas in tank, turn key, and step on pedals," I was intrigued by this sign. What awful things must this brake do (besides having a silly name) that a city feels compelled to ban its use? I assume it's something that trucks have, because I've never seen a car salesman offer me one, but aside from that I have no idea what a "jake brake" is or does. Since you are the master of all knowledge, I've decided to ask you to give me the Straight Dope on this device.

Turns out this is a rather hot issue with the company who holds the trade name for Jake brakes, but more on that later. First an introduction to the world of engine brakes.

Unlike the wheel brakes you have on your car, the Jacobs Engine Brake (TM) is a "compression release" engine brake used by large diesel trucks, especially on steep downgrades. To understand how it works, remember that a diesel engine has much higher compression than a gasoline engine, typically 15:1. The jake brake slightly opens the exhaust valves when the piston is near top dead center (where ignition normally occurs). On the upstroke, the piston compresses the air in the cylinder to 1/15th its original volume. This creates a lot of drag on the engine. The Jacobs Engine Brake then releases the compressed air, and the energy stored in it, before it can push back on the piston during the downstroke. In addition, releasing the compression prevents any fuel in the cylinder from igniting. (Remember, diesels don't have spark plugs like gasoline engines - they rely on compression alone to ignite the fuel.) So, you've got drag on the upstroke, no power on the downstroke.

In short, the jake brake turns a power-producing engine into a power-absorbing air compressor, thus slowing the truck. The brake sits in a box over the engine. The trucker has a switch in the cab where she can choose how many cylinders to cut out; the more cylinders, the more powerful the slowing of the truck. This illustration from the Jacobs Engine Brake website may make the whole thing a little clearer.

A bit of history: In 1931, Clessie Cummins, founder of the Cummins Engine Company, drove across the United States to demonstrate off the viability of his new diesel engine. While descending Cajon pass in California (something that took real cajones in those days), the brakes failed on his truck and he and his pals barely survived the hairy ride. After that experience, Cummins devised the idea of an engine brake to supplement the use of wheel brakes on diesel trucks, especially for use on long downgrades.

Cummins developed the compression release engine brake in 1954 and shopped the idea around, but none of the major engine manufacturers were interested. His brother's son introduced him to the Jacobs Manufacturing Company, established in 1903 by A.I. Jacobs, makers of the world famous three jaw Jacobs Drill Chuck. The Jacobs company ran with the idea and marketed a successful line of compression release and other types of brakes. The firm split in 1986 and chuck manufacturing now takes place in Clemson, South Carolina, while engine brake producition remains in Bloomfield, Connecticut under the Jacobs Vehicle Systems name.

All of the brake systems made by Jacobs Vehicle Systems can accurately be called "jake brakes," not just their compression release engine brakes. They also produce the Jacobs Exhaust Brakes and Jacobs Driveline Brakes, both of which are nearly silent. The engine brakes are the ones that make that distinctive staccato sound, and if I could make a WAV, I'd include my imitation of it here.

The Jacobs company blames the loud noise you hear from passing trucks on the use of engine brakes in vehicles with poorly muffled or unmuffled exhaust systems (straight pipes, for example), exhaust systems that have been illegally modified or are poorly maintained, and/or truckers who simply enjoy making noise. Because of this, the Jacobs company feels that it's inaccurate, unfair, and maybe illegal to use their trademarked name in the generic sounding "NO JAKE BRAKES" signs, especially since some of their other jake brakes are quiet and noisy compression release engine brakes are made by other companies, too. These signs are often seen in residential areas adjacent to a highway. The federal government has required all vehicles manufactured since 1978 to meet noise requirements.

The Jacobs brake people go so far as to ask anyone seeing these "brand specific" signs to E-mail the location to them. If you see a "jake brake" sign, send the route number, city/town, and state where you saw it to jsupport@jakebrake.com.

Now ... who can tell me how to get the grease out from under my fingernails?

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